Born in the first moments of the universe, primordial black holes (PBHs) could provide the answers to many of the biggest open questions in cosmology. How did the universe begin? What is dark matter made of?
Dark matter is believed to make up around 85% of the matter in the Universe. It is unobservable by normal means, but its presence can be detected by its gravitational effects. PBHs are one of the oldest candidates to explain the nature of dark matter, and considered by many to be one of the leading candidates — dark matter may be nothing more than swarms of PBHs clustered in haloes around every galaxy, and even if PBHs only make up a sub-dominant component of DM, their possible existence still has great implications not only for the evolution of the universe, but also its creation.
Over the past decades, there have been many attempts to search for the signs of PBHs, resulting in a large number of different constraints on the abundance of PBHs of different masses — although there are still masses for which PBHs could make up the entire abundance of DM. Whilst there have been no confirmed detections of PBHs, there have been numerous observations which could be explained by their presence. For example, the MaCHO collaboration did observe the presence of compact objects in the galactic halo, and, perhaps most notably, the LIGO-Virgo-KAGRA (LVK) collaboration has made several observations of gravitational waves (GWs) believed to originate from merging binary black hole (BH) systems4. It has been proposed that these BHs may have been primordial in origin (owing partly to the observed masses and low dimensionless effective spins of these BHs), prompting a resurgence of interest in PBHs over recent years.
Historically, PBHs have been used as a tool to place unique constraints on the primordial universe. Constraints on the abundance of PBHs of varying masses are used to place constraints on the primordial power spectrum at scales much smaller than visible by other methods. For example, inflation is typically believed to have lasted at least 50-60 e-folds, but constraints from the CMB and large-scale structure span only 8-10 e-folds. A future observation of even a small abundance of PBHs could rule out WIMPS as a dark matter candidate6, and place constraints on
primordial non-Gaussianity far stronger than available from other sources — granting a better understanding of the mechanism responsible for generating the seeds of all cosmic structure.
The research carried out in this project aims to obtain a better understanding of how PBHs form and cluster in the early universe, enabling more accurate calculations of the gravitational wave signals expected from an abundance of PBHs. Four papers have been produced studying how PBHs could form during the early universe:
1) The first paper takes an updated look at how the abundance of PBHs is affected by different statistical distributions in the early universe.
2) The second paper looks at a model of inflation (the era preceding the Hot Big Bang) known as two-form inflation, and the amount of PBHs and primordial gravitational waves that could be produced.
3) The third paper takes a look at how PBHs could cluster upon their formation.
4) The final paper studies how PBHs could form during the QCD phase transition, which have potentially provided a natural explanation for why the black holes observed by LVK have the masses and spins that they do, as well as how the formation of PBH depends upon the period of cosmic inflation prior to the Big Bang. The attached plot shows how the abundance of PBHs of different masses could be affected by the transition. The black line shows the amount of PBHs formed during the transition, whilst the dashed blue shows a simpler calculation. The red dashed line shows how many PBHs would form in the absence of the phase transition, for the model being studied.
