The Big Bang may not have been the beginning of the universe, according to a theory of cosmology that suggests the universe can “bounce” between phases of contraction and expansion. If that theory is true, then it could have profound implications for the nature of the cosmos, including two of its most mysterious components: black holes and dark matter. With this in mind, a recent study suggests that dark matter could be composed of black holes formed during a transition from the universe's last contraction to the current expansion phase, which occurred before the Big Bang. If this hypothesis holds, the gravitational waves generated during the black hole formation process might be detectable by future gravitational wave observatories, providing a way to confirm this dark matter generation scenario.

Observations of stellar movements in galaxies and the cosmic microwave background — an afterglow of the Big Bang — indicate that about 80% of all matter in the universe is dark matter, a substance that doesn't reflect, absorb, or emit light. Despite its abundance, scientists have not yet identified what dark matter is made of. In the new study, researchers explored a scenario where dark matter consists of primordial black holes formed from density fluctuations that occurred during the universe's last contraction phase, not long before the period of expansion that we observe now. They published their findings in June in the Journal of Cosmology and Astroparticle Physics.

The bouncing universe ~

(Source: Google Images) 

According to the conventional cosmological model, the universe began as a singularity and then experienced a brief, extraordinarily fast expansion known as inflation. However, the authors of the new study examined a more exotic theory that suggests the universe went through a contraction phase first: non-singular matter-bouncing cosmology. The growing density of matter caused a rebound at the end of this phase, which resulted in the Big Bang and the current accelerated expansion. In this bouncing cosmology, the universe contracted to a size that is around 50 orders of magnitude less than it is today. The emergence of photons and other particles after the rebound served as a precursor to the Big Bang. Tiny black holes occurred around the rebound because of the high matter density caused by quantum fluctuations in the matter's density, which makes them tenable candidates for dark matter.

The attributes of this universe mode—such as the curvature of space and the microwave background—accord with the scientists' calculations, which validates their theory. The researchers intend to employ next-generation gravitational wave observatories to test their predictions in more detail. In their model, the scientists computed the characteristics of the gravitational waves created during black hole formation, and they discovered that future gravitational observatories such as the Einstein Telescope and the Laser Interferometer Space Antenna (LISA) may detect them. If primordial black holes are dark matter, these detections may support the theory. But until any facility detects light, more than ten years may pass.

This work is significant because it offers a naturally occurring method of creating small but persistent black holes that form dark matter in a framework different from the standard inflationary one. Current research examines how these tiny black holes behave around stars, which may eventually lead to a method of detecting them.