Sufficiently light primordial black holes (PBH) could evaporate in the very early universe and dilute the preexisting baryon asymmetry and/or the frozen density of stable relics. The effect is especially strong in the case that PBHs decayed if and when they dominated the cosmological energy density. The size of the reduction is first calculated analytically under the simplifying assumption of the delta-function mass spectrum of PBH and instant decay approximation. In the realistic case of exponential decay and for an extended mass spectrum of PBH the calculations are made numerically. Resulting reduction of the frozen number density of the supersymmetric relics opens for them a wider mass window to become viable dark matter candidate.

Particulate dark matter (DM), completely isolated from the Standard Model particle sector, can be produced in the early Universe from primordial black hole (PBH) evaporation. However, big bang nucleosynthesis (BBN) observations put an upper bound on the initial mass of PBH requiring the PBH to evaporate completely before the advent of BBN. DM particles in the mass range $\sim (1–{10}^9)\mathrm{GeV}$ cannot explain the observed relic abundance for an early matter dominated universe due to this BBN constraint. However, this assumes the presence of only one monochromatic PBH mass distribution in the early Universe. In this work, we explore the simple possibility of achieving the observed relic with DM masses from the above mentioned range for an early matter dominated era with two monochromatic evaporating PBH mass distributions and demonstrate that the fermionic DM masses consistent with BBN change slightly.

Primordial black holes (PBHs) with masses below 10 ⁹ grams are typically assumed to have negligible cosmological impact due to their rapid evaporation via Hawking radiation. However, the “memory burden” effect, which is a quantum suppression of PBH evaporation, can dramatically alter their decay dynamics. In this work, we revisit early-Universe constraints on ultralight PBHs in this mass range, demonstrating that memory burden significantly alters previous constraints. We compute new cosmological bounds from BBN that strongly limit the presence of ultralight PBHs in the early Universe. We report that the PBHs in the mass range 10 ⁰ –10 ² g for a suppression parameter k = 2, where k quantifies the strength of the memory burden effect, are unconstrained by observations.

In this work, we consider the simple $Z_2$ symmetric extension to the Standard Model (SM) and proceed to study the nature of electroweak phase transition (EWPT) in the early Universe. We show that the nature of the phase transition changes from a smooth crossover in the SM to a strong first order with this addition of the real scalar. Furthermore, we show that the entropy release in this scenario is higher than that of the SM. This can lead to a strong dilution of the energy density of gravitational waves, as we show in the latter part of the paper.

We revisit the possibility of first order electroweak phase transition (EWPT) in one of the simplest extensions of the Standard Model scalar sector, namely the two-Higgs-doublet model (2HDM). We take into account the ensuing constraints from the electroweak precision tests, Higgs signal strengths and the recent LHC bounds from direct scalar searches. By studying the vacuum transition in 2HDM, we discuss in detail the entropy released in the first order EWPT in various parameter planes of a 2HDM.