Future Perspectives on Primordial Black Holes

Botanical Garden of Sapienza University

Botanical Garden of Sapienza University

Largo Cristina di Svezia, 23 A - 24, 00165 Roma (RM)
Ilia Musco (INFN, Rome), Paolo Pani (Istituto Nazionale di Fisica Nucleare), Alfredo Leonardo Urbano (Istituto Nazionale di Fisica Nucleare)

This workshop will be on Dec 11-13, 2023 at the beautiful location of the Botanic Garden of Sapienza University of Rome, located in Trastevere. It will bring together experts on early universe cosmology, primordial black hole formation and evolution, and gravitational-wave science to discuss how the PBH scenario confronts with current and future data.  

La Sapienza University of Rome - Pava Education

The program will include  discussion sessions and slots for contibuted talks, in particular for young scientists. The dealine for abstract submission is Oct 31st. 

No registration fee. 

The venue can accomodate up to 90 people, therefore we encourage to register in advance: according to the availability of places registrations will be accepted up to Dec 1st.  

A Roma l'Orto Botanico riapre con una sorpresa speciale
Orto Botanico di Roma - Pro Loco Roma









On Tuesday 12 December (3pm) a special event will be helded at the Enrico Fermi Study and Research Center (CREF):

 The Nature of Time: an open question betwen Physics and Philosophy

We are looking forward to see you in Rome to present your work and get involved in stimulating discussions. 

List of topics:

  • Status and future of primordial black hole science 
  • PBH formation and numerical simulations 
  • Synergies with particle dark matter models 
  • Inflationary scenarios  
  • Clustering and N-body PBH simulations 
  • PBH merger rates 
  • Phase Transitions
  • Modelling accretion onto PBHs and its role on constraints 
  • PBH confront GW data: status and future prospects 

List of invited speakers and moderators:

Chris Byrnes (University of Sussex, UK)                          
Marco Bruni (University of Portsmouth, UK)                           
Bernard Carr (Queen Mary University of London, UK)                                                   
Gabriele Franciolini (CERN, CH)                         
Daniele Gaggero (University of Pisa, IT)                     
Tomohiro Harada (Rikkyo University of Tokyo, JP)                 
Andrea Mitridate (DESY, DE)                          
Martti Raidal (NICPB Tallin, EE)                         
Antonio Riotto (University of Geneve, CH)                             
Jun'ichi Yokoyama (University of Tokyo, JP)                    
Sam Young (Univeristy of Leiden, NL)                  

Accommodation: The Grand Hotel del Gianicolo, very close the venue, is offering rooms to the participants for a convenient price. Please beware of fake e-mails attempting to offer you to book accommodation. The only reliable communications are those that come directly from the organisers.

Social Event: We are organizing a conference dinner on Tuesday 12 December.

Scientific Secretariat: Alessandra Curto                          
Organizing Committee: Ilia Musco, Paolo Pani, Alfredo Urbano

Support: This event is supported by INFN Fellini Fellowship DarkPBH "Dark Matter and Gravitational Waves" (H2020-MSCA-COFUND-2016 G.A. n. 754496) and by the ERC project DarkGRA “​Unveiling the dark universe with gravitational waves” (ERC-2017-StG 757480).


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    • 9:00 AM
    • Plenary Session: Introduction and PBH overview
      Convener: Ilia Musco (Istituto Nazionale di Fisica Nucleare, Rome)
      • 1
        Welcome Introduction
        Speakers: Antonio Masiero (Istituto Nazionale di Fisica Nucleare), Paolo Valente (Istituto Nazionale di Fisica Nucleare)
      • 2
        Primordial Black Holes after 50 years: a positive perspective

        This talk presents a historical perspective of the study of primordial black holes (PBHs), my first talk on the subject being 50 years since. PBH papers have usually focused on constraints on their abundance, this having interesting implications for cosmology even if they never formed. However, in recent years attention has turned to the possibility that they might actually exist and solve various cosmological conundra. Numerous arguments will be reviewed, based on observational evidence from a variety of lensing, dynamical, accretion and gravitational-wave effects. This represents what might be termed a positivist perspective. The most exciting possibility is that PBHs provide the dark matter, in which case their Poisson clustering could have important implications for later structure formation. Also if they form at the QCD phase transition, the tiny collapse fraction required might naturally explain the cosmic photon-to-baryon ratio and the comparability of the PBH and baryon densities. Even if PBHs provide only a small fraction of the dark matter, they might still explain some of the galactic and quasar microlensing events, the LIGO/Virgo/KAGRA gravitational wave events, the spatial coherence in the fluctuations of the source-subtracted cosmic infrared and soft X-ray backgrounds, some anomalies associated with Ultra Faint Dwarf galaxies, and the supermassive black holes in galactic nuclei. With a suitable extended mass spectrum, they might even explain all these anomalies. So an exciting new era in PBH research has began and observations are already probing this proposal.

        Speaker: Bernard Carr (Queen Mary University of London)
    • 10:40 AM
      Coffee Break
    • Plenary Session: PBH Formation
      Convener: Ilia Musco (Istituto Nazionale di Fisica Nucleare, Rome)
      • 3
        Primordial black hole formation from cosmological perturbations: recent developments

        Primordial black holes may have been formed from sufficiently large amplitude perturbations in the early Universe. The central aim of the formation studies is to predict the abundance and other properties of PBHs, given the cosmological scenario. Both numerical relativity simulations and analytical investigations play an important role. In this talk I will introduce some of the recent developments, focusing on the dynamics in different situations.

        Speaker: Tomohiro Harada (Rikkyo University (Tokyo))
      • 4
        Primordial black holes: A fine (tuned) opportunity for probing inflation

        From the origin of the merging compact objects seen by LIGO-Virgo-KAGRA to the source of the stochastic gravitational waves seen by pulsar timing arrays via the QCD transition, there is a happy coincidence of scales which could point to primordial black holes. Combined with the long running dark matter search and hunt for new observables from inflation, there are many reasons to hope to learn a lot from PBHs. However, generating such black holes in a simple inflationary model remains far from simple.

        Speaker: Christian Byrnes (University of Sussex)
    • 12:50 PM
      Lunch Break
    • Plenary Session: PBHs from single field inflation
      Convener: Alfredo Leonardo Urbano (Istituto Nazionale di Fisica Nucleare)
      • 5
        Primordial black holes from single field inflation?

        In this talk, I discuss one-loop quantum correction to the spectrum of curvature perturbation in inflationary models with enhanced power spectrum on small scale due to the so-called ultra slow-roll

        Speaker: Prof. Jun'ichi Yokoyama (University of Tokyo)
    • Contributed Talks: PBHs and Inflation
      Convener: Alfredo Leonardo Urbano (Istituto Nazionale di Fisica Nucleare)
      • 6
        Anatomy of single-field inflationary models for primordial black holes

        In this presentation, we will outline the essential features and the phenomenology of single-field inflationary scenarios that can produce enhanced curvature power spectra associated with the production of primordial black holes. We will present a simple analytic set-up capable of capturing the spectral shapes in typical models presented in the literature. We will also discuss models capable of generating sizeable spectral oscillations and the presence or absence of "dips" - features where the power spectrum nearly vanishes.

        Speaker: Hardi Veermäe (NICPB, Tallinn, Estonia)
      • 7
        Back-reaction in the early universe

        Single-field models of inflation that feature a non-attractor phase might lead to enhanced scalar fluctuation on scales much smaller than those seeding the large-scale structure formation. In this scenario, it is possible that the spike of power at high wavenumber might spoil the successful predictions of a nearly Gaussian, scale-invariant power on large scales, e.g. in the form of loop corrections to the large-scale power spectrum. In this talk we discuss analytical estimates for the 1-loop correction. We employ the $\delta N$ formalism, and relate our results to those obtained in the literature by applying the in-in formalism.

        Speaker: Laura Iacconi (Queen Mary University of London)
      • 8
        Does the size of the one-loop correction rule out PBH formation?

        To produce an appreciable quantity of primordial black holes (PBHs), there needs to be a significant enhancement of the amplitude of the primordial power spectrum at short-scales. Large enhancements on short-scales, however, can induce a large contribution to the one-loop correction to the power spectrum on CMB scales. The size of this correction has been hotly debated since Kristiano and Yokoyama’s claim that, in a wide class of PBH-producing models, the loop correction to CMB scales is too large and breaks perturbativity. In this talk we’ll explore some of the key points of contention in the literature and present new results for the loop correction computed numerically from a potential.

        Speaker: Matthew Davies (Queen Mary University of London)
    • 4:00 PM
      Coffee Break
    • Discussion
      Convener: Antonio Riotto (University of Geneva)
    • Plenary Session: PBHs pollution and merger rate
      Convener: Paolo Pani (Istituto Nazionale di Fisica Nucleare)
      • 9
        Merger rate of primordial black holes

        I shall present some details how to compute the PBH merger rate and show results of a dedicated simulation of PBH clustering at the early Universe. I apply the results to fit LIGO data and derive constraints on PBH abundance. I discuss the possibility that LIGO has observed two populations of black holes, astrophysical and primordial.

        Speaker: Martti Raidal (Estonian Academy of Sciences)
      • 10
        Searching for Primordial Black Holes from the QCD Epoch Using Gravitational Waves

        Primordial Black Holes (PBHs) might comprise a significant fraction of dark matter in the Universe and can give rise to observable signatures in current and future gravitational wave (GW) experiments. Focusing on PBHs in the mass range probed by the LIGO/Virgo/Kagra detectors, I will present the results of Bayesian multi-population inference on the most recent dataset. The analysis includes a subpopulation of PBH mergers modeled from first principles, taking into account the softening of the equation of state during the QCD era. These findings allow for setting constraints on both the PBH abundance and the inflationary dynamics underlying PBH formation within the standard scenario. I will then discuss how future observations can improve upon these constraints by searching for specific signatures of PBH mergers.

        Speaker: Gabriele Franciolini (CERN)
    • 10:50 AM
      Coffee Break
    • Plenary Session: PBHs and Gravitational Waves
      Convener: Paolo Pani (Istituto Nazionale di Fisica Nucleare)
      • 11
        Exploring the Gravitational Wave Universe with PTAs: where we are and where we are going

        By tracking the arrival times of radio pulses from a collection of pulsars in the Milky Way, several pulsar timing array collaborations have found evidence for a background of gravitational waves permeating our galaxy. In this talk, I will present this evidence and discuss possible paths forward to discriminate between astrophysical and cosmological interpretations of this background.

        Speaker: Andrea Mitridate (DESY)
    • Contributed Talks
      Convener: Paolo Pani (Istituto Nazionale di Fisica Nucleare)
      • 12
        Timekeepers of the Universe: The recent gravitational wave observation by PTA and PBH.

        The recent data releases by multiple pulsar timing array (PTA) experiments show evidence for Hellings-Downs angular correlations indicating that the observed stochastic common spectrum can be interpreted as a stochastic gravitational wave background. We study whether the signal may originate from gravitational waves induced by high-amplitude primordial curvature perturbations. Such large perturbations may be accompanied by the generation of a sizeable primordial black hole (PBH) abundance. We discuss in which scenarios the inclusion of non-Gaussianities in the computation of the abundance can lead to a signal compatible with the PTA experiments without overproducing PBHs

        Speaker: Antonio Junior Iovino (Istituto Nazionale di Fisica Nucleare)
      • 13
        Subsolar-mass Black Hole Searches

        Detecting a subsolar-mass black hole is one of the two possible ways to prove the existence of PBHs. I will review the recent searches for subsolar-mass black holes and the analysis of the possible candidates, as well as the merger rate predictions for subsolar PBH binaries with a focus on extended PBH mass distributions.

        Speaker: Sébastien Clesse (ULB, Brussels)
      • 14
        Detection of PBH with Einstein Telescope: forecast for statistical methods to distinguish between different BH populations

        In this talk I will present forecasts on the capability of the Einstein Telescope to identify and measure the abundance of a subdominant population of distant PBHs, distinguishing it from Astrophysical Black Holes (ABH), using the difference in the redshift evolution of the merger rate of the two populations as our discriminant. After presenting our model for the merger rates and I will show how we generate realistic mock catalogues of the luminosity distances and errors that would be obtained from GW signals observed by the Einstein Telescope. I will then present two independent statistical methods to analyse the mock data, and I will show in our results the limiting fraction of dark matter in PBHs for which these methods would be able to obtain a detection of their existence.

        Speaker: Matteo Martinelli (INAF - Osservatorio Astronomico di Roma)
    • 1:10 PM
      Lunch Break
    • Special Event: The Nature of Time Aula Fermi (Centro Ricerche Enrico Fermi)

      Aula Fermi

      Centro Ricerche Enrico Fermi

      Via Panisperna 89 A 00184 Roma


      Conveners: Ilia Musco (Istituto Nazionale di Fisica Nucleare, Rome), Roberto Capuzzo Dolcetta (Dep. of Physics, Sapienza, Univ. of Roma)
      • 15
        Welcome Introduction
      • 16
        Mach Principle and the nature of Inertia

        The gravitational mass is a local property of particles like the electric charge, while the inertial mass is the resistance to acceleration, quite a different property. Mach’s principle states that inertial forces should be due to the interaction of a body with all the other masses in the universe. In General Relativity this principle is not fully present and is replaced by the equivalence principle, according to which inertial and gravitational masses are linked by the gravitational constant, which is a fixed number. However, according to a strict interpretation of Mach’s principle, the inertial mass should be non local and depend on all the other masses of the universe and their positions. In this perspective the gravitational constant cannot be just a fixed number. In 1918 Thirring studied the metric inside a rotating spherical mass shell and it was clearly inspired by the conceptual problem of Mach’s principle. This study revealed the appearance of a force with the structure of Coriolis force. No centrifugal like force was present but there was also a curious vertical force. In 1973 we considered a cylindrical rotational symmetry and extended the approach to second order in the gravitational constant. We found that a rotating cylinder leads to a metric which gives exactly the Coriolis term and the centrifugal one with the correct relations. Recently we have reconsidered this problem extending to the case of a linear acceleration. All these results strongly point to the idea that real inertia is the outcome of a relative motion and this would require a generalization of GR theory.

        Speaker: Luciano Pietronero (CREF, Sapienza University)
      • 17
        From the past to the future of time

        Time has been a constant puzzle for mankind and the development of modern science just served to even more pinpoint the tantalising nature of this concept and its apparent clash with the current physical understanding of the world. In this brief seminar I will try to convey a brief history of time and try to point out foreseeable developments of our understanding of its nature within the fabric of reality.

        Speaker: Stefano Liberati (SISSA)
      • 18
        Shapes and a Dynamical Arrow of Time

        I will show that if the philosophically dubious absolute elements which Newton introduced when he created dynamics are eliminated, his theory of gravity applied to a model universe leads to a theory of time and its arrow that is a direct consequence of Newton's laws. The arrow, which does not arise from a special condition of low entropy in the early universe that must be imposed in addition to the dynamical laws, points in the direction of increasing order and not, as widely believed on the basis of the second law of thermodynamics, entropic disorder. I will also draw attention to the dramatic change of perspective that results from the recognition that no ruler exists outside the universe to measure its size. Only the shape of the universe is physically meaningful. Since from the point of view of group theory Newton's and Einstein's theories of gravity have the same architectonic structure, these Newtonian features may well be true of our universe described by Einstein's theory.

        Speaker: Julian Barbour (University of Oxford)
      • 19
        Paradoxes and metaphors of time in between physics and philosophy

        In this contribution I describe the fundamental tension that characterized Western culture in representing time as being fundamental and emergent, especially in fundamental physics and cosmology. I shall discuss the reasons of the persistence of this tension in current physics and then show why in what appears to be a conflict without resolution there is also a stimulating tool for research.

        Speaker: Silvia De Bianchi (University of Milan)
      • 5:30 PM
        Coffee Break
      • 20
        Open moderated discussion
        Speaker: Roberto Capuzzo Dolcetta (Dep. of Physics, Sapienza, Univ. of Roma)
    • 8:00 PM
      Social Dinner Grand Hotel del Gianicolo: Ristornate La Corte degli Archi

      Grand Hotel del Gianicolo: Ristornate La Corte degli Archi

      Via delle Mura Gianicolensi, 107 Roma - Italia - 00152
    • Plenary Session: PBH and Dark Matter
      Convener: Christian Byrnes (University of Sussex)
      • 21
        Calculating the abundance of primordial black holes: complications and opportunities

        With modern and upcoming surveys providing ever tightening constraints, and even potential detections, it is becoming more important than ever to make robust and precise calculations for the abundance of primordial black holes (PBHs). Their abundance depends strongly on the primordial power spectrum, and constraints on the PBH abundance have historically been used to place unique constraints on the power spectrum, although complications such as non-Gaussian distributions and phase transition are often neglected - the effects of which can be degenerate with the effect of the power spectrum. However, by considering other factors, such as the initial clustering mass function, these complications can provide an opportunity to reveal more information about the early universe. In this talk, I will first describe how the abundance and mass function can be calculated, before discussing how phase transitions and non-Gaussianities can leave characteristic signatures in the PBH mass function, abundance and clustering.

        Speaker: Sam Young (University od Leiden)
      • 22
        Dark and shiny dresses around Primordial Black Holes

        I will discuss the interplay between the phenomenology of primordial black holes and the dark matter searches. I will first briefly review the most relevant constraints on PBH abundance, highlighting the caveats and uncertainties. Then, I will discuss how a sub-dominant component of PBHs interacts with the bulk of the DM. In particular, I will describe how a DM "mini-halo" is expected to form around PBHs, with relevant phenomenological consequences. The focus will be on two effects in particular. (i) The dark mini-halo can alter the evolution of a PBH binaries due to dynamical friction. I will discuss the impact of friction on the merger rate of a population of PBHs, and the subsequent impact on current bounds and future high-redshift searches. (ii) If the bulk of the DM is composed of WIMPs, the mini-halos would shine in gamma rays. Hence, a hypotetical future detection of a sub-dominant component of PBHs could allow to set very stringent constraints on the WIMP annihilation cross section.

        Speaker: Daniele Gaggero (Istituto Nazionale di Fisica Nucleare)
    • 10:40 AM
      Coffee Break
    • Contributed Talks: PBH constraints
      Convener: Christian Byrnes (University of Sussex)
      • 23
        On the Cosmological Bound on Primordial Black Holes

        Primordial Black Holes (PBHs) may exist, contributing to the Dark Matter abundance. In this talk, we revisit the cosmological bound on PBHs -- driven by the accurate measurements of the anisotropies in the Cosmic Microwave Background (CMB) -- putting under the spotlight the role of the modeling of accretion physics as well as other sources of uncertainty related to PBH properties and the environmental baryonic gas. We present an up-to-date bound from the CMB on heavy PBHs, i.e. between tens to $\mathcal{O}(10^4) \, M_{\odot}$.

        Speaker: Mauro Valli (Istituto Nazionale di Fisica Nucleare)
      • 24
        Revisiting CMB constraints on PBH + WIMP scenarios

        Primordial black holes (PBHs) may have formed in the early Universe, and yet constitute only a sub-dominant component of the dark matter. In that case, the rest of the dark matter is expected to build up mini-halos with steep density profiles (“spikes”) around the PBHs. If this second dark matter component is in the form of Weakly Interacting Massive Particles (WIMPs), their annihilation rate would be enhanced by the high density environment of the spikes, a scenario that was shown to lead to extremely strong constraints on WIMPs. In this work, we revisit constraints that can be set on this mixed WIMP+PBH scenario from the observation of the cosmic microwave background. In particular, we improve on proevious calculations in the literature thanks to a careful computation of the dark matter spike profile as a function of black hole mass, dark matter particle mass and temperature of kinetic decoupling.

        Speaker: Francesca Scarcella (Laboratoire Univers et Particules de Montpellier (LUPM))
    • Discussion
      Convener: Marco Bruni (University of Portsmouth)
    • 12:40 PM
      Lunch Break (Botanic Garden)
    • PhD student session
      Convener: Marco Bruni (University of Portsmouth)
    • Contributed Talks: PBH miscellanea & conclusions
      Convener: Ilia Musco (Istituto Nazionale di Fisica Nucleare, Rome)
      • 25
        Clustering of PBHs in the stochastic-$\delta N$ formalism

        Primordial black holes may form in the early universe from the collapse of rare, enhanced curvature perturbations. In the presence of such large perturbations, quantum diffusion cannot be neglected. Remarkably, it can be incorporated through the stochastic-$\delta N$ formalism, which can be used to reconstruct the statistics of the curvature perturbation when non negligible quantum diffusion is at play. A general result of this procedure is the presence of heavy exponential tails in the probability density function of cosmological inhomogeneities, which largely affect predictions for PBHs.

        I will present how the stochastic-$\delta N $ formalism can be extended to arbitrary coarse graining and to multiple point statistics. In particular, the latter will be used to derive the two-point statistics of high threshold perturbations, which is needed to derive their spatial correlation. This formalism can be used to investigate whether quantum diffusion affects the spatial distribution of primordial black holes, inducing small-scale clustering at formation. I will present this analysis in single toy models and compare our findings with results obtained by assuming that primordial black holes arise from gaussian perturbations.

        Speaker: Chiara Animali (LPENS, Paris)
      • 26
        Gravitational wave signatures of “magnetised" supermassive primordial black holes

        Primordial black holes (PBH) can account for a wide variety of cosmic conundra, among which the origin of primordial magnetic fields. In this talk, we consider supermassive PBHs furnished with a disk due to the vortex-like motion of the primordial plasma around them at the epoch of their formation. Interestingly enough, we find a novel natural ab initio mechanism for the generation of a battery induced seed magnetic field (MF) which can be later amplified by various dynamo/instability processes and provide the seed for the present day MF on intergalactic scales. We also derive the gravitational-wave (GW) signal induced by the magnetic anisotropic stress of such a population of magnetised PBHs, checking its detectability by future GW detectors. Finally, by avoiding GW overproduction we set upper bound constraints on the abundances of supermassive PBHs as a function of their mass, which are comparable with constraints on from large-scale structure probes; hence promoting the portal of magnetically induced GWs as a new probe to explore the enigmatic nature of supermassive PBHs.

        Speaker: Theodoros Papanikolaou
      • 27
        Compaction function profiles from stochastic inflation

        Strong perturbations from cosmic inflation produce primordial black holes (PBHs). The method of stochastic inflation lets us compute the perturbations beyond linear order. I discuss recent progress in numerical stochastic computations, focusing on the compaction function, a quantity controlling the PBH collapse. The numerical stochastic method allows us, for the first time, to produce full radial profiles of the compaction function needed for accurate estimates of PBH abundance and mass distribution. I discuss the nature of these profiles in an example model of single-field ultra-slow-roll inflation. I highlight their noisy, stochastic nature and raise the question of the correct way to assess the collapse threshold in a realistic model.

        Speaker: Eemeli Tomberg (Lancaster University)
      • 28
        Milky Way Black Holes Hiding in Plain Sight

        We will present our investigation into the abundance and properties of black holes through photometric gravitational microlensing surveys of the Milky Way. We have used the PopSyCLE simulation suite to estimate the abundance and characteristics of black holes in existing and future surveys, both for stellar end-product and primordial formation mechanisms. Based on these simulations we have determined optimal filters for black hole identification in photometric light curve surveys and used these simulations in conjunction with the microlensing survey data to estimate the mass and class (i.e., star, black hole, neutron star, etc.) probability density functions. Our method provides a new means of finding far more black holes than traditional approaches, as well as a new means of constraining the properties of the Milky Way. An underlying thread of the presentation will discuss the microlensing communities historical use of biased estimators (e.g., histograms of single point estimators), introduce alternative unbiased estimators, and discuss the impact to physical interpretations both past and present.

        Speaker: William Dawson (Lawrence Livermore National Laboratory)