Physicist Searches for Largest Black Hole in the Universe

Black holes are colossal entities, with the smallest ones boasting masses several times that of the Sun. Among them, a particularly vast black hole, known as SLAB, has emerged in discussions within the scientific community. These gargantuan black holes can rival, or even surpass, entire galaxies in size.

The concept of SLAB first surfaced years ago, born from astronomers’ pursuit to unravel dark matter’s true nature—this enigmatic substance constitutes approximately 85% of the universe’s total mass. Researchers have since been actively seeking SLABs by striving to detect emitted light or analyze spacetime distortions. Earlier this year, astronomer
Brian Lackey, part of the Breakthrough Listen initiative at the University of Oxford, proposed a novel method: looking for the shadow cast by SLABs on the cosmic microwave background (CMB)—the radiation emitted shortly after the Big Bang that permeates the universe.

New Scientist recently interviewed Lucky about his groundbreaking insights into these massive black holes and the potential implications for cosmology. Curiously, Lackey’s interest in SLABs grew out of his primary focus on the search for extraterrestrial intelligent life.

Matt von Hippel: Your work with the Breakthrough Listen initiative primarily targets alien hunting rather than black holes. Could you elaborate?

Brian Lackey: Breakthrough Listen is the most extensive SETI (Search for Extraterrestrial Intelligence) initiative, dedicated to discovering technosignatures—indications of advanced alien technology. Our primary approach is monitoring radio waves for narrow frequency ranges that are challenging to generate naturally. If we detect such signals and can eliminate terrestrial interference, it may indicate the presence of extraterrestrial technology.

There are also other methods such as monitoring brief laser pulses, as few natural phenomena in the universe seem capable of producing light flashes lasting only nanoseconds. Collaborating with various global programs and facilities, we’re among the largest teams dedicated to this endeavor.

How does the search for extraterrestrial intelligence intersect with the study of massive black holes?

As a theorist, I explore possibilities in the cosmos. Speculative discussions surrounding extraterrestrial life suggest that advanced civilizations may construct massive structures beyond our solar system. One concept, the Dyson swarm, surrounds a star with a cluster of light-absorbing elements to harness energy for societal needs.

A decade ago, thinkers began to theorize grander schemes—envisioning highly advanced societies placing artificial dust particles throughout interstellar space, each housing microcomputers. These particles would absorb light, remaining much colder than nearby stars, thus optimizing energy efficiency for computational tasks.

Taking it a step further, I proposed that if such intelligent beings wanted to maximize the number of tiny computers, they might harness a supermassive black hole—one with a mind-boggling mass, potentially upwards of 1,000 trillion solar masses—to effectively cool their systems. This remains speculative; however, the existence of such a black hole could, theoretically, be detectable.

It’s a fascinating concept: the idea that alien civilizations might utilize these gargantuan black holes as cosmic cooling systems.

Indeed, one possible application is harnessing heat flowing into a black hole, potentially from the cosmic microwave background, creating a heat engine that could generate electricity on a grand scale.

But doesn’t the existence of SLAB contradict our current understanding of the universe?

There are principally two types of black holes: stellar-mass black holes, up to about 100 solar masses, and supermassive black holes, found at galactic centers, which range from around 1 million to tens of billions of solar masses.

Many believe supermassive black holes are the largest in existence. As matter approaches a black hole, immense radiation is generated, producing particle jets or winds that exert pressure, inhibiting further growth—hence the belief that black holes shouldn’t exceed about 100 billion solar masses. However, this remains uncertain.

Who was the pioneer behind the SLAB theory and what mechanisms allow them to attain such enormous sizes?

The first systematic exploration of SLAB and its nomenclature was conducted by Bernard Carr from Queen Mary University of London. In 2020, he and his collaborators speculated that such black holes might have formed shortly after the Big Bang. They theorized that large fluctuations in the universe’s uniform density could have collapsed into these colossal black holes, often referred to as primordial black holes.

Carr postulated: if a population of black holes exceeding a trillion solar masses exists, would we even be aware of their presence? His suggestion stemmed from the possibilities within the laws of physics—are they real, and have we simply overlooked them?

Primordial black holes have intrigued physicists as potential dark matter candidates, correct?

Absolutely. Science has investigated various forms of dark matter, such as weakly interacting massive particles (WIMPs), which were expected to appear in particle experiments but haven’t yet been found. Consequently, primordial black holes have emerged as plausible alternatives.

Could SLAB possibly constitute a significant portion of dark matter?

Not within galactic boundaries; they are intergalactic phenomena. However, they might contribute to diffuse dark matter outside galaxies, which could form a cosmic web linking multiple galaxies.

Is there hope for finding SLAB?

Carr and his collaborators proposed various methods for detecting SLABs. A potential indicator would be their gravitational influence, pulling galaxies toward them. Observing multiple galaxies accelerating towards an unseen mass could be a significant clue. Furthermore, if such black holes were present, matter falling into them would heat up and emit radiation, although no such signatures have been detected thus far.

Your recent research seeks evidence of SLAB within the cosmic microwave background. What’s the approach?

The evidence we are searching for relates to the shadow cast by a SLAB. You may be aware of the images showing Sagittarius A* and M87*, both supermassive black holes appearing as dark “holes” surrounded by luminous material. If a black hole had the mass of a thousand solar masses, it would substantially overshadow a galaxy, creating a shadow against the cosmic microwave background.

So, have you discovered anything?

We analyzed existing cosmic microwave background surveys using highly sensitive telescopes to identify subtle temperature dips or variations indicative of SLABs. Our findings show no detectable signatures; this does not eliminate their possible existence but suggests that they are extraordinarily rare, if they exist at all. According to our data, black holes exceeding 100 quintillion solar masses are exceedingly uncommon, and none have been detected within our observable universe.

What would the implications be if we discovered evidence of SLAB?

Finding SLABs would indicate a significant event shortly after the Big Bang but just before the formation of the cosmic microwave background. This points to unknown physical processes that might have produced such immense black holes, which could revolutionize our understanding of physics.

Apart from SETI and SLAB, what excites you most in astronomy today?

The most ancient galaxies we can observe date back 13.5 billion years. However, there exists a time gap between these galaxies and the cosmic microwave background, the universe’s oldest radiation emitted shortly after the Big Bang—around 300 million years before any of the known galaxies formed. This era, known as the Dark Age of the Universe, is expansive concerning the universe’s age and raises the question of what mysteries it holds. With tools like the James Webb Space Telescope, we can glimpse the earliest galaxies, but this period remains uncharted territory. The search for SLABs is part of this tantalizing exploration, alongside potential remnants waiting to be discovered.