Primordial Black Holes and Cosmological Phase Transitions Report ...

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PBHs and Cosmological Phase Transitions 100 radius. The value of this asymmetry is quantified by the ratio (e.g. Boyanovsky et al., 2006) η = nb − n¯ b nγ (190) where nb, n¯ b and nγ are, respectively, the baryon, antibaryon and photon densities. This is the only free input parameter that enters in nucleosynthesis calculations of the primordial abundance of light elements. The agreement between the WMAP results and the most recent analysis of the primordial deuterium abundance yields (e.g. Boyanovsky et al., 2006) η = (6.1 ± 0.3) × 10 −10 . (191) The origin of this baryon asymmetry is one of the deep mysteries in particle physics and cosmology. One might hope that the baryon asymmetry can be generated at the EW phase transition, if the transition is of strong first–order. If the EW phase transition is second order or a continuous crossover, the associated departure from equilibrium is insufficient to lead to a relevant baryon number production. This means that for EW baryogenesis (EWBG) to succeed, we either need the EW phase transition to be strongly first–order or other methods of destroying thermal equilibrium; for example, topological defects should be present at the phase transition (e.g. Trodden, 1999). The current mass limit for the Higgs is 114.3 GeV at 95% confidence level (e.g. Yao et al., 2006) suggesting that the SMPP does not feature a sharp EW phase transition (either first or second order) but just a smooth Crossover (Section 3.2.1). This means that baryogenesis cannot be explained in the SMPP. One has to explore beyond the SMPP scenarios. The most natural choice is the MSSM (Section 1.9) where a strong first–order phase transition is allowed (e.g. Csikor, 1999). An alternative scenario for baryogenesis proposes that a primordial asymmetry between leptons and antileptons or leptogenesis is responsible for generating the baryon asymmetry. The leptogenesis proposal depends on the details of the origin of neutrino masses and remains a subject of ongoing study (e.g. Boyanovsky et al., 2006). Rangarajan et al. (2002) suggest another alternative scenario for baryogenesis. The baryon asymmetry is created at temperatures much below the EW phase transition temperature during the evaporation of PBHs. When a PBH is evaporating it heats up the plasma around it to a temperature much higher than the ambient temperature, for a short time. This can also happen due to the decay of massive particles. For appropriate PBH masses (or, particle masses) the temperature of the hot region rises above the EW phase transition temperature TEW and the EW symmetry is restored locally. Due to the transfer of energy out of this region, the hot region will cool and the temperature will fall below TEW. Thus, in these hot regions the EW phase transition occurs again and baryon asymmetry is there generated. Brandenberger et al. (1999, 1998) had proposed that baryogenesis may be realized at the QCD phase transition. The scenario is based on the existence

PBHs and Cosmological Phase Transitions 101 of domain walls separating the metastable vacua of low energy QCD from the stable vacuum. The walls acquire a negative fractional baryon charge, leaving behind a compensating positive baryon charge in the bulk. In this sense, this is a charge separation rather than a charge generation mechanism. They claim that it is possible, without fine tuning of parameters, to obtain a reasonable value of the baryon to entropy ratio in the bulk. Another proposed scenario is that of GUT Baryogenesis in which the baryon asymmetry is generated during the GUT epoch at scales of order 10 16 GeV (e.g. Riotto & Trodden, 1999).

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