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Primordial Black Holes and Cosmological Phase Transitions Report ...

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PBHs <strong>and</strong> <strong>Cosmological</strong> <strong>Phase</strong> <strong>Transitions</strong> 28<br />

(71) to (77). Notice, however, that this will not be the case if one is working<br />

locally (i.e., near t− <strong>and</strong> t+). In Section 2.4, for example, we want to determine<br />

numerical values for t− <strong>and</strong> t+. In that case, it does not make sence to consider<br />

t−/t+ = 1. The same idea is valid for the term tEW−/tEW+. According to this,<br />

we can replace equations (69) to (73) by the single equation<br />

<br />

Λ<br />

R(t) = exp c 3 (tSN<br />

2/3 1/2 teq t<br />

− t0)<br />

, te≤t≤ teq (86)<br />

tSN teq<br />

Equations (67) <strong>and</strong> (68) remain unchanged. Equation (76) becomes<br />

<br />

Λ<br />

R(t) = exp c<br />

3 (tSN<br />

2/3 1/2 teq te<br />

− t0)<br />

<strong>and</strong> equation (77) becomes<br />

tSN<br />

teq<br />

exp (Hi (t − ti))<br />

exp (Hi (te − ti)) ,ti ≤ t ≤ te<br />

<br />

Λ<br />

R(t) = exp c 3 (tSN<br />

2/3 1/2 teq te<br />

− t0) tSN teq<br />

1/2 t<br />

exp (−Hi (te − ti)) ,tp ≤ t ≤ ti<br />

ti<br />

(87)<br />

(88)<br />

In Figure 7 we show R(t) for the entire Universe timeline (i.e., from the Planck<br />

time tp up to the present time t0).<br />

1.7 The Cosmic Microwave Background temperature<br />

The existence of the Cosmic Microwave Background (CMB) radiation was first<br />

predicted by Gamow et al. (1948) but it was only in 1964 that it was observed<br />

(serendipitously) by the first time (Penzias & Wilson, 1965).<br />

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE),<br />

<strong>and</strong> the initial findings, released in 1990, were consistent with the Big Bang’s<br />

predictions regarding the CMB. COBE found a residual temperature of 2.726 K<br />

<strong>and</strong> determined that the CMB was isotropic to about one part in 10 5 (Boggess<br />

et al., 1992). During the 1990s, CMB anisotropies were further investigated<br />

by a large number of ground–based experiments <strong>and</strong> the Universe was shown<br />

to be almost geometrically flat, by measuring the typical angular size of the<br />

anisotropies.<br />

The CMB brings us information about the state of the Universe at the<br />

photon decoupling epoch (z ≈ 1090) when the photons that reach us now had<br />

their last scattering (Section 1.2). The spectrum of the CMB at the present<br />

epoch is well described by a blackbody function with (e.g. Boyanovsky et al.,<br />

2006)<br />

T0 =2.725 ± 0.001K (89)

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