Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “Ask a Spaceman” and “Space Radio,” and author of “How to Die in Space.” Sutter contributed this article to Space.com’s Expert Voices: Op-Ed & Insights.
The largest black holes in the universe formed surprisingly quickly, when the cosmos was less than a billion years old. That was so early that these black holes may not have formed from the deaths of massive stars as some theories have proposed but instead may have originated in the first second of the Big Bang.
To test this possibility, a team of astrophysicists has proposed a radical idea: The elements around these giant black holes may be subtly different from the cosmic average, retaining a relic memory of the young universe.
Related: Black holes of the universe (images)
Too big to fail
As the name suggests, supermassive black holes (SMBHs) are gigantic. The smallest ones are millions of times more massive than the sun, and the biggest ones — found in the centers of huge galaxies — reach hundreds of billions of solar masses. Finding such giant black holes in the modern universe isn’t that much of a surprise, since those black holes have had billions of years to gorge on gas and dust (and other black holes).
But recently, astronomers have begun spotting SMBHs in the early universe. Already, we know of over 200 SMBHs that existed when the universe was younger than a billion years and one SMBH that formed when the universe was younger than 700 million years.
That means they formed fast. Too fast. We understand how black holes form in the present era. When a giant star dies, it leaves behind a black hole up to a few dozen times the mass of the sun. That black hole feeds on its surrounding material, finds other black holes and merges with them, and if it’s lucky, eventually gains supermassive status.
The problem is that these processes take time. When the universe was less than a billion years old, the first stars and galaxies were just starting to form. Generating SMBHs in the tight time required stretches the limits of known astrophysical processes.
A primordial origin
So perhaps the SMBHs in the universe didn’t originate from normal astrophysical processes, like the deaths of stars and a steady diet of gas. Perhaps these giant black holes originated in the momentous early moments of the Big Bang.
The early universe was an extreme place. Densities and pressures were high enough to fuse the fundamental forces of nature into unified fields. In the first few seconds, it was even too hot for protons and neutrons to congeal before getting torn apart. In those tumultuous times, it may have been possible for extreme density contrasts to appear spontaneously. And where there are extreme density contrasts — where a lot of mass gets piled into a very tiny volume — black holes can form.
These are the so-called primordial black holes, thought to possibly have formed through exotic interactions in the Big Bang. Astronomers have spent decades searching for them, especially through probes like the cosmic microwave background, the light left over from when the universe was 380,000 years old. All those searches have turned up empty, ruling out almost all models of primordial black hole formation.
The key word there is “almost.” One class of primordial black hole is potentially allowed by observations: black holes with about 100,000 times the mass of the sun that formed within the first second of the Big Bang. Those black holes would quickly gobble up any surrounding matter, gorging themselves until they became the SMBHs we observe in the young cosmos.
But how could we tell the difference between astrophysical and primordial giant black holes?
Something tastes funny
The answer, a team of astrophysicists proposes in a paper published to the preprint server arXiv, is to stare at the black holes really, really hard.
The trick is that primordial black holes didn’t just sit there in the infant universe, minding their own business; they interacted with and affected their surroundings. This is how we can rule out many models, because they would disrupt the hot plasma of the Big Bang so much that they would skew our observations.
Primordial black holes with masses of 100,000 suns would, ironically, have a much subtler effect. There wouldn’t be enough of them to seriously disrupt the physics of the early universe, so they would survive largely undetected. But an important epoch would come just a few minutes after their formation: the nucleosynthesis era, when the first light elements formed out of the hot, dense soup of the Big Bang.
Physicists understand this era remarkably well, because it follows the same kinds of physics as nuclear reactors and atomic bombs. Primordial black holes wouldn’t totally disrupt this process, leaving the amount of hydrogen and helium largely the same throughout the cosmos, but they would influence their surroundings. Nuclear processes would change near the black holes because of their extreme gravity, slightly altering the resulting mix of elements.
If the gas surrounding those black holes could maintain a memory of that era, the material we observe around SMBHs would have a different composition than the cosmic average. For example, the authors of the new paper found that primordial black holes could enhance the amount of helium by around 10% and deplete the amount of lithium by around 10%.
The authors admit that observing this difference would be challenging, but they stress that instruments such as NASA’s soon-to-launch James Webb Space Telescope might be up to the task. Observing this elemental fingerprint might not only reveal the origins of SMBHs themselves but also give astronomers an invaluable window into the earliest moments of the Big Bang.
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