When astrophysicists observe the cosmos, they see different types of black holes. They range from gargantuan supermassive black holes with billions of solar masses to difficult-to-find intermediate-mass black holes (IMBHs) all the way down to smaller stellar-mass black holes.
But there may be another class of these objects: primordial black holes (PBHs) that formed in the very early Universe. If they exist, the Nancy Grace Roman Space Telescope should be able to spot them.
Stellar-mass black holes form when massive stars explode as supernovae. SMBHs grow over time by merging with other black holes. How IMBHs form is still unclear, but it could involve mergers between stellar-mass black holes or multiple stellar collisions in dense star clusters.
Primordial black holes, if they exist, didn’t have any of these mechanisms available to them.
“If we find them, it will shake up the field of theoretical physics.”
William DeRocco, postdoctoral researcher, University of California Santa Cruz.
Nobody knows if primordial black holes exist. They’re theoretical. No physical process we know of can form them. But the early Universe was much different.
New research published in Physical Review D shows how the upcoming Nancy Grace Roman Telescope could detect these primordial Earth-mass objects. It’s titled “Revealing terrestrial-mass primordial black holes with the Nancy Grace Roman Space Telescope.” The lead author is William DeRocco, a postdoctoral researcher at the University of California Santa Cruz.
“Detecting a population of Earth-mass primordial black holes would be an incredible step for both astronomy and particle physics because these objects can’t be formed by any known physical process,” lead author DeRocco said. “If we find them, it will shake up the field of theoretical physics.”
In the modern Universe, only stars with at least eight stellar masses can become black holes. Less massive stars will become neutron stars or white dwarfs. (The Sun will become a white dwarf.)
But things were different in the early Universe. During a period of rapid inflation, space expanded faster than the speed of light. In these unusual conditions, dense areas could have collapsed into PBHs. The scale of these objects is remarkably small. They would be the size of Earth or smaller and have event horizons about as wide as a coin.
The least massive of these ones would’ve disappeared due to evaporation. That’s what Stephen Hawking figured out. But some, the ones as massive as Earth, could’ve survived.
Even though they’re theoretical, there are some evidential hints of their presence. Those hints come from gravitational microlensing.
Two efforts have used microlensing to study objects in the Universe. One is OGLE, the Optical Gravitational Lensing Experiment. Another is MOA, Microlensing Observations in Astrophysics. OGLE found 17 isolated Earth-mass objects in space.
These objects could be PBHs, or they could be rogue planets. Unfortunately, it’s very difficult to differentiate on an individual basis. But since theory predicts the masses and the abundance of rogue planets, that could provide a way for the Roman Telescope to tell them apart from PBHs.
“There’s no way to tell between Earth-mass black holes and rogue planets on a case-by-case basis,” DeRocco said. “Roman will be extremely powerful in differentiating between the two statistically.”
In their research, the authors explain it more fully. “The key point is that though PBH and FFP events cannot be discriminated on an event-by-event basis, the two populations can be distinguished by the statistical distribution of their event durations.” Scientists think that Roman will find 10 times as many objects in this mass range than ground-based efforts like OGLE and MOA.
Finding primordial black holes would create a big upheaval.
“It would affect everything from galaxy formation to the universe’s dark matter content to cosmic history,” said Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore. Sahu wasn’t involved in the research but understands the impact the results would have. “Confirming their identities will be hard work and astronomers will need a lot of convincing, but it would be well worth it.”
If the Roman Space Telescope can find the black holes and confirm them, it could be a defining moment in astronomical history. The discovery would be strong evidence in favour of a period of rapid inflation in the early Universe, an epoch that so far is unproven. Physicists think there must have been a period like this as it helps explain so much else about the Universe.
More excitingly, these primordial black holes could comprise a percentage of dark matter. A small percentage, but a massive improvement over our current understanding of what dark matter is. Scientists keep looking for things like WIMPs (Weakly Interacting Massive Particles) and other particles that could be dark matter, but they never find them.
“The nature of dark matter remains one of the most pressing open questions in fundamental physics. While multiple lines of compelling evidence indicate its existence, its microphysical nature remains unknown,” the authors explain.
The elegant thing about the Roman and PBHs is that it won’t require a special effort to find them. The Roman will already search for planets. “Roman’s Galactic Bulge Time Domain Survey is expected
to observe hundreds of low-mass microlensing events, enabling a robust statistical characterization
of this population,” the authors write in their paper.
Each space telescope we launch is a new window into some aspect of the Universe. The Nancy Grace Roman Space Telescope sure will be. “Though its Galactic Bulge Time Domain Survey targets bound and unbound exoplanets, we have shown that it will have unprecedented sensitivity to physics beyond the Standard Model as well,” DeRocco and his co-researchers write in their paper. That’s because it can “probe the fraction of dark matter composed of primordial black holes,” they write.
“This is an exciting example of something extra scientists could do with data Roman is already going to get as it searches for planets,” Sahu said. “And the results are interesting whether or not scientists find evidence that Earth-mass black holes exist. It would strengthen our understanding of the universe in either case.”
And who doesn’t want a stronger understanding of the Universe?