Black holes are colossal entities in the universe, even the smallest among them boasting a mass many times that of our Sun. However, one particular black hole is capturing considerable attention: the Supermassive Large Astrophysical Black hole (SLAB). These enigmatic objects can rival entire galaxies in size, or even exceed them.

The concept of SLAB originated from astronomers striving to unlock the mysteries of dark matter, a substance that constitutes approximately 85 percent of the universe’s mass. Researchers are testing various methods to find SLAB, including attempts to detect the light they emit, or observe their effects on spacetime. Recently, astronomer Brian Lackey proposed a new approach through the Breakthrough Listen project at the University of Oxford: seeking the shadow SLAB casts on the cosmic microwave background (CMB)—the residual light from the Big Bang.

We engaged in an enlightening conversation with Brian Lackey to discuss his pioneering ideas around these immense black holes, their potential discovery, and the implications for cosmology. Interestingly, Lackey’s journey into this field began through his main work focused on the search for extraterrestrial intelligent life.

<p><strong>Matt von Hippel: Your primary focus is not on black holes, but on the search for aliens through the Breakthrough Listen initiative. Let's start from there.</strong></p>
<p>
    Brian Lackey: Breakthrough Listen represents the most extensive effort to conduct SETI (Search for Extraterrestrial Intelligence), exploring technosignatures or signs of alien technology. Our primary approach involves analyzing radio waves; for instance, we search for unique radio transmissions within narrow frequency ranges, which we believe are challenging to create naturally. If detected and not attributed to human interference, these signals could indicate extraterrestrial technological activity.
</p>
<p>
    Aside from radio waves, we also look for ultra-short laser pulses. Few cosmic phenomena produce flash events lasting mere nanoseconds. Our collaborations with global observatories enable us to survey various technosignatures. We are among the leading groups involved in this search.
</p>
<p><strong>How does the quest for extraterrestrial intelligence intertwine with the fascinating concept of SLAB?</strong></p>
<p>
    As a theorist, I ponder what exists beyond our understanding, shaping our search for life. It is theorized that extraterrestrial intelligence may not only reside on Earth; they could construct vast structures greater than our solar system. One such concept, known as a Dyson swarm, comprises an array of light-absorbing elements encircling a star to harness its energy for their needs, whether for living spaces or computational power.
</p>
<p>
    A decade ago, speculation escalated regarding how advanced societies might operate at a galactic level. I proposed that instead of surrounding stars, these civilizations could deploy engineered dust particles in the interstellar medium, each containing a miniature computer. These dust particles would still capture starlight but remain cooler due to their distance, potentially around 3 or 4 Kelvin. The efficiency of colder environments increases computational performance.
</p>
<p>
    I further contended that, hypothetically, if one were to utilize a massive black hole—one exceeding 1000 trillion solar masses—they could effectively cool a vast array of small computers clustered nearby. This notion is speculative, yet it suggests that should such a colossal black hole exist, it might be detectable.
</p>
<p>
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                 alt="Festival participants dressed as aliens" 
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                 srcset="https://images.newscientist.com/wp-content/uploads/2026/06/17111110/SEI_301636489.jpg?width=300 300w, https://images.newscientist.com/wp-content/uploads/2026/06/17111110/SEI_301636489.jpg?width=400 400w" 
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                 data-caption="Brian Lackey's research involves discovering signs of advanced alien civilizations." 
                 data-credit="Jeff Kravitz/FilmMagic/Getty" />
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                <p class="ArticleImageCaption__Title">Brian Lackey investigates methods to unveil traces of advanced alien civilizations.</p>
                <p class="ArticleImageCaption__Credit">Jeff Kravitz/FilmMagic/Getty</p>
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<p><strong>It's astounding to consider alien societies employing immense black holes as energy sinks, akin to cooling systems in vehicles.</strong></p>
<p>
    Yes, that is indeed one possible application. Another hypothesis suggests that heat could flow into a black hole from surrounding cosmic microwave background radiation, functioning as a cosmic heat engine. This energy could harness heat flows to generate electricity on a galactic scale.
</p>
<p><strong>But doesn't the existence of SLAB conflict with our established understanding of the cosmos?</strong></p>
<p>
    Currently, we recognize two main types of black holes: stellar black holes, generally up to about 100 solar masses, and supermassive black holes found at galaxy centers, said to range from 1 million to tens of billions of solar masses.
</p>
<p>
    The prevailing belief is that supermassive black holes are indeed the universe's largest. As matter approaches a black hole, it generates significant radiation, potentially producing jets or winds that could counteract further growth. Consequently, it was assumed that a black hole exceeding 100 billion solar masses couldn't exist. However, this remains an open question.
</p>
<p><strong>You weren't the first to theorize about SLAB. Who initially considered their existence, and how could they grow so large?</strong></p>
<p>
    The idea was systematically developed by Bernard Carr, an astronomer from Queen Mary University of London, and his collaborators in 2020. They speculated that SLABs may have formed shortly after the Big Bang, occurring from fluctuations in the universe's density that could collapse into black holes. These hypothesized primordial black holes could manifest if such fluctuations spanned extensive cosmic regions.
</p>
<p>
    Carr pondered whether a population of black holes exceeding a trillion solar masses could ever be detected, suggesting it was feasible under the laws of physics—an avenue previously unexplored.
</p>

<p><strong>Primordial black holes also intrigue physicists as potential candidates for dark matter.</strong></p>
<p>
    The search for various dark matter types, such as weakly interacting massive particles (WIMPs), continues, as they have yet to be found in particle experiments. As researchers consider other alternatives, primordial black holes emerge as a compelling option.
</p>
<p><strong>Could SLAB itself constitute a significant portion of dark matter?</strong></p>
<p>
    Not within our galaxy, as they are intergalactic. However, diffuse dark matter may exist in the cosmos, potentially playing a role in the broader cosmic web that links galaxies, even if they don't influence individual galaxy rotations.
</p>
<p><strong>Is there any hope of discovering SLAB?</strong></p>
<p>
    Carr and his team have proposed methodologies to search for them. One potential indication of their presence might be their gravitational influence on nearby galaxies, drawing them together. If such black holes exist within intergalactic space, matter falling into them would heat up and emit radiation. So far, however, no signatures confirming this hypothesis have been found.
</p>
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                 alt="The Event Horizon Telescope (EHT) collaboration's polarized view of the M87 black hole." 
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                 src="https://images.newscientist.com/wp-content/uploads/2026/06/17111117/SEI_301636696.jpg" 
                 srcset="https://images.newscientist.com/wp-content/uploads/2026/06/17111117/SEI_301636696.jpg?width=300 300w, https://images.newscientist.com/wp-content/uploads/2026/06/17111117/SEI_301636696.jpg?width=400 400w" 
                 sizes="(min-width: 1288px) 837px, (min-width: 1024px) calc(57.5vw + 55px), (min-width: 415px) calc(100vw - 40px), calc(70vw + 74px)" 
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                 data-caption="The black hole M87*, imaged by the Event Horizon Telescope collaboration in 2019." 
                 data-credit="EHT collaboration" />
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                <p class="ArticleImageCaption__Title">M87*: The black hole at the core of a neighboring galaxy, captured by the Event Horizon Telescope collaboration in 2019.</p>
                <p class="ArticleImageCaption__Credit">EHT collaboration</p>
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<p><strong>Your recent research seeks different forms of evidence for SLAB within the cosmic microwave background. How do you pursue this?</strong></p>
<p>
    I aim to identify the shadow SLAB may cast. Images of Sagittarius A* and M87* reveal black holes appearing as "holes" surrounded by glowing halos. In principle, if a black hole were to possess the mass of a thousand suns, it could manifest as a sunspot against the cosmic microwave background.
</p>
<p><strong>So, did you uncover anything significant?</strong></p>
<p>
    We utilized existing CMB surveys with highly sensitive telescopes to search for subtle temperature variations. Although no such phenomena have been observed, it does not rule out the existence of SLAB, suggesting they are extremely rare or possibly nonexistent in our observable universe.
</p>

<p><strong>What implications would arise from discovering evidence of SLAB?</strong></p>
<p>
    Finding them would yield insights into events shortly after the Big Bang, possibly revealing unknown physical processes responsible for the formation of these gigantic black holes. This could herald exciting new physics previously unconsidered in our explorations.
</p>
<p><strong>Considering SETI and SLAB, what intrigues you most in current astronomical research?</strong></p>
<p>
    The oldest galaxies observable today date back approximately 13.5 billion years. However, there remains a gap leading to the cosmic microwave background, the earliest detectable radiation emitted shortly after the Big Bang, around 300 million years before the formation of the oldest galaxies. This period, known as the "Dark Age of the Universe," is crucial, yet largely unexplored. While we use tools like the James Webb Space Telescope to observe ancient galaxies, this crucial era remains elusive to us.
</p>
<p>
    It’s thrilling to ponder what treasures might lie within this unexplored era of cosmic history. SLAB is one possibility, but many other remnants of the Big Bang could await discovery.
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