Giant Black Holes Ruled Out as Dark Matter by Big Bang Afterglow

Physicists have struck another suspect from the list of possible explanations for dark matter. According to Universe Today, a new study in Physical Review Letters concludes that stupendously large black holes, known as SLABs, cannot account for a significant share of the invisible material that shapes galaxies.

The finding rests on the oldest light in the universe. If such black holes had existed in large numbers, they would have left a telltale mark on it, and the data show no such mark.

What are stupendously large black holes?

They are a hypothetical subtype of primordial black holes, objects that may have formed in the early universe. Universe Today reported that SLABs, if they exist, would have masses up to 10 billion billion times that of the sun. They are purely theoretical, in contrast to the supermassive black holes that astronomers do observe.

The idea is that they formed directly from dense patches of matter after the Big Bang. They would not have needed to go through the life cycle of a massive star, which ends in a supernova explosion.

How did researchers test the idea?

The team first modeled how SLABs would affect the cosmic microwave background, the relic radiation from the Big Bang. That light was released when the universe was about 380,000 years old. It looks mostly smooth, but sensitive instruments can pick up small fluctuations.

The researchers then compared their model with real observations, largely from the European Space Agency's Planck satellite. According to Universe Today, the actual background lacks the grainy texture that a population of SLABs would have imprinted on it. The authors write that such black holes would produce disturbances larger than current Planck limits allow, which excludes them as a significant part of dark matter.

The paper is titled “Closing the Mass Window for Stupendously Large Black Holes.” The lead author is Christopher Gerlach, a theoretical physicist at Johannes Gutenberg University in Mainz, Germany.

Why is old Planck data still useful?

Planck ended its mission in 2013, yet scientists keep extracting new results from its measurements. Universe Today said better computing power, new datasets that did not exist then and improved ways of filtering out interference from the Milky Way all help. Gerlach said that new insights 13 years after the shutdown show how important the satellite remains.

What does this mean for the search for dark matter?

Earlier work had already all but ruled out ordinary primordial black holes as the sole source of dark matter. Gerlach and his colleagues describe the new result as closing a remaining gap in the range of black hole masses, with Universe Today noting that SLABs ranged from 100 million to 10 billion billion solar masses in the scenario they tested.

Dark matter is still unexplained. Other candidates, including hypothetical particles such as WIMPs and axions, remain on the table. The study narrows the field rather than solving the puzzle, which fits the idea, raised by Universe Today, that dark matter may eventually be identified by a process of elimination.

Does it affect explanations for the early galaxies Webb found?

Yes. When the James Webb Space Telescope found early galaxies that were more massive than models of galaxy formation predicted, researchers looked for explanations. A 2022 study suggested that primordial black holes, making up a fraction of dark matter, could have helped those galaxies grow quickly.

The new authors write that their results challenge scenarios that attribute the early massive galaxy candidates to fluctuations caused by primordial black holes. Universe Today described that explanation as unlikely to be true given the results. The reported work is a published study, and the question of why Webb sees such massive early galaxies remains open.

Around Prague, based on reporting by Universe Today

Cover photo: Event Horizon Telescope / Wikimedia Commons, CC BY 4.0, cropped

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