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 five reviews [19], [31], [38], [40], [41]). 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 [46], 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 [47],
3. a high number of microlensing events by objects between two and five solar masses [48],
4. correlations in the X-ray and cosmic infrared background fluctuations [49],
5. non-observations of certain ultrafaint dwarf galaxies [50],
6. masses, spins, and coalescence rates for black holes found by LIGO/Virgo [51],
7. the relationship between the mass of a galaxy and that of its central supermassive black hole [52],
8. the many high-redshift galaxies (currently up to z = 14) continued to be discovered with JWST [53].

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

With hundreds of merger events expected to be discovered in the coming years [54], I firmly believe that soon we will get the very first evidence for PBHs, and therefore of dark matter!

Research on PBHs can generally be divided into four categories:

• Generation of Overdensities [16]-[19], [25], [26], [31], [33], [36]-[38], [40]
• Constraints and Signatures [17], [19]-[24], [27], [29]-[33], [37], [38], [40], [41], [43]-[45]
• Interaction of Microscopic and Macroscopic Dark Matter [23], [32], [34], [38], [39], [40]
• Quantum Structure [16], [39], [42]

Numbers above refer to my own contributions (see Publications). Additionally, I am currently working on five further projects. Below, I first comment on a selection of my key achievements in each of these fields; this is followed by elaborations on my planned future research.

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Achievements

Generation of Overdensities

 

There is a wealth of mechanisms for generating overdensities that collapse into PBHs. I have worked on inflationary scenarios [17], [19], [26], phase transitions [33], [35], as well as developed a novel mechanism in which PBHs are formed by the confinement of quark pairs [37]. The latter is particularly interesting as it avoids strong coupling and is not plagued by exponential fine-tuning as most of the other scenarios. In Reference [33], we have shown that the thermal history of the Universe naturally yields enhancements in PBH production at astrophysically relevant mass ranges, specifically at (see Figure 1):

1. a) planetary scales,
2. b) around a solar mass,
3. c) an order of magnitude above, and
4. d) around a million solar masses.

As a consequence, regarding b) and c), besides order 10 solar mass mergers, our model [33] predicts high mass-ratio mergers and a larger number of those with both initial masses around one solar mass. Additionally, in Reference [32], we suggested the formation of black holes with masses even above 10¹² solar mass—objects we termed stupendously large black holes (SLABs).

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Constraints and Detection Forecasts

 

Depending on their mass, PBHs can manifest themselves through a large variety of physical effects, such as gravitational lensing, accretion, destruction of neutron stars or white dwarfs, and particularly gravitational waves (see my reviews [19], [31], [38], [40], [41]). For the latter, in Reference [35], we explored how possible lepton-flavour asymmetries alter the forecasted PBH merger detections and could improve predictions of established PBH formation scenarios.

It is often assumed that PBHs have a monochromatic mass spectrum, though this clearly is an oversimplification. In some cases, it can lead to large errors. Hence, it is important to reevaluate monochromatic PBH constraints for the realistic case of extended spectra. My contributions [17], [19], [20] were the first to systematically study this case. We also proposed multimodal PBH mass functions which naturally emerge in theories with non-standard vacua [25].

When dark matter is significantly constituted by PBHs, they might affect the evolution of stars. In Reference [43], we studied the evolution of stars with a black hole at their center and found that such objects can be surprisingly long-lived. Depending on the PBH mass, there could be observable asteroseismological signatures.

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Current & Planned Future Research

 

Building upon my previous work and recent developments in the field, I am pursuing a range of projects that address key open questions in cosmology, dark matter, and gravitational wave physics. These include:

• Gravitational Radiation from Vortices: I aim to perform a realistic determination of the gravitational radiation produced by the generation, destruction, and ejection of vortices. This project will include a detailed comparison of theoretical predictions with current gravitational-wave data.
• Primordial Magnetic Fields from Vortices: Investigating the origin of primordial magnetic fields through their connection to vortices, with the goal of understanding their implications for early-universe magnetogenesis.
• Exploring Light PBH Dark Matter: I plan to conduct an in-depth exploration of light PBH dark matter, particularly examining the effects of PBHs in our Solar System, their potential interference with the cosmic microwave background, and their role during Big Bang nucleosynthesis.
• Simulations of PBH Dark Matter and Early Structure Formation: Large-scale simulations will be developed to model early structure formation driven by PBH dark matter, with a focus on comparing these models to recent observations of high-redshift galaxies.
• Dark Phase Transitions and PBH Abundance: I will investigate the impact of dark phase transitions on the current abundance of PBHs, using PBHs as a potential probe to detect and study these early-universe events.
• Lepton-Flavour Asymmetries and PBH Formation: This project involves a systematic study of how lepton-flavour asymmetries influence PBH formation. I will leverage the latest LIGO/Virgo/KAGRA sensitivities to extend and refine my previous findings in this area.
• Search for Hawking Stars: I will pursue observational strategies for detecting Hawking stars.
• ...

These projects aim to further our understanding of dark matter, early-Universe cosmology, and gravitational phenomena, building a bridge between theoretical predictions and observational data.

References

[1–45]		My own contributions; see Publications.
[46]		H. Niikura et al., Phys. Rev. D 99, 083503 (2019)
[47]		E. Mediavilla et al., Astrophys. J. 836, L18 (2017)
[48]		Ł. Wyrzykowski and I. Mandel, Astron. Astrophys. 636, 12 (2020)
[49]		A. Kashlinsky et al., Nature (London) 438, 45 (2005)
[50]		S. Clesse and J. García-Bellido, Phys. Dark Univ. 22, 137 (2018)
[51]		B. Abbott et al., Phys. Rev. X 13, 011056 (2023)
[52]		D. Kruijssen and N. Lützgendorf, Mon. Not. Roy. Astron. Soc. 434, 41 (2013)
[53]		B. Abbott et al., Living Rev. Rel. 23, 3 (2020)
[54]		G. Dvali and C. Gomez, Fortsch. Phys. 61, 742 (2013)
[55]		G. Dvali and C. Gomez, Eur. Phys. J. C 74, 2752 (2014)
[56]		R. Blandford and R. Znajek, Mon. Not. Roy. Astron. Soc. 179, 433 (1977)