INVITED SPEAKERS

Chiara Animali (U Louvain), Josu Aurrekoetxea (MIT), Albert Escrivà (Nagoya U), Jaume Garriga (U Barcelona),

Shinya Kanemura (Osaka U), Naoya Kitajima (Tohoku U), Kazunori Kohri (NAOJ/SOKENDAI/KEK/Kavli IPMU),

Kai Murai (RIKEN), Luis Padilla (Rikkyo U), Seong Chan Park (Yonsei U), Shao-Jiang Wang (Beijing, Inst. Theor. Phys.) [online], Ying-Li Zhang (Tongji U)

Time table

Time table: PDF

7th Sep.

10:00-11:00 Registration Opens

11:00-11:15 Opening Remarks

11:15-12:15 Invited Talk 1

 Harvesting primordial black holes from stochastic trees: a FOREST of type-I and II PBHs

Stochastic inflation can be implemented on stochastic trees, modelling the inflationary expansion as a branching process. Unlike lattice methods, stochastic trees do not operate on a fixed background, instead new spacetime units dynamically emerge as the trees unfold, naturally incorporating metric fluctuations. Combined with the delta N formalism, this framework allows one to generate real-space maps of the curvature perturbation that fully capture quantum diffusion and its non-perturbative backreaction during inflation. I will show how primordial black holes emerge from unbalanced nodes of the trees, and how their mass distribution can be obtained while automatically accounting for the cloud-in-cloud effect. I will also discuss how the compaction function can be computed on stochastic trees, which also allows to distinguish between type-I and type-II fluctuations. The numerical code FOREST simulates stochastic trees, and I will present its application to simple models of quantum diffusion during inflation, the so-called quantum wells. Overall, when quantum-diffusion effects become relevant, type-II fluctuations become abundant and can also outnumber type-I fluctuations. In this regime, cloud-in-cloud effects are also important, highlighting the need for a better understanding of the evolution and collapse of type-II fluctuations in order to obtain robust predictions for primordial black hole formation when stochastic effects are significant.

Speaker: Chiara Animali

12:15-13:45 Lunch

13:45-14:45 Invited Talk 2

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Speaker: Shinya Kanemura

14:45-15:45 Invited Talk 3

Primordial black hole formation via inverted bubble collapse

We propose a novel mechanism of primordial black hole (PBH) formation through inverted bubble collapse. In this scenario, bubbles nucleate sparsely in an incomplete first-order phase transition, such that they remain isolated and do not percolate or collide with each other due to the extremely low nucleation rate. This is followed by a bulk phase transition in the rest of the universe that inverts these pre-existing bubbles into false vacuum regions. These spherically symmetric false-vacuum bubbles subsequently collapse to form PBHs. Unlike conventional PBH formation mechanisms associated with domain wall collapse or bubble coalescence, our inverted bubble collapse mechanism naturally ensures spherical collapse. We demonstrate that, when applied to the singlet extension of the Standard Model.

Speaker: Kai Murai

15:45-16:15 Coffee Break

16:15-16:45 Contributed Talk 1

Superradiant Clouds around Primordial Black Holes

The superradiant instability can lead to the generation of extremely dense axion clouds around rotating black holes. Despite the long lifetime of the QCD axion, stimulated decay becomes significant above a minimum axion density and leads to extremely bright lasers. Axions with a mass ∼10^{−5} eV  and primordial black holes with mass  ∼10^{24} kg leads to millisecond-bursts in the GHz radio-frequency range, with peak luminosity  ∼10^{42} erg/s, suggesting a possible link to the observed fast radio bursts.

Speaker: Thomas W Kephart

16:45-17:15 Contributed Talk 2

Stringy Aspects of Primordial Black Hole Evolution

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Speaker: Savan Kharel

18:30-20:30 Conference Banquet

8th Sep.

9:15-10:15  Invited Talk 4

Numerical relativity simulations of black holes from inflation

The collapse into black holes of large curvature fluctuations generated during inflation is a nonlinear, strong-gravity process that requires numerical techniques. In this talk, I will describe current progress in using numerical relativity to construct the resulting primordial black hole mass function for a given inflationary model.

Speaker: Josu Aurrekoetxea

10:15-11:15 Invited Talk 5

Gravitational collapse in the early Universe

In this talk, i will talk about different aspects of gravitational collapse in the early Universe, specifically related to PBH formation

Speaker: Albert Escrivà

11:15-11:30 Coffee Break

11:30-12:30 Invited Talk 6

Primordial black hole formation in Kination domination

Primordial black holes provide a powerful probe of the expansion history, matter content, and nonlinear dynamics of the early Universe. After briefly introducing primordial-black-hole formation in non-standard cosmological backgrounds, I will focus on an early period of kination dominated by the kinetic energy of a massless scalar field. Using fully relativistic numerical simulations of superhorizon perturbations in an asymptotically FLRW spacetime, we study black-hole formation close to the threshold and show that the discretely self-similar critical solution familiar from massless scalar-field collapse remains relevant in an expanding cosmological background. The resulting critical behaviour includes a power-law scaling of the black-hole mass together with the characteristic log-periodic modulation associated with discrete self-similarity. I will then discuss how this near-threshold collapse map is incorporated into the primordial-black-hole mass function. Critical behaviour generates an irreducible width even for an extremely narrow primordial spectrum, while discrete self-similarity can produce approximately log-periodic features in the mass distribution. These features remain visible when primordial black holes form within a sufficiently narrow range of horizon masses, but are progressively washed out for broader primordial spectra. The results illustrate how primordial-black-hole mass functions may retain information not only about primordial perturbations and the cosmological background, but also about the critical solution governing gravitational collapse near the formation threshold.

Speaker: Luis Padilla

12:30-14:00 Lunch

14:00-14:30 Contributed Talk 3

An Analytical Model of Smooth Slow-Roll to Ultra-Slow-Roll Transition for Primordial Black Hole Formation

I will present a fully analytical model of a smooth slow-roll to ultra-slow-roll transition in single-field inflation. The model provides analytical predictions for the primordial power spectrum and offers a useful framework for studying primordial black hole formation. I will discuss its main features and compare them with the conventional sharp-transition scenario.

Speaker: Jianing Wang [online]

14:30-15:00 Contributed Talk 4

Numerical Simulations of Primordial Black Hole Formation: Delayed First-Order Phase Transitions and 3+1-Dimensional Radiation-Era Collapse

We present numerical studies of primordial black hole formation in two complementary early-universe settings: delayed first-order phase transitions and the collapse of primordial curvature perturbations during radiation domination. First, we perform fully nonlinear, spherically symmetric simulations of false-vacuum domain collapse in a coupled gravity–scalar–fluid system. We identify type B PBHs containing an interior baby universe, type A PBHs formed through direct wall collapse, and configurations that disperse without forming a black hole. We show that the time-scale ratio (t_{\mathrm H}/t_{\mathrm V}) provides a robust criterion for distinguishing these outcomes.

Second, we present a new relativistic perfect-fluid implementation in GRChombo for 3+1-dimensional simulations of PBH formation from cosmological curvature perturbations in a radiation-dominated universe. The framework combines long-wavelength cosmological initial data, BSSN spacetime evolution, conservative relativistic hydrodynamics, adaptive mesh refinement, and apparent-horizon diagnostics. We also introduce a cosmologically modified gauge evolution that substantially improves the efficiency of long-term simulations after horizon formation. We validate the implementation against established spherical results, providing a basis for future studies of nonspherical PBH formation.

Speaker: Zhuan Ning

15:00-15:30 Contributed Talk 5

Inflaton Accretion onto Primordial Black Holes During Reheating

Primordial Black Holes (PBHs) forming prior to Big Bang Nucleosynthesis evolve during the reheating epoch, an environment dominated by an oscillating inflaton field decaying into a relativistic thermal bath. In this work, we track the complete lifecycle of PBHs within this coupled inflaton-radiation background. Utilizing $\alpha$-attractor E-models, we analytically anchor the reheating initial conditions directly to Cosmic Microwave Background observations. By matching exact scalar field solutions in a Schwarzschild spacetime to the cosmological far-zone, we derive the cycle-averaged mass accretion rate and couple it to the growing radiation bath. We find that this combined accretion induces a highly non-linear enhancement of the final PBH mass. Because the Hawking evaporation timescale scales cubically with mass, PBHs forming near their critical runaway limits experience a massive extension of their lifespans. Surviving deeper into the radiation-dominated era triggers a multi-order-of-magnitude amplification in their emitted Stochastic Gravitational Wave Background (SGWB).

Speaker: Jitumani Kalita

15:30-16:00 Coffee Break

16:00-16:30 Contributed Talk 6

Primordial Black Holes from Cosmology

The standard mechanism for producing Primordial black holes (PBH) tracks large primordial curvature perturbations as they re-enter the horizon.  At the same time, there is a chance that PBH can be formed during violent cosmological phase transitions.  (P)reheating, the phase transition that ended inflation, is one of these violent, nonlinear processes whose dynamics lead to large density fluctuations; it has long been wondered if these sub-horizon fluctuations can dynamically form horizons.  While the answer is complicated, I will show results from the first simulation to show that horizons can form from some models of preheating.  At the same time, this is not generic, and the dynamics of the specific content of the Universe seems to affect whether horizons can be formed.  

I will also briefly comment on ongoing to work that looks at how the standard PBH mechanism is affected by coupling to curvature fluctuations at different scales.

Speaker: Tom Giblin

16:30-17:00 Contributed Talk 7

三つの道: Three Paths to Primordial Black Holes

Using fully nonlinear numerical relativity simulations of cosmic inflation transitioning from slow-roll (SR) to ultra-slow-roll (USR) and then to kination, we investigate the nonlinear dynamics across each phase—SR/USR, USR/kination, and collapse—and show the non-perturbative amplification of curvature fluctuations beyond perturbative reach. We analyse the non-Gaussian influence on the curvature profile and reveal three distinct pathways to primordial black hole formation within the same model, arising from trapped, overdense, and void configurations.

Speaker: Cristian Joana

9th Sep.

9:15-10:15  Invited Talk 7

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Speaker: Jaume Garriga

10:15-11:15 Invited Talk 8

Overview of observational constraints on PBHs in terms of gamma-ray, CMB, BBN, GWs, lensing, etc.
I will provide a comprehensive overview of observational constraints on primordial black holes in terms of gamma rays, the cosmic microwave background (CMB), big bang nucleosynthesis (BBN), gravitational waves (GWs), gravitational lensing, and so on. If time permits, I will discuss the following hot theoretical topics: PBHs as seeds of SMBHs; recent misconceptions in the community regarding the BBN bound and the cosmological accretion.

Speaker: Kazunori Kohri 

11:15-11:30 Coffee Break

11:30-12:30 Invited Talk 9

Higgs-R^2 Inflation and PBH as the whole Dark Matter

I will discuss an extension of the Standard Model of particle physics featuring a realistic scenario where the Higgs boson serves as the inflaton. Notably, this framework accounts for primordial black hole (PBH) dark matter without introducing any new degrees of freedom beyond the known particle spectrum. I will examine two specific PBH production mechanisms—ultra-slow-roll and tachyonic instability—and present the expected PBH mass spectrum.

Speaker: Seong Chan Park

12:30-14:00 Lunch

14:00-14:30 Contributed Talk 8

Peak Profile Anatomy

Primordial Black Holes (PBHs) serve as unique probes of the early universe, yet their theoretical prediction faces a persistent "precision crisis." Because the PBH abundance $f_{\text{PBH}}$ is exponentially sensitive to the amplitude of primordial fluctuations, the spatial morphology, or profile, of these fluctuations plays a decisive role in the collapse process.

Traditional approaches often simplify this problem by either ignoring profile effects or relying on a single "average profile" derived from peak theory. In this talk, I introduce a systematic framework—Peak Profile Anatomy—to move beyond these approximations. By reconstructing the probability distribution of curvature profiles $\zeta(r)$ in functional space, we account for the full stochastic ensemble of fluctuations rather than just the mean.

Our findings reveal that for power spectra with finite width, deviations from the average profile provide non-negligible contributions to the local density contrast. Crucially, these stochastic variations allow peaks with lower central amplitudes to satisfy the collapse criteria, suggesting that previous models relying solely on the average profile may have substantially underestimated the PBH abundance. Finally, I will discuss how this functional approach provides a robust theoretical foundation for future high-precision studies of PBH demographics and numerical relativity simulations.

Speaker: Ao Wang

14:30-15:00 Contributed Talk 9

Primordial black hole formation in excursion set

In this talk, we discuss the mass function of primordial black holes (PBHs) within the excursion-set theory, in which the response of the stochastic density contrast to the variation of the coarse-graining scale is described by colored noises.  For several window functions often used in this literature, we investigate how this choice affects the formation probability as well as the resultant mass function of PBHs. It is found that the low-mass tail of the mass function differs from the one predicted from Carr's formula. The difference comes from the prevalence of correlated noises, by which degeneracy of the formation probabilities ceases to exit. Nevertheless, Carr's formula still provide a practical estimation in the vicinity of the characteristic mass scale, as long as window function in Fourier-space is used.

Speaker: Hayami Iizuka

15:00-15:30 Contributed Talk 10

PBH formation on sub-horizon scales from physics beyond the standard model

I will discuss several recently proposed scenarios for PBH formation on sub-horizon scales.  Most of them them involve physics beyond the standard model, such as supersymmetry (in which PBH turns out to be a generic dark-matter candidate), Yukawa interactions in the dark sector, phase transitions, etc.  

Speaker: Alexander Kusenko 

15:30-16:00 Coffee Break

16:00-16:30 Contributed Talk 11

Initial clustering of primordial black holes generated from local-type non-Gaussianities

We formulate the two-point correlation function of primordial black holes (PBHs) assuming an arbitrary function of local-type non-Gaussianity (LNG) for curvature perturbation. In several inflation scenarios producing the sufficient amount of PBHs, such as the ultra slow-roll scenario and the curvaton model, curvature perturbation is predicted to have LNG in its statistics. LNG induce the initial clustering of the spatial distribution of PBHs and this would significantly affect the current cosmological observables related to PBHs. Our work is applicable no matter whether non-Gaussianity is perturbative or not, such that provide the basis for analysis of the initial clustering of PBHs generated from any LNG. This would facilitate incorporating the effects of LNG into the various constraints on the PBH abundance, particularly in light of their spatial distribution.

Speaker: Hirotaka Ishijima 

16:30-17:00 Contributed Talk 12

Toward faster computation of induced gravitational waves from binned power spectrum

Prior to the dynamics of primordial black hole (PBH) formation, gravitational waves (GWs) are perturbatively induced by primordial curvature perturbations.  The latter is an important observational target to infer the properties of the PBHs.  For analyses of the observation data, parameter estimation, and model discrimination, it is better to have a faster computational method for the power spectrum of the induced GWs with benchmark power spectra of curvature perturbations.  In this work, we study the induced GWs from a top-hat (box shape) power spectrum as a first step.  We find a factorization property of the UV and IR ends of the power spectrum.  

(This is based on sec. 3.1.3 of 2509.18694 [gr-qc].)

Speaker: Takahiro Terada

10th Sep.

9:15-10:15  Invited Talk 10

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Speaker: Ying-Li Zhang 

10:15-11:15 Invited Talk 11

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Speaker: Shao-Jiang Wang [online]

11:15-11:30 Coffee Break

11:30-12:30 Invited Talk 12

Primordial black hole formation from collapsing domain walls

I show the dynamics of isolated closed domain walls with 3+1 numerical relativity. A closed wall shrinks due to its own surface tension, and its surface energy is converted to the kinetic energy, leading to implosion. Then, it can result in the formation of a black hole. First, I will focus on spherically symmetric closed domain walls and clarify whether they finally evolve into black holes. Naively, the wall can collapse if its thickness is smaller than the Schwarzschild radius which is determined by the initial surface energy. Our numerical results support this naive criterion for the black hole formation, and indicate that more than 80% of the initial wall energy falls into the black hole. I will also show the nonspherical collapse by considering the ellipsoidal configurations for the closed domain walls, and it turns out that black holes can be formed even when the ratio of semi-major to semi-minor axes is 1.5.

Speaker: Naoya Kitajima

12:30-14:00 Lunch

14:00-14:30 Contributed Talk 13

Higher Curvature effects to PBH Formation

Higher Curvature terms are usually needed in various gravity models of beyond Einstein. Our goal is to understand the role of the higher curvature terms in PBH. We will approach this goal through studying the specific and simpler Einstein-dilaton-Gauss-Bonnet gravity theory. The essence is in understanding the Misner Equations in this higher gravity theory. This presentation is based on the work in progress.

Speaker: Bum-Hoon Lee

14:30-15:00 Contributed Talk 14

Axisymmetry beyond Einstein's theory

The final equilibrium stage of stellar evolution may result in either a black hole or a compact object such as a white/black dwarf or neutron star. In general relativity, both stationary black holes and stationary stellar configurations are known to be axisymmetric, and black hole rigidity has been extended to several higher curvature modifications of gravity. In contrast, no comparable result had previously been established for stationary stars beyond general relativity. In this talk, I present our work where we extend the stellar axisymmetry theorem to a broad class of diffeomorphism invariant metric theories. Assuming asymptotic flatness and standard smoothness requirements, we show that the Killing symmetry implied by thermodynamic equilibrium inside the star uniquely extends to the exterior region, thereby enforcing rotational invariance. This demonstrates that axisymmetry of stationary stellar configurations is not a feature peculiar to Einstein gravity but a universal property of generally covariant gravitational theories, persisting even in the presence of higher curvature corrections.

Speaker: Nitesh Dubey Kumar

15:00-15:30 Contributed Talk 15

Primordial black holes and scalar-induced gravitational waves from enhanced curvature perturbations in non-standard post-inflationary histories

In this talk, I will present a numerical framework to study the abundance of PBHs and the associated stochastic background of scalar-induced gravitational waves generated by enhanced scalar perturbations in non-standard post-inflationary histories, followed by the transition to radiation domination.

The main goal is to connect the primordial spectrum, the PBH mass function, and the induced gravitational-wave signal with the sensitivity of the Einstein Telescope. I've studied the dependence on the reheating temperature, the difference between rapid and gradual transitions, and the impact of extended PBH mass distributions. 

Speaker: Tadeo Gómez-Aguilar Dariney 

15:30-15:45 Closing Remark