arXiv subjects

Sebastian Garcia-Saenz

Publications and source records attributed to Sebastian Garcia-Saenz.

At least 19 recordsLinked to original sources

arXiv2026

Geodesic completeness of anisotropic cosmologies and the null energy condition

We demonstrate that a Bianchi-I spacetime in general relativity must be geodesically past-incomplete under the assumption of the null energy condition and provided there exists a time in which the universe is expanding in all directions. Spacetimes which are always contracting along at least one direction are not subject to the conclusion, and we provide an explicit example. A distinguishing feature of our theorem is that it makes no assumption on the global topology of the spacetime.

gr-qc↗

arXiv2026

On the logarithmic Love number of black holes beyond general relativity

Tidal Love numbers and other response coefficients of black holes sometimes exhibit a logarithmic dependence on scale, or 'running'. We clarify that this coefficient is directly calculable from the structure of the equation obeyed by the field perturbation, and requires no knowledge of the full solution. The derived formula allows us to establish some general results on the existence of logarithmic running. In particular, we show that any static and spherically symmetric spacetime that modifies the Schwarzschild or Reissner-Nordström solutions in a perturbative way must have non-zero logarithmic Love numbers. This applies for instance to all regular black hole metrics. On the other hand, our analysis highlights the importance of the perturbativity assumption: without it, we find explicit black hole solutions beyond general relativity with exactly zero running. We also illustrate the advantage of our method by recovering and extending the known results for the Hayward metric.

gr-qc↗

arXiv2026

Cosmological tensions in Proca-Nuevo theory

We study the cosmological predictions of (extended) Proca-Nuevo theory. This vector-tensor theory enjoys stable homogeneous and isotropic solutions characterized by an effective dark energy fluid, with behavior that ranges from freezing quintessential to thawing phantom-like, serving as a motivated framework to scrutinize the cosmological tensions that affect the standard $Λ$CDM model. While the model we consider is sufficiently generic to encompass a large class of field theories, it distinguishes itself from scalar dark energy models (quintessential ones, kinetic ones and non-minimally coupled ones) by the presence of what would be classed as a vector degree of freedom which can be for instance inherited from more generic theories of gravity. We improve on previous work in several directions: we consider a general one-parameter class of background models; identify a so-called 'special' model and analyze observational constraints taking also into account perturbations and making use of wide up-to-date catalogs of datasets including recently released ones. We find that the one-parameter Proca-Nuevo model is preferred over $Λ$CDM at $1.5σ$ when fitting CMB and BAO data, and at $2.4σ$ when further adding low-redshift data. The Hubble tension is alleviated, dropping from $5.8σ$ to $2.3σ$ (resp. $1.5σ$) between CMB with (and resp. without) BAO data and local measurements. On the other hand, we find that the vector field generically introduces a significant enhancement of the effective Newton constant for natural values of parameters, so that matching the observed matter power spectrum requires a mild amount of tuning to suppress the impact of perturbations. Since, at the background level, Proca-Nuevo is degenerate with other classes of theories, our results are also relevant to a wider range of set-ups including and beyond vector-tensor models.

hep-th↗

arXiv2026

No-go theorem for spontaneous vectorization

Generalized vector-tensor theories of gravity have drawn attention for admitting hairy black hole solutions, thereby circumventing the standard no-hair theorems. It remains an open question, however, how such black holes may form starting from reasonable initial conditions. It has been suggested that vector hair may grow spontaneously as a result of the field developing a negative effective mass squared $-$ the so-called spontaneous vectorization mechanism. We demonstrate that this is not possible if the initial state is a hairless black hole, a result that applies to essentially all stationary and axisymmetric solutions of interest in general relativity. More precisely, we prove that the appearance of a negative effective mass squared for the vector field must necessarily be accompanied by ghost- or gradient-type instabilities. Demanding the absence of such instabilities translates into interesting bounds on the coupling constants of the theory as functions of the black hole parameters. In particular, we discover that a Kerr black hole may become unstable when the spin increases above a certain critical value.

gr-qc↗

arXiv2026

Cosmology in warped massive gravity

We study the cosmological dynamics and predictions in the theory of warped massive gravity. This set-up postulates a five-dimensional ghost-free massive graviton with a brane-localized four-dimensional massive gravity potential, and has the virtue of raising the strong-coupling scale of the 4D theory. We identify two classes of models that lead to decoupled equations for the scale factor on the brane: one characterized by a particular choice of boundary conditions for the Stückelberg fields and one characterized by a special tuning between the coefficients of the 5D and 4D potentials. In the first case, we find interesting solutions including a cosmological bounce without the need of exotic matter. The second case leads to a modified Friedmann equation, and comparison with data shows the potential of the model to alleviate the Hubble tension.

gr-qc↗

arXiv2025

Loops in inflation with strongly non-geodesic motion

We study loop corrections in the effective field theory of inflation with imaginary speed of sound, which has been shown to provide an effective description of multi-field inflationary models characterized by strongly non-geodesic motion and heavy entropic perturbations. We focus on the one-loop corrections to the scalar and tensor power spectra, taking into account all relevant vertices at leading order in derivatives and in slow-roll. We find a power-law dependence of the scalar two-point function on the scale that defines the range of validity of the effective theory, analogous to the enhancement observed in tree-level correlation functions. Even more dramatic, the relative correction to the tensor spectrum is exponentially enhanced, albeit also suppressed in the slow-roll limit. In spite of these large effects, our results show that this class of models can satisfy the requirement of perturbative control and a consistent loop expansion within a range of parameters of phenomenological interest. On the other hand, models predicting large values of the power spectrum on small scales are found to be under strong tension. As a technical bonus, we carefully explain the prescription for the regularization and manipulation of loop integrals in this set-up, where one has a non-trivial domain of integration for time and momentum integrals owing to the regime of validity of the effective field theory. This procedure is general enough to be of potential applicability in other contexts.

hep-th↗

arXiv2025

Hairless Black Hole by Superradiance

We investigate the interplaying effects of black hole scalarization and superradiance in the context of the Einstein-Maxwell-scalar model, with the scalar field possessing electric charge. Restricted to spherical symmetry, our linear analysis about a Reissner-Nordström background confirms the persistence of tachyonic scalar modes upon inclusion of electric charge. However, fully nonlinear numerical simulations reveal that the system no longer evolves into a scalarized, hairy black hole state. Instead, we find that the superradiance phenomenon (specifically the electromagnetic version of the effect) causes the scalar condensate to become fully depleted through accretion into the black hole and radiation to spatial infinity. The combination of the two effects, which we refer to as "tachyonic superradiance", may thus be seen as a particularly efficient mechanism for the extraction of energy from a black hole, exploiting both the tachyonic growth and superradiant emission. We accurately compute the amounts of energy transfer in different channels by deriving formulae for the energy fluxes of matter fields in the presence of a dynamical horizon, which are amenable for evaluation in numerical relativity calculations.

gr-qc↗

arXiv2025

Black hole destabilization via trapped quasi-normal modes

In the presence of non-minimal gravitational couplings, matter field perturbations on a static black hole spacetime may develop unphysical poles in their linearized equations. Physical solutions confined in the domain between the event horizon and a pole satisfy a boundary value problem, although with boundary conditions which are different from standard quasi-normal modes. We refer to them as "trapped quasi-normal modes". Focusing on a Schwarzschild black hole in Einstein-Proca theory, we find that trapped quasi-normal modes accurately capture the behavior of perturbations under time evolution. In particular, axial-vector modes are unstable, with a growth rate that increases with multipole number. More interestingly, we uncover a new instability that affects monopole perturbations. These results confirm the existence of a novel destabilization mechanism of black holes by non-minimally coupled vector fields, with potential implications to well-studied models of modified gravity and cosmology based on vector particles.

gr-qc↗

arXiv2025

Stable long-term evolution in numerical relativity

We report on the potential occurrence of a numerical instability in the long-time simulation of black holes using the Baumgarte-Shapiro-Shibata-Nakamura formulation of numerical relativity, even in the simple set-up of a Schwarzschild black hole. Through extensive numerical experiments, we identify that this "late-time instability" arises from accumulated violations of the momentum constraint. To address this issue, we propose two modified versions of the so-called conformal covariant Z4 scheme, designed to propagate momentum constraint violations without damping. Our results demonstrate that these alternative formulations, which we refer to as CCZ4' and CCZ3, effectively resolve the late-time numerical instability not only in Schwarzschild spacetimes but also in black hole spacetimes with matter fields. Notably, by preventing damping of the momentum constraint violation, the Hamiltonian constraint damping can be significantly increased, which plays a crucial role in stabilizing long-term evolution in our proposed schemes.

gr-qc↗

arXiv2024

Black hole accretion of scalar clouds with spontaneous symmetry breaking

Spontaneous scalarization of black holes typically occurs through the condensation of a scalar field, with the field evolving from a $U(1)$-symmetric phase into a symmetry-breaking one with lower energy. We show that there exist symmetry-breaking phases which are themselves unstable to the formation of an additional scalar condensate, or `cloud', which is partly accreted into the black hole. By studying the fully nonlinear dynamical evolution of the process, we find that symmetry breaking causes the accretion channels of scalar clouds to be non-degenerate, favoring a dominant channel for evolution. Additionally, the final states form a characteristic energy band due to varying amounts of radiation emitted by clouds in different channels.

gr-qc↗

arXiv2024

Geodesic completeness, cosmological bounces and inflation

The question of geodesic completeness of cosmological spacetimes has recently received renewed scrutiny. A particularly interesting result is the observation that the well-known Borde-Guth-Vilenkin (BGV) theorem may misdiagnose geodesically complete cosmologies. We propose a simple amendment to the BGV theorem which addresses such loopholes while retaining much of its generality. We give straightforward proofs of some recently offered conjectures concerning (generalized) Friedmann-Lemaître-Robertson-Walker spacetimes: geodesic completeness implies (i) the existence of a bounce, loitering phase or an emergent cosmology, and (ii) a phase of accelerated expansion with strictly increasing Hubble rate. Our results are purely kinematic and do not assume general relativity or energy conditions.

gr-qc↗

arXiv2024

Cubic interactions for massless and partially massless spin-1 and spin-2 fields

We perform a complete classification of the consistent two-derivative cubic couplings for a system containing an arbitrary number of massless spin-1, massless spin-2, and partially massless (PM) spin-2 fields in $D$-dimensional (anti-)de Sitter space. In addition to previously known results, we find a unique candidate mixing between spin-1 and PM spin-2 fields. We derive all the quadratic constraints on the structure constants of the theory, allowing for relative ``wrong-sign'' kinetic terms for any of the fields. In the particular case when the kinetic terms in each sector have no relative signs, we find that the unique consistent non-trivial theory is given by multiple independent copies of conformal gravity coupled to a Yang-Mills sector in $D=4$. Our results strengthen the well-known no-go theorems on the absence of mutual interactions for massless and PM spin-2 fields.

hep-th↗

arXiv2023

Scalar-Induced Gravitational Waves from Ghost Inflation and Parity Violation

We calculate the scalar-induced gravitational wave energy density in the theory of Ghost Inflation, assuming scale invariance and taking into account both the power spectrum- and trispectrum-induced contributions. For the latter we consider the leading cubic and quartic couplings of the comoving curvature perturbation in addition to two parity-violating quartic operators. In the parity-even case, we find the relative importance of the trispectrum-induced signal to be suppressed by the requirement of perturbativity, strengthening a no-go theorem recently put forth. The parity-odd signal, even though also bound to be small, is non-degenerate with the Gaussian contribution and may in principle be comparable to the parity-even non-Gaussian part, thus potentially serving as a probe of the Ghost Inflation scenario and of parity violating physics during inflation.

gr-qc↗

arXiv2023

To Half--Be or Not To Be?

It has recently been argued that half degrees of freedom could emerge in Lorentz and parity invariant field theories, using a non-linear Proca field theory dubbed Proca-Nuevo as a specific example. We provide two proofs, using the Lagrangian and Hamiltonian pictures, that the theory possesses a pair of second class constraints, leaving $D-1$ degrees of freedom in $D$ spacetime dimensions, as befits a consistent Proca model. Our proofs are explicit and straightforward in two dimensions and we discuss how they generalize to an arbitrary number of dimensions. We also clarify why local Lorentz and parity invariant field theories cannot hold half degrees of freedom.

hep-th↗

arXiv2022

No-go Theorem for Scalar-Trispectrum-Induced Gravitational Waves

We show that the contribution of the primordial trispectrum to the energy density of the scalar-induced stochastic gravitational wave background cannot exceed the one from the scalar power spectrum in conventional inflationary scenarios. Specifically, we prove in the context of scale-invariant theories that neither regular trispectrum shapes peaking in so-called equilateral configurations, nor local trispectrum shapes diverging in soft momentum limits, can contribute significantly. Indeed, those contributions are always bound to be smaller than an order-one (or smaller) number multiplying the relative one-loop correction to the scalar power spectrum, necessarily much smaller than unity in order for the theory to be under perturbative control. Since a no-go theorem is only worth its assumptions, we also briefly discuss a toy model for a scale-dependent scalar spectrum, which confirms the robustness of our no-go result.

astro-ph.CO↗

arXiv2022

Schwarzschild quasi-normal modes of non-minimally coupled vector fields

We study perturbations of massive and massless vector fields on a Schwarzschild black-hole background, including a non-minimal coupling between the vector field and the curvature. The coupling is given by the Horndeski vector-tensor operator, which we show to be unique, also when the field is massive, provided that the vector has a vanishing background value. We determine the quasi-normal mode spectrum of the vector field, focusing on the fundamental mode of monopolar and dipolar perturbations of both even and odd parity, as a function of the mass of the field and the coupling constant controlling the non-minimal interaction. In the massless case, we also provide results for the first two overtones, showing in particular that the isospectrality between even and odd modes is broken by the non-minimal gravitational coupling. We also consider solutions to the mode equations corresponding to quasi-bound states and static configurations. Our results for quasi-bound states provide strong evidence for the stability of the spectrum, indicating the impossibility of a vectorization mechanism within our set-up. For static solutions, we analytically and numerically derive results for the electromagnetic susceptibilities (the spin-1 analogs of the tidal Love numbers), which we show to be non-zero in the presence of the non-minimal coupling.

gr-qc↗

arXiv2021

Cosmology of Extended Proca-Nuevo

Proca-Nuevo is a non-linear theory of a massive spin-1 field which enjoys a non-linearly realized constraint that distinguishes it among other generalized vector models. We show that the theory may be extended by the addition of operators of the Generalized Proca class without spoiling the primary constraint that is necessary for consistency, allowing to interpolate between Generalized Proca operators and Proca-Nuevo ones. The constraint is maintained on flat spacetime and on any fixed curved background. Upon mixing extended Proca-Nuevo dynamically with gravity, we show that the constraint gets broken in a Planck scale suppressed way. We further prove that the theory may be covariantized in models that allow for consistent and ghost-free cosmological solutions. We study the models in the presence of perfect fluid matter, and show that they describe the correct number of dynamical variables and derive their dispersion relations and stability criteria. We also exhibit, in a specific set-up, explicit hot Big Bang solutions featuring a late-time self-accelerating epoch, and which are such that all the stability and subluminality conditions are satisfied and where gravitational waves behave precisely as in General Relativity.

hep-th↗

arXiv2021

Destabilization of black holes and stars by generalized Proca fields

We demonstrate that black holes and stars in general relativity can be destabilized by perturbations of non-minimally coupled vector fields. Focusing on static and spherically symmetric backgrounds, our analysis shows that black holes with sufficiently small mass and stars with sufficiently high densities are subject to ghost or gradient-type instabilities. This holds for a large class of vector-tensor theories whenever non-minimal couplings contribute to linearized dynamics about a state with vanishing vector field and applies to generalized Proca models that have sparked attention for their potential role in cosmology and astrophysics. The stability criteria translate into bounds of relevance for low-scale theories of dark energy and for ultra-light dark matter scenarios.

gr-qc↗