Research

Brief Non-Specialist-Level Summary

M y research aims at the investigation of the nature of black holes, particularly those produced in the early Universe, so-called primordial black holes. Black holes—objects so compact that not even light can escape from them—are generic predictions of our standard theory of gravitation. There is a plethora of observational evidence for their existence; in particular, supermassive black holes are known to reside in galactic centres (2020 Nobel Prize in Physics), and the direct detection of gravitational waves from merging black holes (2017 Nobel Prize in Physics) has triggered an avalanche of interest in black holes as dark matter candidates. This is an attractive suggestion because it would a priori not require the addition of new particles and interactions; the same mechanism which generates the seeds of cosmic structure may also generate primordial black holes.

The prime objective of my resarch is to gain understanding of important novel effects and signatures related to primordial black holes as a major building block of our Universe. Since they are produced at very early times, they could be used as probes to acquire fundamental new insights into early-Universe physics being presently inaccessible by any other observational or experimental means. I also study the many signatures a possible large population of primordial black holes could have, i.e. how much gravitational radiation they emit, how they interact with light and how the could shape the formation of structures, in particular in the early Universe.

I am also performing investigations to gain understanding of true quantum aspects of black holes. These concerns for instance a new class of structures—vortices—which we recently proposed (see here for media coverage), which might be our first way to to test quantum effects of gravity, constituting a portal to connect the micro and macro worlds.

My research focuses on all aspects of primordial black holes (PBHs). These are black holes formed in the early Universe. Originally proposed by my collaborator Bernard Carr together with Stephen Hawking, they gained considerable interest when it was realised that they are predicted by many inflationary models. Furthermore, they could constitute an appreciable fraction, if not all, of the dark matters (see any of my seven reviews 19, 31, 38, 40, 41, 47, 50). PBHs should not be viewed as a rival candidate to particle dark matter but rather as an additional possibility; dark matter could be both microscopic and macroscopic, with rich interplay.

Moreover, PBHs can address several cosmic conundra as we have pointed out [33], naturally explaining:

1. microlensing events by planetary-mass objects with about 1% of the dark matter density [51], well above most expectations for free-floating planets,
2. microlensing of quasars, having a misalignment with the lensing galaxy such that the probability of lensing by a star is extremely low [52],
3. a high number of microlensing events by objects between two and five solar masses [53],
4. correlations in the X-ray and cosmic infrared background fluctuations [54],
5. non-observations of certain ultrafaint dwarf galaxies [55],
6. masses, spins, and coalescence rates for black holes found by LIGO/Virgo [56],
7. the relationship between the mass of a galaxy and that of its central supermassive black hole [57],
8. the many high-redshift galaxies (currently up to z = 14) continued to be discovered with JWST [58].

These and numerous other hints for the existence of PBHs are extensively discussed in my recent article [41].

Research on PBHs can generally be divided into four categories:

• Formation [17–19, 25, 26, 31, 33, 36–38, 40, 46, 50]
• Astrophysical Signatures & Constraints [17, 19–24, 27, 29–35, 37, 38, 40, 41, 43–45, 47–50]
• Interaction of Microscopic and Macroscopic Dark Matter [23, 32, 34, 38–40]
• Quantum Structure [16, 39, 42, 49]

Numbers refer to my own contributions (see Publications).

References