Black hole week is in full swing, and to celebrate, NASA has explained how its next major astronomical instrument, the Nancy Grace Roman Space Telescope, will hunt for tiny black holes that date back to the Big Bang.
When we think of black holes, we tend to picture vast cosmic monsters like stellar-mass black holes with masses tens to hundreds of times that of the sun. We may even picture supermassive black holes with masses millions (or even billions) of times that of the sun sitting at the hearts of galaxies and dominating their surroundings.
Yet, scientists theorize that the universe could also be populated with vastly less massive, relatively featherweight black holes with masses around that of Earth. These black hole, potentially, could have masses as low as that of a large asteroid. Scientists also suggest such black holes would have existed since the dawn of time, some 13.8 billion years ago.
Aptly named "primordial black holes," these black holes have remained purely theoretical, but Roman, which is set to launch in late 2026, could change that.
Related: Tiny black holes left over from the Big Bang may be prime dark matter suspects
"Detecting a population of Earth-mass primordial black holes would be an incredible step for both astronomy and particle physics because these objects can't be formed by any known physical process," William DeRocco, a postdoctoral researcher at the University of California Santa Cruz who led a team studying how Roman could reveal these ancient tiny black holes., said in a statement "If we find them, it will shake up the field of theoretical physics."
When it comes to event horizons, 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. Once such fusion ceases, these stars collapse under the influence of their own gravity. Typically speaking, the minimum mass a star needs to leave behind a stellar mass black hole is eight times that of the sun — any lighter, and a star will end its life as neutron star or a smoldering white dwarf.
However, conditions in the universe at its onset were very different than those of the modern epoch. When the cosmos was in a hot, dense and turbulent state, it may have allowed much smaller conglomerations of matter to collapse and birth black holes.
All black holes "begin" at an outer boundary called the "event horizon," the point beyond which not even light can escape their gravitational influences. The distance an event horizon is from the black hole's central singularity, the infinitely dense point at which all the laws of physics break down, is determined by the mass of the black hole.
That means, while the event horizon of the supermassive black hole M87*, which has a mass of around 2.4 billion times that of the sun, has a diameter of around 15.4 billion miles (24.8 billion kilometers), a stellar-mass black hole with the mass of 30 suns would have an event horizon just around 110 miles wide (177 kilometers wide). An Earth-mass primordial black hole, on the other hand, 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 with a width smaller than a proton.
Scientists who support the concept of primordial black holes think they would have been born as the universe underwent a bout of initial inflation that we called the Big Bang. As the cosmos raced out at a speed greater than light (this is possible because though nothing can move faster than light within space, space itself can), scientists suggest regions denser than their surroundings could have collapsed to birth low-mass black holes.
However, many researchers don't 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 not even black holes can 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, for supermassive black holes, this process would take longer than the lifetime of the universe. But tiny black holes would leak much faster and thus should die much quicker.
It is thus a challenge to explain how primordial black holes could have hung around for 13.8 billion years without going "poof." If Roman manages discover these cosmic fossils, it would constitute a major rethink of many principles in physics.
"It would affect everything from galaxy formation to the universe's dark matter content 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 identities will be hard work, and astronomers will need a lot of convincing, but it would be well worth it."
Detecting primordial black holes would be no mean feat, either. Like any black hole, these voids would be bound by an event horizon and neither emit nor reflect light. That means the only way to detect them would be to use a principle developed by Albert Einstein in his 1915 theory of gravity known as general relativity.
Teaming up with Einstein
General relativity predicts that all objects with mass cause a curvature in the very fabric of space and time, united as a single four-dimensional entity called "spacetime." When light from a background source passes the warp, its path is curved. The closer to a lensing object that light passes, the more its path is curved. That means that light from the same object can arrive at a telescope at different times. This is called gravitational lensing.
When the lensing object is incredibly massive, like a galaxy, the background source can appear to shift to an apparent position or even appear at multiple places in the same image. If the lensing object is smaller in mass, like a primordial black hole, the lensing effect is smaller, but it can cause a brightening of background sources that can be detected. That is an effect called microlensing.
Currently, microlensing is used to great effect to detect rogue planets, or worlds that drift through the Milky Way without a parent star. This has revealed a large population of roughly Earth-mass rogues — more than theoretica; models predict, in fact. With this pattern, scientists predict Roman will increase detections of Earth-mass rogues tenfold.
The abundance of these objects has led to speculation that some of these Earth-mass objects could actually be primordial black holes. "There's no way to tell between Earth-mass black holes and rogue planets on a case-by-case basis," DeRocco said. "Roman will be extremely powerful in differentiating between the two statistically."
"This is an exciting example of something extra scientists could do with data Roman is already going to get as it searches for planets," Sahu said. "And the results are interesting whether or not scientists find evidence that Earth-mass black holes exist. It would strengthen our understanding of the universe in either case."
The team's research was published in January in the journal Physical Review D.
