A groundbreaking theory in cosmology challenges the conventional view that the Big Bang marked the beginning of the universe. Instead, it proposes that the universe undergoes cyclic phases of contraction and expansion—an idea that, if proven, could redefine our understanding of the cosmos, including the origins of black holes and dark matter.
A recent study builds on this framework, suggesting that dark matter may be composed of black holes formed during the transition from the universe’s last contraction phase to its current expansion—a period preceding the Big Bang. If this hypothesis holds, gravitational waves generated during the formation of these primordial black holes could be detected by next-generation observatories, offering a potential method to confirm this dark matter formation scenario.
Dark Matter and Black Holes: A Deepening Mystery
Observational data from stellar movements in galaxies and the cosmic microwave background—the faint afterglow of the Big Bang—suggests that approximately 80% of all matter in the universe is dark matter, an elusive substance that neither emits nor absorbs light. Despite its dominance, scientists have yet to determine its exact nature.
In the latest study, researchers propose that dark matter consists of primordial black holes formed from density fluctuations during the universe's last contraction phase. Their findings, published in the Journal of Cosmology and Astroparticle Physics, explore how these black holes could have survived the transition into the universe’s current expansion phase and still persist today.
The Bouncing Universe: A New Perspective
Traditional cosmology posits that the universe began as a singularity, followed by an extremely rapid expansion known as inflation. However, the authors of this study investigate an alternative framework called non-singular matter bouncing cosmology. This model suggests that rather than emerging from a singularity, the universe first underwent a contraction phase, reaching an extreme density before bouncing back into expansion—an event that set the stage for the Big Bang.
In this scenario, the universe once shrank to a size 50 orders of magnitude smaller than its current state before rebounding. During this high-density phase, quantum fluctuations in matter could have led to the formation of small black holes, which may now constitute a significant fraction of dark matter.
Primordial Black Holes as Dark Matter Candidates
According to Patrick Peter, director of research at the French National Centre for Scientific Research (CNRS), small black holes formed during the early universe could have persisted due to their resistance to Hawking radiation—a quantum process theorized to cause black holes to slowly evaporate.
"If they are not too small, their decay due to Hawking radiation will not be efficient enough to eliminate them, meaning they could still exist today," Peter explained to Live Science. These black holes, with masses comparable to asteroids, could either contribute significantly to dark matter or potentially explain its entire existence.
Testing the Theory: Gravitational Waves as a Key Evidence
The study’s calculations indicate that this bouncing universe model aligns with current observations of space curvature and the cosmic microwave background, reinforcing its viability. To further validate their predictions, researchers plan to utilize next-generation gravitational wave observatories.
Specifically, they have calculated the expected gravitational wave signals produced during the formation of these primordial black holes. If correct, these waves could be detected by upcoming facilities such as the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope. However, it may take over a decade before these observatories become operational and capable of confirming the theory.
A New Path in Cosmology
Peter emphasized the significance of this research, noting that it offers a novel explanation for dark matter formation outside the traditional inflationary paradigm. Other studies are currently exploring how such tiny black holes might interact with stars, potentially providing additional ways to detect them.
If future observations confirm that primordial black holes are indeed dark matter, it would revolutionize our understanding of both the origins of the universe and the fundamental nature of cosmic structures—marking a paradigm shift in modern cosmology.
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