5th Barcelona Initiative for Gravitation (BIG) Meeting

Friday Mar 7, 2025, 10:00 AM → 5:30 PM Europe/Madrid

Seminar room (IFAE)

Diego Blas (IFAE, ICREA), Fabrizio Rompineve (UAB & IFAE), Oriol Pujolas (IFAE), Valbuena Juan Sebastian (IFAE)

10:00 AM Reception

1 The unexpected shape of the primordial black hole mass function

In a Universe with nearly-Gaussian initial curvature perturbations, the abundance of primordial black holes can be derived from the curvature power spectrum. When the latter is enhanced within a narrow range around a characteristic scale, the resulting mass function has a single distinct peak, corresponding to Schwarzschild radii set by the horizon entry time of that scale. In contrast, we show (both numerically and by providing an analytic estimation) that a broad enhancement - such as a plateau bounded by infrared and ultraviolet scales - produces a bimodal mass function, with a primary peak close to the infrared scale. We find that the typical initial gravitational potential (compaction function), conditioned on meeting the threshold for critical collapse, is generated by a thin spherical shell with infrared radius and a thickness comparable to the ultraviolet scale. This suggests a higher-than-expected abundance of PBH originating from Type II initial fluctuations. Our results significantly impact overproduction bounds on the amplitude of the power spectrum, and tighten the viable mass range for primordial black holes as dark matter.

Speaker: Cristiano Germani (Barcelona U.)

11:30 AM Coffee

2 Dark Radiation, Dark Energy and neutrinos with DESI 2024 BAO, and the H0 tension

I will discuss how the new cosmological Baryon Acoustic Oscillations (BAO) measurements by the DESI
collaboration have changed the status of the so-called Hubble tension, i.e. the mismatch between global
cosmological measurements and local measurements of H0 (by the SH0ES Collaboration). In particular, in
models with Dark Radiation the tension decreases to a moderate level, around 2 sigma and down to 1.7 sigma,
depending on the specific dataset and realization. This allows a combination of cosmological data with the
local SH0ES measurement, leading to a 4-5 sigma evidence for a new Dark Radiation component. I will
also discuss the status of Dark Energy, that points to a time-varying equation of state at very late times,
and neutrino mass fits with the new DESI dataset.

Speaker: Alessio Notari (Barcelona U.)

talk_BIG_DESI.pdf

1:00 PM Lunch

3 Strangeness in Neutron Stars and Mergers

Understanding the dynamics of hadrons with strangeness has received a lot attention over the past decades in connection with the study of exotic atoms, the analysis of strangeness production and propagation in particle and nuclear research facilities, and the investigation of the possible strange phases in the interior of neutron stars. One venue of interest in the field of strangeness is the study of strange baryons, the so-called hyperons. In this talk I will review the dynamics of hyperons with nucleons and nuclear matter. I will also discuss the presence of hyperons in the inner core of neutron stars as well as the consequences for the structure of these compact stars and their dynamics in neutron star mergers.

Speaker: Laura Tolós (ICE CSIC)

3:15 PM Coffee

4 Understanding Semiclassical Backreaction in Quantum Mechanics

The backreaction of quantum degrees of freedom on classical backgrounds is a poorly understood topic in theoretical physics. Understanding how to properly deal with the phenomenon of backreaction is an important problem that can have severe implications. For example, in gravitational physics we often only have a classical description of the background, and it has been argued that backreaction of quantum fields can lead to the halting of Hawking radiation. There are several popular methods to deal with backreaction in quantum field theories. Most often it is treated within the semiclassical approximation with the help of various ad hoc prescriptions accounting for the effect of quantum excitations on the dynamics of the background. It is important to assess when each method offers a good approximation to the full QFT picture, which I will do explicitly in this talk for a QFT in 0 dimensions (a.k.a. quantum mechanics). I will focus on two popular methods: (i) the mean-field approximation whereby quantum degrees of freedom couple to the classical background via their quantum expectation values; (ii) the (stochastic) Truncated Wigner method whereby the fully coupled system is evolved using classical equations of motion for various randomly sampled initial conditions of the quantum degree of freedom, and a statistical average is performed a posteriori. I evaluate the performance of each method in a simple toy model against a fully quantum mechanical treatment, and identify its regime of validity. I will interpret the results in terms of quantum entanglement and loss of classicality of the background.

Speaker: Fabio van Dissel (IFAE)

5 Memory Burden and Ultralight Black Holes : A New Window for Dark Matter

The memory burden effect describes how the information load carried by a system contributes to its stabilization. This phenomenon is particularly significant in systems with a high capacity for information storage, such as black holes and other entities with maximal microstate degeneracy, commonly. The effect has several key implications. Notably, it slows the further decay of a black hole—mainly after it has radiated approximately half of its initial mass. As a result, light primordial black holes, previously thought to have fully evaporated, may persist and serve as viable dark matter candidates.

I will explore the memory burden effect and its role in solitons and black hole dynamics. I will highlight novel features with potential observational relevance, including the model-independent distribution of stabilized masses for initially degenerate primordial black holes.

Speaker: Juan Sebastian Valbuena Bermudez (IFAE)