A Roman telescope’s search for primordial black holes

The upcoming Roman Space Telescope may reveal a new class of „featherweight” black holes that challenge current theories of black hole formation. These Earth-mass black holes, if discovered, could have significant implications for our knowledge of the early universe and the nature of dark matter. Credit: NASA’s Goddard Space Flight Center

NASANancy Grace Roman space telescope May reveal previously undetected „featherweight” black holes with Earth-like masses. These primordial black holes, formed in the early universe, may have a significant impact on our understanding of astronomy and particle physics, explaining some of the dark matter in the universe.

Astronomers have discovered black holes ranging from a few times the mass of the Sun to tens of billions. Now a team of scientists predicts that NASA’s Nancy Grace Roman Space Telescope may discover a class of „featherweight” black holes that have so far eluded detection.

Today, black holes form when a massive star collapses or when massive objects merge. However, scientists suspect that small „primordial” black holes, including those with Earth-like masses, may have formed during the first chaotic moments of the early universe.

„Finding a population of Earth-mass primordial black holes would be an incredible step forward for both astronomy and particle physics, as these objects cannot be formed by any known physical process,” said William Drocco, a postdoctoral researcher at the University of California, Santa. Cruz led the study of how Roman could express them. A paper describing the results Published in the magazine Physical examination d. „If we find them, it will shake up the field of theoretical physics.”

Artistic illustration of the NASA Roman Space Telescope

The discovery of Earth-mass primordial black holes by NASA’s Roman Space Telescope could change our understanding of the universe and dark matter. Credit: NASA’s Goddard Space Flight Center

Primordial Black Hole Recipe

The smallest black holes that form today are born when a massive star runs out of fuel. As nuclear fusion slows its outward pressure decreases, so inward gravity wins the tug-of-war. The star contracts and can become very dense black hole.

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But a minimum mass is required: at least eight times that of our Sun. Lighter stars become white dwarfs or neutron stars.

However, the conditions of the early universe may have allowed the creation of much lighter black holes. One with the mass of Earth would have an event horizon — the point of no return for falling objects — as wide as a US dime.

Scientists think that just as the universe was born, it experienced a brief but intense phase called inflation, when space expanded faster than the speed of light. In these special conditions, regions denser than their surroundings can collapse to form low-mass primordial black holes.

Although theory predicts that the smallest will evaporate before the universe reaches its current age, those with Earth-like masses may have survived.

The discovery of these tiny objects will have an enormous impact on physics and astronomy.

„This affects everything from galaxy formation to the dark matter content of the universe and cosmic history,” said Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore who was not involved in the study. „Confirming their identities will be hard work and require a lot of faith on the part of astronomers, but it will be worth it.”

Primordial Black Hole Lifetimes Infographic

Stephen Hawking hypothesized that black holes might slowly contract as radiation escapes. A slow leak, now known as Hawking radiation, over time, the black hole simply evaporates. This chart shows the estimated lifetimes and event horizon — the point at which a black hole cannot escape its gravitational grip — diameters for black holes of various small masses. Credit: NASA’s Goddard Space Flight Center

Hints of hidden houses

Observations have already revealed clues that such objects may be lurking in our galaxy. Primordial black holes are invisible, but the contractions in spacetime have helped throw some suspects around.

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Microlensing is an observational effect because the presence of mass distorts the fabric of space-time, like the imprint that occurs when a bowling ball is set on a trampoline. Any time an interstellar object appears to be moving near a background star from our point of view, the star’s light must traverse curved spacetime around the object. If the alignment is particularly close, the object acts like a natural lens, focusing and amplifying the light of the background star.

Separate teams of astronomers using data from the MOA (Microlensing Observations in Astrophysics) — and the OGLE (Optical Gravitational Lensing Experiment) — made the findings using microlensing observations using the Mount John University Observatory in New Zealand. An unexpectedly large population of isolated Earth-mass objects.

Theories of planet formation and evolution predict certain masses and abundances of rogue planets—worlds not bound to a star in the galaxy. MOA and OGLE observations suggest that more Earth-mass objects are moving through the galaxy than models predict.


This artist’s concept takes an imaginative approach to imagining tiny primordial black holes. Indeed, such small black holes would have a hard time forming the accretion disks they see here. Credit: NASA’s Goddard Space Flight Center

„There’s no way to tell on a case-by-case basis between Earth-mass black holes and rogue planets,” Drocco said. But scientists expect Roman to find 10 times more objects in this mass range than ground-based telescopes. „Roman would be more powerful in distinguishing the two statistically.”

Drocco made an effort to determine how many rogue planets there must be in that mass range, and how many of them could be primordial black holes that Roman could detect.

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Finding primordial black holes would reveal new information about the early universe and strongly suggest that early inflation did indeed occur. It makes up a small percentage of the mysterious dark matter that scientists say makes up most of our universe, but has so far been unable to identify.

„This is an amazing example of what additional scientists can do with the data that Roman is already going to get when searching for planets,” Sahu said. „And the results are interesting whether or not scientists find evidence for the existence of Earth-mass black holes. They will both strengthen our understanding of the universe.

Reference: William Drocco, Evan Frangipane, Nick Hammer, Stefano Probumo, and Nolan Smith, „Revealing terrestrial-mass primordial black holes with the Nancy Grace Roman Space Telescope,” 8 January 2024. Physical examination d.
DOI: 10.1103/PhysRevD.109.023013

The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with contributions from NASA’s Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team of various scientists. Research institutes. Primary industrial partners include BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and TeleDyne Scientific & Imaging in Thousand Oaks, California.

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