Black holes are about as mysterious as deep space gets, with plenty of puzzles that scientists have yet to solve. One conundrum is how some supermassive black holes became so big early in the universe’s lifespan, but a new study may have solved it by proving that there are a lot more black holes than once thought.
The study, published in The Astrophysical Journal Letters, details how scientists used NASA’s Hubble Space Telescope to discover black holes that they had not previously found. In short, the researchers took pictures of space and compared them to pictures that were taken in the same spots 15 years prior. They then compared the brightness of some celestial bodies to help them identify more black holes.
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This works because black holes don’t have the same brightness all the time. As they feed on nearby space objects — a process known as accretion — they brighten up temporarily as the material gets swallowed up. Once swallowed, black holes become dimmer. Thus, as researchers looked at data over 15 years, they were able to focus on changes in brightness in some celestial bodies and identify them as black holes.
“It turns out that there are several times as many black holes residing in ordinary early galaxies than we originally thought,” said Matthew Hayes, lead author in the study. “Other recent, pioneering work with the James Webb Space Telescope has begun to reach similar conclusions. In total, we have more black holes than can form by direct collapse.”
More black holes help solve a problem
The puzzle that scientists couldn’t figure out was how supermassive black holes existed in early galaxies in the first place. Matthews says that during accretion, a black hole produces a huge amount of radiation and this limits how fast a black hole can grow. Thus, there are supermassive black holes from the infancy of the universe that are bigger than they ought to be since they haven’t yet had enough time to “eat” enough material to grow that large.
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“Many of these objects seem to be more massive than we originally thought they could be at such early times — either they formed very massive or they grew extremely quickly,” Alice Young, co-author of the study, told NASA.
According to Matthews, the existence of so many black holes opens up the possibilities of how they were formed since there are simply too many for it all to be caused by the same method.
“Stars form by gravitational contraction of gas clouds: if significant numbers of dark matter particles can be captured during the contraction phase, then the internal structure could be entirely modified — and nuclear ignition prevented,” says Matthews. “Growth could therefore continue for many times longer than the typical lifetime of an ordinary star, allowing them to become much more massive.”
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In other words, supermassive black holes in the infant universe could have come from a dark star collecting material and then eventually collapsing into a supermassive black hole, which would explain why such big black holes existed back before they were supposed to.
Matthews says the next steps are to use the James Webb Telescope and its increased sensitivity to further study these black holes and find out how many of them really existed in the early universe.
