NEHOP'25 - New Horizons in Primordial Black Hole Physics

19 May 2025, 08:15 → 22 May 2025, 12:40 Europe/Brussels

Brussels

We ask all participants to pay the registration fee by Monday, May 5th. See you soon in Brussels!

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After two very successful editions in Naples and Edinburgh, we are delighted to announce the third New Horizons in Primordial Black Hole Physics (NEHOP'25) workshop. This year, it will take place in the beautiful city of Brussels, Belgium, from May 19th to May 22nd, 2025.

This workshop event aims to nurture discussions and bring together experts on the broad topic of Primordial Black Holes (PBHs). From their phenomenological implications, their possible relationship to extant problems in the SM (dark matter, neutrino masses, baryon asymmetry, inflation), their theoretical underpinnings and origin, to the multi-messenger signals that are motivating state-of-the-art experimental developments.

While most talks will consist of short, technical presentations by young researchers, a small number of expert talks will be included to both provide comprehensive overviews and present new results on key topics.

The registration fee, which we will open soon, is of 150€. This fee covers venue hire, lunch and coffee breaks, and the workshop dinner. 

This is primarily an in-person workshop.

Note that the event will end around 12:00 on Thursday so that participants can travel on Thursday afternoon.

Confirmed speakers
Bernard Carr (Queen Mary University of London)
David Wands (University of Portsmouth)
Florian Kühnel (MPP & LMU Munich)
Gabriele Franciolini (CERN)
Juan Garcia-Bellido (Universidad Autónoma de Madrid, IFT)
Mairi Sakellariadou (King's College London)

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Fishing attempts related to accommodation for NEHOP are possible. Please note that we have not engaged any travel agency to handle hotel reservations for participants.

Monday 19 May ¶

Registration and Welcome¶

PBHs from inflation¶

1 Stochastic inflation beyond slow roll¶

Stochastic inflation offers a non-perturbative framework with which to calculate the distribution of density perturbations and in particular large, but rare fluctuations in the non-Gaussian tail of the distribution that could give rise to primordial black holes. Strongly enhanced density perturbations from single-field inflation require deviations from slow roll, and there are a number of challenges, and conflicting claims, when applying the stochastic formalism beyond the familiar slow roll setting. I will discuss issues including modelling non-adiabatic perturbations on super-Hubble scales and the compare the classical and stochastic delta-N formalisms used to calculate density perturbations.

Speaker: David Wands

2 Calculation of primordial non-Gaussianity from ultra slow-roll inflation¶

The probability of large primordial scalar perturbations (leading to primordial black hole formation and the concurrent GW backgrounds) is sensitive to the tail of the PDF of primordial curvature perturbations. We analyse such tails in inflationary models featuring an ultra slow-roll phase, known to enhance both the amplitude and non-Gaussianity of curvature perturbations at small scales. We first review the computation of the non-Gaussian PDF of primordial curvature perturbations relying on classical non-linear techniques to relate inflaton and curvature perturbations ($\delta N$ formalism). We then assess the corrections introduced in this calculation by potentially sizeable inflaton self-interactions using field theory techniques (in-in formalism). Finally, we present a systematic procedure to simultaneously account for both sources of primordial non-Gaussianity.

Based on 2406.02417 and 2412.14106 with G. Ballesteros, J. Gambín Egea, T. Konstandin, M. Pierre and J. Rey

Speaker: Alejandro Perez Rodriguez (Universidad Autonoma de Madrid)

3 Causality Bounds on the Primordial Power Spectrum¶

Effective field theories (EFTs) parametrize our ignorance of the underlying UV theory through their Wilson coefficients. However, not all values of these coefficients are consistent with fundamental physical principles. In this paper, we explore the consequences of imposing causal propagation on the comoving curvature perturbation in the EFT of inflation, particularly its impact on the primordial power spectrum and the effective sound speed $c_s^{\text{eff}}$. We investigate scenarios where $c_s^{\text{eff}}$ undergoes a transition, remaining consistent with CMB constraints at early times but later experiencing a drastic change, becoming highly subluminal. Such scenarios allow the primordial power spectrum to grow at small scales, potentially leading to the formation of primordial black holes or the generation of scalar-induced gravitational waves. We find the generic feature that in a causal theory, luminal sound speeds imply a free theory, effectively constraining the dynamics. Additionally, we obtain that when considering natural values for the Wilson coefficients, maintaining the validity of the EFT and the weakly coupled regime, and enforcing causal propagation of the EFT modes, the power spectrum cannot increase drastically. This imposes significant constraints on the parameter space of models aiming to produce such features.

Speaker: Sebastian Cespedes (Imperial College London)

10:30 Coffee Break

4 From the inflaton potential to PBH abundance¶

I will discuss how to calculate the PBH abundance and mass distribution starting from the inflationary potential, taking into account stochastic effects and using the compaction function. We determine the radial profiles directly from stochastic calculation, and do not assume Gaussianity. I will outline progress and how to address the remaining open issues.

Speaker: Syksy Räsänen

5 Mapping inflationary loop corrections to boundary terms¶

Both single- and multi-field models of inflation might lead to enhanced scalar fluctuations on scales much smaller than those seeding the large-scale structure formation. In these scenarios, it is possible that the spike of power at high wavenumber might induce large corrections to the scalar power spectrum, e.g. in the form of loop corrections, potentially endangering the perturbativity of the underlying models. In this talk we discuss recent developments in the calculation of the 1-loop correction to a large-scale adiabatic mode. We demonstrate that non-volume-suppressed corrections only contribute at the boundaries of the momentum integral. To achieve this we employ expansion methods, such as the $\delta N$ formalism, as well as more general expansions that do not rely on assumption of validity of the separate universe picture.

Speaker: Dr Laura Iacconi (Queen Mary University of London)

6 Ultraslow-roll inflation on the lattice: backreaction and nonlinear effects¶

Violating the slow-roll regime in the final stages of inflation can significantly enhance curvature perturbations, a scenario often invoked in models producing primordial black holes and small-scale scalar-induced gravitational waves. In this regime, tree-level computations may become insufficient, and nonlinear corrections can play a crucial role. In this talk, I will discuss how lattice simulations can assess the impact of nonlinear effects, both on backreaction to the background and the evolution of nonlinear curvature perturbations. Our systematic study of various USR potentials on the lattice shows that nonlinear corrections are significant in the parameter space relevant for PBH and GW production. These findings demonstrate the need to go beyond leading-order perturbation theory to make robust predictions for inflation models featuring a USR phase.

Speaker: Angelo Caravano

7 Eternal inflation in primordial black hole models¶

Inflationary models with an inflection point potential may produce a significant abundance of primordial black holes. I show that in typical models, inflation is eternal near the inflection point, that is, the volume of the inflating region diverges at late times. The inflating space then has a fractal structure, with eternal inflation taking place inside type II black holes. I discuss the implications for predicting cosmological observables in these models. In particular, eternal inflation challenges the models' CMB predictions, possibly making them incompatible with observations.

Speaker: Eemeli Tomberg (UCLouvain)

8 Stochastic Dynamics during Inflation¶

Quantum fluctuations during inflation in the very early universe inevitably generate inhomogeneities and anisotropies across all observable scales and beyond. In the stochastic approach to modeling inflationary dynamics, these quantum fluctuations are incorporated as stochastic noise. We employ the formalism of stochastic inflation to examine the role of quantum diffusion during inflation. This provides a non-perturbative framework to study the origin of large cosmological fluctuations during inflation, relevant for calculations of primordial black hole abundance. Using the numerical code PyFPT, we investigate the occurrence of extremely rare, large fluctuations in slow-roll inflation models, which may exhibit a highly non-Gaussian distribution. We contrast the case of inflation driven by a simple quadratic scalar field potential with alpha-attractor models favoured by current CMB data. We also compare the results obtained from stochastic inflation with those obtained from the traditional (classical) $\delta N$ formalism used to describe nonlinear evolution on the super-Hubble scales.

Speaker: Parth Bhargava (ICG, Portsmouth)

12:40 Lunch Break

PBHs from inflation & other formation scenarios¶

9 The unexpected shape of the primordial black hole mass function¶

We show that a broad power spectrum—modeled as a plateau between infrared and ultraviolet scales—results in a bimodal mass function, with the dominant peak corresponding to heavy PBHs associated with the infrared scale.

This result, based on compaction function statistics, significantly impacts overproduction bounds on the power spectrum amplitude, underscores the need for further advancements in PBH formation theory, and tightens the viable mass range for primordial black holes as dark matter.

Speaker: Jacopo Fumagalli (ICCUB - Barcelona)

10 Primordial correlates from Multi-point propagators¶

Accurate predictions of correlators of the primordial curvature perturbation are critical for connecting inflationary models to cosmological observations. Numerical methods employing differential equations, such as the transport approach, have been extensively used to compute the evolution of these correlators.
In this talk, I will present a novel numerical implementation of the transport formalism. We use Multi-point propagators (MPPs), that link non-linearly evolved fields to their values at some earlier time.
This method recasts the direct evolution of correlators into a system of differential equations for MPPs.
We benchmark the MPP approach against the established PyTransport code across a range of models, and I will discuss the essential applications of the new method to models than can lead to enhanced fluctuations on small scales.

Speaker: Andrea Costantini (Queen Mary University of London)

11 Scalar fields and primordial black holes¶

In this talk I will discuss scalar fields in cosmology. Specifically, the impact they can have on early universe thermodynamics and primordial black holes.

Speaker: Xavier Pritchard (University of Sussex)

12 Primordial black hole formation in a slow-reheating scenario¶

In scalar field dark matter models, virialized halos form condensed central cores known as solitons. We extend this idea to the reheating phase of the early universe, a critical period that sets the stage for the emergence of structure after inflation. We study the formation of primordial black holes (PBHs) from the gravitational collapse of virialized configurations arising during this era. Focusing on free (pure massive) scalar fields, we derive threshold conditions for collapse—either of the full structure or its central core—in terms of the primordial density contrast. Our results highlight how the dynamics of reheating can influence PBH production and contribute to their overall abundance.

Speaker: Luis Enrique Padilla Albores (Queen Mary University of London)

13 Primordial Black Hole Formation in a Scalar Field Dominated Universe¶

We present a numerical code that solves the Misner-Sharp system for a spherically symmetric cosmological model containing both a scalar field and a perfect fluid. While the code is capable of exploring general scenarios involving an minimally coupled scalar field and perfect fluid, we focus on the regime where the scalar field dominates the dynamics, particularly in the post-inflationary scalar field-dominated scenario, where the universe is governed by a rapidly oscillating scalar field for a period lasting a few $e$-folds. We analyse the threshold for PBH formation under quadratic and quartic potentials, evolving configurations from superhorizon scales. Our results confirm that a quartic potential behavior is similar to the radiation-dominated universe, resulting in a PBH formation threshold close to the well-established value in radiation backgrounds. Conversely, in the quadratic case, we observe a significant deviation from the expected dust-like behaviour, due to wave-like effects opposing the gravitational collapse. While numerical limitations prevent us from evolving a wide range of initial conditions to determine a precise threshold for PBH formation, our findings suggest that PBH formation may be suppressed with respect to the pure dust scenario, potentially allowing the formation of stable solitonic structures instead. This study highlights the importance of properly accounting for wave dynamics in oscillating scalar fields when characterising PBH formation.

Speaker: Ethan Milligan (QMUL)

14 Making PBHs in a Early Matter Dominated Era¶

I present a novel mechanism for creating primordial black holes.   A heavy dissipative dark sector can come to dominate the universe, creating an early matter dominated era prior to Big Bang Nucleosynthesis (BBN).  At this time the dark matter can form halos which persist after the phase transition back to radiation domination, and slowly collapse at late times.  This leads to the late time formation of subsolar mass primordial black holes.  This also leads to the formation of late forming low mass black holes which may Hawking radiate violently at late times.

Speaker: Melissa Diamond

16:00 Coffee Break

15 Spectators Strike Back: Light Scalar Fields Foster PBH Formation¶

I will discuss the effects of adding a scalar spectator field to a primordial black hole (PBH) producing single-field inflation model. Such single field PBH models typically include a small-field feature that leads to a phase of ultra slow-roll inflation (USR). Scalar fields (including spectators—fields which are only coupled to gravity and have small masses and potentials, thus never driving the dynamics) are ubiquitous in Beyond Standard Model (BSM) theories and thus well motivated. I will show how, with the addition of a spectator, the slow-roll parameter is prevented from becoming extremely small, averting any USR phase. While this might suggest that PBH production is suppressed by adding a spectator field, I will further demonstrate that the overall change to the dynamics includes a period of tachyonic isocurvature coupled with a turn in field space. This allows for the transfer of power from isocurvature to adiabatic modes, enhancing the peak in the curvature power spectrum, and eventually leading to critical collapse post-inflation. I will also comment on the problem of fine-tuning in single field models and show that the addition of a spectator partially alleviates the need for such fine tuning while still simultaneously producing PBHs and satisfying the most recent CMB constraints.

Speaker: Sarah Geller (MIT)

16 Primordial black hole formation from a massless scalar field¶

We have investigated primordial black holes formation within a model of the early Universe dominated by a massless scalar field, developing a numerical spherically symmetric code dedicated to this problem. Imposing initial conditions on super horizon scales using the gradient expansion approach, we show that a massless scalar filed is equivalent to a perfect fluid, where the pressure is equal to the total energy density, only in the regime of cosmological expansion, while during the gravitational collapse space-like gradients arises and the comoving slicing is failing. For this reason, moving to a constant mean curvature slicing, we follow entirely the numerical evolution of cosmological perturbations, also during the gravitational collapse, computing the threshold ${\delta }_c$ for primordial black hole formation, and the corresponding mass distribution described by critical collapse.

Speaker: Ilia Musco

17 Primordial Black Holes from an Interrupted Phase Transition¶

I will present a new mechanism of primordial black hole formation via an interrupted phase transition during the early matter-dominated stage of reheating after inflation. In reheating, induced by the decay of a pressureless fluid dominating the Universe at the end of inflation, dubbed as reheaton, the temperature of the radiation bath typically increases, reaching a maximum temperature $T_{\mathrm{m}\mathrm{a}\mathrm{x}}$, and then decreases. We consider a first-order phase transition induced by the increase of the temperature that is aborted as $T_{\mathrm{m}\mathrm{a}\mathrm{x}}$ is higher than the critical temperature but not sufficiently high for the bubble nucleation rate to overcome the expansion of the Universe. Although bubbles never fully occupy the space, some may be nucleated and expand until the temperature once again decreases to the critical temperature. We argue that these bubbles shrink and disappear as the temperature drops further, leaving behind macroscopic spherical regions with positive density perturbations. These perturbed regions accrete the surrounding matter (reheatons) and eventually collapse into primordial black holes whose mass continues to grow until the onset of radiation domination.  We estimate the abundance of these primordial black holes in terms of the bubble nucleation rate at $T_{\mathrm{m}\mathrm{a}\mathrm{x}}$,  and demonstrate that the abundance can be significantly large from a phenomenological perspective.

Speaker: Lucien Heurtier (King's College London)

18 Harvesting primordial black holes from stochastic trees¶

I will present a novel framework for implementing stochastic inflation on stochastic trees, modelling the inflationary expansion as a branching process. The statistical properties of the curvature perturbation and other cosmological fields are encoded within the tree structure in a manner that fully captures quantum diffusion and its non-perturbative backreaction during inflation. Stochastic trees also provide an ideal tool for the study of primordial black holes and I will show how they emerge at unbalanced nodes of the tree and how their mass distribution can be derived while automatically accounting for the “cloud-in-cloud” effect. Stochastic trees do not operate on a fixed background; instead, new spacetime units dynamically emerge as the trees unfold, naturally incorporating metric fluctuations. Their recursive nature offers exceptional numerical efficiency, and I will present the FOrtran Recursive Exploration of Stochastic Trees (FOREST) code. When applied to the “quantum-well” toy model, results reveal broad mass distributions, characterised by mild power laws that are truncated by exponential tails.

Speaker: Chiara Animali (UCLouvain)

19 Stochastic Axion-like Curvaton: Non-Gaussianity and Primordial Black Holes Without Large Power Spectrum¶

We discuss a mechanism of primordial black hole (PBH) formation that does not require specific features in the inflationary potential, revisiting previous literature. In this mechanism, a light spectator field evolves stochastically during inflation and remains subdominant during the post-inflationary era. Even though the curvature power spectrum stays small at all scales, rare perturbations of the field probe a local maximum in its potential, leading to non-Gaussian tails in the distribution of curvature fluctuations, and to copious PBH production. For a concrete axion-like particle (ALP) scenario we analytically determine the distribution of the compaction function for perturbations, showing that it is characterized by a heavy tail, which produces an extended PBH mass distribution. We find the ALP mass and decay constant to be correlated with the PBH mass, for instance, an ALP with a mass ma=5.4×1014 eV and a decay constant fa=4.6×10−5Mpl can lead to PBHs of mass MPBH=1021 g as the entire dark matter (DM) of the universe, and is testable in future PBH observations via lensing in the NGRST and mergers detectable in the LISA and ET Gravitational Waves (GW) detectors. We then extend our analysis to mixed ALP and PBH dark matter and Higgs-like spectator fields. We find that PBHs cluster strongly over all cosmological scales, clashing with CMB isocurvature bounds. We argue that this problem is shared by all PBH production from inflationary models that depend solely on large non-Gaussianity without a peak in the curvature power spectrum and discuss possible remedies.

Speaker: Anish Ghoshal (University of Warsaw, Poland)

Tuesday 20 May ¶

PBH evaporation¶

20 Quantum Quiddity and Correlation Characteristics¶

In the first part of my talk, I will discuss how recent advances in our understanding of quantum effects in black holes impact PBHs. On the one hand, this concerns deviations from Hawking radiation in the form of the memory-burden effect. On the other hand, I will discuss vorticity, which we recently conjectured to be a new characteristic of (near-extremally rotating) black holes. In the second part of my talk, I will present novel results on large-scale simulations of spatially-correlated random fields, being able to resolve events as rare as one in 10^13, and discuss their application to PBHs.

Speaker: Florian Kühnel

21 Impact of general relativistic accretion on ultralight PBHs and dark matter¶

Accretion plays a crucial role in the evolution of primordial black holes (PBHs) in the early Universe. While earlier works focused on Newtonian Bondi-Hoyle models, general relativistic effects become significant in the hot, dense relativistic plasma of the early cosmos. In this study, we consider both relativistic accretion and Hawking evaporation by solving their coupled evolution equations in an expanding background. We present updated constraints on the initial PBH mass spectrum, improved bounds on their abundance, and their contribution to dark matter. We also predict a shift in high-frequency gravitational wave signals due to accretion-driven mass growth.

Speaker: JITUMANI KALITA (INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI)

22 Cogenesis of baryon and dark matter from ultra-light PBH¶

We discuss the possibility of producing the observed baryon asymmetry of the Universe (BAU) and dark matter (DM) from evaporating primordial black holes (PBH) incorporating semi-classical and memory burden regime. In the simplest scenario of baryogenesis via vanilla leptogenesis with hierarchical right handed neutrino (RHN), it is possible to generate the observed BAU with PBH being sole contributor to the production of RHN. While it is not possible to achieve cogenesis in this minimal setup due to structure formation constraints on relic allowed DM parameter space, we show the viability of successful cogenesis in the resonant leptogenesis regime. We also show that successful cogenesis can be achieved in a simple baryogenesis model without taking the leptogenesis route. Due to the possibility of generating asymmetry even below the sphaleron decoupling era, the direct baryogenesis route opens up new parameter space of memory-burdened PBH. The two scenarios of successful cogenesis can also be distinguished by observations of stochastic gravitational waves produced from PBH density fluctuations.

Speaker: Mr Nayan Das (Indian Institute of Technology Guwahati)

23 Boosted sub-GeV Dark Matter from Primordial Black Holes in DarkSide-50¶

The direct detection of sub-GeV dark matter interacting with nucleons is hampered by the low recoil energies induced by scatterings in the detectors. This experimental difficulty is avoided in the scenario of boosted dark matter where a component of dark matter particles is endowed with large kinetic energies. By focusing on the concept of boosted dark matter, wherein a subset of dark matter particles possesses significant kinetic energies, we identify the current evaporation of primordial black holes, ranging in mass from ${10}^{14}$ to ${10}^{16}$\,g, as a potential source of such particles with energies ranging from tens to hundreds of MeV. Specifically, we investigate the implications of this phenomenon on the DarkSide-50 experiment, demonstrating that relativistic dark matter particles originating from primordial black holes could yield signals orders of magnitude larger than existing upper bounds. Consequently, we propose that this avenue enables the constraint of the combined parameter space encompassing primordial black holes and sub-GeV dark matter.

Speaker: Roberta Calabrese

24 Impact of memory-burdened black holes on primordial gravitational waves in light of Pulsar Timing Array¶

Blue-tilted Gravitational Waves (BGWs) have emerged as a promising candidate for explaining the cosmic gravitational wave signals observed by Pulsar Timing Arrays (PTA). Within the standard cosmological model, the frequency range of BGWs is restricted by the Big Bang Nucleosynthesis (BBN) limit on gravitational wave amplitude, which prevents their detection at interferometer scales. However, the inclusion of an early matter-dominated phase can dilute BGWs at higher frequencies, ensuring consistency with both BBN and LIGO constraints on stochastic gravitational waves. This process enables BGWs to match PTA data while generating a unique and testable gravitational wave signature across a wide frequency range. Ultralight Primordial Black Holes (PBHs) could serve as the source of the necessary early matter-dominated phase to facilitate this mechanism. When interpreted through the framework of BGWs, the PTA results provide a means to constrain the parameter space of a novel scenario involving modified Hawking radiation, referred to as the "memory burden" effect, linked to ultralight PBHs. This scenario can be further investigated using high-frequency gravitational wave detectors. In particular, we show that PBHs with masses as light as ${10}^{2-3}\mathrm{g}$ can leave observable imprints on BGWs at higher frequencies while remaining compatible with PTA observations.

Speaker: Satyabrata Datta (Nanjing Normal University)

10:40 Coffee Break

25 Memory-burdened Primordial Black Holes as sources of Ultra-High Energy neutrinos.¶

In this talk I will address how “memory-burdened” primordial black holes (PBHs) of low mass, evaporating today, could in principle be detected via their neutrino emission.
Using the latest IceCube data, we place novel constraints on the combined parameter space of PBH masses and memory burden effects. Additionally, we explore whether the ultra-high-energy neutrino event recently detected by KM3NeT could originate from an evaporating PBH. We systematically examine the parameter space of burdened PBHs under current constraints on PBH dark matter fraction, and predict the occurrence rate of similar events. Future neutrino telescopes such as IceCube-Gen2 and GRAND will provide crucial tests of these scenarios, with the potential to probe highly suppressed evaporation regimes and light PBH masses.

Speaker: Andrea Boccia (Scuola Superiore Meridionale)

26 Transitioning to Memory Burden: Detectable Small Primordial Black Holes as Dark Matter¶

Mounting theoretical evidence suggests that the information stored in black holes suppresses their decay rate. This effect of memory burden opens up a new window for small primordial black holes (PBHs) below ${10}^{15}\mathrm{g}$ as dark matter candidates. In this talk, I show that the smooth transition from semi-classical evaporation to the memory-burdened phase strongly impacts observational bounds on the abundance of small PBHs. The most stringent constraints come from present-day fluxes of astrophysical particles and point towards an early onset of memory burden, after losing only a small fraction of the initial mass. Remarkably, currently-transitioning small PBHs are detectable through high-energetic neutrino events.

Based on:
G. Dvali, M. Zantedeschi, S. Z., Transitioning to Memory Burden: Detectable Small Primordial Black Holes as Dark Matter, arXiv:2503.21740.
M. Michel, S. Z., The Timescales of Quantum Breaking, Fortsch. Phys. 71 (2023) 2300163, arXiv:2306.09410. [Accompanying news article “Where is the boundary to the quantum world?”]
G. Dvali, L. Eisemann, M. Michel, S. Z., Black hole metamorphosis and stabilization by memory burden, Phys. Rev. D 102 (2020) 103523, arXiv:2006.00011.

Speaker: Sebastian Zell (LMU Munich)

27 The impact of memory-burdened primordial black holes on high-scale leptogenesis¶

We explore the impact of the back-reaction of evaporation on the quantum state of Primordial Black Holes (PBHs), known as “memory burden”, on the baryon asymmetry production in the Universe through high-scale leptogenesis. Focusing on PBH masses ranging from 1 to 1000 grams, we investigate the interplay between the non-thermal production of heavy sterile neutrinos and the entropy injection within this non-standard cosmological framework. By assuming appropriate values for the memory-burden parameters, q = 1/2 and k = 1, we derive mutual exclusion limits between PBHs and thermal leptogenesis in the mixed parameter space. Our analysis reveals that the primary contribution of PBHs to baryon asymmetry stems from entropy injection. Indeed, we find that, differently from earlier studies based on the semi-classical Hawking evaporation, the memory- burden effect suppresses the non-thermal source term in the PBH mass range explored. This has significant implications for understanding baryogenesis in such alternative cosmological scenarios.

Speaker: Marco Chianese (Scuola Superiore Meridionale)

28 GrayHawk: A public code for calculating the Gray Body Factors of massless fields around spherically symmetric Black Holes¶

We introduce and describe $\mathtt{G}\mathtt{r}\mathtt{a}\mathtt{y}\mathtt{H}\mathtt{a}\mathtt{w}\mathtt{k}$, a publicly available Mathematica-based tool designed for the efficient computation of gray-body factors for spherically symmetric and asymptotically flat black holes. This program provides users with a rapid and reliable means to compute gray-body factors for massless fields with spin (s = 0, 1/2, 1, 2) in modes specified by the angular quantum number (l), given a black hole metric and the associated parameter values.

$\mathtt{G}\mathtt{r}\mathtt{a}\mathtt{y}\mathtt{H}\mathtt{a}\mathtt{w}\mathtt{k}$ is preloaded with seven different black hole metrics, offering immediate applicability to a variety of theoretical models. Additionally, its modular structure allows users to extend its functionality easily by incorporating alternative metrics or configurations. This versatility makes $\mathtt{G}\mathtt{r}\mathtt{a}\mathtt{y}\mathtt{H}\mathtt{a}\mathtt{w}\mathtt{k}$ a powerful and adaptable resource for researchers studying black hole physics and Hawking radiation.

The codes described in this work are publicly available at https://github.com/marcocalza89/GrayHawk.

Speaker: Dr Marco Calza (University of Trento)

29 Memory-Burdened Black Holes as Sources of High-Energy Particles¶

The “memory burden” effect describes how the information stored in an object resists its own decay. This effect is especially pronounced in “saturons” — systems with maximal entropy consistent with unitarity — of which black holes are prime examples. Importantly, this mechanism is not limited to gravitational systems: it also appears in renormalizable field theories. To illustrate its broader relevance, I will present a soliton model that shares key features with black holes and is similarly stabilized by its memory content. Finally, if black holes stabilized by their memory compose the dark matter, they will undergo mergers in the today's Universe, leading to "young" black holes which evaporate once again. The resulting signal is comparable with observed fluxes of astrophysical high-energy particles.

Speaker: Michael Zantedeschi

12:50 Lunch Break

PBHs signatures¶

30 The detection of observational signatures from a population of dark matter primordial black holes¶

In the last few years the case that primordial black holes make up a significant fraction of the dark matter has received steadily increasing support from a wide variety of observations. If indeed this is the case then there are a number of observational consequences to be expected. In this talk I shall describe situations where such a population of compact bodies should betray their presence with observational signatures. The detection of these signatures is an important consistency check for the idea that primordial black holes are a component of dark matter, and the bulk of my talk will be devoted to presenting observations consistent with the expected signatures.

Speaker: Michael Hawkins (University of Edinburgh)

31 Cosmic evolution of PBH binaries: incorporating radiative feedback into accretion models¶

Understanding the evolution of primordial black hole (PBH) populations across cosmic history is critical for reconciling observational data with theoretical formation models. Previous studies showed that gas accretion can significantly alter the mass and spin distributions of PBH binaries from their formation time to the low-redshift universe where gravitational wave signals are observed. However, such accretion is inherently coupled to radiative feedback: the emitted radiation and localised heating of ambient gas can suppress accretion rates, an effect previously highlighted for isolated PBHs. In this work, we review the accretion-driven evolution of PBH binaries, focusing on the incorporation of radiative heating feedback into our modelling of accretion.

Speaker: Francesca Scarcella (IFCA)

32 21cm constraints on PBHs¶

The 21cm signal, soon to be probed by upcoming experiments, is a powerful late-time cosmological tool for constraining Primordial Black Holes (PBHs). Some of the most stringent limits on PBH abundance in the solar mass range are derived using 21cm forecasts. In this talk, I will critically re-examine these forecasts, highlighting the impact of astrophysical uncertainties on the derived constraints. In particular, I will focus on uncertainties in the modelling of reionisation and stellar astrophysics, as well as those affecting PBH accretion. By reassessing these factors, I will explore the robustness of 21cm-based PBH forecasts.

Speaker: Dominic Agius (IFIC, University of Valencia)

33 Primordial Black Holes in the Solar System¶

If primordial black holes (PBHs) of asteroidal mass make up the entire dark matter, they could be detectable through their gravitational influence in the solar system. In this work, we study the perturbations that PBHs induce on the orbits of planets. Detailed numerical simulations of the solar system, embedded in a halo of PBHs, are performed. We find that the gravitational effect of the PBHs is dominated by the closest encounter. Using the Earth–Mars distance as an observational probe, we show that the perturbations are smaller than the current uncertainties in solar system ephemerides and thus PBHs are not constrained. We estimate that an improvement in the accuracy of the ephemerides by an order of magnitude or the extraction of signals well below the noise level is required to detect the gravitational influence of PBHs in the solar system in the foreseeable future.

Speaker: Valentin Thoss (Ludwig-Maximilians-Universität München)

34 Constraints on asteroid-mass primordial black holes in dwarf galaxies using Hubble Space Telescope photometry¶

Primordial black holes (PBHs) in the asteroid-mass range remain a viable and until now unconstrained dark matter (DM) candidate. If
such PBHs exist, they could be captured by stars in DM-dominated environments with low velocity dispersion such as ultra-faint dwarf
galaxies (UFDs). The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. As
a result, the presence of PBHs in UFDs would alter their stellar mass functions. Using photometric observations of three ultra-faint
dwarf galaxies from the Hubble Space Telescope, we show that it is unlikely that their mass functions have been significantly modified
by PBHs, and we place constraints on the PBH abundance. In the ultra-faint dwarf galaxy Triangulum II, PBHs around 10^19 g are
excluded at the 2σ (3σ) level from constituting more than ∼ 55% (∼ 78%) of the dark matter, while the possibility that PBHs represent
the entirety of the DM is excluded at the 3.7σ level.

Speaker: Mr Nicolas Esser

15:40 Coffee Break

35 Antiprotons and Antideuterons signatures from Primordial Black Holes¶

In this talk, we present our study of the cosmic antiproton and antideuteron fluxes produced by the evaporation of galactic primordial black holes (PBHs) with lognormal initial mass distributions. The antimatter production spectra were obtained using our modified version of the BlackHawk code, which incorporates the new CosmiXs hadronization spectra and a state‐of‐the‐art Wigner function coalescence model for antideuteron formation. The propagation of these fluxes throughout the Galaxy is computed with the USINE code, employing the latest cosmic-ray propagation frameworks. Using a realistic treatment of theoretical uncertainties and experimental errors, we compared our predicted antiproton fluxes with AMS‑02 measurements and background predictions, deriving competitive constraints on the abundance of PBHs up to critical masses of the order of 10²⁰ grams. Finally, we compared the predicted antideuteron fluxes with the expected sensitivity of the GAPS experiment, to probe its potential to detect antideuterons of PBH origin.

Speaker: Lorenzo Stefanuto (University of Turin)

36 Primordial Black Hole Hot Spots and Nucleosynthesis¶

Upon their evaporation via Hawking radiation, primordial black holes (PBHs) may deposit energy in the ambient plasma on scales smaller than the typical distance between two black holes, leading to the formation of hot spots around them.
In this talk (based on the recent work https://arxiv.org/pdf/2501.05531), I will consider how the corresponding rise of the local temperature during the evaporation may act as a shield against the release of low-energy photons, affecting PBH’s capacity to dissociate light nuclei after Big-Bang Nucleosynthesis through photodissociation. Additionally, I will show the different ways PBH hot spots affect the flux of low-energy photons expected from PBH evaporation. We find that such effects turn out to be particularly relevant to the physics of photodissociation during Big-Bang Nucleosynthesis for PBHs with masses between ${10}^{11}g$ and $3\times {10}^{12}g$.
Finally, I will comment on how the magnitude of these effects is highly dependent on the specific shape of the temperature profile around PBHs and its time evolution, and argue that this underscores the necessity for a comprehensive study of PBH hot spots and their dynamics in the future.

Speaker: Clelia Altomonte (King's College London)

37 Could We Observe an Exploding Black Hole in the Near Future?¶

Observation of an exploding black hole would provide the first direct evidence of primordial black holes, of Hawking radiation, and provide definitive information on the particle spectrum of nature. However, indirect constraints suggest that direct observation of an exploding Schwarzschild black hole is implausible. We introduce a dark-QED toy model consisting of a dark photon and a heavy dark electron. For the first time we show that a population of primordial black holes can become quasi-extremal and significantly weaken the indirect constraints, before the black holes discharge towards the end of their lives and exhibit a Schwarzschild-like final explosion. In this scenario the answer is ``yes'', it is possible that we could directly observe an exploding black hole in the near future.
Motivated by this exciting scenario, we perform analyses with photons and neutrinos to quantify the potential of current and future gamma ray and neutrino telescopes to exclude the presence of new dark particles. We also study how a multi-messenger analysis could improve the previous results.

Speaker: Quim Iguaz Juan (UMass Amherst)

38 Non-linear quasi-normal modes of the Schwarzschild black hole from the Penrose limit¶

Quasi-normal modes are a crucial feature of black holes and their study is fundamental to understanding these objects. These excitations are not only linear but it has been shown to have a relevant non-linear counterpart, in general difficult to calculate.
The Penrose limit connects a plane wave geometry to the photon ring of a black hole, where the quasi-normal modes are located in the eikonal limit. With this simplification, we extract the quadratic-level non-linearities in the quasi-normal modes of a Schwarzschild black hole for the (ℓ×ℓ)→2ℓ channel.
This result is independent of ℓ, and the claim can be further confirmed through symmetry arguments.

Speaker: Davide Perrone (Universite de Geneve (CH))

Wednesday 21 May ¶

PBHs, structure formation, cosmological evolution & particle DM¶

39 The Poisson clustering of primordial black holes and implications for whether they provide the dark matter in the asteroidal or solar mass range¶

If most of the dark matter comprises primordial black holes (PBHs), then numerous constraints imply that they must be in either the asteroidal or solar mass range. Although the first possibility arises naturally if the PBHs formed at the QCD transition, it is sometimes claimed that this possibility is excluded by microlensing surveys and the frequency of gravitational-wave events. However, one would expect PBHs to form dark clusters due to the Poisson fluctuations in their initial distribution, these clusters surviving until the present epoch in some circumstances, and this could invalidate such claims. More generally, PBH-induced Poisson fluctuations lead to the formation of virialised bound clusters with PBH halos much earlier than in the standard CDM picture. Such clusters would be disrupted by various dynamical processes (eg. by being subsumed within larger clusters) but they would be expected to survive and reside within the Milky Way for some range of cluster parameters. However, they would be destroyed at sufficiently small Galactocentric distances due to tidal disruption or collisions and some fraction of their mass would be lost even at large Galactocentric distances. This leads to both a smooth and clustered population of PBHs, with important implications for microlensing and gravitational-wave observations. The undisrupted clusters could also be identified with the observed Ultra Faint Dwarf Galaxies, since these have a mass and radius compatible with the PBH scenario.

Speaker: Bernard Carr (Queen Mary University of London)

40 Primordial black holes as cosmic expansion accelerators¶

In this talk, we propose a novel and natural mechanism of cosmic acceleration driven by primordial black holes (PBHs) with repulsive-like interiors. Using a new ``Swiss Cheese'' cosmological approach we show that this cosmic acceleration is quite general by examining three regular black hole spacetimes, namely the Hayward, the Bardeen and the Dymnikova ones as well as the de Sitter-Schwarzschild, which is singular. In particular, by matching the aforementioned black hole spacetimes with an isotropic and homogeneous expanding Universe, we find a stage of cosmic acceleration that can end either due to black hole evaporation or at an energy scale depending on the parameters of the black hole spacetime considered. Remarkably, we find that ultra-light PBHs with masses $m<5\times {10}^8\mathrm{g}$ dominating the energy content of the Univese before Big Bang Nucleosynthesis (BBN) can drive a successful inflationary phase without the use of an inflaton field while PBHs with masses $m\sim {10}^{12}\mathrm{g}$ and abundances $0.107<{\mathrm{\Omega }}_{\mathrm{PBH}}^{\mathrm{eq}}<0.5$ slightly before matter-radiation equality can produce a substantial amount of early dark energy (EDE) relevant for the Hubble tension.

Speaker: Theodoros Papanikolaou

41 Gravothermalizing into Primordial Black Holes¶

Very little is known about the universe’s history from after the end of inflation until the Big Bang nucleosynthesis (BBN), which spans more than ${10}^{39}$ orders of magnitude in time scales. In this work, we show that if there was a long period of matter domination in this unknown period, and if the particle causing the matter domination has moderate self-interactions, the matter particles can undergo gravothermal collapse to form exotic states as primordial black holes (PBHs), boson stars, and cannibal stars. We found that for some choice of parameters, our model can predict an amount of PBHs surviving until today comparable to dark matter. For an optimistic estimate of PBH abundance, we also find that PBHs with masses less than ${10}^9$ g can reheat the universe before BBN. From the bounds on the PBH abundance, we also constrain the models in a large range of parameters.

Speaker: Daniele Perri (Warsaw University)

42 Dark micro-boson stars from primordial black hole superradiance¶

I discuss the production of bosonic dark matter by light primordial black holes via superradiance, alongside Hawking emission. In particular, I will show that the resulting dark matter clouds may, under certain conditions, survive as self-gravitating (microscopic) boson stars after the black holes evaporate completely (before BBN). I will also discuss some of the potential implications of this mechanism for dark matter detection and phenomenology.

Speaker: Joao Rosa (University of Coimbra)

10:30 Coffee Break

43 Prospects for strong interaction and PBH formation in SU(∞) Quantum Gravity (QGR)¶

SU(∞)-QGR is a recently developed foundationally quantum model of
cosmology and gravity (reviewed in arXiv:2409.08932). It treats the Universe as an isolated quantum system having an infinite number of mutually commuting observables. Quantum fluctuations locally fragment the SU(∞) group representing Hilbert space of the Universe to approximately isolated subsystems representing G x SU(∞), where G is a generic finite-rank symmetry and the global SU(∞) symmetry - interpreted as gravity - entangles each subsystem to the rest of the Universe. In addition to parameters of G symmetry, subsystems depend on 4 continuous parameters that their average/effective values are perceived as the classical spacetime with a Lorentzian geometry. In this talk I first highlight the main properties of this model. Then, using the analogy with condensed matter, I present possibilities for the emergence of local strong correlations in the early Universe that may generate - without fine-tuning - over-densities leading to formation of black holes.

Speaker: Houri Ziaeepour

44 Early structure and binary formation with primordial black holes¶

Primordial black holes (PBHs) are a compelling dark matter candidate, with their gravitational interactions shaping the evolution of cosmic structure from the earliest times. In this talk, I will present results from our recent fully-collisional simulations of PBH structure formation and binary evolution. We find that the dynamic interactions between PBHs introduce significant feedback effects, influencing the growth of early structure and altering predictions for large-scale structure formation. Additionally, our simulations confirm that existing semi-analytical models accurately predict the formation of PBH binaries but fail to capture the correct merger rate, highlighting the need for improved treatments of PBH interactions. These findings have important implications for both gravitational wave astronomy and the broader search for PBH dark matter.

Speaker: Samuel Young (University of Sussex)

45 Excursion-set approach for Primordial Black Holes: small-scale clustering and merger rates¶

By extending the so-called excursion-set formalism - often used in the context of Dark Matter halos formation - to two-point statistics, we revisit the initial spatial clustering of Primordial Black Holes (PBHs) originating from the Hubble reentry of large Gaussian density fluctuations in the early Universe. Our work propose a way to correlate the formation of pairs of PBHs, revealing features that the ubiquitously used Poisson model of clustering was unable to capture: the new model effectively includes short-range exclusion effects and proves that PBHs are anticorrelated at short distances. It shows that going beyond point-like treatments for PBHs and taking into account their spatial extension lead to non-trivial volume-exclusion effects.
As a next step, by embedding our formalism within the Tidal Torque Theory (TTT), we show how keeping tracks of correlations impact both the resulting mass and spin distributions of PBHs binaries. From these results, we suggest a novel approach to estimate merger rates from PBHs two-body channel in the early universe, which could apply for models of PBHs formation with broad mass spectrum and spatial clustering, two features that the current analytical treatments of merger rates fail to capture.

Speaker: Baptiste Blachier (UCLouvain (CURL) and LPENS)

46 ALPs production from Light Primordial Black Holes: the role of Superradiance¶

Light Primordial Black Holes (LPBHs) with masses in the range $10$ g $\le M_{\mathrm{B}\mathrm{H}}\le {10}^9$ g, although they evaporate before Big Bang Nucleosynthesis, can play a significant role in the production of both Dark Matter and Dark Radiation. In particular, LPBHs can evaporate into light axions or axion-like particles (ALPs) with masses $m_a\lesssim$ MeV, contributing to the effective number of neutrino species, $\mathrm{\Delta }{N}_{\mathrm{e}\mathrm{f}\mathrm{f}}$. Additionally, heavy scalar particles known as $\mathit{\text{ moduli}}$, predicted by String Theory, can be produced both via Hawking evaporation and through amplification by a mechanism called $\mathit{\text{Superradiance Instability}}$ in the case of spinning PBHs. These moduli can subsequently decay into ALPs, further amplifying their abundance.
In this work, we calculate the number density of ALPs in the presence of moduli enhanced by Superradiance for Kerr PBHs. Using current limits on $\mathrm{\Delta }{N}_{\mathrm{e}\mathrm{f}\mathrm{f}}$ from Planck satellite observations, we derive updated constraints on this scenario.

Speaker: Marco Manno (University of Salento)

47 Primordial black holes in braneworlds¶

Motivated by many interesting features present in braneworld cosmology and higher-dimensional gravity, we have reexamined foundational concepts and processes that have previously been studied, such as gravitational collapse, black hole accretion and Hawking emission. In this talk we delve into the phenomenological signatures characteristic of this framework and consider the possibility of the totality of dark matter being in the form of higher-dimensional black holes formed in the Early Universe. The constraints imposed by (extra)galactic radiation detections and CMB anisotropies substantially differ from the standard case.

Speaker: Itziar Aldecoa Tamayo (Sussex University)

12:40 Lunch Break

PBHs and GWs¶

48 PBH clusters: formation, dynamics and signatures¶

I will review the formation of PBH clusters due to large exponential tails in the curvature perturbation distribution and study the strong non-linear dynamics of PBH in clusters, which produce PBH binaries that could have been detected via their GW emission in LVK.

Speaker: Juan Garcia-Bellido

49 PBH and SIGW: Lessons from PTA¶

Primordial Black Holes (PBHs) are fascinating astrophysical objects that could provide valuable insights into the early universe, cosmology, and gravitational physics.

The standard formation scenario assumes that PBHs originate from the gravitational collapse of large curvature fluctuations generated during inflation. These same large scalar fluctuations, which can lead to PBH formation upon horizon re-entry, also produce a background of scalar-induced gravitational waves (SIGWs) as a second-order effect.

This presentation focuses on calculating the abundance of PBHs and the associated SIGWs, which can be detected through pulsar timing arrays (PTAs) and, in the future, by LISA. A key aspect of the discussion will be the methodologies and uncertainties involved in these calculations, emphasizing recent advancements and outstanding challenges.

Speaker: Antonio Junior Iovino (New York University Abu Dhabi)

50 Searching for gravitational waves from inspiraling planetary-mass primordial black holes in LIGO O3 data¶

Gravitational waves from sub-solar mass inspiraling compact objects would provide almost smoking-gun evidence for primordial black holes (PBHs). We perform the first search for inspiraling planetary-mass compact objects in equal-mass and highly asymmetric mass-ratio binaries using data from the first half of the LIGO-Virgo-KAGRA third observing run. Though we do not find any significant candidates, we determine the maximum luminosity distance reachable with our search to be of $\mathcal{O}(0.1-100)$ kpc, and corresponding model-independent upper limits on the merger rate densities to be $\mathcal{O}({10}^3-{10}^{-7})$ kpc${}^{-3}$yr${}^{-1}$ for systems with chirp masses of $\mathcal{O}({10}^{-4}-{10}^{-2})M_{\odot }$, respectively. Furthermore, we interpret these rate densities as arising from PBH binaries and constrain the fraction of dark matter that such objects could comprise. {For equal-mass PBH binaries, we find that these objects would compose less than 4-100% of DM for PBH masses of ${10}^{-2}{M}_{\odot }$ to $2\times {10}^{-3}{M}_{\odot }$, respectively. For asymmetric binaries, assuming one black hole mass corresponds to a peak in the mass function at 2.5$M_{\odot }$, a PBH dark-matter fraction of 10% and a second, much lighter PBH, we constrain the mass function of the second PBH to be less than 1 for masses between $1.5\times {10}^{-5}{M}_{\odot }$ and $2\times {10}^{-4}{M}_{\odot }$. Our constraints, released on Zenodo, are robust enough to be applied to any PBH or exotic compact object binary formation models, and complement existing microlensing results.

Speaker: Andrew Miller

51 The Irrelevance of Primordial Black Hole Clustering in the LVK mass range¶

The physics of Primordial Black Holes has garnered significant interest in recent years, largely due to the numerous detections of gravitational
waves originating from BH binary mergers and the hypothesis that some of
these may be of primordial origin.
We show that in realistic models where primordial black holes are formed due to the collapse of sizeable inflationary perturbations, their initial spatial clustering beyond Poisson distribution does not play any role in the binary mergers, including sub-solar primordial black holes, responsible for the gravitational waves detectable by LIGO-Virgo-KAGRA.

Speaker: Andrea Ianniccari (University of Geneva)

15:30 Coffee Break

52 Mairi Sakellariadou (TBA)¶

TBA

53 “Scalar-Induced” Gravitational Waves as a probe for Modified Gravity¶

The detection of gravitational waves (GWs) from binary black hole mergers has opened an exciting new avenue for cosmological research. Because GWs interact very weakly with matter, there is potential to observe primordial GWs among the signals captured by current and future detectors. This provides a unique opportunity to probe the early universe physics. Primordial GWs are typically generated by quantum vacuum fluctuations, and due to their quantum nature, they manifest as a stochastic gravitational wave background (SGWB). A notable component of this background comes from the so called ‘scalar-induced’ gravitational waves (SIGWs), which arise from second-order effects and the coupling of scalar perturbations.
In this talk I will discuss the motivations to how detection of SIGW is useful. My main work has been around understanding how we can use SIGWs to probe beyond-gravity theories. We study the SIGW source term with correction w.r.t. to GR using the f(R) gravity theory.
Another motivation is understanding PBHs. The primordial fluctuations responsible for SIGWs can also collapse into primordial black holes (PBHs), presenting an additional route to study the formation and abundance of these black holes in the early universe.

Speaker: Anjali Abirami Kugarajh (Gran Sasso Science Institute)

54 Inflationary and Gravitational Wave Signatures of Small Primordial Black Holes as Dark Matter¶

Mounting evidence suggests that the semi-classical description of a black hole breaks down at the latest after losing an~$O(1)$ fraction of its mass. As a result, effects such as memory burden can slow down evaporation so that small primordial black holes (PBHs), in particular those in the mass range~${10}^6\text{g}$ to~${10}^9\text{g}$, become viable dark matter candidates. I will present our investigation into the production of PBHs from a prototype model of polynomial inflation with a non-minimal coupling to gravity and demonstrate that a sufficiently small PBH mass alleviates any tension with CMB observations. Furthermore, I will present bounds on the scalar-induced stochastic gravitational wave (GW) background generated by small PBH formation. Whilst we identify some prospects for observation with future GW detectors, I will highlight the need to develop new experiments for high-frequency GW detection in the $\sim \text{kHz}$ to $\sim \text{MHz}$ range.

Based on:
Inflationary and Gravitational Wave Signatures of Small Primordial Black Holes as Dark Matter. W. Barker, B. Gladwyn and S. Zell, https://arxiv.org/abs/2410.11948, accepted for publication in Phys. Rev. D

Speaker: Benjamin Gladwyn (University of Oxford (GB))

Workshop Dinner¶

Thursday 22 May ¶

PBHs and GWs¶

55 Gabriele Franciolini (TBA)¶

TBA

56 Bayesian free-form reconstruction of curvature perturbations from scalar induced gravitational waves¶

The formation of primordial black holes (PBHs) from amplified density fluctuations in the early universe may also generate scalar-induced gravitational waves (GW), carrying vital information about the primordial power spectrum and the universe's expansion history. We present a Bayesian approach to reconstruct both the scalar power spectrum and the equation of state from GW observations, using interpolating splines to flexibly capture features in the data. The optimal number of nodes is chosen via Bayesian evidence to balance model complexity and reconstruction fidelity.

Speaker: Ameek Malhotra

57 Towards Accurate Merger Rates of Early Primordial Black Hole Binaries with an Extended Mass Function¶

The dynamics of early primordial black hole (PBH) binaries with an extended mass distribution are investigated. Specifically, we examine the perturbations and disruptions from the low-mass background population induced by such mass functions. Using dedicated Monte-Carlo simulations, new calculations of the so-called merger rate suppression factors are presented. Our results suggest that the limits on the PBH abundance from gravitational wave observations should be rethought for extended mass distributions and re-open the possibility that a sizable dark matter fraction is constituted of solar-mass PBHs.

Speaker: Simon Biot (ULB)

58 Detection prospects for the GW background of Galactic (sub)solar mass PBHs¶

The discovery of subsolar mass black holes would provide compelling evidence
for the primordial origin of these objects. In this talk, I explore
how gravitational wave (GW) signals from a Galactic population of (sub)solar
mass primordial black hole (PBH) binaries could be identified by LISA (arXiv:
2410.04522). By modeling the formation and evolution of PBH binaries that
end up in the Milky Way halo at the present epoch, we find that their highly
eccentric orbits generate a GW background that peaks in the millihertz range,
where LISA’s sensitivity is optimal. While this background is below LISA’s
detection threshold for PBH fractions of 1% of dark matter, it exceeds the detection
limits of DECIGO and BBO for PBH masses of order 0.01-0.1 solar mass.
Additionally, in five years of observation LISA could identify up to O(100) loud
Galactic PBH binaries in the (sub)solar mass range for a PBH fraction of 1%,
or O(1) for a fraction of 0.1%.

Speaker: Frans van Die (Technion)

10:30 Coffee Break

59 Induced gravitational wave probes of the primordial black hole reheating scenario¶

Although there is substantial observational evidence for an
early period of exponential expansion of the Universe, known as
inflation, followed by a subsequent era of radiation domination, the
intermediate period connecting these two epochs, referred to as
reheating, remains challenging to constrain. In this talk, I will
present the primordial black hole (PBH) reheating scenario, where
ultralight black holes temporarily dominate the Universe and reheat it
via Hawking radiation from their evaporation. I will discuss the
gravitational wave (GW) spectrum induced by PBH number density
fluctuations, paying particular attention to how features of the
spectrum depend on the (so far unconstrained) equation of state of the
primordial fluid. The GW signal may enter the observational window of
several future GW detectors, such as LISA and the Einstein Telescope,
indicating that it may soon be possible to directly probe the physics
of reheating via induced GWs.

Speaker: Jan Tränkle (ITP Leibniz University Hanover)

60 Constraints on Primordial Black Holes via Induced Gravitational Waves¶

Primordial black holes (PBHs) serve as a compelling candidate for dark matter and a potential probe of early universe physics. One promising avenue for constraining their abundance is through the stochastic gravitational wave background (SGWB) induced by primordial curvature perturbations. In this talk, I will present how current and future gravitational wave observatories—including Pulsar Timing Arrays (PTA), Astrometry, and sapce-borne detectors—can set limits on the induced SGWB, thereby imposing indirect constraints on the formation and population of PBHs. I will discuss the sensitivity of these observations to different PBH mass ranges.

Speaker: Ao Wang (Institute of Theoretical Physics，Chinese Academy of Sciences; Institute for Theoretical Physics, Leibniz University Hannover)

61 Popcorns in the sky: identifying primordial black holes in the gravitational-wave background¶

Primordial black holes (PBHs) are possible sources of a gravitational-wave background (GWB), detectable with LISA and the next observing runs of LIGO--Virgo--KAGRA. In case of a detection, it will be crucial to distinguish the possible sources of this GWB. One possibility is to exploit the duty cycle that quantifies the number of sources present in the time domain signal, which can be very different depending on the nature and population of the sources. We focus on early PBH binaries, for extended realistic PBH mass distributions including the effects of the QCD epoch on the PBH formation, which is typically boosted in the solar-mass range. We compute the duty cycle and the contribution of the different PBH masses, and we derive the ratio between the shot-noise, popcorn and continuous GWBs. These are compared to other typical sources. We find that PBH models lead to specific and distinguishable predictions compared to astrophysical sources or cosmological backgrounds. Our work therefore emphasises the interest in developing specific analysis tools of the duty cycle that should ideally include a frequency dependence.

Speaker: Federico De Lillo (Universiteit Antwerpen)