arXiv subjects

José T. Gálvez Ghersi

Publications and source records attributed to José T. Gálvez Ghersi.

9 recordsLinked to original sources

arXiv2026

Optimized numerical evolution of perturbations across sharp background trajectory turns in multifield inflation

Features in the primordial power spectrum require numerical methods that are both accurate and scalable across the wide class of multifield inflationary models that produce them. Sharp turns in the background trajectories, induced by either potential or geometric effects, render these computations particularly challenging. In this work, we introduce an efficient method for evolving primordial scalar fluctuations, requiring time steps comparable to those used for the background evolution. We demonstrate that the method accurately tracks perturbations through rapidly turning trajectories in arbitrary field-space geometries, enabling systematic exploration of spectral features across diverse multifield scenarios. Our approach scales robustly to large numbers of degrees of freedom, providing a reliable computational framework for probing regimes that significantly depart from slow-roll or slow-turn dynamics.

gr-qc↗

arXiv2024

Dimensional deformation of sine-Gordon breathers into oscillons

Oscillons are localized field configurations oscillating in time with lifetimes orders of magnitude longer than their oscillation period. In this paper, we simulate non-travelling oscillons produced by deforming the breather solutions of the sine-Gordon model. Such a deformation treats the dimensionality of the model as a real parameter to produce spherically symmetric oscillons. After considering the post-transient oscillation frequency as a control parameter, we probe the initial parameter space to continuously connect breathers and oscillons at various dimensionalities. For sufficiently small dimensional deformations, we find that oscillons can be treated as perturbatively deformed breathers. In $D\gtrsim 2$ spatial dimensions, we observe solutions undergoing intermittent phases of contraction and expansion in their cores. Knowing that stable and unstable configurations can be mapped to disjoint regions of the breather parameter space, we find that amplitude modulated solutions are located in the middle of both stability regimes. These solutions display the dynamics of critical behavior around the stability limit.

hep-th↗

arXiv2021

Numerical renormalization group-based approach to secular perturbation theory

Perturbation theory is a crucial tool for many physical systems, when exact solutions are not available, or nonperturbative numerical solutions are intractable. Naive perturbation theory often fails on long timescales, leading to secularly growing solutions. These divergences have been treated with a variety of techniques, including the powerful dynamical renormalization group (DRG). Most of the existing DRG approaches rely on having analytic solutions up to some order in perturbation theory. However, sometimes the equations can only be solved numerically. We reformulate the DRG in the language of differential geometry, which allows us to apply it to numerical solutions of the background and perturbation equations. This formulation also enables us to use the DRG in systems with background parameter flows, and therefore, extend our results to any order in perturbation theory. As an example, we apply this method to calculate the soliton-like solutions of the Korteweg-de Vries equation deformed by adding a small damping term. We numerically construct DRG solutions which are valid on secular time scales, long after naive perturbation theory has broken down.

hep-th↗

arXiv2021

Integrability of eccentric, spinning black hole binaries up to second post-Newtonian order

Accurate and efficient modeling of the dynamics of binary black holes (BBHs) is crucial to their detection and parameter estimation through gravitational waves, both with LIGO/Virgo and LISA. General BBH configurations will have misaligned spins and eccentric orbits, eccentricity being particularly relevant at early times. Modeling these systems is both analytically and numerically challenging. Even though the 1.5 post-Newtonian (PN) order is Liouville integrable, numerical work has demonstrated chaos at 2PN order, which impedes the existence of an analytic solution. In this article we revisit integrability at both 1.5PN and 2PN orders. At 1.5PN, we construct four (out of five) action integrals. At 2PN, we show that the system is indeed integrable - but in a perturbative sense - by explicitly constructing five mutually-commuting constants of motion. Because of the KAM theorem, this is consistent with the past numerical demonstration of chaos. Our method extends to higher PN orders, opening the door for a fully analytical solution to the generic eccentric, spinning BBH problem.

gr-qc↗

arXiv2020

A fixed point for black hole distributions

Understanding distributions of black holes is crucial to both astrophysics and quantum gravity. Studying astrophysical population statistics has even been suggested as a channel to constrain black hole formation from the quantum vacuum. Here we propose a Gedankenexperiment to show that the non-linear properties of binary mergers (simulated with accurate surrogate models) generate an attractor in the space of distributions. Our results show that the joint distribution of spin magnitude and fractional mass loss evolves to a fixed point, converging in a few generations. The features of this fixed point distribution do not depend on the choice of initial distribution. Since a black hole merger is irreversible it produces entropy - possibly the largest source of entropy in the universe. The fixed-point distributions are neither isothermal nor isentropic, and initially thermodynamic states evolve away from thermality. We finally evaluate the specific entropy production rate per merger from initially thermal and non-thermal distributions, which converges to a constant.

gr-qc↗

arXiv2019

Echoes from the scattering of wavepackets on wormholes

It has been recently shown that observing pulses isolated from the gravitational radiation transient (also known as echoes) would prove the existence of exotic compact objects (ECOs). Many features of the ringdown signal can be reproduced by simulating a scattering problem instead of the full coalescence of ECOs. In this paper, we study the dynamics of scalar and tensor wavepackets colliding against a spherically symmetric Morris-Thorne wormhole. Our aim is to extract the features of the time-dependent scattering solutions inside and outside the effective potential cavity in addition to their asymptotic behavior. Using the geometrical optics approximation, we show that the amplitude of the echoes is only large enough in a narrow bandwidth of frequency space. Additionally, we show that the cavity modifies the polarization of the asymptotic gravitational wave solutions. The computer code used to produce these results is publicly available for further applications, including scattering and accretion processes.

gr-qc↗

arXiv2018

Unbraiding the Bounce: Superluminality around the Corner

We study a particular realization of the cosmological bounce scenario proposed recently by Ijjas and Steinhardt. First, we find that their bouncing solution starts from a divergent sound speed and ends with its vanishing. Thus, the solution connects two strongly coupled configurations. These pathologies are separated from the bouncing regime by only a few Planck times. We then reveal the exact structure of the Lagrangian, which reproduces this bouncing solution. This reconstruction allowed us to consider other cosmological solutions of the theory and analyze the phase space. In particular, we find other bouncing solutions and solutions with superluminal sound speed. These stable superluminal states can be continuously transformed into the solution constructed by Ijjas and Steinhardt. We discuss the consequences of this feature for a possible UV-completion.

gr-qc↗

arXiv2017

Unscreening scalarons with a black hole

It is typically believed that the additional degrees of freedom in any modification of gravity are completely suppressed by the large energy densities coexisting with an astrophysical black hole. In this paper, we find that this might not always be the case. This belief holds for black holes formed via gravitational collapse in very dense environments, whereas the black holes with sufficiently low accretion rates that have low matter densities inside innermost stable circular orbit will generally unscreen chameleons. We develop a novel technique to study the dynamics of accretion of a scalar field onto a Schwarzschild-like black hole which is accurate on both short and long time scales. In particular, we study the behavior of the extra scalar degree of freedom in the Starobinsky and Hu-Sawicki $f(R)$ theories, for the symmetron model, and for the Ratra-Peebles model. Aside from calculating non-trivial static field profiles outside the black hole, we provide the tools to study the (in)stability and evolution towards the equilibrium solution for any generic well behaved set of parameters and initial conditions. Our code is made publicly available for further research and modifications to study other models.

gr-qc↗

arXiv2017

Two-point correlators revisited: Fast and slow scales in multifield models of inflation

We study the structure of two-point correlators of the inflationary field fluctuations in order to improve the accuracy and efficiency of the existing methods to calculate primordial spectra. We present a description motivated by the separation of the fast and slow evolving components of the spectrum which is based on Cholesky decomposition of the field correlator matrix. Our purpose is to rewrite all the relevant equations of motion in terms of slowly varying quantities. This is important in order to consider the contribution from high-frequency modes to the spectrum without affecting computational performance. The slow-roll approximation is not required to reproduce the main distinctive features in the power spectrum for each specific model of inflation.

astro-ph.CO↗