Primordial Black Holes and Cosmological Phase Transitions Report ...

PBHs and Cosmological Phase Transitions 10
when the EW and strong forces separate (e.g. Narlikar & Padmanabhan, 1991).
The inflationary stage is followed by a radiation–dominated era after a short period
of reheating during which the energy stored in the field that drives inflation
decays into quanta of many other fields, which, through scattering processes,
reach a state of local thermodynamic equilibrium (e.g. Boyanovsky et al., 2006).
The period which goes from the end of this reheating process up to t ∼ 10 −6 s
is known as the Quark Era. During this era the Universe consists of a plasma
composed of quarks, leptons, photons, gluons and their antiparticles. Particle–
antiparticle pairs are constantly being created and annihilated. Conversion
between quarks and leptons are not possible because X bosons no longer exist.
When the temperature of the Universe decays to ∼ 180 GeV it is no longer
possible to create top quarks (or anti–top quarks). Top and anti–top quarks
annihilate each other and cease to exist in nature. It is also during the quark
era that the tauon, and the bottom and charm quarks thresholds occur (Table 1).
When the Universe temperature reaches ∼ 100 GeV (corresponding to the mass
of the W ± and Z 0 bosons) another remarkable effect takes place: the weak
force decouples from the electromagnetic force in a process called the EW phase
transition (Section 3). It is only now that the four fundamental interactions are
separarated (e.g. Unsöld & Bascheck, 2002), as we see them today.
When the temperature of the Universe goes from 2 GeV to 1 GeV almost all
the baryons cease to be produced. This applies to the baryons Ω, Ξ, Σ and Λ.
Among the decaying products we have neutrons (mean–life ∼ 600 s (e.g. Jones
& Lambourne, 2004) which is a very long time if compared with the age of the
Universe at this stage) and protons. These were the first stable neutrons and
protons ever produced in the Universe.
As the temperature falls through ∼ 170 MeV the Quark–Hadron phase transition
occurs (Section 2), i.e., quarks and gluons bind into stable hadrons (neutrons
and protons). This marks the beginning of the Hadron Era. During the
hadron era the kaons, pions and muons thresholds take place (Table 1).
When the Universe is 10 −4 s old, and the last pions have just decayed, the
Lepton Era begins. The Universe is now composed, according to the SMPP
(Section 1.8), of photons, protons, neutrons, electrons, positrons, neutrinos and
antineutrinos. Protons and neutrons turn into each other through reactions like:
e − + p ←→ νe + n, e + + n ←→ ¯νe + p, n ←→ p + e − +¯νe (e.g. Lyth, 1993; Jones
& Lambourne, 2004). When the Universe is ≈ 1 s old neutrinos decouple, i.e.,
the Universe becomes transparent to neutrinos. Finally, when the Universe is
≈ 3 s old, the electron threshold occurs marking the end of the Lepton Era.
About 200 s after the singularity, the Universe has cooled to ∼ 10 9 K, allowing
the synthesis of nuclei from protons and neutrons in a process called
Primordial Nucleosynthesis. The first fusion reaction that could occur was that
between a proton and a neutron to form a nucleus of deuterium (deuteron):
p + n ⇆ 2 1H+γ. A deuteron can be broken apart by an incident γ–ray photon
with energy ≥ 2.23 MeV. However at this stage (t ∼ 200 s) the average photon
energy in the universe decreased below that limit and hence, deuterium, once
formed, would no longer be destroyed (e.g. Jones & Lambourne, 2004).
As soon as there was a significant abundance of deuterium, other nuclear