Cosmological theory suggests that the universe can “bounce” between phases of contraction and expansion, which could explain some of the mysteries of dark matter.
The Big Bang, the “Big Explosion” from which the Universe emerged; the story we all know is that, about 14 billion years ago, all the matter and energy, condensed in an infinitely small point in space, the so-called “singularity“, suddenly broke free and reorganized, giving rise to galaxies, stars, nebulae, black holes; and much later, to our planet, one among billions.
However, we do not know what was before the Big Bang: in fact, we do not even know if it makes sense to talk about a before, since we ignore the physical laws of that singularity.
A new study , recently published in the Journal of Cosmology and Astroparticle Physics , suggests that perhaps the Universe had a “existence” prior to the Big Bang , formed by successive phases of contractions and expansions, as if it were an enormous heart beating in the vacuum. If confirmed, the theory could have implications for the behavior of black holes and the nature of dark matter – the mysterious entity that represents around 80% of all matter in the cosmos and that we have not yet been able to observe directly.
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Explosion or rebound?
As we said, traditional cosmological theories explain that the Universe was born from a singularity and that during its first moments of life it experienced very rapid growth, known as “inflation”. A team of scientists from several research institutes; among them the National Institute for Nuclear Physics (INFN), the Scuola Supérieure del Sud and the Department of Physics of the University of Naples Federico II, analyzed a more exotic theory, known as “non-singular matter bounce cosmology”, which refers to the fact that before the Big Bang, the Universe went through a phase of contraction that ended with a recoil , due to the increase in the density of matter, and gave rise to the accelerated expansion that we still observe today.
A few months ago, Salvatore Capozziello, one of the lead authors of the research and a colleague from the Department of Philosophy at the State University of Milan, addressed the problem of the definition of time at the time of the Big Bang, in a study published in the journal Physical Review D. In it they pointed out that ” black holes and the Big Bang are extreme situations in which we lose track of physics as we know it and, with it, the conception of time as a parameter that normally describes past, present and future; which has been a concern for decades, starting with Einstein.”
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Primordial black holes and dark matter
But back to the rebound theory. In this scenario, the authors say, the Universe would have shrunk to a size approximately 50 orders of magnitude smaller than today, and after the “bounce” photons (the particles that make up light) and other elementary particles would have appeared; the extremely high density of matter would also have given rise to small primordial black holes, and here we come full circle, possible candidates for the mysterious dark matter.
Interviewed by Live Science, Patrick Peter, a research director at the French National Centre for Scientific Research (CNR) who was not involved in the study, explained that, indeed, ” small primordial black holes may have been produced during the earliest moments of the Universe, and if they are not too small, their decay due to Hawking radiation is not enough to make them disappear, so they should still exist now, somewhere. And they could be dark matter, or at least part of it.” Hawking radiation is a phenomenon hypothesized by astrophysicist Stephen Hawking that black holes, by virtue of quantum effects, could actually emit a certain amount of particles and energy.
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Interesting results, but we have to wait
According to the calculations of the authors of the new paper, the numbers add up: the scientists have indeed shown that some measurable and measured features of the Universe, including the curvature of space-time and the cosmic background radiation, the “echo” of the Big Bang, are indeed consistent with the predictions of their model, which is certainly a very encouraging observation. But that is not enough. To further corroborate their hypothesis, the researchers hope to be able to compare it with observations from the next generation of gravitational wave detectors.
The model also allows them to estimate some properties of the gravitational waves emitted by primordial black holes in formation, and new-generation detectors could be able to capture these gravitational waves, allowing them to compare predictions with observations and confirm or, if necessary, refute the hypothesis that primordial black holes are indeed made of dark matter. However, it will take a while, as the new detectors could take at least a decade to come into service. “This work is important because it explains in a natural way how small primordial black holes could have formed and how they could have given rise to dark matter, also in a context other than cosmic inflation. There are also other lines of research that are studying the behaviour of these small black holes around stars, and that could suggest how to observe them in the near future,” concludes Parker.