arXiv subjects

P. P. Avelino

Publications and source records attributed to P. P. Avelino.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics of Biased Domain Walls: The Rocket Effect

We investigate the dynamics of domain walls in scalar field theories with degenerate vacua (i.e., vacua of equal energy density) in which the scalar field mass depends on the vacuum state. Using analytical arguments and numerical simulations, we show that this vacuum dependence of the scalar field mass renders the emission of scalar radiation from domain walls anisotropic, preferentially toward regions with smaller scalar field mass. We further show that the resulting recoil (rocket) effect biases the evolution of cosmological domain wall networks in favor of the lower-mass vacuum, thereby promoting network decay. We also demonstrate that the biased evolution of domain walls in theories with degenerate vacua, previously attributed to asymmetries of the potential barrier near the local maximum, is instead primarily controlled by the vacuum dependence of the scalar field mass. More generally, in theories with non-degenerate vacua, this recoil mechanism constitutes an additional source of dynamical bias that can either hasten or delay network decay relative to the standard expectation based solely on differences in vacuum energy density.

astro-ph.CO↗

arXiv2026

Lagrangian Identity and Mass Evolution of Particle-like Objects in Nonminimally Coupled Gravity

We show that the Lagrangian of a Nambu-Goto $p$-brane satisfies the identity $\mathcal{L}_{\rm [\it p \rm]}=T_{\rm [\it p \rm]}/(p+1)$, with $T_{\rm [\it p \rm]}$ denoting the trace of the corresponding energy-momentum tensor, independently of the properties of the gravitational field. While for $p=0$ this reduces to the standard $\mathcal{L}_{\rm [0]}=T_{\rm [0]}$ relation, which determines the on-shell Lagrangian of point particles and their fluids, more generally it depends explicitly on the $p$-brane dimensionality. We explore the implications of this Lagrangian identity for the dynamics of non-self-intersecting cosmic string loops in a homogeneous and isotropic universe within nonminimally coupled scalar-tensor gravity, showing that, unlike in general relativity, their rest mass can evolve in response to the cosmological evolution of the background spacetime, regardless of their small size or tension. We further generalize this analysis to closed $p$-branes in $(N+1)$-dimensional Friedmann-Lemaître-Robertson-Walker spacetimes, showing that the evolution of the rest mass depends explicitly on the dimensionality of the brane, and therefore that the cosmological evolution of particle-like objects in theories of gravity with nonminimal matter couplings is sensitive to their internal structure.

gr-qc↗

arXiv2026

Cosmological Averaging in Nonminimally Coupled Gravity

We address the challenge, commonly referred to as the cosmological averaging problem, of relating the large-scale evolution of an inhomogeneous universe to that predicted by a homogeneous matter distribution in theories of gravity with nonminimal matter-gravity couplings. To this end, we focus on the class of $f(R,T)$ models given by $f(R,T) = R + F(T)$, where $R$ denotes the Ricci scalar and $T$ the trace of the energy-momentum tensor. This framework provides a simple yet theoretically consistent realization of nonminimal coupled gravity and can be recast as General Relativity minimally coupled to a modified matter Lagrangian. Using global K-monopoles as an illustrative toy model, we show that, when $F$ is a nonlinear function of $T$, the ratio between the spatial average of $F$ and $F$ evaluated at the spatial average of $T$ can deviate significantly from unity and depends on the particle number density. We demonstrate that the common assumption that this ratio is equal to unity generally leads to an inaccurate description of cosmological dynamics. We further show that dust in these theories generally exhibits a non-vanishing proper pressure. Our results highlight the importance of properly accounting for spatial averaging in cosmological models with nonminimal matter-gravity couplings.

astro-ph.CO↗

arXiv2025

Note on bulk viscosity as an alternative to dark energy

Bulk viscosity, which characterizes the irreversible dissipative resistance of a fluid to volume changes, has been proposed as a potential mechanism for explaining both early- and late-time accelerated expansion of the Universe. In this work, we investigate two distinct physical scenarios for the origin of bulk viscosity: (1) nonminimal interactions between two fluids, and (2) elastic collisions in an ideal gas. In both cases, we demonstrate that while the associated energy-momentum exchange can significantly influence fluid dynamics, overall energy-momentum conservation precludes such exchange from having any direct gravitational effect in the context of General Relativity. In case (1), we show that the standard bulk viscous energy-momentum tensor can be obtained for the two-fluid system only at the cost of the violation of all classical energy conditions: null, weak, dominant, and strong. In case (2), we consider a single fluid composed of point particles undergoing instantaneous, energy- and momentum-conserving collisions, and find that the proper pressure remains strictly non-negative, with the equation-of-state parameter confined to the interval $[0,1/3]$. In both scenarios, achieving a sufficiently negative effective pressure to drive cosmic acceleration requires assumptions that compromise the physical viability of the model. Our results highlight some of the key physical challenges involved in modeling dark energy through bulk viscous effects.

astro-ph.CO↗

arXiv2025

Deviations from the von Laue condition: Implications for the on-shell Lagrangian of particles and fluids

According to the von Laue condition, the volume integral of the proper pressure inside isolated particles with a fixed structure and finite mass vanishes in the Minkowski limit of general relativity. In this work, we consider a simple illustrative example: nonstandard static global monopoles with finite energy, for which the von Laue condition is satisfied when the proper pressure is integrated over the whole space. We demonstrate, however, that the absolute value of this integral, when calculated up to a finite distance from the center of the global monopole, generally deviates from zero by no more than the energy located outside the specified volume (under the assumption of the dominant energy condition). Furthermore, we find that the maximum deviation from unity of the ratio between the volume averages of the on-shell Lagrangian and the trace of the energy-momentum tensor cannot exceed three times the outer energy fraction. Extending these results to real particles, we demonstrate that these constraints generally hold for finite-mass systems with fixed structure, including stable atomic nuclei, provided the dominant energy condition is satisfied. Specifically, we show that, except in extremely dense environments with energy densities comparable to that of the particles themselves, the volume average of the aforementioned ratio must be extremely close to unity. Finally, we discuss the broader implications of our findings for the form of the on-shell Lagrangian of real fluids, which is often a crucial element for accurately modeling the dynamics of the gravity and matter, especially in scenarios involving nonminimal couplings to other matter fields or gravity. We find that, in general, the ideal gas on-shell Lagrangian provides an accurate approximation of the true on-shell Lagrangian, even for nonideal gases with significant interparticle interactions.

gr-qc↗

arXiv2025

Particles and their fluids in nontrivial matter extensions to general relativity

According to the standard von Laue condition, the volume-averaged pressure inside particles of fixed mass and structure vanishes in the Minkowski limit of general relativity. Here we show that this condition is in general not fulfilled in the context of $f(R,T)$ gravity, or of other theories of gravity in which the linear momentum is not conserved in this limit (here, $R$ and $T$ represent the Ricci scalar and the trace of the energy-momentum tensor, respectively). We derive a generalized von Laue condition valid for the $\mathcal R(R) + \mathcal F(T)$ subclass of $f(R,T)$ theories of gravity and discuss its cosmological implications. In particular, we show that the standard radiation and matter era evolution of the universe is recovered in the context $R + \mathcal F(T)$ gravity independently of the specific properties of the function $\mathcal F(T)$. We also find that dust -- a perfect fluid whose particles are at rest in the fluid's proper frame -- cannot in general be described as pressureless in the context of these theories. We further discuss the implications of our findings for the form of the on-shell Lagrangian of an ideal gas.

gr-qc↗

arXiv2024

Euclid. I. Overview of the Euclid mission

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

astro-ph.CO↗

arXiv2024

Domain walls in light of Cosmic Microwave Background and Pulsar Timing Array data

In this paper, we study the compatibility of biased domain wall scenarios with current gravitational wave data. We show that the Cosmic Microwave Background bounds on the fractional density of gravitational waves at the time of decoupling may only slightly improve on the constraints that result from requiring that domain walls never dominate the cosmic energy budget. We show that, despite this, the range of energy scales of the domain-wall forming phase transitions are already quite constricted, even if the networks decay early in cosmological history. We also show that, if domain walls are to provide an explanation to the stochastic gravitational wave background that was recently detected by pulsar timing arrays, they not only have to decay early in the radiation dominated era but also their energy density would have to be close to dominating the energy density of the universe, which would require some fine tuning of the parameters of the models.

gr-qc↗

arXiv2024

Sound speed as a source of the gravitational field in modified gravity

In the context of $f(R,T)$ gravity and other modified theories of gravity, the knowledge of the first order variation of the trace $T$ of the energy-momentum tensor with respect to the metric is essential for an accurate characterization of the gravitational field. In this paper, by considering a paradigmatic example of a perfect fluid whose dynamics is described by a pure k-essence matter Lagrangian in $f(R,T)=R+\mathcal F(T)$ gravity, we show that the first order variation of the trace of the energy-momentum tensor cannot in general be determined from the proper density, proper pressure and 4-velocity of the fluid alone, and that the sound speed of the fluid can directly influence the dynamics of gravity. We also confirm that the second variation of the matter Lagrangian with respect to the metric should not in general be neglected. These results can be particularly relevant for cosmological studies of $f(R,T)$ gravity in which some of the material content of the Universe is modeled as a perfect fluid.

gr-qc↗

arXiv2024

Stochastic gravitational wave background generated by domain wall networks

In this work we study the power spectrum of the Stochastic Gravitational Wave Background produced by standard and biased domain wall networks, using the Velocity-dependent One-Scale model to compute the cosmological evolution of their characteristic scale and root-mean-squared velocity. We consider a standard radiation + $Λ\rm CDM$ background and assume that a constant fraction of the energy of collapsing domain walls is emitted in the form of gravitational waves. We show that, in an expanding background, the total energy density in gravitational radiation decreases with cosmic time (after a short initial period of quick growth). We also propose a two parameter model for the scale-dependence of the frequency distribution of the gravitational waves emitted by collapsing domain walls. We determine the corresponding power spectrum of the Stochastic Gravitational Wave Background generated by domain walls, showing that it is a monotonic decreasing function of the frequency for frequencies larger than that of the peak generated by the walls that have decayed most recently. We also develop an analytical approximation to this spectrum, assuming perfect linear scaling during both the radiation and matter eras, in order to characterize the dependence of the amplitude, peak frequency and slope of the power spectrum on the model parameters.

gr-qc↗

arXiv2023

Halos of dark energy

We investigate the properties of dark energy halos in models with a nonminimal coupling in the dark sector. We show, using a quasistatic approximation, that a coupling of the mass of dark matter particles to a standard quintessence scalar field $ϕ$ generally leads to the formation of dark energy concentrations in and around compact dark matter objects. These are associated with regions where scalar field gradients are large and the dark energy equation of state parameter is close to $-1/3$. We find that the energy and radius of a dark energy halo are approximately given by $E_{\rm halo} \sim \boldsymbolβ^2 φ\, m$ and $r_{\rm halo} \sim \sqrt{\boldsymbolβ \,φ({R}/{H})}$, where $φ=Gm/(R c^2)$, $m$ and $R$ are, respectively, the mass and radius of the associated dark matter object, $\boldsymbolβ = -(8πG)^{-1/2} d \ln m/d ϕ$ is the nonminimal coupling strength parameter, $H$ is the Hubble parameter, $G$ is the gravitational constant, and $c$ is the speed of light in vacuum. We further show that current observational limits on $\boldsymbolβ$ over a wide redshift range lead to stringent constraints on $E_{\rm halo}/m$ and, therefore, on the impact of dark energy halos on the value of the dark energy equation of state parameter. We also briefly comment on potential backreaction effects that may be associated with the breakdown of the quasistatic approximation and determine the regions of parameter space where such a breakdown might be expected to occur.

astro-ph.CO↗

arXiv2023

Can gravitational vacuum condensate stars be a dark energy source?

Gravitational vacuum condensate stars, also known as gravastars, have been proposed as an alternative to black holes. Their interior contains a perfect fluid with an equation of state akin to that of a cosmological constant. For this reason, they have recently been considered as a possible astrophysical source of dark energy. In this work we argue that gravitational vacuum condensate stars cannot be the source of dark energy and highlight that a direct coupling of their mass to the dynamics of the Universe would lead to an additional velocity dependent acceleration, damping their motion with respect to the cosmological frame. We briefly discuss the potential impact of this additional acceleration in the context of a recent proposal that the observed mass growth of compact objects at the core of elliptical galaxies might result from such a cosmological coupling.

gr-qc↗

arXiv2022

Role of predator-prey reversal in Rock-Paper-Scissors models

In this letter we consider a single parameter generalization of the standard three species Rock-Paper-Scissors (RPS) model allowing for predator-prey reversal. This model, which shall be referred to as $κ$RPS model, incorporates bidirectional predator-prey interactions between all the species in addition to the unidirectional predator-prey interactions of the standard RPS model. We study the dynamics of a May-Leonard formulation of the $κ$RPS model using lattice based spatial stochastic simulations with random initial conditions. We find that if the simulation lattices are sufficiently large for the coexistence of all three species to be maintained, the model asymptotically leads to the formation of spiral patterns whose evolution is qualitatively similar to that of the standard RPS model, albeit with larger characteristic length and time scales. We show that there are in general two distinct scaling regimes: one transient curvature dominated regime in which the characteristic length of the population network grows with time and another where it becomes a constant. We also estimate the dependence of the asymptotic value of the characteristic length of the population network on the likelihood of predator-prey reversal and show that if the simulation lattices are not sufficiently large then predator-prey reversal can have a significant negative impact on coexistence. Finally, we interpret these results by considering the much simpler dynamics of circular domains.

q-bio.PE↗

arXiv2022

Analytical scaling solutions for the evolution of cosmic domain walls in a parameter-free velocity-dependent one-scale model

We derive an analytical approximation for the linear scaling evolution of the characteristic length $L$ and the root-mean-squared velocity $σ_v$ of standard frictionless domain wall networks in Friedmann-Lemaître-Robertson-Walker universes with a power law evolution of the scale factor $a$ with the cosmic time $t$ ($a \propto t^λ$). This approximation, obtained using a recently proposed parameter-free velocity-dependent one-scale model for domain walls, reproduces well the model predictions for $λ$ close to unity, becoming exact in the $λ\to 1^-$ limit. We use this approximation, in combination with the exact results found for $λ=0$, to obtain a fit to the model predictions valid for $λ\in [0, 1[$ with a maximum error of the order of $1 \%$. This fit is also in good agreement with the results of field theory numerical simulations, specially for $λ\in [0.9, 1[$. Finally, we explicitly show that the phenomenological energy-loss parameter of the original velocity-dependent one-scale model for domain walls vanishes in the $λ\to 1^-$ limit and discuss the implications of this result.

astro-ph.CO↗

arXiv2022

On-shell Lagrangian of an ideal gas

In the context of general relativity, both energy and linear momentum constraints lead to the same equation for the evolution of the speed of free localized particles with fixed proper mass and structure in a homogeneous and isotropic Friedmann-Lemaître-Robertson-Walker universe. In this paper we extend this result by considering the dynamics of particles and fluids in the context of theories of gravity nonminimally coupled to matter. We show that the equation for the evolution of the linear momentum of the particles may be obtained irrespective of any prior assumptions regarding the form of the on-shell Lagrangian of the matter fields. We also find that consistency between the evolution of the energy and linear momentum of the particles requires that their volume-averaged on-shell Lagrangian and energy-momentum tensor trace coincide ($\mathcal L_{\rm on-shell}=T$). We further demonstrate that the same applies to an ideal gas composed of many such particles. This result implies that the two most common assumptions in the literature for the on-shell Lagrangian of a perfect fluid ($\mathcal L_{\rm on-shell}=\mathcal{P}$ and $\mathcal L_{\rm on-shell}=-ρ$, where $ρ$ and $\mathcal{P}$ are the proper density and pressure of the fluid, respectively) do not apply to an ideal gas, except in the case of dust (in which case $T=-ρ$).

gr-qc↗

arXiv2021

Distance-duality in theories with a nonminimal coupling to gravity

We show that gravitational theories with a nonminimal coupling (NMC) to the matter fields lead to a violation of Etherington's distance-duality relation, which relates the luminosity and angular diameter distances. We derive constraints on power-law and exponential NMC models using existing measurements of type Ia supernovae and baryon acoustic oscillations throughout the redshift range $0<z<1.5$. These complement previous constrains derived from cosmic-microwave background radiation and big-bang nucleosynthesis data.

gr-qc↗

arXiv2021

Lotka-Volterra versus May-Leonard formulations of the spatial stochastic Rock-Paper-Scissors model: the missing link

The Rock-Paper-Scissors (RPS) model successfully reproduces some of the main features of simple cyclic predator-prey systems with interspecific competition observed in nature. Still, lattice-based simulations of the spatial stochastic RPS model are known to give rise to significantly different results, depending on whether the three state Lotka-Volterra or the four state May-Leonard formulation is employed. This is true independently of the values of the model parameters and of the use of either a von Neumann or a Moore neighborhood. With the objective of reducing the impact of the use of a discrete lattice, in this paper we introduce a simple modification to the standard spatial stochastic RPS model in which the range of the search of the nearest neighbor may be extended up to a maximum euclidean radius $R$. We show that, with this adjustment, the Lotka-Volterra and May-Leonard formulations can be designed to produce similar results, both in terms of dynamical properties and spatial features, by means of an appropriate parameter choice. In particular, we show that this modified spatial stochastic RPS model naturally leads to the emergence of spiral patterns in both its three and four state formulations.

q-bio.PE↗

arXiv2021

On the relation between active-region lifetimes and the autocorrelation function of light curves

Rotational modulation of stellar light curves due to dark spots encloses information on spot properties and, thus, on magnetic activity. In particular, the decay of the autocorrelation function (ACF) of light curves is presumed to be linked to spot/active-region lifetimes, given that some coherence of the signal is expected throughout their lifetime. In the literature, an exponential decay has been adopted to describe the ACF. Here, we investigate the relation between the ACF and the active-region lifetimes. For this purpose, we produce artificial light curves of rotating spotted stars with different observation, stellar, and spot properties. We find that a linear decay and respective timescale better represent the ACF than the exponential decay. We therefore adopt a linear decay. The spot/active-region timescale inferred from the ACF is strongly restricted by the observation length of the light curves. For 1-year light curves our results are consistent with no correlation between the inferred and the input timescales. The ACF decay is also significantly affected by differential rotation and spot evolution: strong differential rotation and fast spot evolution contribute to a more severe underestimation of the active-region lifetimes. Nevertheless, in both circumstances the observed timescale is still correlated with the input lifetimes. Therefore, our analysis suggests that the ACF decay can be used to obtain a lower limit of the active-region lifetimes for relatively long-term observations. However, strategies to avoid or flag targets with fast active-region evolution or displaying stable beating patterns associated with differential rotation should be employed.

astro-ph.SR↗