RER-S8 Cosmology Meeting

Tuesday 26 May 2026, 09:00 → 18:45 Europe/Zurich

Cyclotron Building, Room E349 (Tower E, 3rd floor) (UCLouvain)

09:45 → 10:00 Welcome & Coffee

10:00 → 10:30 Stochastic inflation on the hilltop and beyond

Stochastic inflation allows us to study large inflationary perturbations that may collapse into primordial black holes. I discuss the spectral decomposition of the perturbation distribution in stochastic inflation near a local maximum of the inflaton potential, relevant for primordial black hole production from inflection point models. I compare the stochastic computation with results from classical $\Delta N$ formalism, illustrating, in particular, the difficulties that arise from trajectories that pass `beyond the hilltop,' into the quantum-dominated regime.

Speaker: Eemeli Tomberg

10:30 → 11:00 Time-reversed stochastic inflation

Speaker: Christophe Ringeval

11:00 → 11:15 Coffee Break

11:15 → 11:45 The growth of (primordial) black holes inside main-sequence stars

We study the growth of black holes trapped inside main-sequence stars. Starting from the Bondi accretion framework, we examine the microphysics of the plasma, including nuclear energy generation and energy transport within the accretion flow. We find that the Bondi solution provides an accurate description for black holes with masses between 10^21 and 10^28 g. We also investigate the fate of stars hosting such black holes, including the role of stellar rotation. Importantly, we consider scenarios in which the black holes orbit within the star rather than remaining fixed at its center. We conclude that black holes with masses > 10^21 g will either accrete their host stars on timescales much shorter than the age of the Universe or produce sufficient luminosity to leave observable signatures. The case of lighter black holes remains unclear. We discuss these results in the context of constraints on asteroid-mass primordial black holes from star destruction.

This talk is based on an upcoming paper that completes previous studies on the capture of PBHs by stars and on how asteroid-mass PBHs can be constrained through such processes (2207.07412, 2311.12658, and 2503.03352).

Speaker: Mr Nicolas Esser

11:45 → 12:15 Signatures of loop quantum gravity in primordial black hole cosmologies

The possibility that Dark Matter (DM) is partially or totally constituted by stable Planckian remnants of light Primordial Black Holes (PBHs), suggested for instance by Loop Quantum Gravity (LQG), is investigated. Distinct phenomenological regimes are identified, including scenarios that trigger an early matter-dominated epoch. New constraints are derived on the initial PBH and final remnant abundances. We show that a significant initial abundance of PBHs lighter than $10^3$ kg would overproduce Planckian relics, implying that any observational evidence for such PBHs would challenge models with quasi-stable remnants. Conversely, the products of Hawking radiation from PBHs with masses between $10^3$ and $10^{12}$ kg impose that Planckian relics could only be a highly subdominant DM component. We identify a PBH mass around $10^3$ kg for which Hawking evaporation naturally reheats the Universe while the remnants entirely constitute the present-day DM. Such a scenario does not require fine-tuning the initial abundance of PBH of this mass, which could range from $10^{−10}$ to order one. These early-Universe cosmologies yield distinct observational signatures: scalar-induced gravitational waves sourced by primordial or Poisson fluctuations that are amplified by the early PBH-dominated era. Current and future observations of LIGO/Virgo/KAGRA, the Einstein Telescope and LISA, as well as probes of the effective number of relativistic degrees of freedom, can be used to probe and constrain the initial PBH abundance and the present-day abundance of Planckian relics.

Speaker: Antoine Dierckx

12:15 → 14:15 Lunch

14:15 → 14:45 PBHs in excursion-set formalism

In the excursion-set formalism, various quantities are derived from the first-passage time of a random walk describing the density contrast as the coarse-graining scale varies. In the context of Primordial Black Holes (PBHs) formation, I will show that different choices of hypersurfaces for the sampling of the theory affect the nature of the random walk. Using a synchronous sampling, the random walk becomes a Brownian Motion with moving barrier. I will present quantitative estimates of the mass fraction of PBHs for various power spectra, top-hat, log-normal, both narrow and wide, but also doubly peaked. The effect of cloud-in-cloud as well as the current limitations of the excursion-set approach are discussed. I will then show, in the context of PBHs clustering at formation, how the excursion-set enables to go beyond pointlike treatments (e.g. Poisson model) by including small scales exclusion effects and removing the contribution of the shot noise at zero separation. Quantitative estimates of clustering will be presented.

Speaker: Baptiste Blachier

14:45 → 15:15 Towards Accurate Merger Rates of Early Primordial Black Hole Binaries with an Extended Mass Function

This talk focuses on the dynamics of early primordial black hole binaries with a broad mass distribution. We highlight the subtleties that arise when calculating merger rates for these distributions and propose a framework to account for some of them. While the quest for a definitive rate remains ongoing, we provide critical insights into how mass-function variance impacts theoretical predictions. Finally, we present specific edge cases to demonstrate where current gravitational wave constraints remain robust, where they must be viewed with caution, and where they fail, depending on the underlying mass distribution.

Speaker: Simon Biot

15:15 → 15:45 Orbitographic data from artificial satellites: a valuable resource for probing primordial black holes and the µHz stochastic gravitational wave background

More than a hundred artificial satellites orbiting the Earth have orbits known to within a cm, thanks to laser ranging techniques using retroreflectors and/or microwave ranging. Whether in pairs or entire constellations, they offer a range of different orbital periods and, above all, an incredible wealth of orbital data in free access spanning more than 30 years. In this talk, we present two ways of using these data series to probe primordial black holes (PBH) in the solar system and the stochastic gravitational wave (GW) background. In the first case, we show that the tiny change in the gravitational field when a PBH pass nearby the Earth should affect the orbit of satellites. The use of constellations of satellites perturbed in unison offer a unique signature for such transient events. In the second case, secular evolution of Keplerian orbital parameters induced by GWs strongly amplifies in case of resonance with the satellite's orbit, so becoming detectable. We show that the signature is specific to each satellite, enabling a satellite network to disentangle GW effects from systematics affecting each satellite individually. Finally, we will use the example of the Galileo constellation, the European GPS-like, to make a first assessment of the sensitivity of the detection method in each case.

Speaker: Bruno Bertrand (ROB)

15:45 → 16:00 Coffee Break

16:00 → 16:30 Primordial black holes from stochastic trees

Stochastic inflation can be implemented on stochastic trees, modelling the inflationary expansion as a branching process. Stochastic trees do not operate on a fixed background, instead new spacetime units dynamically emerge as the trees unfold, naturally incorporating metric fluctuations. The tree structure naturally encodes the statistics of curvature perturbations and other cosmological fields, providing a direct tool to study PBH formation. Within this picture, PBHs emerge at unbalanced nodes of the tree, and their mass function can be derived while automatically accounting for the “cloud-in-cloud” effect. A sampled version of the compaction function can also be implemented on such trees, which also allows to distinguish between type I and type II PBH populations. The numerical code FOREST simulates stochastic trees and delivers relevant statistics for PBHs, and I will discuss its application to simple models of quantum diffusion during inflation, the so called quantum wells.

Speaker: Chiara Animali

16:30 → 17:00 Popcorn in the sky: identifying PBHs in the gravitational-wave background

Speaker: Federico De Lillo (UAntwerp)

17:00 → 17:30 The case of stellar-mass primordial black holes as dark matter

Speaker: Sébastien Clesse (Université Libre de Bruxelles (ULB))

17:30 → 18:25 Drink downtown