—> Wen Yin and colleagues proposed that bubble expansion (BE) with ultra-relativistic velocities can produce dark matter.
—> They studied this non-thermal production mechanism in the context of a generic phase transition and the electroweak phase transition (EWPT) and found that a very strong EWPT can lead to the production of a DM candidate up to 10² – 10³ TeV with relatively large interaction couplings, while remaining consistent with observation.
—> They also saw that generically BE mechanism for DM production leads to the stochastic gravitational wave signature in the frequency range, 10^-6 mHz ≤ fpeak ≤ 100 Hz and this can be detected with future technology (like forthcoming XENONnT and DARWIN) experiments and indirect detection (like the CTA) experiments) ..
—> They also claimed that sources of these gravitational waves are (1) bubble collision (2) plasma sound wave.
Wen Yin and colleagues presented a novel mechanism for producing the observed Dark Matter (DM) relic abundance during the First Order Phase Transition (FOPT) in the early universe. They showed that the bubble expansion with ultra-relativistic velocities can lead to the abundance of DM particles with masses much larger than the scale of the transition.
Cosmological observations conspire to suggest the existence of a massive, undetected, dark component permeating the universe, this is the Dark Matter (DM) phenomenon. One of the earliest candidate for this DM, the celebrated WIMP component, demands that the Standard Model (SM) is coupled to the DM, whose stability is guaranteed by a symmetry. This interaction leads to quick thermalisation between the DM and the SM. In this mechanism, known as thermal Freeze-Out (FO), thermal relic density is naturally fixed via the decoupling of the SM-DM sectors, when the rate of interaction can not compete any more with the expansion of the universe. The requirement that this relic density matches the observed abundance imposes a relation between the DM-SM coupling and the mass of the DM candidate. In this context, the surprising and exciting coincidence that weak coupling & TeV scale DM candidate are consistent with observed DM abundance is known as the WIMP miracle. Moreover, unitarity considerations on the coupling governing the scattering of DM provide an upper bound on the mass of the DM candidate, the Griest-Kamionkowski (GK) bound of O(100) TeV. However, today, many WIMP models have been excluded due to the bounds on the DM-nucleon scattering set by the direct detection experiments.
To diversify the range of possibilities inside the (coupling-mass) parameter space, many alternatives to FO have been proposed, as for example; freeze-in, forbidden freeze-in, super-heavy particles decay. Several proposals also take advantage of the possibility of an early First-Order Phase Transitions (FOPT) occurring in the universe, with many different consequences on DM abundance.
If you don’t know about First Order phase transitions (FOPT) let me tell you, first order phase transitions in the early universe are very interesting phenomena which can lead to a plethora of cosmological observations, i.e. production of stochastic gravitational
wave signals, matter-antimatter asymmetry or primordial magnetic fields. During the FOPT the change of phase of the system occurs due to the bubble nucleation and it becomes crucial to understand the dynamics of this process.
Furthermore, phase transitions offer a way to fix the final relic abundance via the VEV flip-flop mechanism, by modifying the stability of DM candidate, through the injection of entropy or also via non-thermal production mechanism.
Now, Web Yin and colleagues presented a new mechanism of DM production, occurring during strong FOPT’s with ultra-relativistic walls and effective when DM is connected via portal coupling to the sector with FOPT. In 2020, Aleksandr Azatov in their paper, showed that an ultra-relativistic wall, with Lorentz factor, γw >> 1, sweeping through the plasma can excite degrees of freedom of mass up to M ∼ √γw × Tnuc, possibly producing out-of-equilibrium particles, mechanism that we call Bubble Expansion (BE) production. In this paper, Web Yin and colleagues showed that those produced particles can be stable and thus constitute viable DM candidates.
“We have shown that the ultra relativistic expansion of the bubbles during the first order phase transition in the early universe can produce a significant amount of the cold relics even if the mass of the DM candidate is much larger than the scale of the Phase transition. This, as a consequence, “brings back to life” components that, due to Boltzmann suppression, did not belong to the plasma any more.”, said Yin, lead author of the study.
They illustrated this mechanism on a simple renormalizable model where DM is a scalar coupled via portal coupling to the field experiencing the phase transition. When the bubble wall reaches velocities,
the exponential suppression of the heavy particle production disappears and bubble expansion (BE) mechanism can become very significant in large ranges of parameter space. Thus the produced DM density can be easily dominant.
They studied this non-thermal production mechanism in the context of a generic phase transition (in which they considered 3 possible like late time annihilation, dilution by supercooling & super-heavy dark matter candidate) and the electroweak phase transition (EWPT) and found that a very strong EWPT can lead to the production of a DM candidate up to 10² – 10³ TeV with relatively large interaction couplings, while remaining consistent with observation.
“In the simple model presented in the paper both bubble expansion ‘BE’ and freeze-out ‘FO’ contributions to the DM relic density were controlled by the same coupling, however this does not have to be the case for more complicated models, where additional interactions can suppress FO contribution further. In the absence of FO produced relics, BE mechanism also provides the possibility of supermassive strongly coupled DM candidate.”, wrote Yin in his paper.
They also saw that generically BE mechanism for DM production leads to the stochastic gravitational wave signature in the frequency range,
which is well in the reach of the current and future experimental studies.
“Our production mechanism can proceed even with very massive DM candidate, thus possibly evading the direct detection experiment bounds, even if the coupling to SM is strong. However an irreducible prediction of the mechanism, which takes advantage of a strong FOPT, is the large imprint left in the Stochastic Gravitational Waves Background (SGWB). Such SGWB signal could be detected in forthcoming GW detectors such as LISA, advanced LIGO, BBO, DECIGO, etc, offering an alternative way to study DM production.”, said Azatov.
But what are the sources of this gravitational wave (GW) signature?
Theoretically, according to authors, two different sources of GW are well understood; the bubble collision, dominating the signal in the case of runaway walls (theories with no gauge bosons), and the plasma sound wave, dominating in the case of Terminal velocity walls, (theories with gauge bosons). We can see that these two scenarios are quite exclusive: runaway behaviour is dominated by bubble component and terminal velocity – by sound waves. This difference in principle allows discrimination between the two bubble expansion scenarios.
We can obtain strong signals for: 1) large α, which is the consequence of long supercooling and large latent heat, 2) small β, which are obtained for slow transitions and thus large bubbles at collision, and 3) relativistic walls vw → 1. Thus, the same conditions necessary for the BE production of Dark Matter will induce the strongest GW signal.
Funding: AA in part was supported by the MIUR contract 2017L5W2PT. WY was supported by JSPS KAKENHI Grant Nos. 16H06490 and 19H05810.
Reference: Aleksandr Azatov, Miguel Vanvlasselaer, Wen Yin, “Dark Matter production from relativistic bubble walls”, ArXiv, pp. 1-20, 2021. https://arxiv.org/abs/2101.05721v1
Copyright of this article totally belongs to our author S. Aman.. One is allowed to reuse it only by giving proper credit either to him or to us..