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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> 174<br />

11 The fraction of the Universe going into PBHs<br />

The fraction of the Universe going into PBHs at a given epoch tk is given by<br />

(e.g. Sobrinho & Augusto, 2007)<br />

β(tk) =<br />

<br />

δmax<br />

1<br />

√ exp −<br />

2πσ(tk) δc<br />

δ2<br />

2σ2 <br />

dδ (286)<br />

(tk)<br />

where σ(tk) represents the mass variance at that epoch, δmax = 1, <strong>and</strong> δc represents<br />

the threshold for PBH formation. The value of δc is, in the case of a<br />

radiation–dominated universe, a constant somewhere between 1/3 <strong>and</strong> 0.7 (see<br />

e.g. Sobrinho & Augusto, 2007). However, if the universe experiences a phase<br />

transition, the value of δc experiences a reduction which favours PBH formation<br />

(Sections 7, 8 <strong>and</strong> 9). In this section, we consider that, during radiation<br />

domination, δc =1/3 <strong>and</strong> that, during the QCD transition, the EW transition,<br />

<strong>and</strong> the electron–positron annihilation epoch, δc assumes the values obtained in<br />

Sections 7, 8, <strong>and</strong> 9, respectively.<br />

In the presence of a Crossover–like transition, such as the QCD Crossover<br />

(Section 2.3.3), the EW Crossover (Section 3.2.1) or the electron–positron annihilation<br />

(Section 4), equation (286) must be replaced by<br />

β1(tk) =<br />

δc<br />

1<br />

√ exp −<br />

2πσ(tk) δc1<br />

δ2<br />

2σ2 <br />

dδ<br />

(tk)<br />

1<br />

+ √<br />

2πσ(tk)<br />

1<br />

δc<br />

<br />

exp − δ2<br />

2σ2 <br />

dδ<br />

(tk)<br />

(287)<br />

where the additional integral accounts for the contribution from the Crossover<br />

epoch. We refer to the second integral, which is equal to the integral in expression<br />

(286), as the contribution from radiation. Denoting this integral by<br />

βRad(tk) equation (287) becomes<br />

β1(tk) =<br />

δc<br />

1<br />

√ exp −<br />

2πσ(tk) δc1<br />

δ2<br />

2σ2 <br />

dδ + βRad(tk) (288)<br />

(tk)<br />

Naturally, if we are dealing with epochs sufficiently apart from the transition<br />

such that δc1 ≈ δc then equation (286) remains valid.<br />

On the other h<strong>and</strong>, in the presence of a Bag Model–like transition, such as<br />

the QCD Bag Model transition (Section 2.3.1) or the EW Bag Model transition<br />

(Section 3.2.2), equation (286) is valid only up to some instant after which there<br />

is an additional window [δc1,δc2] allowing PBH formation (cf. Figure 66 <strong>and</strong><br />

Table 32 for the QCD, Figure 86 <strong>and</strong> Table 37 for the EW). For these cases<br />

equation (286) must be replaced by<br />

β2(tk) =<br />

<br />

δc2<br />

1<br />

√ exp −<br />

2πσ(tk) δc1<br />

δ2<br />

2σ2 <br />

dδ + βRad(tk) (289)<br />

(tk)

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