Frontier Adventure

Roman Could Finally Tell Us if Primordial Black Holes Exist

When the Universe erupted into existence with the Big Bang, all of its matter was compressed into a tiny area. Cosmologists theorize that in some regions, subatomic matter may have been so tightly packed that matter collapsed into primordial black holes. If these primordial black holes exist, they’re small, and they could be hiding among the population of free-floating planets.

Black holes are nature’s most confounding objects. Einstein’s theory of relativity predicted their existence, but he disagreed that black holes could actually form. As other scientists worked with the equations, they eventually showed that black holes could exist.

Now we know they do, and we know they’re so dense that they warp space-time and pull everything towards them. Even light can’t escape.

Researchers have been trying to uncover the family tree of black holes. They know that some massive stars will collapse in on themselves near the end of their lives and form stellar mass black holes. They know that supermassive black holes (SMBHs) exist in the heart of large galaxies like the Milky Way. There’s also increasing evidence for intermediate-mass black holes (IMBHs,) which sit in between the smaller stellar mass black holes and the gargantuan SMBHs.


This chart illustrates the relative masses of super-dense cosmic objects, ranging from white dwarfs to the supermassive black holes encased in the cores of most galaxies. Primordial black holes, if they exist, sit between neutron stars and stellar black holes. Image Credit: NASA/JPL-Caltech

But what about primordial black holes (PBHs)? If they exist, they formed long before the first star burst into life. They can be any size, according to theory, and could have played a role in galaxy formation. In the struggle to understand how black holes become so massive, PBHs could fill in an important blank spot. There are also tantalizing indications that if they do exist, they could be components of dark matter.

New research shows how NASA’s Nancy Grace Roman Space Telescope could find PBHs. They could be hiding among a mysterious population of low-mass objects. The objects could be free-floating planets (FFPs,) also called rogue planets, or they could be PBHs, and the upcoming space telescope could have the power to find them.

The paper is “Rogue worlds meet the dark side: revealing terrestrial-mass primordial black holes with the Nancy Grace Roman Space Telescope.” The lead author is William DeRocco from the Physics Department at UC Santa Cruz.

The Nancy Grace Roman Space Telescope will be a powerful infrared telescope when launched in a few years. It has some large astrophysical meals on its observing menu, like measuring dark energy, relativity, and space-time’s curvature. It’ll also look for low-mass objects as diminutive as Mars that are unanchored to any star. In doing so, it might shed light on primordial black holes.

To look for these low-mass objects and determine if they’re FFPs or PBHs, the Roman Space Telescope needs help. It’ll make use of gravitational micro-lensing to search the sky for these elusive objects. They can be any size, from as small as a single subatomic particle to as large as a small planet.

“Gravitational microlensing is one of the strongest observational techniques to observe nonluminous astrophysical bodies,” the authors write in their paper. “Existing microlensing observations provide tantalizing evidence of a population of low-mass objects whose origin is unknown.”

The question is, are these puzzling objects rogue planets? Or are they PBHs?

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