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NASAThis is Nancy Grace Roman Space Telescope may reveal previously undiscovered “light” black holes with Earth-like masses. These primordial black holes formed in the early universe could greatly impact our understanding of astronomy and particle physics, potentially 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 group of scientists predicts that NASA’s Rome-based Nancy Grace Space Telescope may discover a class of “lightweight” black holes that have never been discovered before.

Today, black holes are formed either by the collapse of a massive star or by the merger of heavy objects. However, scientists suspect that smaller “primordial” black holes, including some with Earth-like masses, may have formed in the first chaotic moments of the early universe.

“Finding a population of Earth-mass primordial black holes would be an incredible step for both astronomy and particle physics, since these objects cannot be formed by any known physical process,” said William DeRocco, a postdoctoral researcher at the Cal Santa Cruz University, who is leading a study on how Roman can reveal them. An article describing the results has been published in the journal Physical examination D. “If we find them, it will shake up the field of theoretical physics.”

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

A recipe for a primordial black hole

The smallest black holes that form today are born when a massive star runs out of fuel. Its outward pressure decreases as nuclear fusion dies down, so the inward gravitational pull wins the battle. The star contracts and can become so dense that it becomes a Black hole.

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

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

Just as the universe was being born, scientists believe it experienced a brief but intense phase known as inflation, when space expanded faster than the speed of light. Under these special conditions, regions that were denser than their surroundings may have collapsed to form low-mass primordial black holes.

While theory predicts that the smallest must evaporate before the universe reaches its current age, those with Earth-like mass could survive.

The discovery of these small objects would have a huge impact on physics and astronomy.

“This will affect everything from galaxy formation to the dark matter content of the universe to cosmic history,” said Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore who was not involved in the study. “Confirming their identity will be hard work and astronomers will need a lot of persuasion, but it would be worth it.”

Stephen Hawking theorizes that black holes may slowly shrink as radiation escapes. The slow leakage of what is now known as Hawking radiation would over time cause the black hole to simply evaporate. This infographic shows the estimated lifetime and event horizon—the point after which infalling objects cannot escape the gravitational pull of a black hole—diameters for black holes of various small masses. Credit: NASA Goddard Space Flight Center

Tips for hidden owners

Observations have already revealed clues that such objects may be lurking in our galaxy. Primordial black holes would be invisible, but the wrinkles in spacetime have helped tie in some possible suspects.

Microlensing is an observational effect that occurs because the presence of mass deforms the fabric of space-time, much like the imprint a bowling ball leaves when placed on a trampoline. Whenever an intervening object appears to drift close to a background star from our point of view, the star’s light must pass through the warped space-time around the object. If the alignment is particularly close, the object can act as a natural lens, focusing and amplifying the light of the background star.

Separate groups of astronomers using data from MOA (Microlensing Observations in Astrophysics) – a collaboration that conducts microlensing observations using Mount John University Observatory in New Zealand – and OGLE (Optical Gravitational Lensing Experiment) have found an unexpectedly large population of isolated Earths – mass items.

Theories of planet formation and evolution predict certain masses and abundances of rogue planets—worlds that wander the galaxy unattached to a star. MOA and OGLE observations suggest that more Earth-mass objects are moving through the galaxy than models predict.

This artist’s concept takes a fantastical approach to representing tiny primordial black holes. In reality, such small black holes would have difficulty forming the accretion disks that make them visible here. Credit: NASA Goddard Space Flight Center

“There is no way to distinguish between Earth-mass black holes and rogue planets on a case-by-case basis,” DeRocco said. But scientists expect Roman to find 10 times more objects in this mass range than ground-based telescopes. “A novel will be extremely strong in distinguishing between the two statistically.”

DeRocco led an effort to determine how many rogue planets there must be in this mass range and how many primordial black holes Roman could discern among them.

The discovery of primordial black holes would reveal new information about the very early universe and suggest that an early period of inflation did indeed occur. It could also explain a small percentage of the mysterious dark matter that scientists believe makes up most of the mass of our universe, but so far have been unable to identify.

“This is an exciting example of something that additional scientists could do with the data that Roman will already receive while searching for planets,” Sahu said. “And the results are interesting regardless of whether scientists find evidence that Earth-mass black holes exist.” This would enhance our understanding of the universe in either case.

Reference: “Discovering primordial Earth-mass black holes with Rome’s Nancy Grace Space Telescope” by William DeRocco, Evan Frangipane, Nick Hammer, Stefano Profumo, and Nolan Smith, 8 Jan 2024, Physical examination D.
DOI: 10.1103/PhysRevD.109.023013

Rome’s Nancy Grace Space Telescope is operated by NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation from NASA’s Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team including Scientist from various research institutions. Major industrial partners are BAE Systems, Inc in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.