Primordial Black Holes and Cosmological Phase Transitions Report ...

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