arXiv subjects

Sergi Sirera

Publications and source records attributed to Sergi Sirera.

8 recordsLinked to original sources

arXiv2026

A Master Equation for Screening in Luminal Horndeski Gravity

Determining the active screening mechanism from a general scalar-tensor Lagrangian remains a challenging problem. As a diagnostic tool, we present a systematic study of nonlinear cosmological perturbations in luminal Horndeski theories. Working in the $α$-basis on a flat FLRW background, we derive and organise the full set of unapproximated second-order perturbation equations and systematically apply the quasistatic and weak-field limits. We find that second-order effects modify only the scalar-field equation. We derive, for static and spherically symmetric configurations, a master screening equation that classifies the nonlinear operators driving screening, recovering the Vainshtein mechanism and the onset of chameleon screening. We also identify a novel candidate regime, which we term Phaedrus screening, characterised by a screening radius that scales linearly with the source mass. For each mechanism, we derive analytical and numerical solutions and clarify the conditions under which they activate. Two new publicly available software packages are introduced: (i) xAlpha, a Mathematica package to compute and organize perturbation equations in scalar-tensor theories, and (ii) escut, a python module to solve the nonlinear scalar equation. In many cases, these tools enable the identification of the active screening type directly from a luminal Horndeski Lagrangian.

gr-qc↗

arXiv2026

Rolling Galileons: Evolving Braiding Strength for Viable Dark Energy

Motivated by growing observational indications that dark energy may be dynamical, we introduce Rolling Galileon gravity: a minimal shift-symmetry-breaking extension of the cubic Galileon in which the coupling coefficients are allowed to vary, giving rise to an evolving braiding strength. The full theory space, shown to be closed under field redefinitions, is characterised by two functions. We derive analytical conditions to satisfy three phenomenological requirements: i) a phantom-crossing equation of state at late times, ii) a positive integrated Sachs-Wolfe signature, and iii) absence of pathologies in the screened scalar force in cosmic voids. We show that these conditions are collectively satisfied by an increasing braiding strength relative to the kinetic sector. A Bayesian analysis of minimal Rolling Galileon models finds that they can satisfy the viability requirements i)-iii) whilst providing an acceptable fit to expansion-history data.

astro-ph.CO↗

arXiv2026

Constraints on Horndeski Gravity with Phantom Crossing

Gravity models in which the dark energy equation of state crosses $w=-1$, also known as the phantom divide, have received extensive interest due to recent analyses favouring this behaviour. We introduce a new subclass of Horndeski scalar-tensor models capable of generating phantom crossing, whilst remaining minimally coupled to matter: the Asymptotic Cubic Galileon (ACG) models. We show that ACG models can jointly fit the expansion history inferred from observations of the Planck cosmic microwave background, baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument, and distance-ladder supernovae measurements from the Dark Energy Survey. We then demonstrate that perturbative observables, including the galaxy-ISW cross-correlation and void force profile, provide powerful constraints that confine viable and testable ACG models to a well-defined region of the broader Horndeski landscape. Model comparison metrics, including $χ^{2}$ and Bayesian evidence, favour both ACG and $w_{0}w_{a}$CDM models over $Λ$CDM, with ACG providing a fit of comparable quality to $w_{0}w_{a}$CDM. Crucially, ACG models ground the observationally preferred $w_{0}w_{a}$CDM behaviour in a robust Lagrangian formulation. This enables interpretation beyond mere phenomenological fits, and motivates further tests of these models on nonlinear scales.

astro-ph.CO↗

arXiv2026

Inverting no-hair theorems: How requiring General Relativity solutions restricts scalar-tensor theories

Black hole solutions in general scalar-tensor theories are known to permit hair, i.e. non-trivial scalar profiles and/or metric solutions different from the ones of General Relativity (GR). Imposing that some such solutions$\unicode{x2013}$e.g. Schwarzschild or de Sitter solutions motivated in the context of black hole physics or cosmology$\unicode{x2013}$should exist, the space of scalar-tensor theories is strongly restricted. Here we investigate precisely what these restrictions are within general quadratic/cubic higher-order scalar-tensor theories for stealth solutions, whose metric is given by that in GR, supporting time-dependent scalar hair with a constant kinetic term. We derive, in a fully covariant approach, the conditions under which the Euler-Lagrange equations admit all (or a specific set of) exact GR solutions, as the first step toward our understanding of a wider class of theories that admit approximately stealth solutions. Focusing on static and spherically symmetric black hole spacetimes, we study the dynamics of linear odd-parity perturbations and discuss possible deviations from GR. Importantly, we find that requiring the existence of all stealth solutions prevents any deviations from GR in the odd-parity sector. In less restrictive scenarios, in particular for theories only requiring the existence of Schwarzschild(-de Sitter) black holes, we identify allowed deviations from GR, derive the stability conditions for the odd modes, and investigate the generic deviation of a non-trivial speed of gravitational waves. All calculations performed in this paper are reproducible via companion $\texttt {Mathematica}$ notebooks.

gr-qc↗

arXiv2026

Testing Dark Energy with Black Hole Ringdown

We show that dynamical dark energy theories can imprint $O(1)$ modifications on the quasi-normal mode (QNM) spectrum characterising black hole ringdown. The time dependence of dynamical dark energy naturally gives rise to cosmological 'hair' around a black hole. Taking the cubic Galileon as a concrete example, which admits the only known stable solution of this kind, we parametrically connect the cosmological and black hole regimes, derive the induced QNM shifts and forecast the resulting dark energy constraints. We find that the dark energy field profile can be constrained with an accuracy of up to $10^{-2}$ for LVK and $10^{-4}$ for LISA.

gr-qc↗

arXiv2025

Black hole ringdown tests of gravity

Understanding gravity is at the heart of some of the biggest questions in modern physics. While General Relativity (GR) is a theoretically unique and experimentally well-tested framework, it remains important to question whether it accurately describes gravity at all scales, motivating the exploration of broader theories. Black holes (BHs) provide ideal natural laboratories for testing gravity in the strong-field regime, and the recent advent of gravitational wave (GW) astronomy has opened a new observational window into these extreme environments. In particular, the final stage of a compact binary merger$\unicode{x2013}$the ringdown phase$\unicode{x2013}$is of great interest. Here, the study of quasinormal modes (QNMs) offers a powerful tool to probe the fundamental nature of gravity and to extract intrinsic properties of BHs. This thesis investigates BH solutions and their QNM spectra within scalar-tensor theories of gravity$\unicode{x2013}$well-motivated extensions of GR that include an additional scalar degree of freedom. In particular, it focuses on stealth BHs, where scalar hair exists without altering the background metric but can modify the QNM spectrum. By analysing perturbations around such spacetimes, we derive forecasted constraints on beyond-GR parameters for current and future GW detectors. Three main investigations are presented: (i) a novel method to constrain the speed of gravity using ringdown signals alone; (ii) a stability and QNM analysis of BHs with linearly time-dependent scalar hair; and (iii) a general classification of stealth solutions in higher-order scalar-tensor (HOST) theories, including a stability analysis and identification of ringdown observational signatures. Together, these studies contribute new theoretical tools and observational forecasts that advance our understanding on fundamental gravitational physics in the era of GW astronomy

gr-qc↗

arXiv2025

Stability and quasinormal modes for black holes with time-dependent scalar hair

We investigate black hole solutions with time-dependent (scalar) hair in scalar-tensor theories. Known exact solutions exist for such theories at the background level, where the metric takes on a standard GR form (e.g. Schwarzschild-de Sitter), but these solutions are generically plagued by instabilities. Recently, a new such solution was identified in arXiv:2310.11919, in which the time-dependent scalar background profile is qualitatively different from previous known exact solutions - specifically, the canonical kinetic term for the background scalar $X$ is not constant in this solution. We investigate the stability of this new solution by analysing odd parity perturbations, identifying a bound placed by stability and the resulting surviving parameter space. We extract the quasinormal mode spectrum predicted by the theory, identifying a shift of quasinormal mode frequencies and damping times compared to GR. We forecast constraints on these shifts (and the single effective parameter $\hatβ$ controlling them) from current and future gravitational wave experiments, finding constraints at up to the ${\cal O}(10^{-2})$ and ${\cal O}(10^{-6})$ level for LVK and LISA/TianQin, respectively. All calculations performed in this paper are reproducible via a companion Mathematica notebook.

gr-qc↗

arXiv2024

Testing the Speed of Gravity with Black Hole Ringdown

We investigate how the speed of gravitational waves, $c_{GW}$, can be tested by upcoming black hole ringdown observations. We do so in the context of hairy black hole solutions, where the hair is associated with a new scalar degree of freedom, forecasting that LISA and TianQin will be able to constrain deviations of $c_{GW}$ from the speed of light at the ${\cal O}(10^{-4})$ level from a single supermassive black hole merger. We discuss how these constraints depend on the nature of the scalar hair, what different aspects of the underlying physics they are sensitive to in comparison with constraints derived from gravitational wave propagation effects, which observable systems will place the most stringent bounds, and that constraints are expected to improve by up to two orders of magnitude with multiple observations. This is especially interesting for dark energy-related theories, where existing bounds from GW170817 need not apply at lower frequencies and where upcoming bounds from lower-frequency missions will therefore be especially powerful. As such, we also forecast analogous bounds for the intermediate-frequency AEDGE and DECIGO missions. Finally, we discuss and forecast analogous black hole ringdown constraints at higher frequencies (so from LVK, the Einstein Telescope and Cosmic Explorer) and in what circumstances they can yield new information on top of existing constraints on $c_{GW}$. All calculations performed in this paper are reproducible via a companion Mathematica notebook.

gr-qc↗