This physicist is hunting for the biggest black hole in the universe

No black hole could ever be described as small. Even the most diminutive are many times the mass of the sun. But there is a now talk of a variety that throws shade on all others: the stupendously large black holes, otherwise known as SLABs. These dark monsters could be as massive as whole galaxies or even bigger.

The idea of SLABs first cropped up a few years ago, partly as a by-product of astronomers’ desperation to unmask dark matter, the mysterious substance that makes up 85 per cent of all matter in the universe. Since then, they have looked for SLABs by trying to detect the light they would emit or the way they would bend space-time. But earlier this year, astronomer Brian Lacki at the Breakthrough Listen project based at the University of Oxford proposed another way to detect SLABs, which involves searching for the shadows they cast on the cosmic microwave background (CMB), the light released just after the big bang that now suffuses the whole universe.

New Scientist spoke with Lacki about the wild new idea of these gigantic black holes, our chances of finding one and how it would shake up cosmology if we did. For his part, Lacki first got interested in the topic in a surprising way: thanks to his day job searching for extraterrestrial intelligence.

Matt von Hippel: You first got interested in SLABs through your work at the Breakthrough Listen initiative. So, your main job is hunting aliens, not black holes. Let’s start there.

Brian Lacki: Breakthrough Listen is the largest effort to conduct SETI, which is the search for extraterrestrial intelligence. It is the scientific search for technosignatures, or signs of evidence of alien technology. The primary way we do this is by looking at radio waves. We look for signs of, for example, radio transmissions that occupy a very narrow range of frequencies. We think those are very hard to produce naturally, so if we find one, and if we can rule out that it’s radio interference from us – which is very hard – then we might have a sign of some kind of extraterrestrial technology.

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There are other possible ways to look for extraterrestrial intelligence – for example, laser pulses that are very short. We don’t really think that there’s much in the universe that produces a flash of light that only lasts for a nanosecond. We partner with a bunch of different facilities and programmes around the world to look for these technosignatures. And we’re pretty much the largest group that does it.

And how does the search for extraterrestrial intelligence connect to these whopping black holes?

I’m a theorist. I consider what might be out there, which informs what kinds of things we can look for in our search for life. People have proposed that extraterrestrial intelligences might not just sit on their planets; they might build very large structures the size of a solar system or even larger. One of them is called a Dyson swarm, where you surround a star with a cluster of elements that absorb the light and use it to power whatever that society needs, whether it’s a habitat, or computation or anything.

About 10 years ago, people were thinking even bigger and looking at how this kind of thing might work on a galactic scale. I suggested that instead of putting swarms around stars, you might have very advanced societies putting artificial dust grains out in the interstellar medium, each containing a microscopic computer. These specks of dust would still absorb starlight, but because they are so much further away, they would be very cold. We’re talking just above the temperature of the cosmic microwave background, perhaps 3 or 4 Kelvin. The reason an intelligence might do this is efficiency; the colder you get, the more computation you can do with a piece of energy.

Then I took things a step further and suggested, if you really want to push things as far as possible, and use as many tiny computers as possible, you might even want to harness a vast black hole that was a quadrillion solar masses to cool your tiny computers because you could fit so many of them in the space around it. All this is speculation, but the point is: if such a black hole did exist, then we would, in principle, be able to detect it.

That is wild. You were imagining alien societies using these stupendously large black holes as a heat sink, like the coolant in a car engine.

Yes, that’s one possible use. Another idea is that they might create a heat engine where heat flows from, say, the cosmic microwave background into the black hole. You could use such a flow of heat to generate power on a cosmic scale.

Hold on, though – is a SLAB not ruled out by what we already know about the universe?

We know of two main types of black holes. There are the stellar-mass black holes, which are typically up to around 100 solar masses, and then there are the supermassive black holes that are at the hearts of galaxies, which range from around a million solar masses to a few tens of billions.

Most people think supermassive black holes are the largest ones. When gas gets really close to the black hole, it produces a lot of radiation. It can produce jets of particles, it can produce winds, and all of these act as a pressure that pushes matter out. And so, it was thought that, because of this choking-off effect, a black hole couldn’t grow to be larger than about 100 billion solar masses. But we don’t actually know that for sure.

You weren’t the first to think about SLABs. Who came up with the idea that they might exist? And how do they get so big anyway?

The first person to think about them systematically – and to coin the name – was astronomer Bernard Carr at Queen Mary University London. Carr and his collaborators proposed in 2020 that SLABs could have been formed shortly after the big bang. It’s thought that maybe there were rare fluctuations in the otherwise uniform background density field of the universe shortly after the big bang, and those could collapse into black holes. These hypothetical black holes from the dawn of time are called primordial black holes. And if those fluctuations were large, spanning huge regions of space, then you might get these stupendously large black holes.

Carr was asking this question: if there was a population of black holes that were a trillion solar masses or larger, would we know about it? The reason he proposed it, I think, is because it’s a possibility, and people just hadn’t considered it. It seems possible under the laws of physics. Are they there and we’ve just not thought to look for them? We should check.

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And primordial black holes in general have interested physicists because they wonder if they could be the true identity of dark matter.

People have searched for more particle types of dark matter, like weakly interacting massive particles, known as WIMPs. It was thought that they should be starting to show up in our particle experiments, but they haven’t appeared yet. So, people have been considering other dark matter candidates, one of which is primordial black holes.

Could SLABs themselves be a large fraction of dark matter?

Certainly not within a galaxy, because they would outweigh the galaxy. But there might be some diffuse dark matter out there, which they can contribute to. They wouldn’t affect how single galaxies rotate, but they could still be part of the cosmic web of dark matter that connects different galaxies.

Have we got any hope of ever finding a SLAB?

Carr and his collaborators proposed several ways you might look for them. One sign is that if you had something really massive out there in intergalactic space, the gravity of that is going to start to pull in other galaxies. We might observe all these galaxies barrelling towards this thing that is invisible. They also considered, if this black hole is sitting in intergalactic space, matter is going to fall into it from the intergalactic medium; it’s going to heat up and release radiation as it falls in. So, they looked for that radiation. But we haven’t seen any hints of these kinds of signatures yet.

Your recent paper looks for a different type of evidence of SLABs in the cosmic microwave background. How do you go about searching for it?

The particular type of evidence I’m looking for is its shadow. You may have seen the images of Sagittarius A* and M87*, which are nearby supermassive black holes, where it literally looks like a black “hole” surrounded by this ring of light from the matter falling in around it. If you have a black hole against a background of light, the black hole absorbs some of that light, so you actually literally see a black “hole” against the sky. Some of these black holes can be very large. If you have a quintillion-solar-mass black hole, then it’s significantly larger than a galaxy, so that forms a black spot against the cosmic microwave background.

So, come on: have you found anything?

We have existing surveys of the CMB from very sensitive telescopes and we have looked at these to see if we can spot little dips and bumps in its temperature, things much more subtle than a black spot. The fact that we don’t see that kind of thing so far doesn’t rule out SLABs, but it does tell us that, if they do exist, they’re very rare. Our analysis has shown that black holes bigger than a hundred quadrillion times the mass of the sun are so rare that there aren’t any within our observable universe.

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If we did ever find evidence for a SLAB, what could we learn?

If we found them, they would, more likely than not, be primordial. That would tell us that something happened a bit before the cosmic microwave background was produced, a few millennia after the big bang. There was some unknown physical process that, once in a while, would produce these stupendously large black holes. And I think that would be exciting, because it would be an indication of some new physics, which we weren’t even aware was something to look for.

Apart from SETI and SLABs, what are you most excited about right now in astronomy?

The oldest, most distant galaxies we can see at the moment are about 13.5 billion years old. But there’s a gap between that and the cosmic microwave background, which is the very oldest thing we can see, having been released shortly after the big bang, around 300 million years before the oldest galaxies. What’s interesting about this period – this cosmic dark age, as it’s called – is that if you consider how far it is expanded out and how fast the universe was expanding at the time, it’s actually a surprisingly large part of the universe. Even with things like the James Webb Space Telescope, where we’re seeing those furthest, oldest galaxies, we can’t see back to this period of cosmic history.

That gets the mind going. What gems might be out there in this unexplored volume of space? SLABs are one thing, but there might be other things out there that are left over from the big bang, just waiting to be found.