arXiv subjects

Josu C. Aurrekoetxea

Publications and source records attributed to Josu C. Aurrekoetxea.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Dynamics near the Threshold of Gravitational Collapse

Perturbation theory is an essential tool to model and interpret gravitational dynamics, for example, binary black hole mergers. Therefore it is also crucial to precisely understand its regimes of validity. The collapse of a scalar field under its own self-gravity provides a clean laboratory to study these questions. By varying the field's initial amplitude we can transition smoothly between a perturbative regime, where the field scatters in an approximately flat spacetime; and a nonperturbative regime, where a black hole forms in finite time. In this work, we use numerical relativity simulations of this set-up to investigate the accuracy of a perturbative expansion around flat spacetime, including next-to-next-to-leading order effects. Our simulations show deviations from these perturbative predictions before black hole formation, once the maximum luminosity of the process is sufficiently large, $L_{peak} \sim 10^{-2} L_{Planck}$. We characterize these nonlinear effects including a redshift of the driving frequency and a power-law spectral amplitude, which we show is consistent with approximate discrete self-similarity. These results provide a step forward towards understanding the limits of perturbative expansions in more realistic strong-gravity phenomena such as non-spherical collapse and high-velocity black hole mergers.

gr-qc↗

arXiv2026

Preheating and oscillon formation in Einstein-scalar-Gauss-Bonnet gravity

Non-perturbative processes in the early universe may create overdense structures in scalar fields like the inflaton, called oscillons. In this work, we explore whether the leading order higher derivative contributions to the scalar-tensor theory change the formation and growth of these structures, and investigate the limits in which the effective field theory (EFT) description breaks down. We find that whilst the properties of the oscillons are not significantly modified, and black holes do not generically form, for large couplings the period of formation can result in the evolution leaving the regime of validity of the EFT, at which point predictivity is lost and the next order terms in the EFT should become relevant. If the oscillons survive their formation, they tend to be stable and the EFT corrections remain bounded. The EFT breakdown is triggered by large curvature terms in the metric in the densest regions of the oscillon, meaning that approximations of such modified theories that neglect the local backreaction and non-linear dynamics of the fields may miss important effects.

gr-qc↗

arXiv2026

GRTresna: An open-source code to solve the initial data constraints in numerical relativity

GRTresna is a multigrid solver designed to solve the constraint equations for the initial data required in numerical relativity simulations. In particular, it is focussed on scenarios with fundamental fields around black holes and inhomogeneous cosmological spacetimes. The code is based on the formalism in Aurrekoetxea, Clough \& Lim arXiv:2207.03125 and can be found at https://github.com/GRTLCollaboration/GRTresna

gr-qc↗

arXiv2026

Scalar fields around black hole binaries in LIGO-Virgo-KAGRA

Light scalar particles arise naturally in many extensions of the Standard Model and are compelling dark-matter candidates. Gravitational interactions near black holes can trigger the growth of dense scalar configurations that, if sustained during inspiral, alter binary dynamics and imprint signatures on gravitational-wave signals. Detecting such effects would provide a novel probe of fundamental physics and dark matter. Here we develop a semi-analytic waveform model for binaries in scalar environments, validate it against numerical relativity simulations, and apply it in a Bayesian analysis of the LIGO-Virgo-KAGRA catalog. We obtain physically meaningful upper limits on scalar densities around most compact binaries. For GW190728 and GW190814, vacuum lies outside the $95\%$ credible region. When including superradiance priors, GW190728 shows tentative evidence for a scalar environment with a Bayes factor of $\ln \mathrm{B}^{\rm env}_{\rm vac} \approx 3.5$, consistent with a light scalar of mass $\sim10^{-12}\,\mathrm{eV}$.

gr-qc↗

arXiv2026

Waveform Modelling for the Laser Interferometer Space Antenna

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome.

gr-qc↗

arXiv2026

On curvature corrections for field theory cosmic strings

We present a combined analytical and numerical study of the effective action of field theory cosmic strings in the Abelian-Higgs model in flat space. Starting directly from the underlying solitonic field theory description, we provide a systematic derivation of the low energy effective action and present evidence for the absence of nontrivial curvature correction terms when only the translational Goldstone modes are retained. Using this framework, we extend the effective theory to include higher energy fluctuations of the soliton profile, which map to massive degrees of freedom propagating on the worldsheet. We show that the leading curvature contribution enters only through the coupling between these massive modes and the worldsheet Ricci scalar. We validate the resulting effective theory via lattice simulations of the full field theory equations of motion in flat space, implemented with Adaptive Mesh Refinement to capture the string dynamics across different scales. The numerical simulations confirm the dynamics obtained using the effective action in its validity range. Furthermore, they also demonstrate the existence of the predicted parametric instability of excited strings that drives the transfer of energy from massive excitations to the Goldstone sector.

hep-th↗

arXiv2025

Cosmology using numerical relativity

This review is an up-to-date account of the use of numerical relativity to study dynamical, strong-gravity environments in a cosmological context. First, we provide a gentle introduction into the use of numerical relativity in solving cosmological spacetimes, aimed at both cosmologists and numerical relativists. Second, we survey the present body of work, focusing on general relativistic simulations, organised according to the cosmological history -- from cosmogenesis, through the early hot Big Bang, to the late-time evolution of universe. We discuss the present state-of-the-art, and suggest directions in which future work can be fruitfully pursued.

gr-qc↗

arXiv2025

Black hole merger simulations in wave dark matter environments

The interaction of binary black hole mergers with their environments can be studied using numerical relativity simulations. These start only a short finite time before merger, at which point appropriate initial conditions must be imposed. A key task is therefore to identify the configuration that is appropriate for the binary and its environment at this stage of the evolution. In this work we study the behaviour of wave dark matter around equal mass black hole binaries, finding that there is a preferred, quasi-stationary profile that persists and grows over multiple orbits, in contrast to heavier mass dark matter where any overdensity tends to be dispersed by the binary motion. Whilst different initial configurations converge to the preferred quasi-stationary one after several orbits, unwanted transient oscillations are generated in the process, which may impact on the signal in short simulation runs. We also point out that naively superimposing the matter onto a circular binary results in artificially eccentric orbits due to the matter backreaction, which is an effect of the initial conditions and not a signature of dark matter. We discuss the further work required so that comparison of waveforms obtained with environments to vacuum cases can be done in a meaningful way.

gr-qc↗

arXiv2024

Robustness of inflation to kinetic inhomogeneities

We investigate the effects of large inhomogeneities in both the inflaton field and its momentum. We find that in general, large kinetic perturbations reduce the number of e-folds of inflation. In particular, we observe that inflationary models with sub-Planckian characteristic scales are not robust even to kinetic energy densities that are sub-dominant to the potential energy density, unless the initial field configuration is sufficiently far from the minimum. This strengthens the results of our previous work. In inflationary models with super-Planckian characteristic scales, despite a reduction in the number of e-folds, inflation is robust even when the potential energy density is initially sub-dominant. For the cases we study, the robustness of inflation strongly depends on whether the inflaton field is driven into the reheating phase by the inhomogeneous scalar dynamics.

astro-ph.CO↗

arXiv2024

Self-interacting scalar dark matter around binary black holes

Gravitational waves can provide crucial insights about the environments in which black holes live. In this work, we use numerical relativity simulations to study the behaviour of self-interacting scalar (wave-like) dark matter clouds accreting onto isolated and binary black holes. We find that repulsive self-interactions smoothen the ``spike'' of an isolated black hole and saturate the density. Attractive self-interactions enhance the growth and result in more cuspy profiles, but can become unstable and undergo explosions akin to the superradiant bosenova that reduce the local cloud density. We quantify the impact of self-interactions on an equal-mass black hole merger by computing the dephasing of the gravitational-wave signal for a range of couplings. We find that repulsive self-interactions saturate the density of the cloud, thereby reducing the dephasing. For attractive self-interactions, the dephasing may be larger, but if these interactions dominate prior to the merger, the dark matter can undergo bosenova during the inspiral phase, disrupting the cloud and subsequently reducing the dephasing.

gr-qc↗

arXiv2024

The effect of wave dark matter on equal mass black hole mergers

For dark matter to be detectable with gravitational waves from binary black holes, it must reach higher than average densities in their vicinity. In the case of light (wave-like) dark matter, the density of dark matter between the binary can be significantly enhanced by accretion from the surrounding environment. Here we show that the resulting dephasing effect on the last ten orbits of an equal mass binary is maximized when the Compton wavelength of the scalar particle is comparable to the orbital separation, $2π/μ\sim d$. The phenomenology of the effect is different from the channels that are usually discussed, where dynamical friction (along the orbital path) and radiation of energy and angular momentum drive the dephasing, and is rather dominated by the radial force (the spacetime curvature in the radial direction) towards the overdensity between the black holes. Whilst our numerical studies limit us to scales of the same order, this effect may persist at larger separations and/or particle masses, playing a significant role in the merger history of binaries.

gr-qc↗

arXiv2024

Revisiting the cosmic string origin of GW190521

For the first time we analyse gravitational-wave strain data using waveforms constructed from strong gravity simulations of cosmic string loops collapsing to Schwarzschild black holes; a previously unconsidered source. Since the expected signal is dominated by a black-hole ringdown, it can mimic the observed gravitational waves from high-mass binary black hole mergers. To illustrate this, we consider GW190521, a short duration gravitational-wave event observed in the third LIGO--Virgo--KAGRA observing run. We show that describing this event as a collapsing cosmic string loop is favoured over previous cosmic string analyses by an approximate log Bayes factor of $22$. The binary black hole hypothesis is still preferred, mostly because the cosmic string remnant is non-spinning. It remains an open question whether a spinning remnant could form from loops with angular momentum, but if possible, it would likely bring into contention the binary black hole preference. Finally, we suggest that searches for ringdown-only waveforms would be a viable approach for identifying collapsing cosmic string events and estimating their event rate. This work opens up an important new direction for the cosmic-string and gravitational-wave communities.

gr-qc↗

arXiv2024

Symmetry restoration and vacuum decay from accretion around black holes

Vacuum decay and symmetry breaking play an important role in the fundamental structure of the matter and the evolution of the universe. In this work we study how the purely classical effect of accretion of fundamental fields onto black holes can lead to shells of symmetry restoration in the midst of a symmetry broken phase. We also show how it can catalyze vacuum decay, forming a bubble that expands asymptotically at the speed of light. These effects offer an alternative, purely classical mechanism to quantum tunnelling for seeding phase transitions in the universe.

gr-qc↗

arXiv2023

GRDzhadzha: A code for evolving relativistic matter on analytic metric backgrounds

GRDzhadzha is an open-source code for relativistic simulations of matter fields on curved spacetimes that admit an analytic description (e.g. stationary black holes). It is based on the publicly available 3+1D numerical relativity code GRChombo. Such a description is valid where the density of the matter is small compared to the curvature scale of the spacetime, which is the case for many physical scenarios - for example, dark matter environments. The approach offers significant savings on memory and speed compared to running full numerical relativity simulations, since the metric variables and their derivatives are calculated analytically, and therefore are not evolved or stored on the grid. This brief paper introduces the code and gives details of some applications for which it has already been used.

gr-qc↗

arXiv2023

Spinning primordial black holes formed during a matter-dominated era

We study the formation of spinning primordial black holes during an early matter-dominated era. Using non-linear 3+1D general relativistic simulations, we compute the efficiency of mass and angular momentum transfer in the process -- which we find to be $\mathcal{O}(10\%)$ and $\mathcal{O}(5\%)$, respectively. We show that subsequent evolution is important due to the seed PBH accreting non-rotating matter from the background, which decreases the dimensionless spin. Unless the matter era is short, we argue that the final dimensionless spin will be negligible.

astro-ph.CO↗

arXiv2023

Oscillon formation during inflationary preheating with general relativity

We study the non-perturbative evolution of inflationary fluctuations during preheating using fully non-linear general-relativistic field-theory simulations. We choose a single-field inflationary model that is consistent with observational constraints and start the simulations at the end of inflation with fluctuations both in the field and its conjugate momentum. Gravity enhances the growth of density perturbations, which then collapse and virialize, forming long-lived stable oscillon-like stars that reach compactnesses $\mathcal{C}\equiv GM/R \sim 10^{-3}-10^{-2}$. We find that $\mathcal{C}$ increases for larger field models, until it peaks due to the interplay between the overdensity growth and Hubble expansion rates. Whilst gravitational effects can play an important role in the formation of compact oscillons during preheating, the objects are unlikely to collapse into primordial black holes without an additional enhancement of the initial inflationary fluctuations.

gr-qc↗

arXiv2023

CTTK: A new method to solve the initial data constraints in numerical relativity

In numerical relativity simulations with non-trivial matter configurations, one must solve the Hamiltonian and momentum constraints of the ADM formulation for the metric variables in the initial data. We introduce a new scheme based on the standard Conformal Transverse-Traceless (CTT) decomposition, in which instead of solving the Hamiltonian constraint as a 2nd order elliptic equation for a choice of mean curvature $K$, we solve an algebraic equation for $K$ for a choice of conformal factor. By doing so, we evade the existence and uniqueness problem of solutions of the Hamiltonian constraint without using the usual conformal rescaling of the source terms. This is particularly important when the sources are fundamental fields, as reconstructing the fields' configurations from the rescaled quantities is potentially problematic. Using an iterative multigrid solver, we show that this method provides rapid convergent solutions for several initial conditions that have not yet been studied in numerical relativity; namely (i) periodic inhomogeneous spacetimes with large random Gaussian scalar field perturbations and (ii) asymptotically flat black hole spacetimes with rotating scalar clouds.

gr-qc↗

arXiv2023

Electromagnetic instability of compact axion stars

If the dark matter is composed of axions, then axion stars are expected to be abundant in the Universe. We demonstrate in fully non-linear (3+1) numerical relativity the instability of compact axion stars due to the electromagnetic Chern-Simons term. We show that above the critical coupling constant $g_{aγ}^\mathrm{crit} \propto M_s^{-1.35}$, compact axion stars of mass $M_s$ are unstable. The instability is caused by parametric resonance between the axion and the electromagnetic field. The existence of stable compact axion stars requires approximately Planck-suppressed couplings to photons. If the coupling exceeds the critical value, then all stable axion stars are necessarily non-compact. Unstable axion stars decay leaving behind a less massive, less compact, remnant. The emitted radiation peaks at frequency $ω\sim 1/R_s$, where $R_s$ is the axion star radius.

hep-ph↗