arXiv subjects

Raissa F. P. Mendes

Publications and source records attributed to Raissa F. P. Mendes.

At least 19 recordsLinked to original sources

arXiv2026

Inspirals into bosonic dark matter stars and chirp mimickers

We investigate extreme--mass--ratio inspirals in which a stellar-mass compact object orbits a supermassive bosonic dark matter star, modeled as a boson star, using fully relativistic perturbative methods. Unlike inspirals around electro-vacuum black holes, these systems can shed scalar matter through dynamical friction which significantly alters the inspiral dynamics. We show that this additional dissipation can induce a chirp-like gravitational-wave signal closely resembling that of black hole binaries, allowing boson stars to act as gravitational-wave chirp mimickers even when they are not ultracompact. The inspiral evolution and resulting waveform depend sensitively on the compactness of the central boson star: highly compact configurations trigger dipolar scalar radiation, leading to a rapid plunge, whereas less compact stars yield smoother inspirals dominated by gravitational and quadrupolar scalar waves. To support waveform modeling, we derive semi-analytical prescriptions for the gravitational and scalar energy fluxes that remain accurate deep into the relativistic regime. Our findings indicate that future space-based detectors such as LISA could distinguish these mimicker signals from true black hole inspirals through measurable phase dephasings induced by scalar dissipation.

gr-qc↗

arXiv2026

Breakdown of Hydrodynamic Universality in Neutron Star Oscillations

Hydrodynamic universality refers to the property that different formulations of relativistic dissipative hydrodynamics yield identical predictions in the asymptotic long-wavelength regime. We show that neutron-star oscillations need not reach this regime. Because the finite stellar radius limits the accessible wavelengths and causality imposes a lower bound on microscopic relaxation times, the separation between microscopic and macroscopic scales can become insufficient for hydrodynamic universality to emerge. Comparing linear oscillations in relativistic Navier--Stokes and Israel--Stewart theories, we find that realistic bulk viscosities can produce sizeable modifications of the oscillation spectrum, even at the longest wavelengths. These results identify neutron-star oscillations as a direct probe of microscopic nonequilibrium dynamics beyond the leading hydrodynamic description.

gr-qc↗

arXiv2026

Radial Oscillations of Viscous Stars at Finite Temperature

We study the radial oscillation spectrum of relativistic stars within Israel-Stewart and Navier-Stokes theories, extending previous analyses to include heat diffusion and a thermodynamically consistent finite-temperature equation of state. The inclusion of heat flux gives rise to a distinct thermal sector in the mode spectrum, whose structure closely mirrors the dispersion relations of an infinite dissipative fluid. Within Israel-Stewart theory, the thermal modes transition from purely damped to propagating behavior above a critical overtone number, providing a finite-size realization of relativistic second sound in compact stars. Remarkably, the finite stellar geometry can push even the fundamental thermal mode into the propagating regime -- a feature with no continuum analogue. For the class of equations of state considered here, where finite-temperature corrections enter as controlled, Sommerfeld-type perturbations of a cold polytrope, the thermal sector couples only weakly to the ordinary fluid oscillation spectrum, with the coupling being of second order in a suitable temperature parameter. We further show that the discrete stellar spectrum is well captured by an analytic ansatz constructed from the flat-spacetime dispersion relations, with the star's finite radius discretizing the continuous mode structure. Our results complete the analysis of radial oscillations of viscous stars by incorporating the last remaining dissipative degree of freedom within the Israel-Stewart framework.

gr-qc↗

arXiv2026

Radial Oscillations of Viscous Neutron Stars: Zero Diffusion Case

The spectrum of radial oscillations of neutron stars is systematically studied within two frameworks of viscous relativistic hydrodynamics: the relativistic Navier-Stokes and Israel-Stewart theories. A correspondence is established between the discrete stellar eigenmodes and the continuous dispersion relation of perturbations around a homogeneous fluid, providing a basis for interpreting our numerical results. We analyze the Newtonian limit and assess the impact of relativistic corrections, such as the gravitational redshifting of microscopic relaxation timescales. We show that bulk viscosity can significantly affect the behavior of both hydrodynamic and nonhydrodynamic fundamental modes, and that, depending on the magnitude of the viscous effects, it is the nonhydrodynamic mode that becomes unstable beyond the turning point in a sequence of equilibrium configurations. These results provide a useful step toward systematic studies of neutron star quasinormal modes in the presence of viscosity.

gr-qc↗

arXiv2025

On the phase transition mechanism of spontaneous scalarization

In certain modified gravity theories that include additional scalar degrees of freedom, compact objects such as black holes and neutron stars may undergo a process known as spontaneous scalarization, in which the scalar field is suddenly activated beyond a certain critical point. Since its discovery, it has been clear that this effect can be understood in many cases as a continuous phase transition, well described by the phenomenological Landau model. Recently, it has been pointed out that spontaneous scalarization can also manifest as a first-order phase transition. In this paper, we take a closer look at the nature of spontaneous scalarization as a phase transition, analyzing in detail cases where it occurs as either a second- or first-order transition, as well as a more unconventional scenario characterized by a negative scalar susceptibility. Critical exponents are explicitly computed, and implications for dynamical scalarization are discussed. Moreover, the dynamics of a first-order phase transition is probed through fully nonlinear numerical simulations.

gr-qc↗

arXiv2024

Exceeding the conformal limit inside rotating neutron stars: Implications to modified theories of gravity

At the supranuclear densities achieved inside a neutron star, matter may exhibit extreme properties. In particular, it may be the case that a suitable average of the speed of sound squared exceeds the so-called conformal limit, i.e., $\langle c_s^2 \rangle > 1/3$, a condition that is equivalent to the positiveness of the trace of the energy-momentum tensor at the stellar center. This property, that holds for highly compact neutron stars obeying many (but not any) realistic equations of state, would turn these objects into interesting laboratories for tests of several scalar extensions of general relativity. In this paper, we investigate how rapid rotation influences the superconformality of the averaged speed of sound squared and modified gravity effects that depend thereupon, paying particular attention to scalar-tensor theories prone to the spontaneous scalarization effect.

gr-qc↗

arXiv2024

I-Love-$\langle c_s^2 \rangle$: Approximately universal relations for the average neutron star stiffness

The accurate observations of neutron stars have deepened our knowledge of both general relativity and the properties of nuclear physics at large densities. Relating observations to the microphysics that govern these stars can sometimes be aided by approximate universal relations. One such relation connects the ratio of the central pressure to the central energy density and the compactness of the star, and it has been found to be insensitive to realistic models for the equation of state to a $\sim 10\%$ level. In this paper, we clarify the meaning of the microscopic quantity appearing in this relation, which is reinterpreted as the average of the speed of sound squared in the interior of a star, $\langle c_s^2 \rangle\!$. The physical origin of the quasi-universality of the $\langle c_s^2 \rangle - C$ relation is then investigated. Making use of post-Minkowskian expansions, we find it to be linked to the Newtonian limit of the structure equations, as well as to the fact that the equations of state that describe NSs are relatively stiff. The same post-Minkowskian approach is also applied to the relations between $\langle c_s^2 \rangle\!$, the moment of inertia, and the tidal deformability of a neutron star, arriving at similar conclusions.

gr-qc↗

arXiv2022

Effective-action model for dynamical scalarization beyond the adiabatic approximation

In certain scalar-field extensions to general relativity, scalar charges can develop on compact objects in an inspiraling binary -- an effect known as dynamical scalarization. This effect can be modeled using effective-field-theory methods applied to the binary within the post-Newtonian approximation. Past analytic investigations focused on the adiabatic (or quasi-stationary) case for quasi-circular orbits. In this work, we explore the full dynamical evolution around the phase transition to the scalarized regime. This allows for generic (eccentric) orbits and to quantify nonadiabatic (e.g., oscillatory) behavior during the phase transition. We also find that even in the circular-orbit case, the onset of scalarization can only be predicted reliably when taking the full dynamics into account, i.e., the adiabatic approximation is not appropriate. Our results pave the way for accurate post-Newtonian predictions for dynamical scalarization effects in gravitational waves from compact binaries.

gr-qc↗

arXiv2022

Equation-of-state-insensitive measure of neutron star stiffness

Universal relations (i.e., insensitive to the equation of state) between macroscopic properties of neutron stars have proven useful for a variety of applications -- from providing a direct means to extract observables from data to breaking degeneracies that hinder tests of general relativity. Similarly, equation-of-state-insensitive relations directly connecting macroscopic and microscopic properties of neutron stars can potentially provide a clean window into the behavior of nuclear matter. In this work, we uncover a tight correlation between certain macroscopic properties of a neutron star - its compactness $C$, moment of inertia $\bar{I}$ and tidal deformability $\barΛ$ - and the ratio $α_c \equiv p_c/ε_c$ of central pressure to central energy density, which can be interpreted as a mean notion of the stiffness of nuclear matter inside that object. We describe interesting properties of this stiffness measure, quantify the (approximate) universality of the $α_c - C/\bar{I}/\barΛ$ relations, and explore its consequences in the face of recent and future neutron star observations.

gr-qc↗

arXiv2022

Nonlinear dynamics of oscillating neutron stars in scalar-tensor gravity

The spectrum of oscillating compact objects can be considerably altered in alternative theories of gravity. In particular, it may be enriched by modes with no counterpart in general relativity, tied to the dynamics of additional degrees of freedom generically present in these theories. Detection of these modes, e.g. in the gravitational-wave signal from a binary compact object coalescence, could provide a powerful tool to probe the underlying theory of gravity. To access the potential of such a detection, it is crucial to understand the linear and nonlinear spectral features of dynamically formed, oscillating compact objects in alternative theories of gravity. As a step towards that goal, in this work we present a suite of 1+1 numerical relativity simulations of neutron stars in scalar-tensor theories, we carefully analyze the spectrum of stellar pulsations, and we compare results with expectations from linear perturbation theory. This allows us to build intuition for the case of binary neutron star mergers. Additionally, the models investigated in this work are representatives of two broad classes, in which the scalar field couples either strongly or weakly with the fluid. The distinct phenomenology of the nonlinear dynamics that we identify for each class of models, may find counterparts also in other alternative theories of gravity.

gr-qc↗

arXiv2021

Neutron stars in the symmetron model

Screening mechanisms are often deployed by dark energy models in order to conceal the effects of their new degrees of freedom from the scrutiny of terrestrial and solar system experiments. However, extreme properties of nuclear matter may lead to a partial failure of screening mechanisms inside the most massive neutron stars observed in Nature, opening up the possibility of probing these theories with neutron star observations. In this work we explore equilibrium and stability properties of neutron stars in two variants of the symmetron model. We show that around sufficiently compact neutron stars, the symmetron is amplified with respect to its background, cosmological value by several orders of magnitude, and that properties of such unscreened stars are sensitive to corrections to the leading linear coupling between the symmetron and matter.

gr-qc↗

arXiv2020

Highly compact neutron stars and screening mechanisms. I. Equilibrium and stability

Modified theories of gravity that offer viable models for dark energy often rely on mechanisms that screen their effects in high density environments. From this perspective, it would appear that, once solar system constraints are satisfied, these theories would predict a trivial phenomenology for (much denser) neutron stars. In this work we explore the fact that in scalar-tensor theories the scalar degree of freedom does not couple to the mass density alone, but to the trace of the energy-momentum tensor - which can increase and eventually change sign as density and pressure build up in the core of neutron stars -, and investigate whether there could be a partial unscreening of the scalar field inside the most compact stars found in Nature. For this purpose, we construct neutron star solutions with realistic equations of state in theories with screening mechanisms and study their stability under radial perturbations. In particular, we find that stable solutions with a unscreened core can exist in chameleon models, while for the environmentally-dependent dilaton model a wealth of new, scalarized equilibrium solutions are found, some of which can be stable.

gr-qc↗

arXiv2019

Scalar charges and pulsar-timing observables in the presence of nonminimally coupled scalar fields

Pulsar-timing has become a celebrated tool for probing modifications to General Relativity in the strong-field surroundings of neutron stars. Here we investigate whether scalar-tensor theories that incorporate a nonminimally coupled scalar degree of freedom may pass pulsar-timing tests, by computing the scalar charges entering such observables. In particular we show that for positive values of the nonminimal coupling $ξ$, pulsar-timing constraints may be evaded even in the presence of spontaneous scalarization.

gr-qc↗

arXiv2018

Trace of the energy-momentum tensor and macroscopic properties of neutron stars

A generic feature of scalar extensions of general relativity is the coupling of the scalar degrees of freedom to the trace $T$ of the energy-momentum tensor of matter fields. Interesting phenomenology arises when the trace becomes positive---when pressure exceeds one third of the energy density---a condition that may be satisfied in the core of neutron stars. In this work, we study how the positiveness of the trace of the energy-momentum tensor correlates with macroscopic properties of neutron stars. We first show that the compactness for which $T=0$ at the stellar center is approximately equation-of-state independent, and given by $C = 0.262_{-0.017}^{+0.011}$ (90% confidence interval). Next, we exploit Bayesian inference to derive a probability distribution function for the value of $T$ at the stellar center given a putative measurement of the compactness of a neutron star. This investigation is a necessary step in order to use present and future observations of neutron star properties to constrain scalar-tensor theories based on effects that depend on the sign of $T$.

gr-qc↗

arXiv2018

Evolution of Highly Eccentric Binary Neutron Stars Including Tidal Effects

This work is the first in a series of studies aimed at understanding the dynamics of highly eccentric binary neutron stars, and constructing an appropriate gravitational-waveform model for detection. Such binaries are possible sources for ground-based gravitational wave detectors, and are expected to form through dynamical scattering and multi-body interactions in globular clusters and galactic nuclei. In contrast to black holes, oscillations of neutron stars are generically excited by tidal effects after close pericenter passage. Depending on the equation of state, this can enhance the loss of orbital energy by up to tens of percent over that radiated away by gravitational waves during an orbit. Under the same interaction mechanism, part of the orbital angular momentum is also transferred to the star. We calculate the impact of the neutron star oscillations on the orbital evolution of such systems, and compare these results to full numerical simulations. Utilizing a Post-Newtonian flux description we propose a preliminary model to predict the timing of different pericenter passages. A refined version of this model (taking into account Post-Newtonian corrections to the tidal coupling and the oscillations of the stars) may serve as a waveform model for such highly eccentric systems.

gr-qc↗

arXiv2018

New class of quasinormal modes of neutron stars in scalar-tensor gravity

Detection of the characteristic spectrum of pulsating neutron stars can be a powerful tool not only to probe the nuclear equation of state, but also to test modifications to general relativity. However, the shift in the oscillation spectrum induced by modified theories of gravity is often small and degenerate with our ignorance of the equation of state. In this Letter, we show that the coupling to additional degrees of freedom present in modified theories of gravity can give rise to new families of modes, with no counterpart in general relativity, which could be sufficiently well resolved in frequency space as to allow for a clear detection. We present a realization of this idea by performing a thorough study of radial oscillations of neutron stars in massless scalar-tensor theories of gravity. We anticipate astrophysical scenarios where the presence of this class of quasinormal modes could be probed with electromagnetic and gravitational wave measurements.

gr-qc↗

arXiv2017

Swimming in spacetime: the view from a Fermi observer

An extended test body moving in a curved spacetime does not typically follow a geodesic, because of forces that arise from couplings between its multipole moments and the ambient curvature. An illustration of this fact was provided by Wisdom, who showed that the motion of a quasi-rigid body undergoing cyclic changes of shape in a curved spacetime deviates, in general, from a geodesic. Wisdom's analysis, however, was recently challenged on the grounds that the body's motion should be described by the Mathisson-Papapetrou-Dixon equations, and that these predict geodesic motion for the kind of body considered by Wisdom. We attempt to shed some light on this matter by examining the motion of an internally-moving tripod in Schwarzschild spacetime, as viewed by a Fermi observer moving on a timelike geodesic. We find that the description of the motion depends sensitively on a choice of cycle for the tripod's internal motions, but also on a choice of "center of mass" for the tripod; a sensible (though not unique) prescription for this "center of mass" produces a motion that conforms with Wisdom's prediction: the tripod drifts away from the observer, even when they are given identical initial conditions. We suggest pathways of reconciliation between this conclusion and the null result that apparently follows from the Mathisson-Papapetrou-Dixon equations of motion.

gr-qc↗

arXiv2017

Tidal deformability of boson stars and dark matter clumps

In this work we consider minimally-coupled boson stars immersed in a tidal environment and compute their tidal deformability to leading order. We also describe an approximate correspondence between Newtonian boson star configurations (described by the Schrödinger-Poisson equations) and dynamical dark matter clumps (described by the collisionless Boltzmann equation). This allows us to map our results for the tidal deformability of boson stars to approximate statements for dark matter clumps.

astro-ph.CO↗