by Black hole week is in full swing, and to celebrate, NASA explained how its next major astronomical instrument, the Nancy Grace Roman Space Telescope, will search for tiny black holes dating back to the Big Bang.
When we think of black holes, we tend to picture massive cosmic monsters as stellar-mass black holes tens to hundreds of times the mass of the sun. We can even make excellent pictures of supermassive black holes millions (or even billions) of times the size of the sun sitting at the heart of galaxies and dominating their environment.
However, scientists theorize that there could be far less massive, featherweight black holes in the universe with masses around Earth. These black holes may have masses as low as the length of a large asteroid. Scientists also suggest that such black holes existed from the beginning of time, around 13.8 billion years ago.
Aptly called “primordial black holes”, these black holes have only remained theoretical, but Roman, due to launch in late 2026, could change that.
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“The detection of a population of primordial Earth-mass black holes was a major step for astronomy and particle physics because these objects cannot be formed by any known physical process,” said William DeRocco, a postdoctoral researcher at the University of California Santa Cruz. who led a team that studied how the Romans could reveal these ancient black holes., said in a statement “If we find them, it will destroy the field of theoretical physics.”
When it comes to the area of interest of the events, the mass matters
The smallest black holes ever confirmed to exist are stellar mass black holes, which are created when massive stars run out of the fuel needed for nuclear fusion in their cores. When such fusion ceases, these stars will collapse under the influence of their own gravity. Typically, the minimum mass a star must leave behind in a stellar-mass black hole is eight times that of the sun – any lighter, and a star will end its life as a neutron star or a smoldering white dwarf.
However, the conditions in the universe at the beginning were very different than in the modern era. When the cosmos was in a hot, dense and turbulent state, it may have allowed conglomerates of much smaller matter to collapse and give birth to black holes.
All black holes “start” at an outer boundary called the “event field”, beyond which even light cannot escape their gravitational effects. The mass of the black hole determines the distance between the event horizon and the black hole’s central singularity, the point of infinite density at which all the laws of physics break down.
That means that while the event horizon of the supermassive black hole M87*, which has a mass of about 2.4 billion times that of the sun, is a stellar-mass black hole with a diameter of about 15.4 billion miles (24.8 billion kilometers). a departure horizon at a mass of 30 suns would be just about 110 miles wide (177 kilometers wide). On the other hand, a primordial black hole would not have an exit horizon wider than a dime. A primordial black hole with the mass of an asteroid would have an exit horizon as wide smaller than a proton.
Scientists who support the concept of primordial black holes believe that they were born as the universe underwent an initial inflation known as the Big Bang. As the cosmos went faster than light (this is possible because although nothing can move faster than light within space, space itself can), scientists suggest regions closer than as their environment could have collapsed to give birth to low-mass black holes.
However, many researchers do not support the concept of primordial black holes in the current universe, including Stephen Hawking.
Do black holes die?
One of Stephen Hawking’s most revolutionary theories suggested that even black holes could not live forever. The great physicist thought that black holes “leak” a form of thermal radiation, a concept later called “Hawking radiation” in his honor.
As black holes emit Hawking radiation, they lose mass and eventually explode. The less massive the black hole, the faster it should emit Hawking radiation. That means that for supermassive black holes, this process would take longer than the lifetime of the universe. But small black holes would spawn much faster and therefore should die much faster.
So it’s a challenge to explain how primordial black holes could hang around for 13.8 billion years without going “poof.” If the Romans succeed in finding these cosmic fossils, it would be a major rethinking of many principles in physics.
“It affected everything from galaxy formation to the universe’s dark matter to cosmic history,” Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore who was not involved in the study, said in the statement. “Confirming their identity will be hard work, and will require a lot of convincing from astronomers, but it will be well worth it.”
Detecting primordial black holes is no mean feat, either. Like any black hole, these voids would be bound by an exit horizon and neither emit nor reflect light. That means the only way to detect them is to use a principle developed by Albert Einstein in his 1915 theory of gravity called general relativity.
Working together with Einstein
General relativity predicts that all objects with mass cause a curvature in the fabric of space and time, unified as a single four-dimensional entity called “space-time.” When light from a background source passes over the warp, its path is curved. The closer light travels to a lensed object, the more its path is curved. This means that light from the same object can reach a telescope at different times. This is called gravitational lensing.
When the lensed object is extremely massive, such as a galaxy, the background source can appear to move to an apparent location or even appear in multiple locations in the same image. If the mass of the lensing object is smaller, such as a primordial black hole, the lensing effect is smaller, but it can brighten the detectable background sources. That is an effect called microlensing.
Currently, microlensing is used very effectively to detect rogue planets, or worlds drifting through the Milky Way without a parent star. This indicated that there is a large population of pirates around the World roughly – more than theoretica; models to predict, in fact. With this pattern, scientists predict that the Romans will increase pirate detection on the mass of the Earth tenfold.
The abundance of these objects has led to speculation that some of these cosmic objects may be primordial black holes. “There is no way to tell between Earth-mass black holes and rogue planets on a case-by-case basis,” DeRocco said. “Romans will be extremely powerful in distinguishing the two statistically.”
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“This is an exciting example of something additional scientists could do with the data that Roman is already getting in his search for planets,” Sahu said. “And the results are interesting whether or not scientists find evidence of Earth-mass black holes. Either way, it would strengthen our understanding of the universe.”
The team’s research was published in January in the journal Physical Review D.
