Gross and colleagues computed one predictive model: a gauge theory with a dark quark relic heavier than the confinement scale. They showed that, during the first-order phase transition to confinement, dark quarks remain in the false vacuum and get compressed, forming Fermi balls that can undergo gravitational collapse to stable dark dwarfs (bound states analogous to white dwarfs) near the Chandrasekhar limit, or to primordial black holes.
Dark Matter (DM) might be an accidentally stable dark baryon made of dark quarks q colored under a new dark gauge group. In models with an appropriate number of light dark quark flavours the dark confinement phase transition is first-order and has interesting cosmological implications: relic dark quarks tend to remain in the false vacuum (because they are lighter than dark baryons in the true vacuum), so expanding bubbles of the true vacuum compress them down to small pockets. In the presence of a dark asymmetry this process can lead to macroscopic DM relics.
A similar first-order phase transition takes place in models with no light dark quarks. Heavy relic dark quarks remain in the false vacuum because they cannot access the confined phase as free quarks (until they meet and form dark baryons) and get compressed to small pockets. If dark quarks are only mildly heavy, such pockets/fermi balls evaporate leaving no macroscopic remnants when dark baryon formation occurs.
If relic dark quarks are heavy enough that their gravity becomes relevant, after the initial stage of compression, a gravitational collapse can take over and lead to a new kind of macroscopic DM relic. This is one of the main new points Gross and colleagues pointed out in their recently published paper.
They considered a dark gauge group that becomes strongly coupled at a confinement scale “Λ” in the presence of one heavy dark quark q with mass m >> Λ. The phase transition to confinement is of first order: bubbles of the true confined vacuum appear and expand (refer fig 1 above for whole process). The large latent heat reheats the universe back up to the critical temperature Tcr ≈ Λ keeping the expansion of the existing bubbles slow and stopping nucleation of new bubbles. Relic heavy quarks cannot enter the confined true-vacuum phase, unless they meet other dark quarks and form dark baryons. When the bubbles meet and coalesce, it forms fermi balls in the false unconfined vacuum. They (fermi balls) keep shrinking compressing the heavy quarks in them. If dark quarks are heavy enough, fermi balls can gravitationally collapse under their weight before evaporating, forming stable dark dwarfs (bound states analogous to white dwarfs) near the Chandrasekhar limit, or primordial black holes. But, this dynamics depends on mass ‘m’, on confinement scale ‘Λ’ and on the dark-sector temperature Tdark/TSM.
“Depending on the dark quark mass m, pockets can form stable relic dark dwarfs or black holes.”
They argued that this kind of DM candidates — pockets of false vacua relics compressed by a first order phase transition — can arise in a multiverse context, taking into account that vacuum transitions after slow-roll inflation can involve some vacua near the physical SM vacuum. For example, DM could be pockets of compressed particles that in the false vacuum are lighter than in the SM.
Reference: Christian Gross, Giacomo Landini, Alessandro Strumia, Daniele Teresi, “Dark Matter as dark dwarfs and other macroscopic objects: multiverse relics?”, Arxiv, pp. 1-23, 2021. https://arxiv.org/abs/2105.02840
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