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Friday, 21 February 2020

Special gravitational waves could explain the mystery of the existence of the Universe

According to the standard cosmological model, at the time of the Big Bang, matter and antimatter are created in identical proportions. They should therefore have annihilated themselves, leaving an empty universe. However, today, it is a universe composed mainly of matter that we observe. This defect in antimatter is called matter-antimatter asymmetry and constitutes one of the most active research themes in particle physics. Several hypotheses have been put forward: one of them suggests that neutrinos played a key role in this mechanism. And recently, physicists have proposed a way to test this hypothesis.

The neutrino hypothesis suggests that about a million years after the Big Bang, the Universe has cooled and undergone a phase transition. This phase change caused the neutrinos to decay into more matter than antimatter, implying a violation of CP symmetry.

But according to Jeff Dror, lead author of the new study and postdoctoral researcher at the University of California at Berkeley, there is no simple way to probe this theory and understand if this process actually happened in the Universe. primitive. The results of the study were published in the journal Physical Review Letters.

A phase transition at the origin of the creation of cosmic strings

But Dror and his team, through theoretical models and calculations, found a way to see this phase transition. They proposed that the change would have created extremely long and thin strands of energy called cosmic cords , which are still present in the Universe today.

According to the authors, the phase transition at the origin of the matter-antimatter asymmetry would also have created particular structures called cosmic cords which, today, would be at the origin of detectable gravitational waves. Credits: R. Hurt / Caltech-JPL, NASA, and ESA Credit: Kavli IPMU

Cosmic cords are topological defects. Topological defects are hypothetical structures presumed stable which would have formed in the first moments of the Universe.

Theories implying the formation of topological defects predict that they would have appeared at the end of the inflationary period. More precisely, topological defects are deemed to have formed during the various phase transitions of the primitive universe.

In the Standard Model, these phase transitions are accompanied by different spontaneous symmetry breaks. The cosmic cords therefore appeared when, at the end of the inflationary period, cylindrical and axial symmetries were broken.

These are one-dimensional topological defects of linear form. The number of cosmic strings in the universe cannot be determined with certainty, however, Kibble's calculations indicate that there would be approximately one cosmic cord per Hubble volume, i.e., one cosmic cord every 10^31 cubic light years .

Cosmic strings: potential sources of gravitational waves

Dror and his team suggest that these cosmic cords are very likely to be the source of gravitational waves. The strongest gravitational waves occur when a supernova emerges; when two massive stars rotate around one another; or when two black holes merge. But the potential gravitational waves caused by cosmic cords would be much weaker than those detected so far.

Frequency and amplitude at which gravitational waves produced by cosmic strings can be detected. The detection sensitivities of current and future instruments are also indicated. Credits: Jeff A. Dror et al. 2020

However, when the team modeled this hypothetical phase transition under various temperature conditions that could have occurred during this phase transition, they made an encouraging discovery: In all cases, cosmic strings would create gravitational waves that would be detectable by future observatories, such as NASA's Laser Interferometer Space Antenna (LISA), the European Space Agency's proposed Big Bang Observer and the Japan Aerospace Exploration Agency's Deci-hertz Interferometer Gravitational wave Observatory (DECIGO).

"If these strings are produced at sufficiently high energy scales, they will indeed produce gravitational waves that can be detected by planned observatories," concludes Tanmay Vachaspati, a theoretical physicist at Arizona State University


Testing the Seesaw Mechanism and Leptogenesis with Gravitational Waves

Jeff A. Dror, Takashi Hiramatsu, Kazunori Kohri, Hitoshi Murayama, and Graham White

Phys. Rev. Lett. 124, 041804


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