Microlensing constraints on clustered primordial black holes

Understand

The discovery of black-hole-binary mergers through their gravitational wave (GW) emission has reopened the exciting possibility that dark matter is made, at least partly, of primordial black holes (PBHs).

• However, this scenario is challenged by many observational probes that set bounds on the relative PBH abundance across a broad range of viable PBH masses.
• Among these bounds, the ones coming from microlensing surveys lead to the strongest constraints in the mass range from $\sim 10^{-10}$ to a few M$_{\odot}$.
• The upper part of this range precisely corresponds to the mass window inside which the formation of PBHs should be boosted due to the QCD phase transition in the early Universe, which makes the microlensing probes particularly important.