Black Hole Week is in full swing and to celebrate, NASA has explained how its next big astronomical instrument, the Nancy Grace Roman Space Telescope, will hunt for small black holes dating back to the Big Bang.
When we think of black holes, we tend to imagine enormous cosmic monsters, such as stellar-mass black holes with masses tens to hundreds of times greater than that of the Sun. We can even imagine supermassive black holes with masses millions (or even billions) times that of the Sun, sitting at the hearts of galaxies and dominating their environments.
Yet scientists theorize that the universe could also be populated with much less massive, relatively feather-light black holes with masses around that of Earth. These black holes could potentially have a mass as low as that of a large asteroid. Scientists also suggest that such black holes have existed since the beginning of time, about 13.8 billion years ago.
These black holes, aptly called “primordial black holes,” have remained purely theoretical, but Roman, launching in late 2026, could change that.
Related: Small black holes left over from the Big Bang could be prime dark matter suspects
“Detecting a population of primordial black holes with Earth’s mass would be an incredible step for both 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 studying how Roman could reveal these ancient small black holes, said in a statement: “If we find them, it will shake up the field of theoretical physics.”
When it comes to the horizon of events, mass matters
The smallest black holes ever confirmed to exist are stellar-mass black holes, which form when massive stars run out of fuel needed for nuclear fusion in their cores. Once such a merger stops, these stars collapse under the influence of their own gravity. Normally, the minimum mass required for a star to leave behind a stellar-mass black hole is eight times that of the Sun. If it is lighter, a star will end its life as a neutron star or a smoldering white dwarf.
However, the conditions in the universe at its creation were very different from those of the modern era. When the cosmos was in a hot, dense, and turbulent state, it was possible for many smaller conglomerates of matter to collapse and form black holes.
All black holes “start” at an outer boundary called the “event horizon,” the point beyond which not even light can escape their gravitational influences. The distance between an event horizon and the black hole’s central singularity, the infinitely dense point at which all the laws of physics break down, is determined by the black hole’s mass.
This 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, has a diameter of about 24.8 billion kilometers, a stellar mass black hole with the mass of 30 suns would have an event horizon only about 110 miles wide (177 kilometers wide). In contrast, an Earth-mass primordial black hole would have an event horizon no wider than a dime. A primordial black hole with the mass of an asteroid would have an event horizon of width smaller than a proton.
Scientists who support the concept of primordial black holes think they might have been born when the universe underwent a period of initial inflation called the Big Bang. As the cosmos rushed outwards at speeds greater than light (this is possible because nothing can move faster than light in space, but space itself can), scientists suggest that regions denser than their surroundings could have collapsed and have produced low-mass black holes.
However, many researchers do not support the concept of primordial black holes existing in the current universe, and that is because of Stephen Hawking.
Do black holes die?
One of Stephen Hawking’s most revolutionary theories suggested that even black holes cannot last forever. The great physicist thought that black holes “leak” a form of thermal radiation, a concept later named “Hawking radiation” in his honor.
As black holes leak Hawking radiation, they lose mass and eventually explode. The smaller a black hole’s mass, the faster it should leak 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 leak much faster and therefore die much faster.
So the challenge is to explain how primordial black holes could have persisted for 13.8 billion years without going ‘poof’. If Roman succeeds in discovering these cosmic fossils, it would mean a major rethinking of many physical principles.
“It would affect everything from the formation of galaxies 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, in the declaration. “Confirming their identity will be hard work, and astronomers will need a lot of convincing, but it would be worth it.”
Detecting pristine black holes would also be no easy feat. Like any black hole, these voids would be bounded by an event horizon and would not emit or 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, known as general relativity.
Collaborating with Einstein
General relativity predicts that all objects with mass cause a warp in the fabric of space and time, unified as a single four-dimensional entity called “spacetime.” When light from a background source passes through the warp, the path is curved. The closer the light comes to a lens object, the more the path is curved. This means that light from the same object can arrive at a telescope at different times. This is called gravitational lensing.
When the lens object is incredibly large, such as a galaxy, the background source may appear to shift to an apparent position or even appear in multiple places in the same image. If the lens object has a smaller mass, such as a primordial black hole, the lensing effect is smaller, but it can cause a brightening of background sources that can be detected. That’s an effect called microlensing.
Currently, microlensing is being used with great success to detect rogue planets, or worlds drifting through the Milky Way without a parent star. This has revealed a large population of Earth-sized villains – more than theoretical; models actually predict. With this pattern, scientists predict that Roman will increase the detection of Earth villains tenfold.
The abundance of these objects has led to speculation that some of these terrestrial objects could actually be primordial black holes. “There is no way to distinguish between Earth-mass black holes and rogue planets on a case-by-case basis,” DeRocco said. “Roman will be extremely powerful in statistically distinguishing between the two.”
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“This is an exciting example of something extra that scientists could do with the data Roman is already getting from his search for planets,” Sahu said. “And the results are interesting whether or not scientists find evidence that Earth-mass black holes exist. In either case, it would strengthen our understanding of the universe.”
The team’s research was published in January in the journal Physical Review D.