Space and Physics
PUBLISHED
In November, a gravitational wave signal was captured by LIGO, and it made researchers very excited: a collision between two dense objects that appeared to be a lot smaller than anything we have ever seen collide and a lot smaller than the predicted minimum mass for stellar black holes or neutron stars. Two researchers have now put forward a radical idea as to what it might be: a primordial black hole.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Known black holes in the universe are either of huge masses, like the supermassive ones at the center of galaxies and the intermediate-mass black holes, or they are stellar-sized, a few times or a few tens of solar masses. These are the products of supernovae, the deadly explosion of massive stars at the end of their lives. The smallest black hole you can make that way is at least over 1.4 times the mass of the Sun.
Candidate gravitational wave event S251112cm is a problem. Its mass is a lot smaller than the Sun's – between 10 and 87 percent of it, and that is after the collision. This means that, if the signal is real, the two incredibly dense objects must have been very light. A hypothesis proposed in November suggested a neutron star that somehow lost material in the supernova that formed it.
The new hypothesis instead put forward the idea that these were primordial black holes. It has been theorized, but never proven, that black holes of various masses could have formed in the instants after the Big Bang. These objects did not form from stars, so they could be lighter than our Sun. What S251112cm represents in that case would be the first direct evidence of a collision between two such objects.
“[The research] suggests that the most plausible explanation for the LIGO signal, which lacks any conventional astrophysical explanation, is the detection of a primordial black hole,” senior author Nico Cappelluti, associate professor at the University of Miami, said in a statement. “And our research indicates that these primordial black holes could account for a significant portion, if not all, of dark matter.”
Dark matter is an invisible hypothetical form of matter. If it were made by small black holes, that could, in part or totality, explain why we have not been able to find it.
The LIGO-Virgo-KAGRA collaboration, as the consortium of the four detectors is called, has massively increased the number of detections of gravitational waves in just a decade. If S251112cm is neither a fluke nor a mirage, more events such as this will eventually be spotted. This could be a confirmation of the primordial black hole hypothesis.
“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,” added lead author Alberto Magaraggia, a graduate researcher at the University of Miami. “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 paper has been accepted for publication in The Astrophysical Journal, and a preprint version is posted to arXiv.
