How could the first black holes have formed immediately after the Big Bang, even before the ignition of stars? This question resurfaces with a recent observation that could change our understanding of the Universe.
On November 12, the LIGO and Virgo gravitational wave detectors recorded a very particular signal, named S251112cm. Analysis indicates it would come from the merger of two objects, one of which has a mass lower than that of our Sun. Such a characteristic is incompatible with classical black holes from dead stars or with neutron stars, making the event extremely unusual. A physicist from Durham University stated in the pages of
Science that this would be a major discovery, as no conventional astrophysical process can explain it.
Primordial black holes are a hypothesis to explain this type of signal. Unlike their stellar counterparts, they are not born from the collapse of a massive star. Scientists believe they could have formed in the first hot and dense seconds following the Big Bang, from density fluctuations in the primordial plasma. Their mass could extend over a wide range, from a tiny fraction of that of a paperclip to a hundred thousand times that of the Sun, thus covering the so-called "substellar mass range."
The existence of these compact objects could have profound implications, notably for elucidating the nature of dark matter (see the explanation at the end of the article). This invisible component would constitute about 85% of the matter in the Universe, but it does not interact with light, making it very difficult to study directly. Primordial black holes represent an attractive candidate, as their formation would be possible within the framework of current cosmological models, without requiring new fundamental physics beyond the standard model.
However, the detection remains surrounded by caution. Researchers from the LIGO-Virgo project note that the probability of this signal being a false alarm due to noise in the instruments is not negligible, with an estimated rate of about one every four years. Furthermore, the localization of the source is very imprecise, complicating the search for an associated light signal that could confirm the event.
Despite these uncertainties, this observation opens a new line of research. If similar signals were confirmed in the future, they could provide the first direct evidence of the existence of primordial black holes. This quest illustrates the ability of gravitational waves to reveal cosmic phenomena that totally escape traditional telescopes.
Dark Matter
Dark matter is a hypothetical form of matter that does not emit, absorb, or reflect light, making it invisible to classical telescopes. Its existence is deduced indirectly from its gravitational effects on visible matter, such as the abnormally rapid rotation of stars around galactic centers or the bending of light from distant objects, a phenomenon called gravitational lensing.
Estimates indicate that this component would represent about 85% of all matter contained in the cosmos. Without it, galaxies could not maintain their cohesion and would disperse. Yet, its fundamental nature remains one of the great open questions in modern physics, as it does not correspond to any known particle described by the standard model of particle physics.
Several theoretical candidates have been proposed to explain dark matter, ranging from exotic particles like WIMPs (Weakly Interacting Massive Particles) to primordial black holes. The latter have the advantage of not requiring new physics beyond the known laws of gravity and cosmology, making them a particularly economical hypothesis from a theoretical standpoint.
Research continues actively through underground experiments seeking to capture rare interactions, targeted astronomical observations, and numerical simulations. Identifying the nature of dark matter is essential to fully understand the formation and evolution of large-scale structures in the Universe, from galaxy clusters to cosmic filaments.