research paper about black hole

Black Holes

Black holes are some of the most fascinating and mind-bending objects in the cosmos. The very thing that characterizes a black hole also makes it hard to study: its intense gravity. All the mass in a black hole is concentrated in a tiny region, surrounded by a boundary called the “event horizon”. Nothing that crosses that boundary can return to the outside universe, not even light. A black hole itself is invisible.

But astronomers can still observe black holes indirectly by the way their gravity affects stars and pulls matter into orbit. As gas flows around a black hole, it heats up, paradoxically making these invisible objects into some of the brightest things in the entire universe. As a result, we can see some black holes from billions of light-years away. For one large black hole in a nearby galaxy, astronomers even managed to see a ring of light around the event horizon, using a globe-spanning array of powerful telescopes.

Center for Astrophysics | Harvard & Smithsonian scientists participate in many black hole-related projects:

Using the Event Horizon Telescope (EHT) to capture the first image of a black hole’s “shadow”: the absence of light that marks where the event horizon is located. The EHT is composed of many telescopes working together to create one Earth-sized observatory , all monitoring the supermassive black hole at the center of the galaxy M87, leading to the first image ever captured of a black hole. CfA Plays Central Role In Capturing Landmark Black Hole Image

Observing supermassive black holes in other galaxies to understand how they evolve and shape their host galaxies. CfA astronomers use telescopes across the entire spectrum of light, from radio waves to X-rays to gamma rays. A Surprising Blazar Connection Revealed

Studying the infall of matter — called “accretion” — onto black holes, using NASA’s Chandra X-ray Observatory and other telescopes. In addition, CfA researchers use cutting-edge supercomputers to create theoretical models for the disks and jets of matter that black holes create around themselves. Supermassive Black Hole Spins Super-Fast

Hunting for black hole interactions with other astronomical objects. That includes “disruption” events, where black holes tear stars or other objects apart, creating bursts of intense light. Black Hole Meal Sets Record for Length and Size

Observing clusters of stars to find intermediate mass black holes, and modeling how they shape their environments. A Middleweight Black Hole is Hiding at the Center of a Giant Star Cluster

Hunting for and characterizing stellar mass black holes, which can include information about their birth process and evolution. NASA's Chandra Adds to Black Hole Birth Announcement

The Varieties of Black Holes

Black holes come in three categories:

Stellar Mass Black Holes are born from the death of stars much more massive than the Sun. When some of these stars run out of the nuclear fuel that makes them shine, their cores collapse into black holes under their own gravity. Other stellar mass black holes form from the collision of neutron stars , such as the ones first detected by LIGO and Virgo in 2017. These are probably the most common black holes in the cosmos, but are hard to detect unless they have an ordinary star for a companion. When that happens, the black hole can strip material from the star, causing the gas to heat up and glow brightly in X-rays.

Supermassive Black Holes are the monsters of the universe, living at the centers of nearly every galaxy. They range in mass from 100,000 to billions of times the mass of the Sun, far too massive to be born from a single star. The Milky Way’s black hole is about 4 million times the Sun’s mass, putting it in the middle of the pack. In the form of quasars and other “active” galaxies , these black holes can shine brightly enough to be seen from billions of light-years away. Understanding when these black holes formed and how they grow is a major area of research. Center for Astrophysics | Harvard & Smithsonian scientists are part of the Event Horizon Telescope (EHT) collaboration, which captured the first-ever image of the black hole: the supermassive black hole at the center of the galaxy M87.

Intermediate Mass Black Holes are the most mysterious, since we’ve hardly seen any of them yet. They weigh 100 to 10,000 times the mass of the Sun, putting them between stellar and supermassive black holes. We don’t know exactly how many of these are, and like supermassive black holes, we don’t fully understand how they’re born or grow. However, studying them could tell us a lot about how the most supermassive black holes came to be.

Black holes can seem bizarre and incomprehensible, but in truth they’re remarkably understandable. Despite not being able to see black holes directly, we know quite a bit about them. They are …

Simple . All three black hole types can be described by just two observable quantities: their mass and how fast they spin. That’s much simpler than a star, for example, which in addition to mass is a product of its unique history and evolution , including its chemical makeup. Mass and spin tell us everything we need to know about a black hole: it “forgets” everything that went into making it. Those two quantities determine how big the event horizon is, and the way gravity affects any matter falling onto the black hole.

Compact . Black holes are tiny compared to their mass. The event horizon of a black hole the mass of the Sun would be no more than 6 kilometers across, and the faster it spins, the smaller that size is. Even a supermassive black hole would fit easily inside our Solar System.

Powerful . The combination of large mass and small size results in very strong gravity. This gravity is strong enough to pull a star apart if it gets too close, producing powerful bursts of light. A supermassive black hole heats gas falling onto it to temperatures of millions of degrees, making it glow brightly enough in X-rays and other types of radiation to be seen across the universe.

Very common . From theoretical calculations based on observations, astronomers think the Milky Way might have as many as a hundred million black holes, most of which are stellar mass. And with at least one supermassive black hole in most galaxies, there could be hundreds of billions of supermassive black holes in the observable universe.

Very important . Black holes have a reputation for eating everything that comes by, but they turn out to be messy eaters. A lot of stuff that falls toward a black hole gets jetted away, thanks to the complicated churning of gas near the event horizon. These jets and outflows of gas called “winds” spread atoms throughout the galaxy, and can either boost or throttle the birth of new stars, depending on other factors. That means supermassive black holes play an important role in the life of galaxies, even far beyond the black hole’s gravitational pull.

And yes, mysterious . Along with astronomers, physicists are interested in black holes because they’re a laboratory for “quantum gravity”. Black holes are described by Albert Einstein’s general relativity, which is our modern theory of gravity, but the other forces of nature are described by quantum physics. So far, nobody has developed a complete quantum gravity theory, but we already know black holes will be an important test of any proposed theory.

The first image of a black hole in human history, captured by the Event Horizon Telescope, showing light emitted by matter as it swirls under the influence of intense gravity. This black hole is 6.5 billion times the mass of the Sun and resides at the center of the galaxy M87.

• What do black holes look like?
• What happens to space time when cosmic objects collide?
• The Energetic Universe
• The Milky Way Galaxy
• Extragalactic Astronomy
• Stellar Astronomy
• Theoretical Astrophysics
• Einstein's Theory of Gravitation
• Radio and Geoastronomy

Related News

Astronomers unveil strong magnetic fields spiraling at the edge of milky way’s central black hole, black hole fashions stellar beads on a string, m87* one year later: proof of a persistent black hole shadow, unexpectedly massive black holes dominate small galaxies in the distant universe, unveiling black hole spins using polarized radio glasses, a supermassive black hole’s strong magnetic fields are revealed in a new light, nasa telescopes discover record-breaking black hole, new horizons in physics breakthrough prize awarded to cfa astrophysicist, cfa selects contractor for next generation event horizon telescope antennas, sheperd doeleman awarded the 2023 georges lemaître international prize, dasch (digital access to a sky century @ harvard), sensing the dynamic universe, champ (chandra multiwavelength project) and champlane (chandra multiwavelength plane) survey, telescopes and instruments, einstein observatory, event horizon telescope (eht), large aperture experiment to explore the dark ages (leda), the greenland telescope, very energetic radiation imaging telescope array system (veritas).

Black Holes

Black holes are some of the most fascinating and mind-bending objects in the cosmos. The very thing that characterizes a black hole also makes it hard to study: its intense gravity. All the mass in a black hole is concentrated in a tiny region, surrounded by a boundary called the “event horizon”. Nothing that crosses that boundary can return to the outside universe, not even light. A black hole itself is invisible.

But astronomers can still observe black holes indirectly by the way their gravity affects stars and pulls matter into orbit. As gas flows around a black hole, it heats up, paradoxically making these invisible objects into some of the brightest things in the entire universe. As a result, we can see some black holes from billions of light-years away. For one large black hole in a nearby galaxy, astronomers even managed to see a ring of light around the event horizon, using a globe-spanning array of powerful telescopes.

Center for Astrophysics | Harvard & Smithsonian scientists participate in many black hole-related projects:

Using the Event Horizon Telescope (EHT) to capture the first image of a black hole’s “shadow”: the absence of light that marks where the event horizon is located. The EHT is composed of many telescopes working together to create one Earth-sized observatory , all monitoring the supermassive black hole at the center of the galaxy M87, leading to the first image ever captured of a black hole. CfA Plays Central Role In Capturing Landmark Black Hole Image

Observing supermassive black holes in other galaxies to understand how they evolve and shape their host galaxies. CfA astronomers use telescopes across the entire spectrum of light, from radio waves to X-rays to gamma rays. A Surprising Blazar Connection Revealed

Studying the infall of matter — called “accretion” — onto black holes, using NASA’s Chandra X-ray Observatory and other telescopes. In addition, CfA researchers use cutting-edge supercomputers to create theoretical models for the disks and jets of matter that black holes create around themselves. Supermassive Black Hole Spins Super-Fast

Hunting for black hole interactions with other astronomical objects. That includes “disruption” events, where black holes tear stars or other objects apart, creating bursts of intense light. Black Hole Meal Sets Record for Length and Size

Observing clusters of stars to find intermediate mass black holes, and modeling how they shape their environments. A Middleweight Black Hole is Hiding at the Center of a Giant Star Cluster

Hunting for and characterizing stellar mass black holes, which can include information about their birth process and evolution. NASA's Chandra Adds to Black Hole Birth Announcement

The Varieties of Black Holes

Black holes come in three categories:

Stellar Mass Black Holes are born from the death of stars much more massive than the Sun. When some of these stars run out of the nuclear fuel that makes them shine, their cores collapse into black holes under their own gravity. Other stellar mass black holes form from the collision of neutron stars , such as the ones first detected by LIGO and Virgo in 2017. These are probably the most common black holes in the cosmos, but are hard to detect unless they have an ordinary star for a companion. When that happens, the black hole can strip material from the star, causing the gas to heat up and glow brightly in X-rays.

Supermassive Black Holes are the monsters of the universe, living at the centers of nearly every galaxy. They range in mass from 100,000 to billions of times the mass of the Sun, far too massive to be born from a single star. The Milky Way’s black hole is about 4 million times the Sun’s mass, putting it in the middle of the pack. In the form of quasars and other “active” galaxies , these black holes can shine brightly enough to be seen from billions of light-years away. Understanding when these black holes formed and how they grow is a major area of research. Center for Astrophysics | Harvard & Smithsonian scientists are part of the Event Horizon Telescope (EHT) collaboration, which captured the first-ever image of the black hole: the supermassive black hole at the center of the galaxy M87.

Intermediate Mass Black Holes are the most mysterious, since we’ve hardly seen any of them yet. They weigh 100 to 10,000 times the mass of the Sun, putting them between stellar and supermassive black holes. We don’t know exactly how many of these are, and like supermassive black holes, we don’t fully understand how they’re born or grow. However, studying them could tell us a lot about how the most supermassive black holes came to be.

Black holes can seem bizarre and incomprehensible, but in truth they’re remarkably understandable. Despite not being able to see black holes directly, we know quite a bit about them. They are …

Simple . All three black hole types can be described by just two observable quantities: their mass and how fast they spin. That’s much simpler than a star, for example, which in addition to mass is a product of its unique history and evolution , including its chemical makeup. Mass and spin tell us everything we need to know about a black hole: it “forgets” everything that went into making it. Those two quantities determine how big the event horizon is, and the way gravity affects any matter falling onto the black hole.

Compact . Black holes are tiny compared to their mass. The event horizon of a black hole the mass of the Sun would be no more than 6 kilometers across, and the faster it spins, the smaller that size is. Even a supermassive black hole would fit easily inside our Solar System.

Powerful . The combination of large mass and small size results in very strong gravity. This gravity is strong enough to pull a star apart if it gets too close, producing powerful bursts of light. A supermassive black hole heats gas falling onto it to temperatures of millions of degrees, making it glow brightly enough in X-rays and other types of radiation to be seen across the universe.

Very common . From theoretical calculations based on observations, astronomers think the Milky Way might have as many as a hundred million black holes, most of which are stellar mass. And with at least one supermassive black hole in most galaxies, there could be hundreds of billions of supermassive black holes in the observable universe.

Very important . Black holes have a reputation for eating everything that comes by, but they turn out to be messy eaters. A lot of stuff that falls toward a black hole gets jetted away, thanks to the complicated churning of gas near the event horizon. These jets and outflows of gas called “winds” spread atoms throughout the galaxy, and can either boost or throttle the birth of new stars, depending on other factors. That means supermassive black holes play an important role in the life of galaxies, even far beyond the black hole’s gravitational pull.

And yes, mysterious . Along with astronomers, physicists are interested in black holes because they’re a laboratory for “quantum gravity”. Black holes are described by Albert Einstein’s general relativity, which is our modern theory of gravity, but the other forces of nature are described by quantum physics. So far, nobody has developed a complete quantum gravity theory, but we already know black holes will be an important test of any proposed theory.

The first image of a black hole in human history, captured by the Event Horizon Telescope, showing light emitted by matter as it swirls under the influence of intense gravity. This black hole is 6.5 billion times the mass of the Sun and resides at the center of the galaxy M87.

• What do black holes look like?
• What happens to space time when cosmic objects collide?
• The Energetic Universe
• The Milky Way Galaxy
• Extragalactic Astronomy
• Stellar Astronomy
• Theoretical Astrophysics
• Einstein's Theory of Gravitation
• Radio and Geoastronomy

Related News

Astronomers unveil strong magnetic fields spiraling at the edge of milky way’s central black hole, black hole fashions stellar beads on a string, m87* one year later: proof of a persistent black hole shadow, unexpectedly massive black holes dominate small galaxies in the distant universe, unveiling black hole spins using polarized radio glasses, a supermassive black hole’s strong magnetic fields are revealed in a new light, nasa telescopes discover record-breaking black hole, new horizons in physics breakthrough prize awarded to cfa astrophysicist, cfa selects contractor for next generation event horizon telescope antennas, sheperd doeleman awarded the 2023 georges lemaître international prize, dasch (digital access to a sky century @ harvard), sensing the dynamic universe, champ (chandra multiwavelength project) and champlane (chandra multiwavelength plane) survey, telescopes and instruments, einstein observatory, event horizon telescope (eht), large aperture experiment to explore the dark ages (leda), the greenland telescope, very energetic radiation imaging telescope array system (veritas).

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Insight  08 January 2019

Black Holes

Black holes were theoretically established in 1915, shortly after Albert Einstein published his theory of General Relativity. Since then, observations have confirmed black holes as actual astrophysical objects. In this Insight we publish ten long- and short-format pieces (in the “Reviews & Perspectives” and “Views & Comments” tabs, respectively) discussing key aspects of black holes, from their masses, to their spins, to the ways in which they impact their surroundings and are studied. This collection also showcases some of the black hole-related content that Nature Astronomy has published since our launch (in the “Primary research” and “Further reading” tabs). Also, please view our  previous collection  of landmark black hole discoveries published in  Nature and other Springer Nature journals.

Reviews & Perspectives

Observing black holes spin

Current black hole spin measurements, in X-rays, radio and gravitational waves, are already constraining models for the growth of black holes, the dynamics of stellar core-collapse and the physics of relativistic jet production.

• Christopher S. Reynolds

Observational constraints on the feeding of supermassive black holes

Supermassive black holes are fed through galaxy interactions and mergers, chaotic cold accretion in galaxy clusters and secular processes that may include stellar bars. Observations constraining these mechanisms at different scales are reviewed.

• Thaisa Storchi-Bergmann
• Allan Schnorr-Müller

KAGRA: 2.5 generation interferometric gravitational wave detector

KAGRA is a new gravitational wave detector being built in Japan. Unlike LIGO/Virgo, it will operate at cryogenic temperatures with sapphire mirrors. KAGRA will help improve the localization of gravitational wave detections and determination of the source parameters.

• KAGRA collaboration

The many definitions of a black hole

Examining and comparing many of the definitions of a black hole, it is concluded that the profusion of different definitions is a virtue that makes the investigation of black holes possible and fruitful in many different kinds of problems.

• Erik Curiel

Views & Comment

Cosmic celebrities with gravitas.

Black holes have the distinct honour of being the most popular and potentially the least well-understood objects in the Universe. This issue’s Insight explores how far black hole research has come since its inception, though it still has a long way to go.

Black holes

Mitchell C. Begelman, Professor in the Department of Astrophysical and Planetary Sciences at the University of Colorado Boulder and a black hole expert, discusses the start of the field with Nature Astronomy .

• Marios Karouzos

Dwarf galaxies might not be the birth sites of supermassive black holes

Intermediate-mass black holes (BHs) in local dwarf galaxies are considered the relics of the early seed BHs. However, their growth might have been impacted by galaxy mergers and BH feedback so that they cannot be treated as tracers of the early seed BH population.

Black hole masses in active galactic nuclei

The masses of supermassive black holes, key to many cosmological studies, are highly uncertain beyond our local Universe. The main challenge is to establish the spatial and kinematic structure of the broad-line emitting gas in active galactic nuclei.

• Marianne Vestergaard

Probing supermassive black hole binaries with pulsar timing

The detection of a gravitational-wave background at nanohertz frequencies can tell us if and how supermassive black holes merge, and inform our knowledge of galaxy merger rates and supermassive black hole masses. All we have to do is time pulsars.

• Chiara M. F. Mingarelli

MUSEings of the highest quality

Primary research.

Observational signatures of massive black hole formation in the early Universe

The James Webb Space Telescope may detect and distinguish a young galaxy that hosts a direct-collapse black hole and nearby massive metal-free star formation at redshift 15 with as little as a 20,000-second total exposure time across four filters.

• Kirk S. S. Barrow
• Aycin Aykutalp
• John H. Wise

The local nanohertz gravitational-wave landscape from supermassive black hole binaries

We calculate the continuous nanohertz gravitational-wave emission from individual supermassive black hole binaries and the gravitational-wave background they generate, which will be observable with pulsar timing arrays.

• T. Joseph W. Lazio
• Stephen R. Taylor

A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674

A candidate dual supermassive black hole system with a projected separation of 0.35 pc is found in the gas-rich interacting spiral galaxy NGC 7674, evidenced by a ∼0.7 kpc Z-shaped radio jet and two, possibly inverted-spectrum, compact radio cores.

Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry

The X-ray polarization properties of the black hole binary Cygnus X-1 in its hard state, combined with spectral and timing data, reveals that the accretion disk corona is either an extended structure or located far from the black hole.

• H.-G. Florén

A luminous X-ray outburst from an intermediate-mass black hole in an off-centre star cluster

A bright X-ray outburst from a massive star cluster 12.5 kpc from a galactic centre fits the profile of a tidal disruption event (TDE), indicating the likely presence of an intermediate-mass black hole (IMBH). TDEs could be the most effective way of identifying IMBHs.

• Dacheng Lin
• Jay Strader
• Stephen D. J. Gwyn

The current ability to test theories of gravity with black hole shadows

The authors predict the ability of the Event Horizon Telescope (in its 2017 campaign) to distinguish between different theories of gravity based on images of Sagittarius A*; they suggest that it will not be possible.

• Yosuke Mizuno
• Ziri Younsi
• Luciano Rezzolla

A wide and collimated radio jet in 3C84 on the scale of a few hundred gravitational radii

Interferometric observations of 3C84 reveal a broad cylindrical jet a few hundred gravitational radii from the black hole, implying that the jet either undergoes a rapid lateral expansion on even smaller scales or is launched from the accretion disk.

• G. Giovannini
• T. Savolainen

Reconfinement and loss of stability in jets from active galactic nuclei

During reconfinement in unmagnetized relativistic jets, a centrifugal instability develops that leads to a turbulent state. This instability likely lies behind the division of active galactic nuclei jets into the two Fanaroff–Riley classes.

• Konstantinos N. Gourgouliatos
• Serguei S. Komissarov

The effect of nuclear gas distribution on the mass determination of supermassive black holes

Black hole masses derived from the properties of the accretion disk and virial mass estimates differ by a factor that is inversely proportional to the width of the broad emission lines. An inclined planar gas distribution may account for this effect.

• J. E. Mejía-Restrepo
• D. M. Capellupo

A population of highly energetic transient events in the centres of active galaxies

The discovery of the most energetic transient event to date is reported. Its spectroscopic properties and temporal evolution imply it is powered by shock interaction between expanding material and large quantities of surrounding dense matter.

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

A delay between rapid optical and X-ray flux variations from an accreting black-hole binary is reported together with a brightening radio jet, indicating a characteristic elevation of the radiative jet base of 0.1 light-seconds above the black hole.

• M. Bachetti

Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way

A candidate intermediate-mass black hole is reported within a molecular cloud near Sgr A*, the centre of our Galaxy. High-resolution observations with ALMA reveal extreme gas kinematics and a compact source consistent with a quiescent black hole.

• Tomoharu Oka
• Shiho Tsujimoto
• Shunya Takekawa

A universal minimal mass scale for present-day central black holes

Low-mass black holes that accrete stars from locally dense environments grow over the Hubble time above a minimal mass of 10 5 solar masses, independently of their initial mass. This explains why there are no convincing cases of intermediate-mass black holes to date.

• Tal Alexander

Rapid formation of massive black holes in close proximity to embryonic protogalaxies

The key ingredients for a massive cloud of gas to collapse and directly form a black hole without fragmenting and forming stars are a strong ionizing background emission and a closely timed burst of star formation in its vicinity.

• John A. Regan
• Greg L. Bryan

Magnetic origin of black hole winds across the mass scale

A magnetohydrodynamic model for outflows around supermassive black holes can also reproduce the X-ray properties of an outflow around a stellar black hole. This indicates that magnetic forces have a universal role to play in driving these winds.

• Keigo Fukumura
• Demosthenes Kazanas
• Ioannis Contopoulos

The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole

Transient object ASASSN-15lh was previously cast as the most luminous supernova ever discovered. Now, however, there is convincing evidence that its flare was a tidal disruption event: a rapidly-spinning black hole tearing apart a neighbouring star.

• G. Leloudas
• D. R. Young

Further Reading

Fourth time’s a XARM

Arguably, no mission changed X-ray astronomy in as short a time as did Hitomi. The planned X-ray Astronomy Recovery Mission, XARM, will carry its legacy forward.

• Poshak Gandhi

AGN outflows and feedback twenty years on

• C. M. Harrison

Tests for the existence of black holes through gravitational wave echoes

Black holes and spacetime singularities are fundamental in science. While observational proof for black holes is hard to come by, alternatives can be ruled out or confirmed to exist through precision gravitational wave observations.

• Vitor Cardoso

Nuclear obscuration in active galactic nuclei

The material surrounding accreting supermassive black holes connects them with their hosts. From studies in the infrared and X-rays, the structure of this material is found to be complex, clumpy and dynamic.

• Cristina Ramos Almeida
• Claudio Ricci

Archaeology of active galaxies across the electromagnetic spectrum

Quantifying the effect of active galactic nuclei (AGNs) on their hosts requires knowledge of their life cycle. This review on AGN archaeology summarizes the main recent findings regarding the AGN life cycle from optical and radio observations.

• Raffaella Morganti

Impact of supermassive black hole growth on star formation

Feedback from actively accreting supermassive black holes is thought to be important in the evolution of galaxies. Theoretical and observational results are reviewed with regard to the impact of this feedback on star formation in galaxies.

Interpreting radiative efficiency in radio-loud AGNs

Radiative efficiency in radio-loud active galactic nuclei is governed by the accretion rate onto the central black hole rather than directly by the type of accreted matter; while it correlates with real differences in host galaxies and environments, it does not provide unambiguous information about particular objects.

• Martin Hardcastle

Observing positive and negative AGN feedback

Galaxy-scale outflows powered by actively accreting supermassive black holes are routinely detected, and they have been associated with both the suppression and triggering of star formation. Recent observational evidence and simulations are favouring a delayed mechanism that connects outflows and star formation.

• Giovanni Cresci
• Roberto Maiolino

Questions and challenges of what powers galactic outflows in active galactic nuclei

Different mechanisms can drive outflows in active galactic nuclei (AGNs), but it is often unclear which mechanism dominates, if any. To quantify the impact of AGN feedback on galaxy evolution, the driving mechanism of outflows must be better understood.

• Dominika Wylezalek

The largely unconstrained multiphase nature of outflows in AGN host galaxies

Observations and simulations show that outflows in active galactic nuclei contain gas in different phases. To understand their true impact on galaxy evolution, we advocate consistent and unbiased investigation of these multiphase winds in large active galactic nuclei samples.

• Claudia Cicone
• Marcella Brusa
• Vincenzo Mainieri

On the two main classes of active galactic nuclei

Active galactic nuclei (AGNs) are empirically divided into ‘radio-loud’ and ‘radio-quiet’. These 50-year-old labels are obsolete, misleading and wrong. I argue that AGNs should be classified as ‘jetted’ and ‘non-jetted’ based on a physical difference — the presence (or lack) of strong relativistic jets.

• Paolo Padovani

Beyond the event horizon or altogether without it?

Millimetre-wavelength interferometry and gravitational-wave detectors currently provide the most stringent tests for the existence of cosmic black holes. Complementary measurements of magnetic fields near their event horizon would be decisive.

• Andrei Lobanov

Astronomical tests for quantum black hole structure

Black holes present a profound challenge to our current foundations of physics, and an exciting era of astronomy is just opening in which gravitational-wave observation and very-long-baseline interferometry may provide important hints about the new principles of physics needed.

• Steven B. Giddings

Planck stars as observational probes of quantum gravity

A phenomenon recently studied in theoretical physics may hold considerable interest for astronomers: the explosive decay of primordial black holes through quantum tunnelling. Their detection would be of major theoretical importance.

• Carlo Rovelli

Reality and myths of AGN feedback

Feedback from active galactic nuclei (AGNs) remains controversial despite its wide acceptance as necessary to regulate massive galaxy growth. Consequently, we held a workshop in October 2017, at Leiden’s Lorentz Center, to distinguish between the reality and myths of feedback.

• Bernd Husemann
• Chris M. Harrison

Seeing the unseeable

The Event Horizon Telescope, an Earth-sized interferometer, aims to capture an image of a black hole’s event horizon to test the theory of general relativity and probe accretion processes, explains project director Shep Doeleman.

• Sheperd S. Doeleman

A new window on black hole accretion

Recent polarization measurements of the stellar-mass black hole in Cygnus X-1 reveal an extended corona in the inner parts of the accretion flow and open the path for a new era in high-energy astrophysics.

• Julien Malzac

Jet precession in binary black holes

Supermassive binary black holes are thought to lie at the centres of merging galaxies. The blazar OJ 287 is the poster child of such systems, showing strong and periodic variability across the electromagnetic spectrum. A new study questions the physical origin of this variability.

• Zulema Abraham

Weighing supermassive black holes

A study suggests that the gas clouds in the vicinity of rapidly accreting supermassive black holes are distributed in a planar distribution, impacting the estimation of the mass of the black hole based on the motion of these clouds.

Tuning pulsars to listen in on massive galaxies

Orbiting supermassive black holes in the centres of nearby galaxies contribute to a gravitational-wave background over the whole sky. Networks of millisecond pulsars are sensitive to this signal. Creating maps of this background using information from known galaxies can help us to project when (and how) we may observe it.

• Leonidas A. Moustakas

How to light up a black hole

The first detection of electromagnetic emission from a gravitational wave source bridges the gap between one of the most energetic phenomena in the Universe and their dark, difficult to detect progenitors.

• Tyler Pritchard

Black Holes: The making of a monster

The biggest black holes in the Universe were in place soon after the Big Bang. Explaining how they formed so rapidly is a daunting challenge, but the latest simulations give clues to how this may have occurred.

• Lucio Mayer

Black holes: Crime in search of a crime scene

The destruction of stars by supermassive black holes appears to be rarer than predicted. A candidate stellar disruption in a kind of galaxy that is usually obscured may explain why.

• James Guillochon

Accretion physics: It's not U , it's B

Black holes grow by accreting mass, but the process is messy and redistributes gas and energy into their environments. New evidence shows that magnetic processes mediate both the accretion and ejection of matter.

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Exploring the Black Hole Population with an Open Mind

• Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Canada

Since the first groundbreaking observation of gravitational waves from a black hole merger [ 1 ], a worldwide network of observatories–LIGO, Virgo, and KAGRA—has discovered nearly a hundred mergers involving black holes and neutron stars (Fig. 1 ). The nature of this population of compact objects has implications for nearly every aspect of astrophysics and cosmology. However, understanding how gravitational-wave sources fit into our astrophysical theories has proved challenging. Many of the discoveries have confirmed our expectations, but some—such as those of asymmetric black hole binaries or of unexpectedly massive black holes—defy them. How can we understand new observations in the context of existing knowledge while remaining open to surprises that challenge our understanding? Thomas Callister of the University of Chicago and Will Farr of Stony Brook University, New York, offer a solution to this question: a “parameter-free” model for describing the black hole binary population [ 2 ]. Unlike most present-day models, their approach describes this population in an “agnostic” way—without assuming that the black hole masses, spins, and redshifts follow parametrized mathematical distributions derived from theoretical predictions. As such, the new model may enable the discovery of truly unexpected features of the black hole population.

Gravitational waves give us unprecedented insight into the life and death of stars, allowing access to some of the youngest, faintest galaxies in the Universe. They have revealed a population of black holes with masses between a few and 150 times the mass of the Sun. These black holes are likely skeletons left over by the deaths of the Universe’s most massive stars in relatively chemically pristine (“low-metallicity”) pockets of the Universe [ 3 ]. These environments cannot be accessed with traditional astronomy observations, which typically probe big, chemically rich galaxies that have undergone periods of extensive star formation.

The masses, spins, and redshifts of black hole binaries encode information about how, where, and when the binaries were made. For example, “pileups” and “gaps” in the black hole mass distribution can be interpreted as resulting from exotic phenomena such as pulsational pair-instability supernovae [ 4 , 5 ]. The distribution of black hole spin orientations can be used to distinguish between scenarios in which two black holes spend their whole lives in a binary system and those in which they exchange companions multiple times before merging [ 6 ]. The redshift dependence of the merger rate probes how star formation varies across cosmic time and how long it takes for black holes to merge after the birth of their progenitor stars [ 7 ]. In these examples, the task might seem simple: extract physical insight by fitting the binary black hole mass, spin, and redshift distributions. In reality, this process is challenging because we lack complete, reliable first-principles models to analyze the data.

In the absence of these models, a common approach has been to fit the binary black hole population with simple, phenomenological models. Guided by theoretical predictions, gravitational-wave data analysts propose various features expected to describe the black hole population. They typically start by using a power law to describe the black hole mass distribution, because the mass distribution of the black holes’ stellar progenitors is often described as a power law. However, our understanding of supernovae suggests that the relationship between initial stellar mass and black hole mass is nonlinear. In particular, pulsational pair-instability supernovae should lead to a wide range of initial stellar masses collapsing to a narrow range of black hole masses. To account for this effect, the power law is usually combined with a Gaussian peak, whose location, height, and width are constrained by the data [ 5 ]. However, currently available data favor a Gaussian peak at lower masses than predicted from pair instability, casting doubts on the peak’s origin [ 8 ]. Should we then look for more Gaussian peaks? We could come up with countless combinations of power laws and Gaussians to throw at the data, but why stop at just power laws and Gaussians? Callister and Farr offer a natural solution through their flexible, nonparametric model.

The model fits the binary black hole merger rate as a function of mass, spin, and redshift through a so-called autoregressive process. Compared with parametric models, the autoregressive model only assumes that the merger rate is a continuous function. It is also more flexible than previous models (such as those based on “splines” or on a “binned Gaussian process”) that are dubbed nonparametric but are based on underlying parametrized functions describing a priori black hole distributions. The new model’s freedom allows any variety of features—including peaks, dips, and breaks in the black hole property distributions—to be recovered from the data, without biases deriving from expectations about these features. However, the results from a parameter-free model are much more complex to interpret than those from a parametric one. The model’s output is simply a bunch of squiggly lines that are not inherently meaningful.

To tackle this problem, Callister and Farr introduce statistical techniques to extract meaningful features from their nonparametric inference (Fig. 2 ). Some of the extracted features—such as a peak in the primary black hole mass distribution at 35 solar masses—had previously been identified by a serendipitous choice of parameterized model. Other features confirm hints delivered by previous nonparametric approaches [ 8 – 10 ]. Such features include a second peak at 10 solar masses and a merger rate that increases more steeply for large redshifts (between 0.4 and 1) than for redshifts corresponding to the “local Universe” (between 0 and 0.4). This finding could imply a nontrivial evolution of black hole progenitors and of their merger times.

Callister and Farr don’t envision that their flexible, nonparametric model will entirely replace parametric ones, but they propose combining the two approaches in an iterative process. Nonparametric models could first be applied to pinpoint possible new features, which parametric models could characterize and interpret. The results could then be rechecked and refined using nonparametric models as more observations become available. Ultimately, the features extracted from such a process could be used to place constraints on the underlying astrophysics. This approach holds great promise for strengthening the link between observations and theoretical predictions.

LIGO-Virgo-KAGRA’s ongoing observing run is expected to triple the sample of binary black hole merger events, and the next decade might bring thousands, if not millions, of binary black hole detections. At the same time, our theoretical models will improve as we better understand the physical processes behind the Universe’s production of binary black holes. With these prospects, we should stay open minded to surprises, and flexible models such as that proposed by Callister and Farr will help us explore “unknown unknowns” that might otherwise be missed.

• B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of gravitational waves from a binary black hole merger,” Phys. Rev. Lett. 116 , 061102 (2016) .
• T. A. Callister and W. M. Farr, “Parameter-free tour of the binary black hole population,” Phys. Rev. X 14 , 021005 (2024) .
• A. Heger et al. , “Black holes as the end state of stellar evolution: Theory and simulations,” The Encyclopedia of Cosmology (2023) .
• M. Fishbach and D. E. Holz, “Where are LIGO’s big black holes?” Astrophys. J. Lett. 851 (2017) .
• C. Talbot and E. Thrane, “Measuring the binary black hole mass spectrum with an astrophysically motivated parameterization,” Astrophys. J. Lett. 856 (2018) .
• S. Vitale et al. , “Use of gravitational waves to probe the formation channels of compact binaries,” Class. Quantum Grav. 34 (2017) .
• M. Chruślińska, “Chemical evolution of the Universe and its consequences for gravitational‐wave astrophysics,” Ann. Phys. 536 (2022) .
• R. Abbott et al. (LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration), “Population of merging compact binaries inferred using gravitational waves through GWTC-3,” Phys. Rev. X 13 (2023) .
• A. M. Farah et al. , “Things that might go bump in the night: Assessing structure in the binary black hole mass spectrum,” Astrophys. J. Lett. 955 (2023) .
• B. Edelman et al. , “Cover your basis: Comprehensive data-driven characterization of the binary black hole population,” Astrophys. J. Lett. 946 (2023) .

About the Author

Maya Fishbach is an assistant professor in the Canadian Institute for Theoretical Astrophysics at the University of Toronto. She is a gravitational-wave astrophysicist and member of the LIGO Scientific Collaboration. Fishbach uses gravitational waves to study black holes and neutron stars and their implications for stellar evolution, the growth of galaxies, and the cosmic expansion history.

Parameter-Free Tour of the Binary Black Hole Population

Thomas A. Callister and Will M. Farr

Phys. Rev. X 14 , 021005 (2024)

Published April 8, 2024

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Teachable Moments | April 19, 2019

How scientists captured the first image of a black hole.

By Ota Lutz

Find out how scientists created a virtual telescope as large as Earth itself to capture the first image of a black hole's silhouette.

Accomplishing what was previously thought to be impossible, a team of international astronomers has captured an image of a black hole’s silhouette.

Evidence of the existence of black holes – mysterious places in space where nothing, not even light, can escape – has existed for quite some time, and astronomers have long observed the effects on the surroundings of these phenomena. In the popular imagination, it was thought that capturing an image of a black hole was impossible because an image of something from which no light can escape would appear completely black. For scientists, the challenge was how, from thousands or even millions of light-years away, to capture an image of the hot, glowing gas falling into a black hole.

An ambitious team of international astronomers and computer scientists has managed to accomplish both. Working for well over a decade to achieve the feat, the team improved upon an existing radio astronomy technique for high-resolution imaging and used it to detect the silhouette of a black hole – outlined by the glowing gas that surrounds its event horizon, the precipice beyond which light cannot escape. Learning about these mysterious structures can help students understand gravity and the dynamic nature of our universe, all while sharpening their math skills.

How They Did It

Why it's important.

Though scientists had theorized they could image black holes by capturing their silhouettes against their glowing surroundings, the ability to image an object so distant still eluded them. A team formed to take on the challenge, creating a network of telescopes known as the Event Horizon Telescope, or the EHT. They set out to capture an image of a black hole by improving upon a technique that allows for the imaging of far-away objects, known as Very Long Baseline Interferometry, or VLBI.

Telescopes of all types are used to see distant objects. The larger the diameter, or aperture, of the telescope, the greater its ability to gather more light and the higher its resolution (or ability to image fine details). To see details in objects that are far away and appear small and dim from Earth, we need to gather as much light as possible with very high resolution, so we need to use a telescope with a large aperture.

That’s why the VLBI technique was essential to capturing the black hole image. VLBI works by creating an array of smaller telescopes that can be synchronized to focus on the same object at the same time and act as a giant virtual telescope. In some cases, the smaller telescopes are also an array of multiple telescopes. This technique has been used to track spacecraft and to image distant cosmic radio sources, such as quasars.

Making up one piece of the EHT array of telescopes, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile has 66 high-precision antennas. Image credit: NRAO/AUI/NSF | + Expand image

The aperture of a giant virtual telescope such as the Event Horizon Telescope is as large as the distance between the two farthest-apart telescope stations – for the EHT, those two stations are at the South Pole and in Spain, creating an aperture that’s nearly the same as the diameter of Earth. Each telescope in the array focuses on the target, in this case the black hole, and collects data from its location on Earth, providing a portion of the EHT’s full view. The more telescopes in the array that are widely spaced, the better the image resolution.

This video shows the global network of radio telescopes in the EHT array that performed observations of the black hole in the galaxy M87. Credit: C. Fromm and L. Rezzolla ( Goethe University Frankfurt )/ Black Hole Cam / EHT Collaboration | Watch on YouTube

To test VLBI for imaging a black hole and a number of computer algorithms for sorting and synchronizing data, the Event Horizon Telescope team decided on two targets, each offering unique challenges.

The closest supermassive black hole to Earth, Sagittarius A*, interested the team because it is in our galactic backyard – at the center of our Milky Way galaxy, 26,000 light-years (156 quadrillion miles) away. (An asterisk is the astronomical standard for denoting a black hole.) Though not the only black hole in our galaxy, it is the black hole that appears largest from Earth. But its location in the same galaxy as Earth meant the team would have to look through “pollution” caused by stars and dust to image it, meaning there would be more data to filter out when processing the image. Nevertheless, because of the black hole’s local interest and relatively large size, the EHT team chose Sagittarius A* as one of its two targets.

A close-up image of the core of the M87 galaxy, imaged by the Chandra X-ray Observatory. Image credit: NASA/CXC/Villanova University/J. Neilsen | + Expand image

This image from NASA's Hubble Space Telescope shows a jet of subatomic particles streaming from the center of M87*. Image credits: NASA and the Hubble Heritage Team (STScI/AURA) | + Expand image

The second target was the supermassive black hole M87*. One of the largest known supermassive black holes, M87* is located at the center of the gargantuan elliptical galaxy Messier 87, or M87, 53 million light-years (318 quintillion miles) away. Substantially more massive than Sagittarius A*, which contains 4 million solar masses, M87* contains 6.5 billion solar masses. One solar mass is equivalent to the mass of our Sun, approximately 2x10^30 kilograms. In addition to its size, M87* interested scientists because, unlike Sagittarius A*, it is an active black hole, with matter falling into it and spewing out in the form of jets of particles that are accelerated to velocities near the speed of light. But its distance made it even more of a challenge to capture than the relatively local Sagittarius A*. As described by Katie Bouman, a computer scientist with the EHT who led development of one of the algorithms used to sort telescope data during the processing of the historic image, it’s akin to capturing an image of an orange on the surface of the Moon.

By 2017, the EHT was a collaboration of eight sites around the world – and more have been added since then. Before the team could begin collecting data, they had to find a time when the weather was likely to be conducive to telescope viewing at every location. For M87*, the team tried for good weather in April 2017 and, of the 10 days chosen for observation, a whopping four days were clear at all eight sites!

Each telescope used for the EHT had to be highly synchronized with the others to within a fraction of a millimeter using an atomic clock locked onto a GPS time standard. This degree of precision makes the EHT capable of resolving objects about 4,000 times better than the Hubble Space Telescope. As each telescope acquired data from the target black hole, the digitized data and time stamp were recorded on computer disk media. Gathering data for four days around the world gave the team a substantial amount of data to process. The recorded media were then physically transported to a central location because the amount of data, around 5 petabytes, exceeds what the current internet speeds can handle. At this central location, data from all eight sites were synchronized using the time stamps and combined to create a composite set of images, revealing the never-before-seen silhouette of M87*’s event horizon. The team is also working on generating an image of Sagittarius A* from additional observations made by the EHT.

This zoom video starts with a view of the ALMA telescope array in Chile and zooms in on the heart of M87, showing successively more detailed observations and culminating in the first direct visual evidence of a supermassive black hole’s silhouette. Credit: ESO/L. Calçada, Digitized Sky Survey 2, ESA/Hubble, RadioAstron, De Gasperin et al., Kim et al., EHT Collaboration. Music: Niklas Falcke | Watch on YouTube

As more telescopes are added and the rotation of Earth is factored in, more of the image can be resolved, and we can expect future images to be higher resolution. But we might never have a complete picture, as Katie Bouman explains here (under “Imaging a Black Hole”).

To complement the EHT findings, several NASA spacecraft were part of a large effort to observe the black hole using different wavelengths of light. As part of this effort, NASA’s Chandra X-ray Observatory, Nuclear Spectroscopic Telescope Array (NuSTAR) and Neil Gehrels Swift Observatory space telescope missions – all designed to detect different varieties of X-ray light – turned their gaze to the M87 black hole around the same time as the EHT in April 2017. NASA’s Fermi Gamma-ray Space Telescope was also watching for changes in gamma-ray light from M87* during the EHT observations. If the EHT observed changes in the structure of the black hole’s environment, data from these missions and other telescopes could be used to help figure out what was going on.

Though NASA observations did not directly trace out the historic image, astronomers used data from Chandra and NuSTAR satellites to measure the X-ray brightness of M87*’s jet. Scientists used this information to compare their models of the jet and disk around the black hole with the EHT observations. Other insights may come as researchers continue to pore over these data.

Learning about mysterious structures in the universe provides insight into physics and allows us to test observation methods and theories, such as Einstein’s theory of general relativity. Massive objects deform spacetime in their vicinity, and although the theory of general relativity has directly been proven accurate for smaller-mass objects, such as Earth and the Sun, the theory has not yet been directly proven for black holes and other regions containing dense matter.

One of the main results of the EHT black hole imaging project is a more direct calculation of a black hole’s mass than ever before. Using the EHT, scientists were able to directly observe and measure the radius of M87*’s event horizon, or its Schwarzschild radius, and compute the black hole’s mass. That estimate was close to the one derived from a method that uses the motion of orbiting stars – thus validating it as a method of mass estimation.

The size and shape of a black hole, which depend on its mass and spin, can be predicted from general relativity equations. General relativity predicts that this silhouette would be roughly circular, but other theories of gravity predict slightly different shapes. The image of M87* shows a circular silhouette, thus lending credibility to Einstein’s theory of general relativity near black holes.

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. Image credit: ESO | + Expand image

The data also offer some insight into the formation and behavior of black hole structures, such as the accretion disk that feeds matter into the black hole and plasma jets that emanate from its center. Scientists have hypothesized about how an accretion disk forms, but they’ve never been able to test their theories with direct observation until now. Scientists are also curious about the mechanism by which some supermassive black holes emit enormous jets of particles traveling at near light-speed.

These questions and others will be answered as more data is acquired by the EHT and synthesized in computer algorithms. Be sure to stay tuned for that and the next expected image of a black hole – our Milky Way’s own Sagittarius A*.

Update: May 12, 2022 – Scientists have released the first image of Sagittarius A* captured by the Event Horizon Telescope. › Read more from Teachable Moments

Capture your students’ enthusiasm about black holes by challenging them to solve these standards-aligned math problems .

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TAGS: Black Hole , Teachable Moments , Science , K-12 Education , Teachers , Educators , Universe

Ota Lutz , K-12 Education Group Manager, NASA-JPL Education Office

Ota Lutz is the manager of the K-12 Education Group at NASA’s Jet Propulsion Laboratory. When she’s not writing new lessons or teaching, she’s probably cooking something delicious, volunteering in the community, or dreaming about where she will travel next.

April 1, 2007

12 min read

The Reluctant Father of Black Holes

Albert Einstein's equations of gravity are the foundation of the modern view of black holes; ironically, he used the equations in trying to prove these objects cannot exist

By Jeremy Bernstein

G reat science sometimes produces a legacy that outstrips not only the imagination of its practitioners but also their intentions. A case in point is the early development of the theory of black holes and, above all, the role played in it by Albert Einstein. In 1939 Einstein published a paper in the journal Annals of Mathematics with the daunting title On a Stationary System with Spherical Symmetry Consisting of Many Gravitating Masses. With it, Einstein sought to prove that black holes--celestial objects so dense that their gravity prevents even light from escaping--were impossible.

The irony is that, to make his case, he used his own general theory of relativity and gravitation, published in 1916--the very theory that is now used to argue that black holes are not only possible but, for many astronomical objects, inevitable. Indeed, a few months after Einstein's rejection of black holes appeared--and with no reference to it--J. Robert Oppenheimer and his student Hartland S. Snyder published a paper entitled On Continued Gravitational Contraction. That work used Einstein's general theory of relativity to show, for the first time in the context of modern physics, how black holes could form.

Perhaps even more ironically, the modern study of black holes, and more generally that of collapsing stars, builds on a completely different aspect of Einstein's legacy--namely, his invention of quantum-statistical mechanics. Without the effects predicted by quantum statistics, every astronomical object would eventually collapse into a black hole, yielding a universe that would bear no resemblance to the one we actually live in.

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Bose, Einstein and Statistics

EINSTEIN'S CREATION of quantum statistics was inspired by a letter he received in June 1924 from a then unknown young Indian physicist named Satyendra Nath Bose. Along with Bose's letter came a manuscript that had already been rejected by one British scientific publication. After reading the manuscript, Einstein translated it himself into German and arranged to have it published in the prestigious journal Zeitschrift fr Physik .

Why did Einstein think that this manuscript was so important? For two decades he had been struggling with the nature of electromagnetic radiation--especially the radiation trapped inside a heated container that attains the same temperature as its walls. At the start of the 20th century German physicist Max Planck had discovered the mathematical function that describes how the various wavelengths, or colors, of this black body radiation vary in intensity. It turns out that the form of this spectrum does not depend on the material of the container walls. Only the temperature of the radiation matters. (A striking example of black-body radiation is the photons left over from the big bang, in which case the entire universe is the container. The temperature of these photons has been measured at 2.726 0.002 kelvins.)

Somewhat serendipitously, Bose had worked out the statistical mechanics of black-body radiation--that is, he derived the Planck law from a mathematical, quantum-mechanical perspective. That outcome caught Einstein's attention. But being Einstein, he took the matter a step further. He used the same methods to examine the statistical mechanics of a gas of massive molecules obeying the same kinds of rules that Bose had used for the photons. He derived the analogue of the Planck law for this case and noticed something absolutely remarkable. If one cools the gas of particles obeying so-called Bose-Einstein statistics, then at a certain critical temperature all the molecules suddenly collect themselves into a degenerate, or single, state. That state is now known as Bose-Einstein condensation (although Bose had nothing to do with it).

An interesting example is a gas made up of the common isotope helium 4, whose nucleus consists of two protons and two neutrons. At a temperature of 2.18 kelvins, this gas turns into a liquid that has the most uncanny properties one can imagine, including frictionless ow (that is, superuidity). More than a decade ago U.S. researchers accomplished the difficult task of cooling other kinds of atoms to several billionths of a kelvin to achieve a Bose-Einstein condensate.

Not all the particles in nature, however, show this condensation. In 1925, just after Einstein published his papers on the condensation, Austrian-born physicist Wolfgang Pauli identified a second class of particles, which includes the electron, proton and neutron, that obey different properties. He found that no two such identical particles--two electrons, for example--can ever be in exactly the same quantum-mechanical state, a property that has since become known as the Pauli exclusion principle. In 1926 Enrico Fermi and P.A.M. Dirac invented the quantum statistics of these particles, making them the analogue of the Bose-Einstein statistics.

Because of the Pauli principle, the last thing in the world these particles want to do at low temperatures is to condense. In fact, they exhibit just the opposite tendency. If you compress, say, a gas of electrons, cooling it to very low temperatures and shrinking its volume, the electrons are forced to begin invading one another's space. But Pauli's principle forbids this, so they dart away from one another at speeds that can approach that of light. For electrons and the other Pauli particles, the pressure created by these eeing particles--the degeneracy pressure--persists even if the gas is cooled to absolute zero. It has nothing to do with the fact that the electrons repel one another electrically. Neutrons, which have no charge, do the same thing. It is pure quantum physics.

Quantum Statistics and White Dwarfs

BUT WHAT HAS quantum statistics got to do with the stars? Before the turn of the century, astronomers had begun to identify a class of peculiar stars that are small and dim: white dwarfs. The one that accompanies Sirius, the brightest star in the heavens, has the mass of the sun but emits about 1/360 the light. Given their mass and size, white dwarfs must be humongously dense. Sirius's companion is some 61,000 times denser than water. What are these bizarre objects? Enter Sir Arthur Eddington.

When I began studying physics in the late 1940s, Eddington was a hero of mine but for the wrong reasons. I knew nothing about his great work in astronomy. I admired his popular books (which, since I have learned more about physics, now seem rather silly to me). Eddington, who died in 1944, was a neo-Kantian who believed that everything of significance about the universe could be learned by examining what went on inside one's head. But starting in the late 1910s, when Eddington led one of the two expeditions that confirmed Einstein's prediction that the sun bends starlight, until the late 1930s, when Eddington really started going off the deep end, he was truly one of the giants of 20th-century science. He practically created the discipline that led to the first understanding of the internal constitution of stars, the title of his classic 1926 book. To him, white dwarfs were an affront, at least from an aesthetic point of view. But he studied them nonetheless and came up with a liberating idea.

In 1924 Eddington proposed that the gravitational pressure that was squeezing a dwarf might strip some of the electrons off protons. The atoms would then lose their boundaries and might be squeezed together into a small, dense package. The dwarf would eventually stop collapsing because of the Fermi-Dirac degeneracy pressure--that is, when the Pauli exclusion principle forced the electrons to recoil from one another.

The understanding of white dwarfs took another step forward in July 1930, when Subrahmanyan Chandrasekhar, who was 19, was onboard a ship sailing from Madras to Southampton. He had been accepted by British physicist R. H. Fowler to study with him at the University of Cambridge (where Eddington was, too). Having read Eddington's book on the stars and Fowler's book on quantum-statistical mechanics, Chandrasekhar had become fascinated by white dwarfs. To pass the time during the voyage, Chandrasekhar asked himself: Is there any upper limit to how massive a white dwarf can be before it collapses under the force of its own gravitation? His answer set off a revolution.

A white dwarf as a whole is electrically neutral, so all the electrons must have a corresponding proton, which is some 2,000 times more massive. Consequently, protons must supply the bulk of the gravitational compression. If the dwarf is not collapsing, the degeneracy pressure of the electrons and the gravitational collapse of the protons must just balance. This balance, it turns out, limits the number of protons and hence the mass of the dwarf. This maximum is known as the Chandrasekhar limit and equals about 1.4 times the mass of the sun. Any dwarf more massive than this number cannot be stable.

Chandrasekhar's result deeply disturbed Eddington. What happens if the mass is more than 1.4 times that of the sun? He was not pleased with the answer. Unless some mechanism could be found for limiting the mass of any star that was eventually going to compress itself into a dwarf, or unless Chandrasekhar's result was wrong, massive stars were fated to collapse gravitationally into oblivion.

Eddington found this intolerable and proceeded to attack Chandrasekhar's use of quantum statistics--both publicly and privately. The criticism devastated Chandrasekhar. But he held his ground, bolstered by people such as Danish physicist Niels Bohr, who assured him that Eddington was simply wrong and should be ignored.

A Singular Sensation

AS RESEARCHERS explored quantum statistics and white dwarfs, others tackled Einstein's work on gravitation, his general theory of relativity. As far as I know, Einstein never spent a great deal of time looking for exact solutions to his gravitational equations. The part that described gravity around matter was extremely complicated, because gravity distorts the geometry of space and time, causing a particle to move from point to point along a curved path. More important to Einstein, the source of gravity--matter--could not be described by the gravitational equations alone. It had to be put in by hand, leaving Einstein to feel the equations were incomplete. Still, approximate solutions could describe with sufficient accuracy phenomena such as the bending of starlight. Nevertheless, he was impressed when, in 1916, German astronomer Karl Schwarzschild came up with an exact solution for a realistic situation--in particular, the case of a planet orbiting a star.

In the process, Schwarzschild found something disturbing. There is a distance from the center of the star at which the mathematics goes berserk. At this distance, now called the Schwarzschild radius, time vanishes, and space becomes infinite. The equation becomes what mathematicians call singular. The Schwarzschild radius is usually much smaller than the radius of the object. For the sun, for example, it is three kilometers, whereas for a one-gram marble it is 10 28 centimeter.

Schwarzschild was, of course, aware that his formula went crazy at this radius, but he decided that it did not matter. He constructed a simplified model of a star and showed that it would take an infinite gradient of pressure to compress it to his radius. The finding, he argued, served no practical interest.

But his analysis did not appease everybody. It bothered Einstein, because Schwarzschild's model star did not satisfy certain technical requirements of relativity theory. Various people, however, showed that one could rewrite Schwarzschild's solutions so that they avoided the singularity. But was the result really nonsingular? It would be incorrect to say that a debate raged, because most physicists had little regard for these matters--at least until 1939.

To make his point, Einstein focused on a collection of small particles moving in circular orbits under the inuence of one another's gravitation--in effect, a system resembling a spherical star cluster. He then asked whether such a configuration could collapse under its own gravity into a stable star with a radius equal to its Schwarzschild radius. He concluded that it could not, because at a somewhat larger radius the stars in the cluster would have to move faster than light in order to keep the configuration stable. Although Einstein's reasoning is correct, his point is irrelevant: it does not matter that a collapsing star at the Schwarzschild radius is unstable, because the star collapses past that radius anyway. I was much taken by the fact that the then 60-year-old Einstein presents in this paper tables of numerical results, which he must have gotten by using a slide rule. But the paper, like the slide rule, is now a historical artifact.

From Neutrons to Black Holes

WHILE EINSTEIN was doing this research, an entirely different enterprise was unfolding in California. Oppenheimer and his students were creating the modern theory of black holes. The curious thing about the black hole research is that it was inspired by an idea that turned out to be entirely wrong. In 1932 British experimental physicist James Chadwick found the neutron, the neutral component of the atomic nucleus. Soon thereafter speculation began--most notably by Fritz Zwicky of the California Institute of Technology and independently by the brilliant Soviet theoretical physicist Lev D. Landau--that neutrons could lead to an alternative to white dwarfs.

When the gravitational pressure got large enough, they argued, an electron in a star could react with a proton to produce a neutron. (Zwicky even conjectured that this process would happen in supernova explosions; he was right, and these neutron stars we now identify as pulsars.) At the time of this work, the actual mechanism for generating the energy in ordinary stars was not known. One solution placed a neutron star at the center of ordinary stars, in somewhat the same spirit that many astrophysicists now conjecture that black holes power quasars.

The question then arose: What was the equivalent of the Chandrasekhar mass limit for these stars? Determining this answer is much harder than finding the limit for white dwarfs. The reason is that the neutrons interact with one another with a strong force whose specifics we still do not fully understand. Gravity will eventually overcome this force, but the precise limiting mass is sensitive to the details. Oppenheimer published two papers on this subject with his students Robert Serber and George M. Volkoff and concluded that the mass limit here is comparable to the Chandrasekhar limit for white dwarfs. The first of these papers was published in 1938 and the second in 1939. (The real source of stellar energy--fusion--was discovered in 1938 by Hans Bethe and Carl Friedrich von Weizscker, but it took a few years to be accepted, and so astrophysicists continued to pursue alternative theories.)

To simplify matters, Oppenheimer told Snyder to make certain assumptions and to neglect technical considerations such as the degeneracy pressure or the possible rotation of the star. Oppenheimer's intuition told him that these factors would not change anything essential. (These assumptions were challenged many years later by a new generation of researchers using sophisticated high-speed computers--poor Snyder had an old-fashioned mechanical desk calculator--but Oppenheimer was right. Nothing essential changes.) With the simplified assumptions, Snyder found out that what happens to a collapsing star depends dramatically on the vantage point of the observer.

Two Views of a Collapse

LET US START with an observer at rest a safe distance from the star. Let us also suppose that there is another observer attached to the surface of the star--co-moving with its collapse--who can send light signals back to his stationary colleague. The stationary observer will see the signals from his moving counterpart gradually shift to the red end of the electromagnetic spectrum. If the frequency of the signals is thought of as a clock, the stationary observer will say that the moving observer's clock is gradually slowing down.

Indeed, at the Schwarzschild radius the clock will slow down to zero. The stationary observer will argue that it took an infinite amount of time for the star to collapse to its Schwarzschild radius. What happens after that we cannot say, because, according to the stationary observer, there is no after. As far as this observer is concerned, the star is frozen at its Schwarzschild radius.

Indeed, until December 1967, when physicist John A. Wheeler of Princeton University coined the name black hole in a lecture he presented, these objects were often referred to in the literature as frozen stars. This frozen state is the real significance of the singularity in the Schwarzschild geometry. As Oppenheimer and Snyder observed in their paper, the collapsing star tends to close itself off from any communication with a distant observer; only its gravitational field persists. In other words, a black hole has been formed.

But what about observers riding with collapsing stars? These observers, Oppenheimer and Snyder pointed out, have a completely different sense of things. To them, the Schwarzschild radius has no special significance. They pass right through it and on to the center in a matter of hours, as measured by their watches. They would, however, be subject to monstrous tidal gravitational forces that would tear them to pieces.

The year was 1939, and the world itself was about to be torn to pieces. Oppenheimer was soon to go off to war to build the most destructive weapon ever devised by humans. He never worked on the subject of black holes again. As far as I know, Einstein never did, either. In peacetime, in 1947, Oppenheimer became the director of the Institute for Advanced Study in Princeton, N.J., where Einstein was a professor. From time to time they talked. There is no record of their ever having discussed black holes. Further progress would have to wait until the 1960s, when discoveries of quasars, pulsars and compact x-ray sources reinvigorated thinking about the mysterious fate of stars.

JEREMY BERNSTEIN is professor emeritus of physics at the Stevens Institute of Technology in Hoboken, N.J. He was a staff writer for the New Yorker from 1961 to 1995 and is the recipient of many science writing awards. He is a former adjunct professor at the Rockefeller University and a vice president of the board of trustees of the Aspen Center for Physics, of which he is now an honorary trustee. Bernstein has written 12 books on popular science and mountain travel. This article is adapted from his collection of essays, A Theory for Everything , published by Copernicus Books in 1996.

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Scientists find first evidence that black holes are the source of dark energy

by RAL Space

Observations of supermassive black holes at the centers of galaxies point to a likely source of dark energy—the 'missing' 70% of the universe.

The measurements from ancient and dormant galaxies show black holes growing more than expected, aligning with a phenomenon predicted in Einstein's theory of gravity. The result potentially means nothing new has to be added to our picture of the universe to account for dark energy: black holes combined with Einstein's gravity are the source.

The conclusion was reached by a team of 17 researchers in nine countries, led by the University of Hawai'i and including Imperial College London and STFC RAL Space physicists. The work is published in two papers in the journals The Astrophysical Journal and The Astrophysical Journal Letters .

Study co-author Dr. Dave Clements, from the Department of Physics at Imperial, said, "This is a really surprising result. We started off looking at how black holes grow over time, and may have found the answer to one of the biggest problems in cosmology."

Study co-author Dr. Chris Pearson, from STFC RAL Space, said, "If the theory holds, then this is going to revolutionize the whole of cosmology, because at last we've got a solution for the origin of dark energy that's been perplexing cosmologists and theoretical physicists for more than 20 years."

Gravity versus dark energy

In the 1990s, it was discovered that the expansion of the universe is accelerating—everything is moving away from everything else at a faster and faster rate. This is difficult to explain—the pull of gravity between all objects in the universe should be slowing the expansion down.

To account for this, it was proposed that a 'dark energy' was responsible for pushing things apart more strongly than gravity. This was linked to a concept Einstein had proposed but later discarded—a ' cosmological constant ' that opposed gravity and kept the universe from collapsing.

This concept was revived with the discovery of the accelerating expansion of the universe, with its main component being a kind of energy included in spacetime itself, called vacuum energy. This energy pushes the universe further apart, accelerating the expansion.

Black holes posed a problem though—their extremely strong gravity is hard to oppose, especially at their centers, where everything seems to break down in a phenomenon called a 'singularity.'

The new result shows that black holes gain mass in a way consistent with them containing vacuum energy, providing a source of dark energy and removing the need for singularities to form at their center.

Black hole growing pains

The conclusion was made by studying nine billion years of black hole evolution. Black holes are formed when massive stars come to the end of their life. When found at the centers of galaxies, they are called supermassive black holes . These contain millions to billions of times the mass of our Sun inside them in a comparatively small space, creating extremely strong gravity.

Black holes can increase in size by accreting matter, such as by swallowing stars that get too close, or by merging with other black holes. To discover whether these effects alone could account for the growth of supermassive black holes, the team looked at data spanning nine billion years.

The researchers looked at a particular type of galaxy called giant elliptical galaxies, which evolved early in the universe and then became dormant. Dormant galaxies have finished forming stars, leaving little material for the black hole at their center to accrete, meaning any further growth cannot be explained by these normal astrophysical processes.

Comparing observations of distant galaxies (when they were young) with local elliptical galaxies (which are old and dead) showed growth much larger than predicted by accretion or mergers: the black holes of today are 7—20 times larger than they were nine billion years ago.

Cosmological coupling

Further measurements with related populations of galaxies at different points in the universe's evolution show good agreement between the size of the universe and the mass of the black holes. These show that the measured amount of dark energy in the universe can be accounted for by black hole vacuum energy.

This is the first observational evidence that black holes actually contain vacuum energy and that they are 'coupled' to the expansion of the universe, increasing in mass as the universe expands—a phenomenon called 'cosmological coupling.' If further observations confirm it, cosmological coupling will redefine our understanding of what a black hole is.

Study first author Duncan Farrah, University of Hawai'i Astronomer and former Imperial Ph.D. student, said, "We're really saying two things at once: that there's evidence the typical black hole solutions don't work for you on a long, long timescale, and we have the first proposed astrophysical source for dark energy."

"What that means, though, is not that other people haven't proposed sources for dark energy, but this is the first observational paper where we're not adding anything new to the universe as a source for dark energy: black holes in Einstein's theory of gravity are the dark energy ."

The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acb704 . iopscience.iop.org/article/10. … 847/2041-8213/acb704

Journal information: Astrophysical Journal , Astrophysical Journal Letters

Provided by RAL Space

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We finally know why Stephen Hawking's black hole equation works

Stephen Hawking and Jacob Bekenstein calculated the entropy of a black hole in the 1970s, but it took physicists until now to figure out the quantum effects that make the formula work

By Leah Crane

5 April 2024

An artist’s visualisation of a black hole

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We finally understand how black holes get their entropy. Physicists have been struggling to figure this out since the early 1970s, when Stephen Hawking and Jacob Bekenstein calculated how much entropy, or disorder, should be present in a black hole. Now, with a little help from quantum mechanics, researchers may have finally solved the problem.

“For a long time, people have thought that to solve this problem, you have to do all kinds of fancy things in string theory. But what we show is…

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Physicists observationally confirm Hawking’s black hole theorem for the first time

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There are certain rules that even the most extreme objects in the universe must obey. A central law for black holes predicts that the area of their event horizons — the boundary beyond which nothing can ever escape — should never shrink. This law is Hawking’s area theorem, named after physicist Stephen Hawking, who derived the theorem in 1971.

Fifty years later, physicists at MIT and elsewhere have now confirmed Hawking’s area theorem for the first time, using observations of gravitational waves. Their results appear today in Physical Review Letters .

In the study, the researchers take a closer look at GW150914, the first gravitational wave signal detected by the Laser Interferometer Gravitational-wave Observatory (LIGO), in 2015. The signal was a product of two inspiraling black holes that generated a new black hole, along with a huge amount of energy that rippled across space-time as gravitational waves.

If Hawking’s area theorem holds, then the horizon area of the new black hole should not be smaller than the total horizon area of its parent black holes. In the new study, the physicists reanalyzed the signal from GW150914 before and after the cosmic collision and found that indeed, the total event horizon area did not decrease after the merger — a result that they report with 95 percent confidence.

Their findings mark the first direct observational confirmation of Hawking’s area theorem, which has been proven mathematically but never observed in nature until now. The team plans to test future gravitational-wave signals to see if they might further confirm Hawking’s theorem or be a sign of new, law-bending physics.

“It is possible that there’s a zoo of different compact objects, and while some of them are the black holes that follow Einstein and Hawking’s laws, others may be slightly different beasts,” says lead author Maximiliano Isi, a NASA Einstein Postdoctoral Fellow in MIT’s Kavli Institute for Astrophysics and Space Research. “So, it’s not like you do this test once and it’s over. You do this once, and it’s the beginning.”

Isi’s co-authors on the paper are Will Farr of Stony Brook University and the Flatiron Institute’s Center for Computational Astrophysics, Matthew Giesler of Cornell University, Mark Scheel of Caltech, and Saul Teukolsky of Cornell University and Caltech.

An age of insights

In 1971, Stephen Hawking proposed the area theorem, which set off a series of fundamental insights about black hole mechanics. The theorem predicts that the total area of a black hole’s event horizon — and all black holes in the universe, for that matter — should never decrease. The statement was a curious parallel of the second law of thermodynamics, which states that the entropy, or degree of disorder within an object, should also never decrease.

The similarity between the two theories suggested that black holes could behave as thermal, heat-emitting objects — a confounding proposition, as black holes by their very nature were thought to never let energy escape, or radiate. Hawking eventually squared the two ideas in 1974, showing that black holes could have entropy and emit radiation over very long timescales if their quantum effects were taken into account. This phenomenon was dubbed “Hawking radiation” and remains one of the most fundamental revelations about black holes.

“It all started with Hawking’s realization that the total horizon area in black holes can never go down,” Isi says. “The area law encapsulates a golden age in the ’70s where all these insights were being produced.”

Hawking and others have since shown that the area theorem works out mathematically, but there had been no way to check it against nature until LIGO’s first detection of gravitational waves .

Hawking, on hearing of the result, quickly contacted LIGO co-founder Kip Thorne, the Feynman Professor of Theoretical Physics at Caltech. His question: Could the detection confirm the area theorem?

At the time, researchers did not have the ability to pick out the necessary information within the signal, before and after the merger, to determine whether the final horizon area did not decrease, as Hawking’s theorem would assume. It wasn’t until several years later, and the development of a technique by Isi and his colleagues, when testing the area law became feasible.

Before and after

In 2019, Isi and his colleagues developed a technique to extract the reverberations immediately following GW150914’s peak — the moment when the two parent black holes collided to form a new black hole. The team used the technique to pick out specific frequencies, or tones of the otherwise noisy aftermath, that they could use to calculate the final black hole’s mass and spin.

A black hole’s mass and spin are directly related to the area of its event horizon, and Thorne, recalling Hawking’s query, approached them with a follow-up: Could they use the same technique to compare the signal before and after the merger, and confirm the area theorem?

The researchers took on the challenge, and again split the GW150914 signal at its peak. They developed a model to analyze the signal before the peak, corresponding to the two inspiraling black holes, and to identify the mass and spin of both black holes before they merged. From these estimates, they calculated their total horizon areas — an estimate roughly equal to about 235,000 square kilometers, or roughly nine times the area of Massachusetts.

They then used their previous technique to extract the “ringdown,” or reverberations of the newly formed black hole, from which they calculated its mass and spin, and ultimately its horizon area, which they found was equivalent to 367,000 square kilometers (approximately 13 times the Bay State’s area).

“The data show with overwhelming confidence that the horizon area increased after the merger, and that the area law is satisfied with very high probability,” Isi says. “It was a relief that our result does agree with the paradigm that we expect, and does confirm our understanding of these complicated black hole mergers.”

The team plans to further test Hawking’s area theorem, and other longstanding theories of black hole mechanics, using data from LIGO and Virgo, its counterpart in Italy.

“It’s encouraging that we can think in new, creative ways about gravitational-wave data, and reach questions we thought we couldn’t before,” Isi says. “We can keep teasing out pieces of information that speak directly to the pillars of what we think we understand. One day, this data may reveal something we didn’t expect.”

This research was supported, in part, by NASA, the Simons Foundation, and the National Science Foundation.

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Researchers from MIT and other institutions have been able to observationally confirm one of Stephen Hawking’s theorems about black holes, measuring gravitational waves before and after a black hole merger to provide evidence that a black hole’s event horizon can never shrink, reports Caroline Delbert for Popular Mechanics . “This cool analysis doesn't just show an example of Hawking's theorem that underpins one of the central laws affecting black holes,” writes Delbert, “it shows how analyzing gravitational wave patterns can bear out statistical findings.”

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Black holes

B. brügmann.

* Max Planck Institute for Gravitational Physics, Albert Einstein Institute, Am Mühlenberg 1, 14476 Golm, Germany; ‡ University of California, Los Angeles, CA 90095-1562; and § Astrophysical Institute Potsdam, An der Sternwarte 16, 14482, Potsdam, Germany

Recent progress in black hole research is illustrated by three examples. We discuss the observational challenges that were met to show that a supermassive black hole exists at the center of our galaxy. Stellar-size black holes have been studied in x-ray binaries and microquasars. Finally, numerical simulations have become possible for the merger of black hole binaries.

Black holes are a striking example of a prediction of Einstein's theory of gravity, general relativity. Although it took many decades before the physical concept of a black hole was fully understood and widely accepted, recent years have seen rapid advances on both the observational and theoretical side, which we want to illustrate in this brief note with three examples. Black holes have become an astrophysical reality. Solid observational evidence exists for black holes in two mass ranges. Supermassive black holes of 10 6 -10 9 solar masses have been observed at the centers of many galaxies, and here we discuss the observational challenges that were met to show that there exists a black hole at the center of our own galaxy. Stellar-size black holes of about 3–20 solar masses have been studied in x-ray binaries and microquasars. Finally, numerical simulations have become possible for the merger of black hole binaries.

Recent high-resolution imaging studies of stars at the center of our Galaxy have produced strong dynamical evidence for a central concentration of dark matter, establishing the Milky Way as the most convincing case of a galaxy containing a central supermassive black hole ( 1 , 2 ). In those experiments, images obtained over 2–6 years at the Keck telescope ( 1 ) and European Southern Observatory's New Technology Telescope ( 2 ) provided measurements of the stars' velocities in the plane of the sky, from which a statistical analysis revealed the existence of 2−3 × 10 6 solar masses of dark matter contained within a radius of 0.015 parsec (1 parsec = 3.09 × 10 16 m), or 2.6 light weeks. At this meeting, new results from the Keck telescope were reported. With this new data set, which triples the number of maps obtained and doubles the time baseline for the Keck experiment, the velocity uncertainties are reduced by a factor of 3 compared with the earlier Keck work ( 1 ), primarily as a result of the increased time baseline and, in the central square arcsecond, by a factor of 6 compared with ref. 2 , due to the higher angular resolution (0."05 vs. 0."15). In addition to simply increasing the time baseline for velocity measurements, the new measurements have advanced this experiment in two significant ways: ( i ) the first Keck adaptive optics (AO) images of the galactic center have been obtained (Fig. ​ (Fig.1), 1 ), allowing a more complete census of stars in this region to be obtained ( 3 ), and ( ii ) the first measurements of stellar accelerations in this field have now been achieved ( 4 ).

A ≈3" × 3" region showing the Sgr A* cluster (the faint stars located just to the right of the center of the field of view). Both images were taken in May 1999 at 2.2 μm (K-band); however, the image on the left was produced by shift-and-adding Keck I speckle data, and the image on the right was obtained with the new Keck II adaptive optics system. The adaptive optics image represents a large improvement.

With longer integration times, AO should probe a yet larger sample of fainter stars, place stringent limits on Sgr A*, and explore the possibility of a gravitational lensing experiment ( 5 ). For the time being, the AO map has increased the number of stars in the proper motion study ( 3 ). With relative positional accuracies of ≈3 milliarcseconds, the motions of stars are now fit with a second-order polynomial as opposed to a simple linear fit, which was done in earlier work. Among the 90 stars in the original Keck proper motion sample ( 1 ), accelerations of 2–5 milliarseconds/yr 2 , or equivalently 3–6 × 10 −6 km/sec 2 , are now detected for three stars, S0–1, S0–2, and S0–4 ( 4 ). These three stars are independently distinguished in this sample as being among the fastest moving stars ( v = 565 to 1,383 km/sec) and among the closest to the nominal position of Sgr A* (< r > = 0.003 to 0.015 parsec). Acceleration vectors, in principle, are more precise tools than velocity vectors for studying the properties of the central dark mass. These acceleration measurements improve the localization of our Galaxy's dynamical center by a factor of 3, which is critical for reliably associating any near-infrared source with the black hole, given the complexity of the region. In addition, these acceleration measurements increase the minimum mass density inferred by a factor of 8 over previous results, thereby strengthening the case for a black hole.

X-Ray Binaries and Microquasars.

In contrast to the need for measuring dozens of stars to determine the mass of the black hole in the Galactic Center, that of black holes in x-ray binary systems can be deduced either from optical/IR measurements of just one star, namely the companion of the stellar-mass black hole, or from x-ray observations of the binary. In x-ray binaries, a black hole of typically 3–10 solar masses and a normal star (1–30 solar masses) orbit each other. Matter is pulled off the companion star and, because of its angular momentum, is forming an accretion disk as it moves toward the black hole. Before finally falling into the black hole, the matter heats up to several million degrees at the inner part of the disk and emits luminous x-ray radiation. Because the whole accretion process is highly variable, numerous such black hole binaries have been found over the last decade, thanks to x-ray detectors on satellites, such as Compton Gamma-Ray Observatory and Rossi X-Ray Timing Explorer, constantly monitoring the whole sky.

Although optical/IR spectroscopic measurements of the velocity of the companion star can readily determine the mass of the black hole, x-ray measurements promise to be a sharper and even more flexible diagnostic tool as they reach down to the inner edge of the accretion disk at a few Schwarzschild radii of the black hole. High time-resolution observations of black hole binaries have revealed quasiperiodic oscillations in x-ray emission at a stable minimum period, e.g., at 67 Hz for GRS 1915 + 105 ( 6 ), which may very well be related to the period of the innermost stable orbit of the accretion disk. The Kerr metric fixes this period as a function of mass and spin of the black hole. Because the maximum temperature of the innermost disk, as discernable from x-ray spectroscopy, is also thought to be just a function of black hole mass and spin, detailed x-ray observations can be used to determine both the mass AND spin of a black hole ( 7 ).

A small fraction of black hole binaries also eject matter at relativistic speeds into two opposite jets that are observable in the radio band as knots moving apart at superluminal speed. Actually, GRS 1915 + 105 is the most famous representative of this class of object, called microquasars ( 8 ). Simultaneous observations of these microquasars in the x-ray, optical/IR, and radio band have for the first time revealed a relation between accretion disk instabilities and jet ejections ( 9 , 10 ). Theorists now face the challenge of modeling the highly dynamical processes of nonsteady accretion and jet formation, acceleration, and collimation, with all of the complications of three-dimensional magnetohydrodynamics and general relativity.

Another example in which the full Einstein equations have to be solved in the highly dynamic and nonlinear regime is the collision and merger of two black holes. In fact, although single black holes are comparatively simple exact solutions of the Einstein equations, the two-body problem of general relativity for black holes, or neutron stars, is unsolved. As opposed to Newtonian theory, where the Kepler ellipses provide an astrophysically relevant example for the analytic solution of the two-body problem, in Einsteinian gravity there are no corresponding exact solutions. The failure of Einstein's theory to lead to stable orbits is due to the fact that, in general, two orbiting bodies will emit gravitational waves that carry away energy and momentum from the system, leading to an inspiral. Of course, this “leak” is not considered detrimental. It is expected that gravitational wave astronomy will open a new window onto the universe ( 11 ), and binary black hole mergers are considered to be among the most likely candidates for first detection.

Numerical relativity is only now approaching a state where the evolution of rather general three-dimensional data sets can be simulated on a computer to solve the Einstein equations (see, e.g., ref. 12 ). After early computations for the axisymmetric head-on collision of two black holes in the 1970s, it was in 1995 that, for the first time, spherically symmetric data for a single Schwarzschild black hole was evolved with a three-dimensional computer code ( 13 ). The first fully three-dimensional binary black hole evolutions, the grazing collision of nearby spinning and moving black holes, is reported in ref. 14 . Fig. ​ Fig.2 2 shows a visualization of such a black hole merger [M. Alcubierre, W. Benger, B. Brügmann, G. Lanfermann, L. Nerger, E. Seidel & R. Takahashi, R. http://jean-luc.aei.mpg.de/Press/BH1999/ ]. These simulations are still severely limited in achievable evolution time (300 μs for a final black hole of 10 solar masses), i.e., one can evolve through the very last moments of the inspiral when the two black holes merge, but even a single full orbit is not yet possible. Concretely, the computer code crashes when the space–time distortion becomes too severe. The recent computer simulations not only reflect an increase in raw computer power but also are due to theoretical work on how to construct good coordinates dynamically to deal with strong and even singular gravitational fields, and a new way to compute black hole initial data was developed. Work is in progress to obtain at least one orbit and to compute the gravitational waves generated in a black hole merger.

The evolution of the apparent horizon during a grazing black hole collision. Initially there are two separate horizons, which, during the merger, become enclosed by a third one. The coloring represents the curvature of the surface. The black holes appear to grow, because numerical grid points are falling toward and into the black hole.

In conclusion, we believe that black hole physics will be a very dynamic field in the coming years.

Acknowledgments

A.M.G. was supported by the National Science Foundation and the Packard Foundation. J.G. was partly supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DLR) under contract 50 QQ 9602 3. The simulations in refs. 14 and 15 were performed at the Albert-Einstein-Institut and at National Center for Supercomputing Applications.

This paper is a summary of a session presented at the sixth annual German–American Frontiers of Science symposium, held June 8–10, 2000, at the Arnold and Mabel Beckman Center of the National Academies of Science and Engineering in Irvine, CA.

5 Years After the First Black Hole Image, Physicists Unveil A New Experiment To Map Space-Time

The first video of a churning black hole would be revolutionary.

Five years ago, on April 10, 2019, the Event Horizon Telescope — a matrix of radio observatories situated around the world that turn the Earth into one giant radio telescope — captured something incredible: The first-ever image of the hollow depths of a black hole. The clear, almost simplistic image provides the strongest evidence that black holes actually exist and that they match what theoretical physicists predicted. Now, astronomers are attempting something even more surreal: The first-ever movie of a black hole — and it might look like a flipbook.

More than 400 scientists are now collaborating on the next generation of the Event Horizon Telescope (EHT). They are taking snapshots of the supermassive black hole at the center of galaxy Messier 87 (M87) — which is the black hole they took that original image of five years ago — to see how it changes over time. EHT has set a goal of making the video by 2026. The movie will probably be a synopsis of scenes taken over two months.

The appearance of black hole M87* changes from one image to the next, taken a year apart. EHT published the April 11, 2017 observation two years later in 2019. EHT published the 2018 observation (right) in January 2024. In one Earth year, the brighest peak of the ring shifted by about 30 degrees, according to EHT.

If EHT succeeds at its current negotiations for funding and booking global telescope access during the optimal observation months of March and April, the researchers working on the project could turn the images taken every third day into a moving picture. “We have half a year left to convince everyone that we can do this,” Remo Tilanus , research professor at the University of Arizona and EHT operations manager, tells Inverse .

Vlogging a black hole

EHT works by turning the Earth into a giant radio telescope. EHT facilities around the world act like parts of a lens. When our planet turns on its axis, the telescopes shift their position relative to the black hole, acting as different parts of that lens.

Scientists have once again set their eyes on M87*. The black hole was a Goldilocks candidate for an image, Paulina Lira , an astrophysics professor at the University of Chile who was not involved with the work, tells Inverse. She studies the most powerful supermassive black holes, which she says would have likely been too bright to allow the central shadow to appear.

M87* is also more robust than smaller, tamer supermassive black holes, so it will be ideal for a video. Material orbits the black hole on the order of days or weeks. That’s a slow enough time to capture a still image of it but also fast enough for scientists to see changes over 60 days.

That’s a much longer timescale than the other major EHT target, the Milky Way’s supermassive black hole called Sagittarius A*. Our neighborhood monster is much smaller, with much less stuff going around it that takes just minutes to orbit.

A video of a black hole will require significant upgrades both to the EHT’s technology and the number of arrays involved. Instead of the handful of days required to take a still image, a video of a black hole will take about 60 days, which is far more challenging, says Tilanus. In addition to booking the right arrangements at the observatories, a video requires EHT to observe in more wavelengths, which requires improvements to the telescopes. Taking the video will also get easier as more observatories join EHT. The larger the array, and the more radio wavelengths that can be collected, the higher the quality of the black hole images.

“We have completed an initial design of the [next generation] EHT upgrades and are now sourcing the funds required to build the array. We hope to obtain movies from the upgraded array in the next several years,” Lindy Blackburn , an astronomer and EHT data scientist at the Center for Astrophysics | Harvard and Smithsonian, tells Inverse via email.

The updates will allow EHT to reconstruct models of the black hole’s dynamics from existing data, and the new telescopes over the next several years will be key to making “high fidelity movies that show the inflow and outflow dynamics near the boundary of a black hole,” Blackburn says.

A video is worth a thousand images

The University of Chile’s Lira says the original black hole image far surpassed her expectations. Its impact on scientists, and popularity amongst the public, she says, came from its simplicity: a clear, non-pixelated, shadow of a black hole against a bright ring. Memes, like her personal favorite of the donut-shaped black hole held by Homer Simpson, highlight the universal appeal of the visual.

The video would catapult black hole work. In the video, astronomers say they might see bright spots, in the ring of super-hot plasma, changing as they orbit the black hole M87*. Astrophysicists could better understand M87*’s jet, which stretches out 5,000 light-years long , yet originates from a black hole smaller than the size of the Solar System.

Tilanus says a video would show how a black hole ingests and ejects matter, how that in turn might be creating the magnetic field lines that funnel matter into a jet, and how that phenomenon affects star formation in the galaxy or halts them altogether. On a more macro-level, the video could reveal what the first black holes were like, and how the supermassive ones that lurk in most if not all the universe’s galaxies come to be.

The video could give clues about a major cosmic evolution debate. It's a “chicken or the egg” question: Did black holes emerge first from the gas and dust of the early universe, and did galaxies grow around them? Or, did galaxies form first and then create these black hole behemoths at their hearts?

In addition, a video could also help theoretical physicists learn by what degree time slows down if an object were to fall into a black hole.

Chi-kwan Chan , an associate research professor at the University of Arizona and EHT data scientist, tells Inverse that there are already predictions for a black hole’s time dilation, given Albert Einstein’s theory of general relativity. “But, unless you can do an experiment, it’s not really confirmed. It’s not really science. One of the biggest contributions for the EHT [work] that’s coming up is really to confirm these theoretical predictions,” Chan says.

A diagram of a rapidly spinning supermassive black hole and its accretion disk.

Einstein’s theory of general relativity says that time is a dimension in the universe: Time can change; it is not absolute. That’s in contrast to the Newtonian view of physics, where time and space are in the background. Einstein predicted that extreme mass could modify time.

Space-time can be thought of as an elastic surface, which will stretch if a weight is on it. Likewise, a massive object like a black hole will stretch space-time, and thus slow down time.

How fast material moves in a black hole depends on the curvature of space-time, says Chan. The more curved, the more distorted it will be. Only a video could give insight into this phenomenon.

A video would allow theoretical physicists to map the time dilation. An image of a black hole, however spectacular, lacks time-dependent information.

Time dilation data in a movie would be a major win for theoretical physicists because black holes are complicated systems, and most of the current work on black holes is done on supercomputers and simulations.

If humanity achieves this incredible feat, physicists will take a major step toward understanding the chaotic environment of a black hole. “The first time I heard about the project and its goals, it sounded fantastically challenging bordering on unachievable,” says Blackburn. “That is also a very fun place to be.”

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General Relativity and Quantum Cosmology

Title: gravitational lensing by spinning black holes in astrophysics, and in the movie interstellar.

Abstract: Interstellar is the first Hollywood movie to attempt depicting a black hole as it would actually be seen by somebody nearby. For this we developed a code called DNGR (Double Negative Gravitational Renderer) to solve the equations for ray-bundle (light-beam) propagation through the curved spacetime of a spinning (Kerr) black hole, and to render IMAX-quality, rapidly changing images. Our ray-bundle techniques were crucial for achieving IMAX-quality smoothness without flickering. This paper has four purposes: (i) To describe DNGR for physicists and CGI practitioners . (ii) To present the equations we use, when the camera is in arbitrary motion at an arbitrary location near a Kerr black hole, for mapping light sources to camera images via elliptical ray bundles. (iii) To describe new insights, from DNGR, into gravitational lensing when the camera is near the spinning black hole, rather than far away as in almost all prior studies. (iv) To describe how the images of the black hole Gargantua and its accretion disk, in the movie \emph{Interstellar}, were generated with DNGR. There are no new astrophysical insights in this accretion-disk section of the paper, but disk novices may find it pedagogically interesting, and movie buffs may find its discussions of Interstellar interesting.

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