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Scientists may have "heard" the first tantalizing hints of long-theorized primordial black holes born during the Big Bang. The potential detection of these tiny black holes that could be the size of a coin or even as small as a fraction of the size of an atom came from the detection of ripples in spacetime called gravitational waves by two Earth-based detectors, the Laser Interferometer Gravitational-wave Observatory (LIGO), and Virgo.

The LIGO-Virgo collaboration has been routinely detecting gravitational waves launched through the fabric of space by mergers between black holes and collisions between extreme stellar remnants called neutron stars, since 2012. On Nov. 12, however, the LIGO-Virgo-KAGRA collaboration issued an automated alert for a black hole merger that was anything but routine.

When the signal from the event designated S251112cm was observed, it revealed that one of the objects involved had a mass way too small to be either a stellar-mass black hole or a neutron star, both of which are stellar remnants born from the collapsing core of a dying massive star and have masses greater than that of the sun. "If this turns out to be real, then it's enormous," Durham University theoretical physicist Djuna Croon, who was not involved in the gravitational wave observation, told Science. "This is not an event we can explain by conventional astrophysical processes." However, that "if" is a very, very big one.

Gravitational wave astronomer and LIGO team member, Christopher Berry, shared the LIGO-Virgo alert of Bluesky, writing: "Interesting #GravitationalWave candidate #S251112cm potentially from a *subsolar* mass source."

Later, the University of Glasgow researcher added that there was still a significant chance of this being a false alarm, resulting from noise in the detectors. Current estimates suggest that the rate for false alarms in terms of this type of detection is around one every four years. For signals from "ordinary" black hole and neutron star mergers, which are detected frequently, this is a small margin of error, but for a signal as rare as S251112cm, it casts a large shadow of doubt.

However, primordial black holes have been speculated about for a long time but have thus far proved elusive, and that makes even the slightest chance of a potential detection something that is very exciting indeed.

What are primordial black holes?

Generally, when we use the term "black hole," what we are referring to is a stellar-mass black hole. That is a black hole with a mass between 5 and 100 times the mass of the sun that is born when the core of a massive star with at least 10 solar masses collapses, triggering a supernova that blows away that star's outer layers.

The other common usage of the term "black hole" refers to the supermassive black holes that lurk at the hearts of all large galaxies. With masses larger than millions to billions of suns, supermassive black holes are much too massive to have formed from a single star, so scientists theorize that these cosmic titans grow from repeated mergers between successively larger and larger black holes.

Primordial black holes are believed to have formed long before even the first stars, directly from overly dense pockets in the steaming-hot "soup" of plasma that filled the universe in the first few seconds after the Big Bang.

Primordial black holes have been proposed to have masses ranging from 1/100,000th that of a paperclip to 100,000 times that of the sun, a mass range that encompasses "sub-stellar masses." They are often referred to as "non-astrophysical black holes," which is based on the fact that they don't rely on stars to be created.

If primordial black holes exist, then they could be major players in how the universe has evolved over time, or they could explain one of the most pressing mysteries in modern cosmology: the nature of dark matter.

Dark matter is puzzling to scientists because, despite accounting for around 85% of the matter in the universe, it is effectively invisible because it doesn't interact with electromagnetic radiation. That lack of interaction means that we can only infer the existence of dark matter via its interaction with gravity, which affects space-time and subsequently impacts ordinary matter and light. The lack of direct interaction with light has prompted scientists to search for potential candidates for dark matter outside the Standard Model of particle physics.

Primordial black holes are an appealing dark matter candidate because their existence is within the remit of our current models of the universe, meaning they don't require physics beyond the standard model. Thus far, however, if they exist, primordial black holes have evaded any attempts to detect them, and that may be because they simply aren't around in the modern cosmos anymore.

According to Stephen Hawking, black holes leak heat into the universe in the form of "Hawking Radiation." This causes them to gradually evaporate before a final explosion, but this process slows as the mass of a black hole increases. That would give supermassive black holes lifetimes that exceed the predicted lifespan of the universe. However, it would mean that very light primordial black holes could have evaporated within seconds of their formation, while larger examples could still be evaporating in the cosmos today.

A needle in a cosmic haystack

If this signal is more than a false alarm, then researchers currently can't account for it with the collision of any known astrophysical bodies. The alert from LIGO-Virgo has enabled astronomers to begin searching for an explosion that accompanied the gravitational wave signal. However, the gravitational wave detectors were only able to narrow down the source of this signal to a region of sky equivalent to around 6,000 times the width of the moon, which makes looking for an accompanying electromagnetic signal akin to searching for a needle in a cosmic haystack.

That means for the moment, researchers only have this gravitational wave signal to assess in order to investigate the nature of this merger. That provides more information than it may initially seem, however. Gravitational wave scientists have the opportunity to study the "hum" of gravitational waves preceding the merger to determine the identity of the two objects that were spiraling together.

Unfortunately, we may never know if this really is a signal from a primordial black hole. That is, unless more similar signals are detected, something scientists say is a slim possibility.

"It seems unlikely that we’ll actually know with certainty whether this alert was real or not," Croon concluded.