Scientists have detected ripples in space-time from the violent collision of two massive black holes that spiralled into one another far beyond the distant edge of the Milky Way.

The black holes, each more than 100 times the mass of the sun, began circling each other long ago and finally slammed together to form an even more massive black hole about 10bn light years from Earth.

The event is the most massive black hole merger ever recorded by gravitational wave detectors and has forced physicists to rethink their models of how the enormous objects form. The signal was recorded when it hit detectors on Earth sensitive enough to detect shudders in space-time thousands of times smaller than the width of a proton.

“These are the most violent events we can observe in the universe, but when the signals reach Earth, they are the weakest phenomena we can measure,” said Prof Mark Hannam, the head of the Gravity Exploration Institute at Cardiff University. “By the time these ripples wash up on Earth they are tiny.”

Evidence for the black hole collision arrived just before 2pm UK time on 23 November 2023 when two US-based detectors in Washington and Louisiana, operated by the Laser Interferometer Gravitational-wave Observatory (Ligo), twitched at the same time.

View image in fullscreen The Laser Interferometer Gravitational-wave Observatory (Ligo) detector. Photograph: Caltech/MIT/LIGO Lab

The sudden spasm in space-time caused the detectors to stretch and squeeze for one tenth of a second, a fleeting moment that captured the so-called ringdown phase as the merged black holes formed a new one that “rang” before settling down.

Analysis of the signal revealed that the colliding black holes were 103 and 137 times the mass of the sun and spinning about 400,000 times faster than Earth, close to the theoretical limit for the objects.

“These are the highest masses of black holes we’ve confidently measured with gravitational waves,” said Hannam, a member of the Ligo scientific collaboration. “And they’re strange, because they are slap bang in the range of masses where, because of all kinds of weird things that happen, we don’t expect black holes to form.”

Most black holes form when massive stars run out of nuclear fuel and collapse at the end of their life cycle. The incredibly dense objects warp space-time so much that they create an event horizon, a boundary within which even light cannot escape.

View image in fullscreen Gravitational wave signature detected at the two US instruments. Photograph: Caltech/MIT/LIGO Lab

Physicists at Ligo suspect the black holes that merged were themselves products of earlier mergers. That would explain how they came to be so massive and why they were spinning so fast, as merging black holes tend to impart spin on the object they create. “We’ve seen hints of this before, but this is the most extreme example where that’s probably what’s happening,” Hannam said.

Scientists have detected about 300 black hole mergers from the gravitational waves they generate. Until now, the most massive merger known produced a black hole about 140 times the mass of the sun. The latest merger produced a black hole up to 265 times more massive than the sun. Details are to be presented on Monday at the GR-Amaldi meeting in Glasgow.

Before the first gravitational wave detectors were built in the 1990s, scientists could observe the universe only through electromagnetic radiation such as visible light, infrared and radio waves. Gravitational wave observatories provide a new view of the cosmos, allowing researchers to see events that were otherwise hidden from them.

“Usually what happens in science is, when you look at the universe in a different way, you discover things you didn’t expect and your whole picture is transformed,” said Hannam. “The detectors we have planned for the next 10 to 15 years will be able to see all the black hole mergers in the universe, and maybe some surprises we didn’t expect.”

Black holes are spinning faster than expected, researchers find

Credit: Left: NASA/JPL-Caltech Right: Logan Fries and the SDSS collaboration

There’s a universe full of black holes out there, spinning merrily away—some fast, others more slowly. A recent survey of supermassive black holes reveals that their spin rates reveal something about their formation history.

If you want to describe a supermassive black hole’s characteristics, there are two important numbers to use. One is its mass and the other is its spin rate. Some black hole spin rates are thought to be very close to the speed of light.

According to Logan Fries, a Ph.D. student at the University of Connecticut, those numbers are tough to get. “The problem is that mass is hard to measure, and spin is even harder,” he said. Yet, having accurate numbers is important if we want to understand black hole evolution.

Fries and his colleagues in the Sloan Digital Sky Survey’s Reverberation Mapping Project took on a tough job. They measured the spin rates of black holes over cosmic history. “We have studied the giant black holes found at the centers of galaxies, from today to as far back as 7 billion years ago,” said Fries, a primary author of a paper about this work.

The mapping project also made detailed observations of the associated accretion disks. Those are the areas nearest the black hole where matter accumulates and heats up as it spirals in. Measuring that region is important since knowing the black hole’s mass and its accretion disk’s structure provides data that allows them to measure the spin rate. Astronomers typically estimate the spin rate by observing how matter behaves as it falls into the black hole.

Black holes and their archaeology

The results of the SDSS Survey of mass measurements of hundreds of black holes were a surprise, according to Fries. That’s because the spin rates reveal something about the black holes’ formation history. “Unexpectedly, we found that they were spinning too fast to have been formed by galaxy mergers alone,” he said. “They must have formed in large part from material falling in, growing the black hole smoothly and speeding up its rotation.”

Fries described his work at the 245th meeting of the American Astronomical Society. “I have read research papers that examine black hole spin, theoretically, from the lens of like black hole mergers, and I was curious if spin could be observationally measured,” said Fries. He pointed out that the history of black hole growth requires more precise measurements than have been available.

And, they’re not easy, according to Fries’s thesis advisor, Physics professor Jonathan Trump. “The challenge lies in separating the spin of the black hole from the spin of the accretion disk surrounding it,” said Trump. “The key is to look at the innermost region, where gas is falling into the black hole’s event horizon. A spinning black hole drags that innermost material along for the ride, which leads to an observable difference when we look at the details in our measurements.”

Digging into the mass and spin of a black hole requires spectral measurements. Those made by the SDSS contain subtle shifts in the spectra toward shorter wavelengths of light. That shift is a major clue to the black hole’s rotation rate. “I call this approach ‘black hole archaeology,'” said Fries, “because we’re trying to understand how the mass of a black hole has grown over time. By looking at the spin of the black hole, you’re essentially looking at its fossil record.”

What the black holes tell us

So what does that fossil record tell us? First of all, it challenges the prevailing wisdom that black holes are always created in galaxy collisions. In other words, when galaxies merged, so did their central black holes. Each galaxy brings a rotation rate and orientation to the merger. The rotations could just as easily cancel each other out as they are to add together. If that is true, then the astronomers should have seen a wide range of spins. Some black holes should have a lot of spin, others… not so much.

The big surprise is that many black holes appear to spin very quickly. Even more amazing, the most distant ones seem to be spinning faster than the ones nearest to us (i.e., the “nearby” universe). It’s as if they spin faster in the early universe, and more slowly in more recent epochs. “We find that about 10 billion years ago, black holes acquired their mass primarily through eating things,” Fries explained.

The early fast spin rate implies that most supermassive black holes (like the one in our own Milky Way galaxy) built up over time by taking in gas and dust in a very smooth and controlled manner. In other words, the more they eat (in the way of stars and gas), the faster their spin rate. It also turns out that merger growth actually slows the spin of supermassive black holes. That could explain why those we measure today have a mix of spin rates, rather than the more uniform rates of earlier epochs.

The idea of black holes forming smoothly over time provides a new direction for black hole research. Observations by JWST will help give more targets to study. Surveys such as the SDSS Reverberation Mapping project will follow up with more precise measurements of the huge supermassive black holes JWST continually finds as it studies the universe.

Black holes are some of the strangest and most fascinating objects in the universe. They’re extremely dense, with gravitational forces so strong, that nothing, not even lights, can escape once it crosses the boundary known as the event horizon.

The Milky Way could contain over 100 million black holes, though detecting these gluttonous beasts is very difficult. At the heart of the Milky Way lies a supermassive black hole — Sagittarius A*. The colossal structure is about 4 million times the mass of the sun and lies approximately 26,000 light-years away from Earth , according to a statement from NASA.

The first image of a black hole was captured in 2019 by the Event Horizon Telescope (EHT) collaboration. The striking photo of the black hole at the center of the M87 galaxy 55 million light-years from Earth thrilled scientists around the world.

Related: White holes: What we know about black holes’ neglected twins

Black hole FAQs answered by an expert

We asked theoretical astrophysicist Priyamvada Natarajan a few commonly asked questions about black holes.

Priyamvada Natarajan Social Links Navigation Theoretical Astrophysicist Chair of the Department of Astronomy, Joseph S. and Sophia S. Fruton Professor of Astronomy and Professor of Physics, Yale University. Natarajan’s research explores how black holes form, grow, and shape the universe, along with mapping dark matter in cosmic structures.

How do black holes form? Black holes are expected to form via two distinct channels. According to the first pathway, they are stellar corpses, so they form when massive stars die. Stars whose birth masses are above roughly 8 to 10 times mass of our sun, when they exhaust all their fuel — their hydrogen — they explode and die leaving behind a very compact dense object, a black hole. The resulting black hole that is left behind is referred to as a stellar mass black hole and its mass is of the order of a few times the mass of the sun. Not all stars leave behind black holes, stars with lower birth masses leave behind a neutron star or a white dwarf. Another way that black holes form is from the direct collapse of gas, a process that is expected to result in more massive black holes with a mass ranging from 1000 times the mass of the sun up to even 100,000 times the mass of the sun. This channel circumvents the formation of the traditional star, and is believed to operate in the early universe and produce more massive black hole seeds.

Who discovered black holes? Black holes were predicted as an exact mathematical solution to Einstein’s equations. Einstein’s equations describe the shape of space around matter. The theory of general relativity connects the geometry or shape of shape to the detailed distribution of matter. The black hole solution was found was by Karl Schwarzschild in 1915, and these regions — black holes — were found to distort space extremally and generate a puncture in the fabric of spacetime. It was unclear at the time if these corresponded to real objects in the universe. Over time, as other end products of stellar death were detected, namely, neutron stars seen as pulsars it became clear that black holes were real and ought to exist. The first detected black hole was Cygnus-X1.

Do black holes die? Black holes do not die per se, but they are theoretically predicted to eventually slowly evaporate over extremely long time scales. Black holes grow by the accretion of matter nearby that is pulled in by their immense gravity. Hawking predicted that black holes could also radiate away energy and shrink very slowly. Quantum theory suggests that there exist virtual particles popping in and out of existence all the time. When this happens, a particle and its companion anti-particle appear. However, they can also recombine and disappear again. When this process occurs near the event horizon of a black hole, strange things can happen. Instead of the particle antiparticle pair existing for a moment and then annihilating each other, one of them can get by gravity and fall into the black hole, while the other particle can fly off into space. Over very long timescales, we are speaking about timescales that are much much longer than the age of our universe, the theory states that this trickle of escaping particles will cause the black hole to slowly evaporate.

Are black holes wormholes? No black holes are not wormholes. Wormholes can be thought of as tunnels that connect two separate points in space and time. It is believed that the interior of black holes could contain a wormhole, the puncture is spacetime, that could offer a portal to another point in spacetime potentially even in a different universe.

First black hole discovered

Albert Einstein first predicted the existence of black holes in 1916, with his general theory of relativity. The term “black hole” was coined many years later in 1967 by American astronomer John Wheeler. After decades of black holes being known only as theoretical objects.

The first black hole ever discovered was Cygnus X-1, located within the Milky Way in the constellation of Cygnus, the Swan. Astronomers saw the first signs of a black hole in 1964 when a sounding rocket detected celestial sources of X-rays according to NASA. In 1971, astronomers determined that the X-rays were coming from a bright blue star orbiting a strange dark object. It was suggested that the detected X-rays were a result of stellar material being stripped away from the bright star and “gobbled” up by the dark object — an all-consuming black hole.

How many black holes are there?

At the center of the Milky Way lies a supermassive black hole Sagittarius A* (Sgr A*). (Image credit: NASA/UMass/D.Wang et al., IR: NASA/STScI)

According to the Space Telescope Science Institute (STScI) approximately one out of every thousand stars is massive enough to become a black hole. Since the Milky Way contains over 100 billion stars, our home galaxy must harbor some 100 million black holes.

Though detecting black holes is a difficult task and estimates from NASA suggest there could be as many as 10 million to a billion stellar black holes in the Milky Way.

The closest black hole to Earth is called Gaia-BH1, and it sits only 1,560 light-years away from us.

Related: How many black holes are there in the universe?

Black hole images

The Event Horizon Telescope, a planet-scale array of eight ground-based radio telescopes forged through international collaboration, captured this image of the supermassive black hole in the center of the galaxy M87 and its shadow. (Image credit: EHT Collaboration)

In 2019 the Event Horizon Telescope (EHT) collaboration released the first image ever recorded of a black hole. The EHT saw the black hole in the center of galaxy M87 while the telescope was examining the event horizon or the area past which nothing can escape from a black hole. The image maps the sudden loss of photons (particles of light). It also opens up a whole new area of research in black holes, now that astronomers know what a black hole looks like.

In 2021, astronomers revealed a new view of the giant black hole at the center of M87, showing what the colossal structure looks like in polarized light. As polarized light waves have a different orientation and brightness compared to unpolarized light, the new image shows the black hole in even more detail. Polarization is a signature of magnetic fields and the image makes it clear that the black hole’s ring is magnetized.

Following the release of the first image of a black hole in 2019, astronomers captured a new polarized view of the black hole. (Image credit: EHT Collaboration)

In May 2022, scientists revealed the historic first image of the supermassive black hole at the center of our galaxy — Sagitarrius A*.

Related: The 1st photo of the Milky Way’s monster black hole explained in images

What do black holes look like?

Black holes have three “layers”: the outer and inner event horizon, and the singularity.

The event horizon of a black hole is the boundary around the mouth of the black hole, past which light cannot escape. Once a particle crosses the event horizon, it cannot leave. Gravity is constant across the event horizon.

The inner region of a black hole, where the object’s mass lies, is known as its singularity, the single point in space-time where the mass of the black hole is concentrated.

Scientists can’t see black holes the way they can see stars and other objects in space. Instead, astronomers must rely on detecting the radiation black holes emit as dust and gas are drawn into the dense creatures. But supermassive black holes, lying in the center of a galaxy, may become shrouded by the thick dust and gas around them, which can block the telltale emissions.

Sometimes, as matter is drawn toward a black hole, it ricochets off the event horizon and is hurled outward, rather than being tugged into the maw. Bright jets of material traveling at near-relativistic speeds are created. Although the black hole remains unseen, these powerful jets can be viewed from great distances.

The EHT’s image of a black hole in M87 (released in 2019) was an extraordinary effort, requiring two years of research even after the images were taken. That’s because the collaboration of telescopes, which stretches across many observatories worldwide, produces an astounding amount of data that is too large to transfer via the internet.

With time, researchers expect to image other black holes and build up a repository of what the objects look like. The next target is likely Sagittarius A*, which is the black hole in the center of our own Milky Way galaxy. Sagittarius A* is intriguing because it is quieter than expected, which may be due to magnetic fields smothering its activity, a 2019 study reported. Another study that year showed that a cool gas halo surrounds Sagittarius A*, which gives unprecedented insight into what the environment around a black hole looks like.

ESO’s black hole anatomy diagram shows what a black hole looks like and labels the different components. (Image credit: ESO)

Types of black holes

So far, astronomers have identified three types of black holes: stellar black holes, supermassive black holes and intermediate black holes.

Stellar black holes — small but deadly

When a star burns through the last of its fuel, the object may collapse, or fall into itself. For smaller stars (those up to about three times the sun ‘s mass), the new core will become a neutron star or a white dwarf. But when a larger star collapses, it continues to compress and creates a stellar black hole.

Black holes formed by the collapse of individual stars are relatively small but incredibly dense. One of these objects packs more than three times the mass of the sun into the diameter of a city. This leads to a crazy amount of gravitational force pulling on objects around the object. Stellar black holes then consume the dust and gas from their surrounding galaxies, which keeps them growing in size.

Supermassive black holes — the birth of giants

Small black holes populate the universe, but their cousins, supermassive black holes, dominate. These enormous black holes are millions or even billions of times as massive as the sun but are about the same size in diameter. Such black holes are thought to lie at the center of pretty much every galaxy, including the Milky Way.

Scientists aren’t certain how such large black holes spawn. Once these giants have formed, they gather mass from the dust and gas around them, material that is plentiful in the center of galaxies, allowing them to grow to even more enormous sizes.

Supermassive black holes may be the result of hundreds or thousands of tiny black holes that merge. Large gas clouds could also be responsible, collapsing together and rapidly accreting mass. A third option is the collapse of a stellar cluster, a group of stars all falling together. Fourth, supermassive black holes could arise from large clusters of dark matter. This is a substance that we can observe through its gravitational effect on other objects; however, we don’t know what dark matter is composed of because it does not emit light and cannot be directly observed.

Intermediate black holes

Scientists once thought that black holes came in only small and large sizes, but research has revealed the possibility that midsize, or intermediate, black holes (IMBHs) could exist. Such bodies could form when stars in a cluster collide in a chain reaction. Several of these IMBHs forming in the same region could then eventually fall together in the center of a galaxy and create a supermassive black hole.

In 2014, astronomers found what appeared to be an intermediate-mass black hole in the arm of a spiral galaxy . And in 2021 astronomers took advantage of an ancient gamma-ray burst to detect one.

“Astronomers have been looking very hard for these medium-sized black holes,” study co-author Tim Roberts, of the University of Durham in the United Kingdom, said in a statement. “There have been hints that they exist, but IMBHs have been acting like a long-lost relative that isn’t interested in being found.”

Research, from 2018, suggested that these IMBHs may exist in the heart of dwarf galaxies (or very small galaxies). Observations of 10 such galaxies (five of which were previously unknown to science before this latest survey) revealed X-ray activity — common in black holes — suggesting the presence of black holes of from 36,000 to 316,000 solar masses. The information came from the Sloan Digital Sky Survey, which examines about 1 million galaxies and can detect the kind of light often observed coming from black holes that are picking up nearby debris.

Binary black holes: double trouble

Artist’s illustration of a supermassive black hole with a companion black hole orbiting around it. (Image credit: Caltech-IPAC)

In 2015, astronomers using the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from merging stellar black holes.

“We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses — these are objects we didn’t know existed before LIGO detected them,” David Shoemaker, the spokesperson for the LIGO Scientific Collaboration (LSC), said in a statement. LIGO’s observations also provide insights into the direction a black hole spins. As two black holes spiral around one another, they can spin in the same direction or the opposite direction.

There are two theories on how binary black holes form. The first suggests that the two black holes in a binary form at about the same time, from two stars that were born together and died explosively at about the same time. The companion stars would have had the same spin orientation as one another, so the two black holes left behind would as well.

Under the second model, black holes in a stellar cluster sink to the center of the cluster and pair up. These companions would have random spin orientations compared to one another according to LIGO Scientific Collaboration. LIGO’s observations of companion black holes with different spin orientations provide stronger evidence for this formation theory.

“We’re starting to gather real statistics on binary black hole systems,” said LIGO scientist Keita Kawabe of Caltech, who is based at the LIGO Hanford Observatory. “That’s interesting because some models of black hole binary formation are somewhat favored over the others even now, and in the future, we can further narrow this down.”

Black hole facts

If you fell into a black hole, theory has long suggested that gravity would stretch you out like spaghetti, though your death would come before you reached the singularity. But a 2012 study published in the journal Nature suggested that quantum effects would cause the event horizon to act much like a wall of fire, which would instantly burn you to death.

Black holes don’t suck. Suction is caused by pulling something into a vacuum, which the massive black hole definitely is not. Instead, objects fall into them just as they fall toward anything that exerts gravity, like the Earth.

The first object considered to be a black hole is Cygnus X-1. Cygnus X-1 was the subject of a 1974 friendly wager between Stephen Hawking and fellow physicist Kip Thorne, with Hawking betting that the source was not a black hole. In 1990, Hawking conceded defeat.

Miniature black holes may have formed immediately after the Big Bang. Rapidly expanding space may have squeezed some regions into tiny, dense black holes less massive than the sun.

If a star passes too close to a black hole, the star can be torn apart.

Astronomers estimate that the Milky Way has anywhere from 10 million to 1 billion stellar black holes, with masses roughly three times that of the sun.

Black holes remain terrific fodder for science fiction books and movies. Check out the movie “Interstellar,” which relied heavily on Thorne to incorporate science. Thorne’s work with the movie’s special effects team led to scientists’ improved understanding of how distant stars might appear when seen near a fast-spinning black hole.

Additional resources

Dive deeper into the mystery of black holes with NASA Science. Watch videos and read more about black holes from NASA’s Hubblesite. Discover more about black holes with the National Science Foundation.

Bibliography

Hubblesite: Black holes: Gravity’s relentless pull interactive : Encyclopedia. STScI Home. Retrieved May 6, 2022.

NASA. Imagine the universe! NASA. Retrieved May 6, 2022.

Boen, B. ( 2013, August 29 ). Supermassive black hole Sagittarius A*. NASA. Retrieved May 6, 2022.

Supermassive Black Holes (SMBHs) can have billions of solar masses, and observational evidence suggests that all large galaxies have one at their centres. However, the JWST has revealed a foundational cosmic mystery. The powerful space telescope, with its ability to observe ancient galaxies in the first billion years after the Big Bang, has shown us that SMBHs were extremely massive even then. This contradicts our scientific models explaining how these behemoths became so huge.

How did they get so massive so early?

Black holes of all masses are somewhat mysterious. We know that massive stars can collapse and form stellar-mass black holes late in their lives. We also know that pairs of stellar-mass black holes can merge, and we’ve detected the gravitational waves from those mergers. So, it’s tempting to think that SMBHs also grow through mergers when galaxies merge together.

The problem is, in the early Universe, there wasn’t enough time for black holes to grow large enough and merge often enough to produce the SMBHs. The JWST has shown us the errors in our models of black hole growth by finding quasars powered by black holes of 1-10 billion solar masses less than 700 million years after the Big Bang.

Astrophysicists are busy trying to understand how SMBHs became so massive so soon in the Universe. New research titled ” Primordial black holes as supermassive black holes seeds ” attempts to fill in the gap in our understanding. The lead author is Francesco Ziparo from the Scuola Normale Superiore di Pisa, a public university in Italy.

There are three types of black holes: Stellar-mass black holes, intermediate-mass black holes (IMBHs), and SMBHs. Stellar-mass black holes have masses ranging from about five solar masses up to several tens of solar masses. SMBHs have masses ranging from hundreds of thousands of solar masses up to millions or billions of solar masses. IMBHs are in between, with masses ranging from about one hundred to one hundred thousand solar masses. Researchers have wondered if IMBHs could be the missing link between stellar-mass black holes and SMBHs. However, we only have indirect evidence that they exist.

There’s a fourth type of black hole that is largely theoretical, and some researchers think they can help explain how the early SMBHs were so massive. They’re called primordial black holes (PBHs.) Conditions in the very early Universe were much different than they are now, and astrophysicists think that PBHs could’ve formed by the direct collapse of dense pockets of subatomic matter. PBHs formed before there were any stars, so aren’t limited to the rather narrow mass range of stellar-mass black holes.

“The presence of supermassive black holes in the first cosmic Gyr (gigayear) challenges current models of BH formation and evolution,” the researchers write. “We propose a novel mechanism for the formation of early SMBH seeds based on primordial black holes (PBHs).”

Ziparo and his co-authors explain that in the early Universe, PBHs would’ve clustered and formed in high-density regions, the same regions where dark matter halos originated. Their model takes into account PBH accretion and feedback, the growth of dark matter halos, and dynamical gas friction.

In this model, the PBHs are about 30 solar masses and are in the central region of dark matter (DM) halos. As the halos grow, baryonic matter settles in their wells as cooled gas. “PBHs both accrete baryons and lose angular momentum as a consequence of the dynamical friction on the gas, thus gathering in the central region of the potential well and forming a dense core,” the authors explain. Once clustered together, a runaway collapse occurs that ends up as a massive black hole. Its mass depends on the initial conditions.

Planted soon enough, these seeds can explain the early SMBHs the JWST has observed.

There’s a way to test this model, according to the authors.

“During the runaway phase of the proposed seed formation process, PBH-PBH mergers are expected to copiously emit gravitational waves. These predictions can be tested through future Einstein Telescope observations and used to constrain inflationary models,” they explain.

The Einstein Telescope or Einstein Observatory is a proposal from several European research agencies and institutions for an underground gravitational wave (GW) observatory that would build on the success of the laser-interferometric detectors Advanced Virgo and Advanced LIGO. The Einstein Telescope would also be a laser interferometer but with much longer arms. While LIGO has arms four km long, Einstein would have arms 10 km long. Those longer arms, combined with new technologies, would make the Telescope much more sensitive to GWs.

The Einstein Telescope should open up a GW window into the entire population of stellar and intermediate-mass black holes over the entire history of the Universe. “The Einstein Telescope will make it possible, for the first time, to explore the Universe through gravitational waves along its cosmic history up to the cosmological dark ages, shedding light on open questions of fundamental physics and cosmology,” the Einstein website says.

A thorough understanding of SMBHs is a ways away, but it’s important to understand them because of their role in the Universe. They help explain the universe’s large-scale structure by influencing the distribution of matter on large scales. The fact that they appeared so much earlier in the Universe than we thought possible shows that we have a lot to learn about SMBHs and how the Universe has evolved to the state it’s in now.

I would argue that the most fascinating and mysterious objects in the cosmos are black holes. These pockets in the fabric of spacetime are anchored by an infinitely dense and infinitesimally small concentration of mass: A singularity. We simply do not know what lies beyond a black hole’s event horizon — the boundary beyond which light can’t cross — and perhaps never will. These objects are simply too extreme for our brains to lightly comprehend and for our bodies to withstand.

But in the 25 years since 1999, when Space.com was founded, the science of black holes has come on leaps and bounds — especially as it relates to bringing these cosmic titans from their theoretical origins into observational reality. In fact, a comprehensive list of black hole breakthroughs made since the foundation of Space.com would require a dedicated website of its own.

However, what we can do to celebrate our silver anniversary is bring you, in no particular order, some of the most important, wondrous and even confusing discoveries made in black hole science since 1999. Let’s dig in.

The first image of a black hole

SEE MORE 25TH ANNIVERSARY FEATURES: Check out a list of Space.com’s special 25th anniversary week stories in our hub linked here!

Like all black holes, supermassive black holes at the hearts of galaxies are bounded by one-way, light-trapping surfaces called event horizons. Thus, no light can escape a black hole, and no black hole can really ever be seen. What can be seen, however, is the shadow these voids cast on the glowing material surrounding them. It is upon this material that black holes gradually feed.

Related: Hubble Space Telescope finds closest massive black hole to Earth — a cosmic clue frozen in time

Even still, capturing an image of a black hole is no mean feat. One project that endeavored to do this is the Event Horizon Telescope (EHT), a global network of observatories that coordinates to act like a telescope the size of Earth. In April 2019, sure enough, the EHT collaboration revealed to the public that they had succeeded in imaging a black hole using data collected in 2017.

The object in question was the supermassive black hole at the heart of the distant galaxy Messier 87 (M87). The golden ring in the image is material racing around the black hole at near-light speeds.

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“I think the first images of a black hole are really the first direct evidence that we have of the existence of black holes,” Sara Issaoun, an observational astronomer at Harvard & Smithsonian’s Center of Astrophysics (CfA) and member of the EHT collaboration, told Space.com. “We get to actually see their shadows — their impact on light and gas around them directly. I think that’s been a big shift in science, especially because of the visual aspect of the result.”

The Event Horizon Telescope captured this image of the supermassive black hole in the center of the galaxy M87 and its shadow. (Image credit: EHT Collaboration)

The black hole in question is M87*, located around 55 million light-years away with a mass of about 6.5 billion suns, making it much more massive than our galaxy’s supermassive black hole Sagittarius A* (Sgr A*). Little wonder this cosmic titan’s image caught the public’s imagination.

“The image of M87* brought a lot of broader interest to black holes and science, and astronomy, in general,” Issaoun said.

Mass of the Milky Way’s black hole, measured

At the heart of the Milky Way, our home galaxy is the cosmic titan Sagittarius A* (Sgr A*), which was first detected in strong radio waves by Karl Jansky in the 1930s and isolated to a more compact region in 1974 by astronomers Bruce Balick and Robert L. Brown. By the 1980s, astronomers had officially proposed this object was a tremendously large black hole, but Sgr A* remained somewhat shrouded in mystery.

That was until 2008, when astronomers Reinhard Genzel and Andrea Ghez determined Sgr A* to be a supermassive black hole with a mass 4.3 million times that of the sun. The discovery was ingeniously made not by looking at Sgr A* directly (that’s coming up, don’t worry), but by measuring the velocity of fast-moving stars called the “S-group” that whip around it.

NASA’s Chandra X-ray Observatory images the center of the Milky Way and Sgr A*. (Image credit: X-ray- NASA/UMass/D.Wang et al., IR- NASA/STScI)

“Tracking these stars over two decades, looking at the signals of these stars as they approach this dark mass and leap away from it, Genzel and Ghez were able to measure the mass and size of this region to really great accuracy,” Issaoun said. “The most obvious explanation is that this particular object had to be a black hole.”

Since then, astronomers have also calculated the diameter of the Sgr A* to be around 14.6 million miles (23.5 million kilometers) , which is extremely tiny compared to the Milky Way itself, which is 100,000 light-years wide and 1,000 light-years thick.

This discovery revealed that, like other galaxies, the Milky Way revolves around a black hole with an almost incomprehensible mass, cementing our understanding of the morphology of our galaxy and our wider place in the cosmos.

Imaging Sagittarius A*

Following the groundbreaking reveal of the supermassive black hole at the heart of M87, space fans began to grow impatient for an image of the black hole at the heart of the Milky Way, Sagittarius A* (Sgr A*).

On May 12, 2022, the EHT Collaboration managed to reveal the first image of Sgr A* created using data collected in 2017. Despite Sgr A* being much closer to Earth, it was tougher to image because the material surrounding it also races around at near light-speed, but Sgr A* is much smaller than M87*, so full orbits were completed almost quicker than the eye of the EHT could see.

An image of the supermassive black hole at the center of the Milky Way, a behemoth dubbed Sagittarius A*, revealed by the Event Horizon Telescope on May 12, 2022. (Image credit: Event Horizon Telescope collaboration)

One of the astounding things about the images of M87* and Sgr A*, when compared, was that both black holes are so similar in appearance despite the former having a mass billions of times that of the sun, and the latter having a mass equivalent to just millions of suns.

“What’s interesting about these two black holes is that, although they’re both supermassive black holes, they’re also quite different,” Issaoun said. “M87* lives inside the M87 galaxy, which is a giant elliptical galaxy. It’s quite old. It’s gone through many mergers, and it’s very large. On the other hand, Sgr A* lives in our Milky Way, which is very common among galaxies and, in galactic terms, very small. It’s a spiral galaxy that’s not that old.”

The fastest-growing black hole ever discovered

We’ve already discussed supermassive black holes with very different diets: the revenously feeding M87* and the less greedy Sgr A*, which consumes so little matter it is akin to a human eating one grain of rice every million years. But a supermassive black hole discovered in 2024 really takes the cake, quite literally.

J0529-4351 is a quasar powered by a supermassive black hole that is located so far from Earth its light has taken about 12 billion years to reach us. With a brightness equivalent to 500 trillion suns, this is the brightest quasar seen to date.

Existing when the universe was less than 2 billion years old, J0529-4351 has a mass between 17 billion and 19 billion suns, and it eats, or “accretes,” at least one solar mass worth of gas and dust every single day. While many records on this list exist merely to be broken, it is hard to imagine a black hole monstrous enough to displace J0529-4351.

Gravitational waves detected from black hole mergers

John Regan, a Royal Society University research fellow at Maynooth University who specializes in black hole science, told Space.com that one of the most revolutionary black hole discoveries in the last quarter of a century was the detection of gravitational waves from merging black holes.

Gravitational waves are tiny ripples in spacetime caused when objects accelerate; they were first suggested to exist by Albert Einstein’s 1915 theory of gravity, general relativity. As binary black holes spiral around one another, they set the fabric of space ringing with gravitational waves. When they eventually collide, they create a high-frequency screech of gravitational waves, then a final gravitational wave “ringdown,” lasting a fraction of a second.

However, Einstein believed that even the most intense gravitational waves would be too faint and emitted at a distance too great to ever be detected on Earth. Yet, on Sept. 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected the gravitational wave signal GW150914 from the merger of stellar mass black holes about one billion light years away. The detection proved Einstein’s fears unnecessary, while the signal simultaneously proved his theory of general relativity correct.

An artist’s illustration of two black holes spiraling together, creating gravitational waves in the process. (Image credit: NASA)

“The story behind that was just so incredible. They started building LIGO in the 90s when I was doing my Ph.D., and I remember people thinking the idea of working on gravitational waves was pointless. Then, that breakthrough happened in 2015, and the field opened up completely,” Reagan said. “Now, if you’re not working with gravitational waves, people think you’re crazy. It’s totally changed the field. The sheer determination of what they did and how rigorous they were in their detections is unbelievable.”

Since 2015, LIGO and its collaborating instruments, Virgo in Italy and KAGRA in Japan, have detected a multitude of gravitational wave signals from colliding black hole pairs, merging neutron stars, and even mixed mergers between black holes and neutron stars.

“Seeing the ring-down signal, as predicted from the theory of two pretty massive solar mass black holes merging together, was a pretty incredible feat,” Issaoun agreed.

Intermediate-mass black holes finally show themselves

The discoveries discussed so far have concentrated on supermassive black holes, or black holes that sit at the hearts of galaxies and influence the realms’ development. These cosmic titans are born from a merger chain of increasingly larger and larger black holes. This means they end up with incredibly huge masses.

There are more diminutive black holes, however (relatively speaking, of course). Stellar-mass black holes are born when massive stars, with about eight times more mass than the sun or more, run out of the fuel supply needed for nuclear fusion in their cores and collapse, triggering a supernova. According to NASA, the masses of these black holes start at about five solar masses and range up to around 100 solar masses.

That means there is a vast mass gap between stellar mass black holes and supermassive black holes. But, in this gap, you’d expect the intermediate-mass black holes to dwell. Yet, much less is known about these medium-sized black holes, which should have a mass range of around a 100 solar masses to hundreds of thousands of solar masses. They’ve simply remained elusive.

An illustration showing the three types of astrophysical black holes, staring from the most massive on the left to the least massive on the right (Image credit: Robert Lea (created with Canva))

Several potential intermediate black hole discoveries have been made over the last 25 years, including GCIRS 13E in 2004. This was suspected to be the first intermediate-mass black hole found in the Milky Way galaxy, orbiting Sgr A* at a distance of around three light-years away. This, like many other potential sightings of intermediate mass black holes, has been disputed.

The most well-founded evidence of the existence of intermediate black holes came in 2020, when LIGO detected its biggest gravitational signal to date. The source of the signal, designated GW190521, was a merger of two stellar-mass black holes birthing a 142-solar-mass black hole located around 7 billion light-years away.

The James Webb Space Telescope finds ancient black holes

The method by which supermassive black holes grow to cosmic titans has already been discussed, but there is a bit of confusion about this process. Both mergers of smaller black holes and black holes feeding on surrounding matter to become bigger black holes should take billions of years.

That isn’t too problematic when we see supermassive black holes in the close and “recent” universe, but explaining large black holes starts to get challenging is when we see black holes with millions or billions of solar masses that existed before the universe was 1 billion years old. Though astronomers have been seeing this for some time, the James Webb Space Telescope (JWST), which launched on Christmas Day in 2021, has turned the conundrum into an issue that really needs to be addressed.

A timeline of the universe. Finding supermassive black holes billions of years after the Big Bang is expected, but discovering them around the time the first stars formed is more surprising. (Image credit: ESA)

If scientists were worried when other telescopes were turning up with results of supermassive black holes existing 800 million years after the Big Bang, they started getting very concerned when the JWST found such ultramassive black holes as early as when the universe was only 500 million to 600 million years old.

“The JWST launched just two years ago, and what it’s done in that time is quite extraordinary. It’s seeing what we think are supermassive black holes at very, very early times,” Reagan said. “The observations it’s making are both electrifying and confusing. There are questions arising about black holes because we’re probing into regions of the universe we haven’t probed before.”

While Reagan thinks this confusion could continue for the next two years, he suspects that the mystery of supermassive black hole growth in the early universe will be solved before the JWST completes its 10-year primary mission.

This could possibly be the result of the confirmation of heavy black hole seeds in the infant universe that gave supermassive black holes a “head start” in their growth process. Alternatively, the JWST may help reveal something about the environments in which these rapidly growing black holes are sitting that helps facilitate their rapid growth.

“I suspect things will start to even out, and we’ll start to get better statistics,” Reagan said. “It’s not a problem; it’s a challenge. This is a very interesting and exciting time in black hole physics.”

It is indeed, and Space.com is excited to be here after 25 years and to discover what the next quarter of a century holds for our understanding of black holes and all the mysteries they hide within themselves.