Mini workshop on Primordial Black Holes as Dark Matter, or not...

Tuesday 2 Apr 2019, 09:45 → 17:15 Europe/Brussels

E/3rd floor-E.349 - Seminar room (E.349) (Marc de Hemptinne (chemin du Cyclotron, 2, Louvain-la-Neuve))

After about twenty years in the shadows, primordial black holes (PBH) are today seing a huge revival of interest.   The goal of this meeting, organized one day before the Solvay Workshop on "The dark side of Black Holes" in Brussels (but not officially related to it), is to gather a few experts on PBH and to discuss recent developments in the field and important questions like:

How primordial black holes (PBH) form in the early Universe?  What are their mass distribution and merger rates?  Can PBH explain part of, or even the totality of the dark matter?   How to detect or constrain their abundance with gravitational wave observations and/or other electromagnetic probes, such as microlensing?  What are the implications of PBH for high energy physics?

After the discussion session, it will be proposed to participants to go for a drink and/or dinner in Louvain-la-Neuve.

1 Welcome and coffee

2 Threshold and abundance of primordial black holes: dependence on the profile of the cosmological perturbations

Primordial black holes can form in the early Universe from the collapse of cosmological perturbations after the cosmological horizon crossing. They are possible candidates for the dark matter as well as for the seeds of supermassive black holes observed today in the centre of galaxies. In calculations of spherically symmetric collapse, using a large curvature perturbation in the super horizon regime, the initial conditions are specified using the gradient expansion approximation in the long wave length limit. The non linear evolution is then simulated using a Lagrangian relativistic hydrodynamical code. If the perturbation is larger than a threshold depending on the equation of state and on the specific shape of the perturbation, a black hole is formed. In this talk I will discuss the dependence of PBH formation from the initial shape of the curvature profile showing the relation between the threshold amplitude and the steepness of the perturbation which is linked to the amplitude of the pressure gradients that are developing during the collapse. I will show how to derive the initial curvature profile form the shape of the inflationary power spectrum, which affects also the abundance of PBHs. Depending on the model, a proper calculation, using the shape of the power spectrum, shows that the abundance of PBHs is significantly increased by several order of magnitudes compared to previous estimations.

Speaker: Dr Ilia Musco (ICC, U. Barcelona)

3 Hidden universality in the merger rate distribution in the primordial black hole scenario

It has been proposed that primordial black holes (PBHs) form binaries in the radiation dominated era. Once formed, some fraction of them may merge within the age of the Universe by gravitational radiation reaction. We investigate the merger rate of the PBH binaries when the PBH mass function is not monochromatic, which is a generalization of the previous studies where the PBHs are assumed to have the same mass. After deriving a formula for the merger time probability distribution in the PBH mass plane, we evaluate it under two different approximations. We identify a quantity constructed from the mass-distribution of the merger rate density whose value is close to unity for all binary masses independently of the PBH mass function. This result suggests that the measurement of this quantity is useful for testing the PBH scenario.

Speaker: Prof. Teruaki Suyama (TITech Tokyo)

4 Lunch

5 Primordial Black Holes - Formation, Constraints, Uncertainties

Primordial black holes are black holes that may have formed in the early Universe. Their masses potentially span a range from as low as the Planck mass up to many orders of magnitude above the solar mass. This, in particular, includes black holes of the order of 10 solar masses, like those recently discovered by LIGO. These may be of primordial origin. In order to quantitatively asses this and related scenarios, a profound understanding of the holes' formation mechanism necessary. After a general introduction on primordial black holes, I will discuss the most consequential aspects of their formation, and elaborate on the observable imprints these may leave. I will give an overview about recent abundance limits, discuss the uncertainty of these constraints.

Speaker: Dr Florian Kuhnel (Oscar Klein Centre, Stockholm U.)

6 The signature of primordial black holes in galaxy dark matter haloes

There are several well-known gravitationally lensed quasar systems where the quasar images appear to lie well clear of the stars in the lensing galaxy. I shall show that in some such cases the stellar population cannot account for the observed microlensing. In these cases the most plausible bodies responsible for thr lensing are primordial black holes.

Speaker: Prof. Michael Hawkins (Edimburgh U./Royal Obs.)

7 Coffee Break + Discussion