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Gorton, Matthew (2025) Observational Constraints on Primordial Black Hole Dark Matter. PhD thesis, University of Nottingham.

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Abstract

In spite of compelling evidence for its existence and great experimental efforts to detect it, the nature of dark matter remains unknown. One scenario is that some or all of the dark matter consists of black holes formed from the collapse of highly overdense regions in the early Universe. Observational constraints on these `primordial black holes' (PBHs) appear to exclude PBHs from making up all of the dark matter unless their mass, MPBH, lies in the range 1017 g ≲ MPBH ≲ 1022 g, often known as the `asteroid-mass window'. In this thesis we investigate the impact of assumptions made when calculating observational constraints on PBHs.

Firstly, we consider the effect of PBH clustering on microlensing constraints. Clustering of PBHs occurs to a greater extent than for standard cold dark matter on small scales. For PBHs formed from the collapse of large gaussian fluctuations, we find clustering has only a small effect on microlensing constraints even for very massive PBHs (MPBH ∼ 103M⊙) for which the effect of clustering is largest.

Constraints on PBHs are usually obtained assuming all PBHs have the same mass, though accounting for critical collapse shows they would have an extended mass function. A lognormal fit has been widely used to parameterise the PBH mass function, though recent work has shown that other functions provide a better fit. We recalculate both current and prospective future constraints on PBHs with these improved fitting functions, to assess to what extent the asteroid-mass window remains open. For current constraints, the window is narrowed, though there remains a region where PBHs can make up all of the dark matter. Future constraints from evaporation and microlensing may together exclude all of dark matter being made of PBHs if the PBH mass function is sufficiently wide.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Green, Anne
Keywords:	Dark Matter, Primordial Black Holes, Cosmology
Subjects:	Q Science > QB Astronomy
Faculties/Schools:	UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID:	80560
Depositing User:	Gorton, Matthew
Date Deposited:	31 Jul 2025 04:40
Last Modified:	31 Jul 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/80560

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