Albert Einstein and Hubble may have helped us find an invisible black hole

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Adam McMaster is a PhD student at The Open University. He studies astronomy. Andrew Norton is a professor at The Open University. His main field of study is astrophysics education. This story was originally published on The Conversation.

Astronomers famously captured the first ever direct image (or “light map”) of a black hole in 2020, thanks to material glowing from its presence. But many black holes aren’t actually detectable. Now another team using Hubble has apparently finally discovered something nobody has ever seen before: a blackhole which is completely invisible. The research, which was posted online and submitted for review in the Astrophysical journal, has not been peer-reviewed.

Black holes are what remains after large stars die and the core collapses. They are incredibly dense. Gravity is so strong that nothing can escape them, even light. Black holes are important for astronomers because they can tell us about the way that stars die. By measuring the masses and sizes of black holes, we learn about what happened in stars’ final moments, when their core collapsed and their outer layers were expelled.

Black holes are by definition invisible because they’re able to trap light. But we can still tell if they’re there by looking at the way they interact with the other objects. Hundreds of small black hole candidates have been discovered by their interactions with other stars.

There are two different ways to detect such attacks. A black hole’s strong gravity can cause material pulled from its companion to spiral into it. As the material gets closer to the black hole, it heats up by friction. The hot material glowed brightly in X-ray lights, making the black hole appear, before being sucked into it and disappearing. You can also detect mergers of black holes by detecting their gravitational waves, which are tiny ripples in space time.

There are many rogue asteroids floating through space without interacting with any planets, however, making them difficult to detect. That’s not a problem, because if there aren’t any isolated black holes, then it means we won’t be able to learn anything about them.

New, dark horizons

To discover such an unseen black hole, the team had to combine two different kinds of observations over several years of time. This impressive achievement promises an entirely new way of finding the elusive class of isolated black hole.

Albert Einstein’s General Theory of Relativity predicts that massive objects will bend the path of light as it passes by them. That means that any bright light passing near an invisible black hole—not close enough to fall into it—will be bent similarly to light passing through a mirror. This is called gravitational lensesing, and can be seen when a foreground object align with a background object, causing its light to bend. It’s already been used to study clusters of galaxies, planets around other stars, and even human behavior.

The authors of this study combined two types of gravitational lenses in their search for black hole candidates. It began when they spotted a faint light from a faraway star suddenly appear brighter for a few seconds before returning to its previous state. They couldn’t see anything that was causing the magnification through gravitational lensing, though It suggested the object might be an isolated black hole, something which has never been seen before. The problem was that there could be a faint star nearby.

To figure out whether it was a black hole, or a faint star, required a lot of work. And that’s when the second type of gravitational lenses came in. The authors repeatedly took pictures of the star using Hubble for six years, tracking how far the star appeared from one picture to another.

Eventually they were able to calculate the mass and distance from the object which caused the gravitational lensing effect. They found it was approximately 7 times the mass of our sun, located about 5,500 light years away, which seems far away but is actually quite close. A star that large and that close should be obvious to us. Since we can’t see it, they concluded that it must be an isolated “black hole.”

It’s not easy to take so many observations with an observitory like Hubble. The telescope is very popular and there is a lot of competition for its time. Given the difficulty of confirming an item like this, you might conclude that the prospects for finding more items like this aren’t great Fortunately, we’re at the beginning of a new era in astronomy. Thanks to a new generation of telescopes, including the ongoing Gaia mission, and upcoming Vera Rubin observatory and Nancy Grace Roman space telescope, all of which will measure large parts of the sky repeatedly in unprecedented detail.

That’s a big deal for all areas of astronomy, too. We’re going to measure the entire sky regularly, allowing us to investigate changes on very short timescals. We’ll study things as varied as asteroid, exploding stars known as Supernovas, and planets orbiting other stars in new ways

When it comes to searching for invisible black holes, it means that instead of celebrating finding just one, it might become routine to find many. It will allow us to fill in some of the gaps in our understanding regarding the deaths of stars and their creation into black holes.

Eventually, the galaxies’ invisible black holes will find it much harder to conceal themselves.

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