It may well take years to prove, but a pair of University of Miami astrophysicists could be on the verge of a cosmic breakthrough that will confirm the existence of primordial black holes and the role they play in one of cosmology's greatest mysteries.
Believed to have formed within the first fraction of a second after the Big Bang, primordial black holes are purely theoretical. But if confirmed, these hypothetical cosmic phenomena, which could range from asteroid-sized to massive, could explain a lot, including the nature of dark matter—the invisible substance that constitutes about 85 percent of all matter in the universe, acting as "gravitational glue" that holds galaxies together.
"We believe our study will aid in confirming that they actually do exist," Nico Cappelluti, an associate professor in the College of Arts and Sciences' Department of Physics, said of the research he and Ph.D. student Alberto Magaraggia have conducted.
That research builds directly on the recent potential discovery of a subsolar black hole by the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which late last year detected an unusual signal of a gravitational wave, an invisible ripple in the fabric of spacetime caused by violent processes such as the collision of two black holes.
"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 last November, LIGO issued an automated alert for a merger in which at least one of the objects weighed less than 1 solar mass, suggesting the potential of a primordial black hole.
Was it a cosmic breakthrough, or perhaps a false alarm that is the result of noise in LIGO's massive detectors, as some astrophysicists believe?
Cappelluti and Magaraggia are convinced that what LIGO detected could not be anything but the signature of a primordial black hole born in the early universe's high-density environment long before stars even formed. And they are hopeful that their research will prove it.
"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."
The study, which will be published in an upcoming issue of the Astrophysical Journal, "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."
But much more work is still required to fully understand the LIGO detection and how it relates to the nature of dark matter, the two researchers agreed.
So, for now, it's a waiting game, hinging on whether LIGO and its two international partners will detect another unusual signal from what could be a primordial black hole. "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."
An aerial view of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston, Louisiana, which last year detected an unusual wave signal from the far reaches of space. Image: Courtesy of LIGO
It was pioneering soviet scientists Yakov Zeldovich and Igor Novikov, working under the constraints of the Cold War, who were the first to propose the existence of primordial black holes. In the early 1970s, renowned theoretical physicist Stephen Hawking expanded on their work, suggesting that these mysterious objects exist in large numbers, radiate energy, and could explain the mystery of dark matter.
When LIGO became operational, it helped provide the earliest evidence of their theories. The instrument first detected gravitational waves on Sept. 14, 2015, ushering in a new era of astronomy and providing direct evidence of Albert Einstein's general theory of relativity.
The massive observatory is actually two facilities in Hanford, Washington, and Livingston, Louisiana. It operates in coordination with the Virgo gravitational-wave detector in Italy and the underground KAGRA observatory in Japan, forming a network known as LVK that hunts for black holes—regions of space so compact that their gravity prevents anything, including light, from escaping.
Future upgrades to LIGO will make the observatory more sensitive, perhaps allowing it to detect additional such signals. But the instrument, which consists of two L-shaped detectors with 2.5-mile-long vacuum arms, still won't be able to see actual gravitational waves from the Big Bang. It was designed to detect high-frequency waves from relatively recent violent stellar events.
Gravitational wave detectors of the future will be able to see much deeper into the cosmos, Cappelluti pointed out. Launching into space in 2035, the European Space Agency's Laser Interferometer Space Antenna, or LISA, promises to detect gravitational waves from the earliest epochs after the Big Bang.
Meanwhile, Cosmic Explorer, a U.S.- and ground-based gravitational-wave observatory now in the design phase, will be 10 times more sensitive than LIGO, detecting black hole and neutron star mergers back to the dawn of the first stars.
