In the last 25 years, black hole physicists have uncovered the unimaginable

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

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.

"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 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.

"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.

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.

"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.

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.

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."

— How do some black holes get so big? The James Webb Space Telescope may have an answer

— Brightest quasar ever seen is powered by black hole that eats a 'sun a day'

— Black hole-like 'gravastars' could be stacked like Russian tea dolls

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.