6th Barcelona Initiative for Gravitation (BIG) Meeting

Friday 4 Jul 2025, 10:00 → 17:10 Europe/Madrid

Alberto Lobo Conference Room (Institute of Space Sciences (ICE-CSIC))

Carlos Sopuerta (ICE-CSIC), Ivan Martin Vilchez (ICE-CSIC), Pablo Fernández de Alarcón Muguruza (ICE-CSIC), Sascha Husa (ICE-CSIC)

The main goal of this meeting is to bring together researchers working in the different aspects of gravitation in the great area of Barcelona.

10:00 → 10:30 Welcome to the BIG meeting: Registration

Registration and coffee

2025.06.24_REDONGRA-announcement.pdf

10:30 → 13:00 Morning Session

10:30 First ICCUB Numerical Relativity Waveform Catalog of Eccentric Black Hole Binaries

We release the first Numerical Relativity catalog of Institut de Ciencies del Cosmos at University of Barcelona (ICCUB) consisting of 128 simulations for black hole binaries. All simulations in this first release correspond to highly eccentric binaries with eccentricity ϵ = (0.62, 0.79) which develop zoom-whirls up to three close passages before merger. We consider aligned, equal spin configurations and equal mass ratios. Moreover, we provide metadata and the parameter files required to reproduce our results using the open-source code Einstein Toolkit.

Speaker: Ana Climent Catalá (ICCUB)

Ana_Climent_BIG_meeting.pdf

11:10 Dynamical formation of regular black holes

Recently, it has been shown that the addition of an infinite tower of higher-curvature terms to the Einstein-Hilbert action in dimensions greater or equal than five regularises the Schwarzschild black hole singularity. In this talk, I will present a dynamical process of thin-shell collapse which leads to the formation of these regular black holes. Starting at some finite radius, the shell collapses until it reaches a minimum radius inside the regular black hole inner horizon. This is followed by a bounce which makes it emerge in a new universe until reaching a turning point, from which the process of collapse is restarted.

Speaker: Ángel Jesús Murcia Gil (ICCUB)

Talk-AngelMurcia.pdf

11:30 Generalized entropic cosmology

The thermodynamics of black holes inspired the concept of black hole entropy, linking gravity and thermodynamics. When applied to cosmology, this leads to the standard Friedmann equations, which face challenges in explaining the universe’s evolution. Modified entropies have been proposed to resolve these issues. A recently introduced four-parameter generalized entropy unifies several known forms. Studying this framework offers insights into which entropic models align with observational data. It also allows for consistent cosmological evolution, including inflation, potentially matching Planck results even with scalar field potentials previously ruled out by standard models.

Speaker: Simone D'Onofrio (ICE-CSIC)

Big_Generalized_Entropic_Cosmology.pdf

11:50 Coffee Break

12:10 Results from DESI

Speaker: Prof. Francisco Castander (ICE-CSIC)

13:00 → 14:30 Lunch Break

14:30 → 17:10 Afternoon Session

14:30 Gravitational waves from the Axiverse

Models with axion like particles (ALPs) often predict the formation of a string-domain wall network in the early universe. In a string theory motivated scenario with several axions, we show how a bias term in the axion potential arises naturally from the effects of multiple instantons, leading to the eventual decay of the domain walls. The annihilation of the network leads to the generation of a stochastic gravitational wave background (SGWB) with contributions from both walls and strings, that can be probed at existing and upcoming observatories. The extinction of the network is also accompanied by the production of different axion mass eigenstates. In a region of the parameter space, the lightest eigenstate can be long lived and make up the dark matter in the universe.

Speaker: Francesc Ferrer

Francesc_Ferrer_BIG.pdf

15:10 LISA’s Potential to Test the Kerr Hypothesis: Symmetry-Breaking Effects in Extreme-Mass-Ratio Inspirals

Extreme-Mass-Ratio Inspirals (EMRIs), consisting of a stellar-mass compact object spiraling into a massive black hole, are key sources of gravitational waves for space-based detectors like LISA. Due to their precise waveforms, EMRIs offer a unique opportunity to test the nature of massive black holes and explore potential deviations from General Relativity, such as non-Kerr geometries. This work explores how modifications to the primary object’s mass multipole moments affect gravitational radiation and EMRI waveforms. Using the Analytic Kludge model and Fisher matrix analysis, the study assesses LISA’s ability to constrain these multipole deviations. The results highlight LISA’s potential to detect symmetry-breaking effects and provide insights into the fundamental nature of black holes.

Speaker: Pablo F. Muguruza (ICE-CSIC)

6th BIG Meeting - Pablo F. Muguruza.pdf

15:30 Using gravitational self-lensing to distinguish the environment of compact binary mergers

Gravitational waves from stellar-mass binary black hole mergers can come from different formation environments, such as star clusters, active galactic nuclei (AGN) disks, or isolated binary evolution in the field. Some intrinsic characteristics of the gravitational-wave signal, such as eccentricity, spins and phase shifts, can give clues about formation channels. However, some of these characteristics can be found in more than one environment, which can make it difficult to tell them apart. Gravitational lensing by an object inside the environment (self-lensing) can help distinguish their origin by imprinting additional signatures on the waveform. In this talk, we quantify the probability of self-lensing and its detectability for LIGO-Virgo-KAGRA and Einstein Telescope sources. While most self-lensing by stellar-mass black holes is undetectable, central massive black hole lenses can imprint detectable signatures and a few (1-10) of these self-lensed events could be detected in next generation detectors. The combination of self-lensing imprints with other characteristic signatures of each environment can give additional information to reveal their origin.

Speaker: Helena Ubach Raya (ICCUB)

2025-Ubach-self-lensing-BIG-meeting.pdf

15:50 An effective Hubble constant in f(R) modified gravity

The precise determination of the current expansion rate of the Universe, the Hubble constant ($H_0$), remains a central challenge in cosmology. To address the persistent $H_0$ tension and reconcile the discrepant values inferred from observations at different redshifts ($z$), we investigate $𝑓(𝑅)$ modified gravity theories in the Jordan frame. In this context, an additional degree of freedom arises from a scalar field non-minimally coupled to gravity, naturally inducing a redshift-dependent effective Hubble constant, $H_0^{EFF}(z)$. We reconstruct the scalar field potential to ensure that $H_0^{EFF}$ decreases with z, potentially resolving the mismatch between early- and late-time measurements. More broadly, $H_0^{EFF}(z)$ offers a diagnostic tool, especially in redshift-binned analyses, for testing cosmological models.

Speaker: Tiziano Schiavone (ICE-CSIC)

6th-BIG-Meeting2025-talk-Schiavone.pdf

16:10 Discussion - All