Primordial black holes have remained one of astronomy’s most intriguing ideas for decades. Now, researchers at the University of Miami believe a recent gravitational wave detection may bring scientists closer to confirming that these ancient objects are real, a breakthrough that could also help solve the enduring mystery of dark matter.
Primordial black holes are thought to have formed during the first fraction of a second after the Big Bang, long before the first stars or galaxies existed. Unlike the black holes created by collapsing stars, these hypothetical objects could range in size from something as small as an asteroid to much larger bodies.
Although no primordial black hole has ever been confirmed, scientists believe they could answer several major questions about the universe. One of the biggest is the nature of dark matter, the invisible substance that makes up about 85 percent of all matter and provides the gravitational pull that helps hold galaxies together.
“We believe our study will aid in confirming that they actually do exist,” said Nico Cappelluti, an associate professor in the University of Miami’s Department of Physics, referring to research he conducted with Ph.D. student Alberto Magaraggia.
An Unusual LIGO Signal
Their work builds on a possible discovery reported by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which late last year detected an unusual gravitational wave signal. Gravitational waves are ripples in spacetime produced by some of the universe’s most violent events, including collisions between black holes.
Most known black holes form after massive stars explode as supernovas. Their masses typically range from several times the mass of the Sun to billions of solar masses.
“The most common black holes form as the result of a supernova, the death of a massive star. So, their masses can range from a few times the Sun’s mass to billions of solar masses,” Cappelluti explained.
But in November, LIGO issued an automated alert for a merger in which at least one object appeared to have less than one solar mass. Such a small black hole would be difficult to explain through conventional stellar evolution and instead could point to a primordial black hole.
Not everyone is convinced. Some astrophysicists have suggested the signal may simply be noise within LIGO’s extremely sensitive detectors rather than evidence of a remarkable new discovery.
Could This Explain Dark Matter?
Cappelluti and Magaraggia argue that the detected object is best explained as a primordial black hole that formed in the dense conditions of the early universe, long before stars existed.
To test that idea, the researchers estimated how many primordial black holes might exist throughout the cosmos and how frequently LIGO should detect them.
“We attempted to estimate how many primordial black holes may exist in the universe and how many of them LIGO should be able to detect,” Magaraggia said. “And our results are encouraging. We predict that subsolar black holes like the one LIGO may have observed should indeed be rare, consistent with how infrequently such events have been seen so far.”
Their findings, published in The Astrophysical Journal, suggest that the mysterious LIGO signal has no conventional astrophysical explanation and is most consistent with a primordial black hole.
The study “suggests that the most plausible explanation for the LIGO signal, which lacks any conventional astrophysical explanation, is the detection of a primordial black hole,” Cappelluti said. “And our research indicates that these primordial black holes could account for a significant portion, if not all, of dark matter.”
Even so, both researchers emphasize that one detection is not enough to settle the question.
For now, scientists must wait to see whether LIGO and its international partners record additional events that match the same pattern.
“LIGO picked up what is very strong evidence that these types of black holes exist. But we’ll need to detect another such signal or even several others to get the smoking-gun confirmation that they are real,” Cappelluti said. “But what is clear is that they cannot be excluded as being real.”
A Theory Decades in the Making
The concept of primordial black holes dates back to the Cold War era, when Soviet scientists Yakov Zeldovich and Igor Novikov first proposed their existence. In the early 1970s, Stephen Hawking expanded on the idea, arguing that these objects could be abundant throughout the universe, emit radiation, and possibly explain dark matter.
LIGO later provided the first opportunity to search for evidence supporting those theories. On Sept. 14, 2015, the observatory made history by detecting gravitational waves for the first time, confirming a major prediction of Albert Einstein’s general theory of relativity and opening an entirely new way to study the universe.
The Future of Gravitational Wave Astronomy
LIGO consists of two observatories located in Hanford, Washington, and Livingston, Louisiana. Together with the Virgo detector in Italy and the underground KAGRA observatory in Japan, they form the international LVK collaboration, which searches for black holes, regions of space where gravity is so strong that not even light can escape.
Planned upgrades will make LIGO even more sensitive, increasing its chances of finding additional candidate primordial black holes. However, the observatory’s two L shaped detectors, each with 2.5 mile long vacuum arms, were designed to detect the high frequency gravitational waves produced by relatively recent cosmic collisions, not the waves generated directly during the Big Bang itself.
Future observatories will extend that reach much farther back in time. The European Space Agency’s Laser Interferometer Space Antenna (LISA), scheduled for launch in 2035, is expected to detect gravitational waves from the universe’s earliest epochs after the Big Bang.
Another planned facility, Cosmic Explorer, is currently in the design phase in the United States. Researchers expect it to be about 10 times more sensitive than LIGO, allowing it to detect black hole and neutron star mergers stretching back to the era when the first stars formed.