The Big Bang may not have been the definitive beginning of the universe.

According to a provocative theory in cosmology, the universe might cyclically "bounce" between phases of contraction and expansion.

If this theory, known as the Big Bounce, is correct, it could fundamentally reshape our understanding of the cosmos, including its most enigmatic components: black holes and dark matter.

Recent research delves into this idea, proposing that dark matter might consist of black holes formed during a critical transition from the universe's last contraction to its current expansion phase—an era predating what we commonly refer to as the Big Bang.

If this hypothesis holds true, the gravitational waves produced during the formation of these black holes could potentially be detected by future observatories, offering a groundbreaking method to confirm this dark matter origin theory.

The Mysterious Nature of Dark Matter

Astrophysical observations, such as the movement of stars in galaxies and the cosmic microwave background—the faint afterglow of the Big Bang—indicate that approximately 80% of all matter in the universe is dark matter.

This elusive substance neither reflects, absorbs, nor emits light, making it invisible and extremely challenging to study. Despite its abundance, the true nature of dark matter remains one of the biggest mysteries in modern physics.

In a recent study, researchers explored a scenario where dark matter is composed of primordial black holes. These black holes, according to the theory, originated from density fluctuations during the universe's last contraction phase, just before the expansion that gave rise to our current universe.

Their findings were published in June in the Journal of Cosmology and Astroparticle Physics.

A Universe That Bounces

Traditional cosmology posits that the universe began from a singularity, followed by a brief period of rapid expansion known as inflation. However, the new study revisits a more unconventional theory—non-singular matter bouncing cosmology.

This theory suggests that before the universe expanded, it first underwent a contraction phase. As the universe contracted, the density of matter increased, eventually leading to a rebound, or "bounce," which triggered the Big Bang and the subsequent expansion we observe today.

In this bouncing cosmology model, the universe shrank to a size about 50 orders of magnitude smaller than its current state.

After the bounce, photons and other particles emerged, marking the event we call the Big Bang. During this incredibly dense phase, quantum fluctuations in matter density could have led to the formation of small black holes, which are now considered strong candidates for dark matter.

Primordial Black Holes as Dark Matter

Patrick Peter, a director of research at the French National Centre for Scientific Research (CNRS), who was not involved in the study, explains the potential longevity of these primordial black holes. "Small primordial black holes could have been produced in the very early universe.

If they are not too small, their decay through Hawking radiation [a theoretical process where black holes emit particles due to quantum effects] would be insufficient to eliminate them, meaning they could still exist today," says Peter shared.

"With masses comparable to asteroids, these black holes might not only contribute to dark matter but could also solve the mystery entirely." he added.

The researchers' calculations indicate that the properties of this bouncing universe, including space curvature and the cosmic microwave background, align well with current observations, lending support to their hypothesis.

The Road Ahead: Gravitational Wave Detection

To test their predictions further, the researchers aim to utilize next-generation gravitational wave observatories. They have calculated the characteristics of the gravitational waves that would have been produced during the black hole formation process in their model.

If detected by upcoming observatories like the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope, these waves could provide compelling evidence that primordial black holes are indeed the elusive dark matter. However, it may take more than a decade before these observatories are operational and able to provide the necessary data.

"This research is significant as it offers a natural mechanism for forming small, yet enduring, black holes that constitute dark matter within a framework that diverges from the conventional inflation-based model," Peter remarked. "Current studies are also exploring how these tiny black holes might interact with stars, potentially offering new ways to detect them in the future."

The implications of this work are profound, offering a fresh perspective on the origins and nature of the universe, while bringing us a step closer to unraveling the mystery of dark matter.

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