searching for – A physicist tied to Breakthrough Listen says the hunt for “stupendously large black holes” could shift from radio searches and gravitational effects to the cosmic microwave background—by looking for the vast shadows such objects might cast. If such SLABs were
The idea starts with a contradiction that feels almost unfair: you can’t make a black hole small. Even the smallest ones still outweigh the sun by many times. And yet the newest twist in black-hole hunting asks people to imagine something even more extreme—so large that it could be as massive as entire galaxies. or bigger.
They’re being discussed as “stupendously large black holes,” or SLABs. The speculation doesn’t come from a fantasy of science fiction; it grew from astronomers’ very real desperation to understand the universe’s invisible matter. Dark matter makes up 85 per cent of all matter in the universe, but it doesn’t shine. So researchers have been trying to catch what it might be—using light that should exist. the way gravity should bend space-time. and signals that might not be natural.
Earlier this year. astronomer Brian Lacki—working with the Breakthrough Listen project based at the University of Oxford—proposed a different kind of search. Instead of looking for light a SLAB might emit. or the distortion it might create. Lacki suggested scanning the cosmic microwave background (CMB): the faint afterglow released just after the Big Bang. which still suffuses the universe. The method hinges on a simple visual idea. If a black hole sits in front of a background of light, it can carve out a literal dark patch.
The conversation about SLABs lands in an unexpected place when Lacki explains how he first became interested. His day job, after all, is hunting for signs of extraterrestrial intelligence. Breakthrough Listen is the largest effort to conduct SETI. the search for extraterrestrial intelligence. focused on technosignatures—evidence of alien technology. The primary approach is to look at radio waves, searching for radio transmissions in a very narrow range of frequencies. Such signals are considered hard to produce naturally. If a narrow-band transmission is found—and radio interference from Earth can be ruled out. which Lacki says is very hard—then it could be a sign of something technological.
There are other routes too. The project also considers laser pulses that last for extremely short durations. though Lacki notes that there probably isn’t much in the universe that produces a flash of light only lasting for a nanosecond. Breakthrough Listen partners with a range of facilities and programmes worldwide. but Lacki describes Breakthrough Listen as “pretty much the largest group” doing the work.
And that’s where the SLABs begin. Lacki is a theorist—someone who thinks about what could exist out there and what kinds of things humanity might be able to detect. He points to one idea that could connect civilizations and black holes: extraterrestrials might not simply build on planets. They could construct enormous structures—on scales as large as a solar system or beyond.
One famous concept is the Dyson swarm. where elements are arranged around a star to absorb its light and use it to power a society’s needs. from habitats to computation. But Lacki says that about 10 years ago, people started thinking even bigger and pushed this to a galactic scale. His own suggestion took a different direction. Rather than swarms around stars, he proposed that extremely advanced societies might distribute artificial dust grains throughout the interstellar medium. Each grain would contain a microscopic computer. These specks would absorb starlight. but because they would be far away. they would stay very cold—around 3 or 4 Kelvin. just above the temperature of the cosmic microwave background.
The pitch is efficiency. The colder the system, the more computation you can extract from a given amount of energy. Then Lacki took it further still: if you wanted to push the idea to the extreme—using as many tiny computers as possible—you might “harness a vast black hole” with a mass of a quadrillion solar masses to cool them. This is all speculation, but the claim is precise: if such a black hole existed, it would be detectable.
In that vision, the SLAB becomes a kind of cosmic infrastructure. Lacki compares one use to a coolant system, like the way a car engine relies on cooling. He also describes another possibility: a heat engine in which heat flows from the cosmic microwave background into the black hole. generating power on a cosmic scale.
But the question immediately turns to plausibility. Are SLABs ruled out by what we already know?
Black holes come in two main categories. There are stellar-mass black holes, typically up to around 100 solar masses. Then there are supermassive black holes at the hearts of galaxies. ranging from around a million solar masses to a few tens of billions. People tend to assume supermassive black holes are the biggest. The traditional reasoning goes like this: when gas comes too close to a black hole. the infalling material produces lots of radiation and can launch jets and winds. That radiation and outflow act like a pressure that pushes matter away. For that reason, many researchers believed growth would choke off before the black hole could exceed roughly 100 billion solar masses.
Lacki pushes back on the certainty. “We don’t actually know that for sure,” he says.
SLABs weren’t born with Lacki either. The idea of SLABs, and the name itself, traces back to astronomer Bernard Carr at Queen Mary University London. Carr and collaborators proposed in 2020 that SLABs could have formed shortly after the Big Bang. They argued that rare fluctuations in an otherwise uniform density field shortly after the Big Bang could have collapsed into black holes. Such black holes are called primordial black holes.
Carr’s focus, Lacki says, was on scale. The question was: if there were a population of black holes of a trillion solar masses or larger. would anyone know?. His reasoning, Lacki says, wasn’t a claim that they definitely exist. It was an argument that the laws of physics might allow it—and that people might not have considered looking for them.
That kind of “could it be?” thinking has also made primordial black holes interesting to physicists searching for dark matter. If another class of particle dark matter—such as weakly interacting massive particles. or WIMPs—were the answer. particle experiments should have started seeing it. But WIMPs have not shown up. So attention has drifted to other candidates, including primordial black holes.
Could SLABs themselves be a large fraction of dark matter?. Lacki’s answer is limited. Within a galaxy, certainly not. The mass would outweigh the galaxy. But there could be some diffuse dark matter outside galaxies that SLABs might contribute to. They wouldn’t change how single galaxies rotate. but they could still be part of the cosmic web—the structure that connects galaxies.
So where would the evidence show up?
Carr and collaborators proposed several ways to look. One sign relies on gravity in intergalactic space: a really massive object would begin pulling on other galaxies. and researchers might see galaxies barrelling toward something invisible. Another is radiation. If matter falls into such a black hole from the intergalactic medium, it would heat up and release radiation. Carr’s team searched for these signatures. Lacki says no hints have emerged.
That brings the focus back to Lacki’s own recent work and the central idea: don’t just hunt for what the black hole emits—hunt for what it blocks.
He points to how nearby black holes like Sagittarius A* and M87* have been imaged by the Event Horizon Telescope collaboration. The point isn’t the size of those objects; it’s the geometry. In those pictures. the black hole appears as a dark “hole” surrounded by a ring of light from matter falling in. A black hole absorbs light behind it. If you put that effect against the cosmic microwave background, the result can be a black spot in the CMB.
The math matters, but the picture is intuitive: if there were a quintillion-solar-mass black hole, it would be significantly larger than a galaxy, and it could leave a detectable shadow against the cosmic microwave background.
So, have surveys found it?
Lacki says existing CMB surveys from very sensitive telescopes have been analyzed for subtle dips and bumps in temperature—patterns much more subtle than an obvious black spot. The absence of that kind of signal doesn’t disprove SLABs. Instead, it narrows the odds. Lacki says the analysis shows 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.
The result is not the final word, but it does change the stakes. If SLAB evidence were found, Lacki expects it would most likely be primordial. That would suggest that something produced these stupendously large black holes a bit before the cosmic microwave background was produced—after a few millennia following the Big Bang. It would also imply “some new physics” that scientists hadn’t been looking for.
Lacki’s curiosity doesn’t end with black holes. He says he’s also drawn to a dark period in cosmic history—one that sits between the oldest galaxies we can currently observe and the moment when the CMB was released. The oldest, most distant galaxies are about 13.5 billion years old. The CMB, by contrast, was released around 300 million years before those galaxies. That gap is called the cosmic dark age.
What makes it difficult to reach isn’t just distance. Lacki says that when you factor in how the universe expanded and how fast it was expanding at the time. this dark age occupies a surprisingly large part of the universe. Even with the James Webb Space Telescope, humanity still can’t see back to this period of cosmic history. For Lacki. that makes it feel like an open door—an unexplored volume of space where “gems” might still be waiting.
SLABs are one possibility. They might not be the only remnants from the Big Bang, either—just the ones that, for now, are pushing the CMB into the spotlight again.
SLABs black holes cosmic microwave background CMB primordial black holes dark matter Breakthrough Listen SETI Brian Lacki technosignatures Sagittarius A* M87* James Webb Space Telescope