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P. P. Avelino

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

At least 37 records · Page 2Linked to original sources
arXiv2021

Impact of parity in rock-paper-scissors type models

We investigate the impact of parity on the abundance of weak species in the context of the simplest generalization of the rock-paper-scissors model to an arbitrary number of species -- we consider models with a total number of species ($N_S$) between 3 and 12, having one or more (weak) species characterized by a reduced predation probability (by a factor of ${\mathcal P}_w$ with respect to the other species). We show, using lattice based spatial stochastic simulations with random initial conditions, large enough for coexistence to prevail, that parity effects are significant. We find that the performance of weak species is dependent on whether the total number of species is even or odd, especially for $N_S \le 8$, with odd numbers of species being on average more favourable to weak species than even ones. We further show that, despite the significant dispersion observed among individual models, a weak species has on average a higher abundance than a strong one if ${\mathcal P}_w$ is sufficiently smaller than unity -- the notable exception being the four species case.

q-bio.PE↗
arXiv2021

Weak species in rock-paper-scissors models

In this letter, we investigate the population dynamics in a May-Leonard formulation of the rock-paper-scissors game in which one or two species, which we shall refer to as "weak", have a reduced predation or reproduction probability. We show that in a nonspatial model the stationary solution where all three species coexist is always unstable, while in a spatial stochastic model coexistence is possible for a wide parameter space. We find, that a reduced predation probability results in a significantly higher abundance of "weak" species, in models with either one or two "weak" species, as long as the simulation lattices are sufficiently large for coexistence to prevail. On the other hand, we show that a reduced reproduction probability has a smaller impact on the abundance of "weak" species, generally leading to a slight decrease of its population size -- the increase of the population size of one of the "weak" species being more than compensated by the reduction of the other, in the two species case. We further show that the species abundances in models where both predation and reproduction probabilities are simultaneously reduced may be accurately estimated from the results obtained considering only a reduction of either the predation or the reproduction probability.

q-bio.PE↗
arXiv2020

Lagrangian description of cosmic fluids: mapping dark energy into unified dark energy

We investigate the appropriateness of the use of different Lagrangians to describe various components of the cosmic energy budget, discussing the degeneracies between them in the absence of nonminimal couplings to gravity or other fields, and clarifying some misconceptions in the literature. We further demonstrate that these degeneracies are generally broken for nonminimal coupled fluids, in which case the identification of the appropriate on-shell Lagrangian may become essential in order characterize the overall dynamics. We then show that models with the same on-shell Lagrangian may have different proper energy densities and use this result to map dark energy models into unified dark energy models in which dark matter and dark energy are described by the same perfect fluid. We determine the correspondence between their equation of state parameters and sound speeds, briefly discussing the linear sound speed problem of unified dark energy models as well as a possible way out associated to the nonlinear dynamics.

astro-ph.CO↗
arXiv2020

Domain walls and other defects in Eddington-inspired Born-Infeld gravity

We investigate domain wall and other defect solutions in the weak-field limit of Eddington-inspired Born-Infeld gravity as a function of $κ$, the only additional parameter of the theory with respect to General Relativity. We determine, both analytically and numerically, the internal structure of domain walls, quantifying its dependency on $κ$ as well as the impact of such dependency on the value of the tension measured by an outside observer. We find that the pressure in the direction perpendicular to the domain wall can be, in contrast to the weak-field limit of General Relativity, significantly greater or smaller than zero, depending, respectively, on whether $κ$ is positive or negative. We further show that the generalized von Laue condition, which states that the average value of the perpendicular pressure is approximately equal to zero in the weak-field limit of General Relativity, does not generally hold in EiBI gravity not only for domain walls, but also in the case cosmic strings and spherically symmetric particles. We argue that a violation of the generalized von Laue condition should in general be expected in any theory of gravity whenever geometry plays a significant role in determining the defect structure.

gr-qc↗
arXiv2020

On using dipolar modes to constrain the helium glitch in red-giant stars

The space-borne missions CoRoT and Kepler have revealed numerous mixed modes in red-giant stars. These modes carry a wealth of information about red-giant cores, but are of limited use when constraining rapid structural variations in their envelopes. This limitation can be circumvented if we have access to the frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole modes that would exist in the absence of coupling between gravity and acoustic waves. We present a pilot study aimed at evaluating the implications of using these pure acoustic mode frequencies in seismic studies of the helium structural variation in red giants. The study is based on artificial seismic data for a red-giant-branch stellar model, bracketing seven acoustic dipole radial orders around vmax. The pure acoustic dipole-mode frequencies are derived from a fit to the mixed-mode period spacings and then used to compute the pure acoustic dipole-mode second differences. The pure acoustic dipole-mode second differences inferred through this procedure follow the same oscillatory function as the radial modes second differences. The additional constraints brought by the dipolar modes allow us to adopt a more complete description of the glitch signature when performing the fit to the second differences. The amplitude of the glitch retrieved from this fit is 15% smaller than that from the fit based on the radial modes alone. Also, we find that thanks to the additional constraints, a bias in the inferred glitch location, found when adopting the simpler description of the glitch, is avoided.

astro-ph.SR↗
arXiv2020

Rock-paper-scissors models with a preferred mobility direction

We investigate a modified spatial stochastic Lotka-Volterra formulation of the rock-paper-scissors model using off-lattice stochastic simulations. In this model one of the species moves preferentially in a specific direction -- the level of preference being controlled by a noise strength parameter $η\in [0, 1]$ ($η= 0$ and $η= 1$ corresponding to total preference and no preference, respectively) -- while the other two species have no referred direction of motion. We study the behaviour of the system starting from random initial conditions, showing that the species with asymmetric mobility has always an advantage over its predator. We also determine the optimal value of the noise strength parameter which gives the maximum advantage to that species. Finally, we find that the critical number of individuals, below which the probability of extinction becomes significant, decreases as the noise level increases, thus showing that the addition of a preferred mobility direction studied in the present paper does not favour coexistence.

q-bio.PE↗
arXiv2020

Boltzmann's $H$-theorem, entropy and the strength of gravity in theories with a nonminimal coupling between matter and geometry

In this paper we demonstrate that Boltzmann's $H$-theorem does not necessarily hold in the context of theories of gravity with nonminimally coupled matter fields. We find sufficient conditions for the violation of Boltzmann's $H$-theorem and derive an expression for the evolution of Boltzmann's $H$ in terms of the nonminimal coupling function, valid in the case of a collisionless gas in a homogeneous and isotropic Friedmann-Lemaître-Robertson-Walker universe. We highlight the implications of this result for the evolution of the entropy of the matter fields, briefly discussing the role played by collisions between particles whenever they are relevant. We also suggest a possible link between the high entropy of the Universe and the weakness of gravity in the context of these theories.

gr-qc↗
arXiv2020

Comparing parametric and non-parametric velocity-dependent one-scale models for domain wall evolution

We perform a detailed comparison between a recently proposed parameter-free velocity-dependent one-scale model and the standard parametric model for the cosmological evolution of domain wall networks. We find that the latter overestimates the damping of the wall motion due to the Hubble expansion and neglects the direct impact of wall decay on the evolution of the root-mean-square velocity of the network. We show that these effects are significant but may be absorbed into a redefinition of the momentum parameter. We also discuss the implications of these findings for cosmic strings. We compute the energy loss and momentum parameters of the standard parametric model for cosmological domain wall evolution using our non-parametric velocity-dependent one-scale model in the context of cosmological models having a power law evolution of the scale factor $a$ with the cosmic time $t$ ($a \propto t^λ$, $0 < λ< 1$), and compare with the results obtained from numerical field theory simulations. We further provide simple linear functions which roughly approximate the dependence of the energy loss and momentum parameters on $λ$.

astro-ph.CO↗
arXiv2020

Performance of weak species in the simplest generalization of the rock-paper-scissors model to four species

We investigate the problem of the predominance and survival of "weak" species in the context of the simplest generalization of the spatial stochastic rock-paper-scissors model to four species by considering models in which one, two, or three species have a reduced predation probability. We show, using lattice based spatial stochastic simulations with random initial conditions, that if only one of the four species has its probability reduced then the most abundant species is the prey of the "weakest" (assuming that the simulations are large enough for coexistence to prevail). Also, among the remaining cases, we present examples in which "weak" and "strong" species have similar average abundances and others in which either of them dominates -- the most abundant species being always a prey of a "weak" species with which it maintains a unidirectional predator-prey interaction. However, in contrast to the three-species model, we find no systematic difference in the global performance of "weak" and "strong" species, and we conjecture that the same result will hold if the number of species is further increased. We also determine the probability of single species survival and coexistence as a function of the lattice size, discussing its dependence on initial conditions and on the change to the dynamics of the model which results from the extinction of one of the species.

q-bio.PE↗
arXiv2020

Second law of thermodynamics in nonminimally coupled gravity

In the present work we show that the second law of thermodynamics does not generally hold if the matter and gravitational fields are nonminimally coupled. We demonstrate this result by explicitly computing the evolution of the entropy of the matter fields in the case of a closed homogeneous and isotropic universe filled with dust and radiation, showing that, in this case, the sign of the entropy variation is determined by the evolution of the universe. The preservation of the second law of thermodynamics in these modified theories would require its generalization to account for a gravitational entropy contribution.

gr-qc↗
arXiv2020

Modelling Stochastic Signatures in Classical Pulsators

We consider the impact of stochastic perturbations on otherwise coherent oscillations of classical pulsators. The resulting dynamics are modelled by a driven damped harmonic oscillator subject to either an external or an internal forcing and white noise velocity fluctuations. We characterize the phase and relative amplitude variations using analytical and numerical tools. When the forcing is internal the phase variation displays a random walk behaviour and a red noise power spectrum with a ragged erratic appearance. We determine the dependence of the root mean square phase and relative amplitude variations ($σ_{Δφ}$ and $σ_{ΔA/A}$, respectively) on the amplitude of the stochastic perturbations, the damping constant $η$, and the total observation time $t_{\rm obs}$ for this case, under the assumption that the relative amplitude variations remain small, showing that $σ_{Δφ}$ increases with $t_{\rm obs}^{1/2}$ becoming much larger than $σ_{ΔA/A}$ for $t_{\rm obs} \gg η^{-1}$. In the case of an external forcing the phase and relative amplitude variations remain of the same order, independent of the observing time. In the case of an internal forcing, we find that $σ_{Δφ}$ does not depend on $η$. Hence, the damping time cannot be inferred from fitting the power of the signal, as done for solar-like pulsators, but the amplitude of the stochastic perturbations may be constrained from the observations. Our results imply that, given sufficient time, the variation of the phase associated to the stochastic perturbations in internally driven classical pulsators will become sufficiently large to be probed observationally.

astro-ph.SR↗
arXiv2019

Parameter-free velocity-dependent one-scale model for domain walls

We develop a parameter-free velocity-dependent one-scale model for the evolution of the characteristic length $L$ and root-mean-square velocity $σ_v$ of standard domain wall networks in homogeneous and isotropic cosmologies. We compare the frictionless scaling solutions predicted by our model, in the context of cosmological models having a power law evolution of the scale factor $a$ as a function of the cosmic time $t$ ($a \propto t^λ$, $0< λ< 1$), with the corresponding results obtained using field theory numerical simulations. We show that they agree well (within a few $\%$) for root-mean-square velocities $σ_v$ smaller than $0.2 \, c$ ($λ\ge 0.9$), where $c$ is the speed of light in vacuum, but significant discrepancies occur for larger values of $σ_v$ (smaller values of $λ$). We identify problems with the determination of $L$ and $σ_v$ from numerical field theory simulations which might potentially be responsible for these discrepancies.

astro-ph.CO↗
arXiv2019

Analytical modelling of period spacings across the HR diagram

The characterisation of stellar cores may be accomplished through the modelling of asteroseismic data from stars exhibiting either gravity-mode or mixed-mode pulsations, potentially shedding light on the physical processes responsible for the production, mixing, and segregation of chemical elements. In this work we validate against model data an analytical expression for the period spacing that will facilitate the inference of the properties of stellar cores, including the detection and characterisation of buoyancy glitches (strong chemical gradients). This asymptotically-based analytical expression is tested both in models with and without buoyancy glitches. It does not assume that glitches are small and, consequently, predicts non-sinusoidal glitch-induced period-spacing variations, as often seen in model and real data. We show that the glitch position and width inferred from the fitting of the analytical expression to model data consisting of pure gravity modes are in close agreement (typically better than 7$\%$ relative difference) with the properties measured directly from the stellar models. In the case of fitting mixed-mode model data, the same expression is shown to reproduce well the numerical results, when the glitch properties are known a priori. In addition, the fits performed to mixed-mode model data reveal a frequency dependence of the coupling coefficient, $q$, for a moderate-luminosity red-giant-branch model star. Finally, we find that fitting the analytical expression to the mixed-mode period spacings may provide a way to infer the frequencies of the pure acoustic dipole modes that would exist if no coupling took place between acoustic and gravity waves.

astro-ph.SR↗
arXiv2019

Probing gravity at sub-femtometer scales through the pressure distribution inside the proton

Recently, a measurement of the pressure distribution experienced by the quarks inside the proton has found a strong repulsive (positive) pressure at distances up to 0.6 femtometers from its center and a (negative) confining pressure at larger distances. In this paper we show that this measurement puts significant constraints on modified theories of gravity in which the strength of the gravitational interaction on microscopic scales is enhanced with respect to general relativity. We consider the particular case of Eddington-inspired Born-Infeld gravity, showing that strong limits on $κ$, the only additional parameter of the theory with respect to general relativity, may be derived from the quark pressure measurement ($|κ| \lsim 10^{-1} \, {\rm m^5 \, kg^{-1} \, s^{-2}}$). Furthermore, we show how these limits may be significantly improved with precise measurements of the first and second moments of the pressure distribution inside the proton.

gr-qc↗
arXiv2019

Expanding spatial domains and transient scaling regimes in populations with local cyclic competition

We investigate a six-species class of May-Leonard models leading to formation two types of competing spatial domains, each one inhabited by three-species with their own internal cyclic rock-paper-scissors dynamics. We study the resulting population dynamics using stochastic numerical simulations in two-dimensional space. We find that as three-species domains shrink, there is an increasing probability of extinction of two of the species inhabiting the domain, with the consequent creation of one-species domains. We determine the critical initial radius beyond which these one-species spatial domains are expected to expand. We further show that a transient scaling regime, with a slower average growth rate of the characteristic length scale $L$ of the spatial domains with time $t$, takes place before the transition to a standard $L \propto t^{1/2}$ scaling law, resulting in an extended period of coexistence.

nlin.AO↗
arXiv2019

Predominance of the weakest species in Lotka-Volterra and May-Leonard implementations of the rock-paper-scissors model

We revisit the problem of the predominance of the 'weakest' species in the context of Lotka-Volterra and May-Leonard implementations of a spatial stochastic rock-paper-scissors model in which one of the species has its predation probability reduced by $0 < \mathcal{P}_w < 1$. We show that,despite the different population dynamics and spatial patterns, these two implementations lead to qualitatively similar results for the late time values of the relative abundances of the three species (as a function of $\mathcal{P}_w$), as long as the simulation lattices are sufficiently large for coexistence to prevail --- the 'weakest' species generally having an advantage over the others (specially over its predator). However, for smaller simulation lattices, we find that the relatively large oscillations at the initial stages of simulations with random initial conditions may result in a significant dependence of the probability of species survival on the lattice size and total simulation time.

q-bio.PE↗
arXiv2019

Death by starvation in May-Leonard models

We consider the dynamics of spatial stochastic May-Leonard models with mutual predation interactions of equal strength between any two individuals of different species. Using two-dimensional simulations, with two and three pecies, we investigate the dynamical impact of the death of individuals after a given threshold number of successive unsuccessful predation attempts. We find that the death of these individuals can have a strong impact on the dynamics of population networks and provide a crucial contribution to the preservation of coexistence.

q-bio.PE↗