arXiv subjects

P. P. Avelino

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

At least 127 records · Page 7Linked to original sources

arXiv2008

On the cosmological evolution of alpha and mu and the dynamics of dark energy

We study the cosmological evolution of the fine structure constant, $α$, and the proton-to-electron mass ratio, $μ=m_p/m_e$, in the context of a generic class of models where the gauge kinetic function is a linear function of a quintessence-type real scalar field, $ϕ$, described by a Lagrangian with a standard kinetic term and a scalar field potential, $V(ϕ)$. We further assume that the scalar field potential is a monotonic function of $ϕ$ and that the scalar field is always rolling down the potential. We show that, for this class of models, low-redshift constrains on the evolution of $α$ and $μ$ can provide very stringent limits on the corresponding variations at high-redshift. We also demonstrate that these limits may be relaxed by considering more general models for the dynamics of $α$ and $μ$. However, in this case, the ability to reconstruct the evolution of the dark energy equation of state using varying couplings could be seriously compromised.

astro-ph↗

arXiv2008

Linear and non-Linear Instabilities in Unified Dark Energy Models

We revisit the paradigm of unified dark energy discussing in detail the averaging problem in this type of scenarios, highlighting the need for a full non-linear treatment. We also address the question of if and how models with one or several dark fluids can be observationally distinguished. Simpler and physically clearer derivations of some key results, most notably on the relation between the generalized Chaplygin gas and the standard ($Λ$CDM) `concordance' model and on a Jeans-type small-scale instability of some coupled dark energy/dark matter models are presented.

astro-ph↗

arXiv2008

Clustering Properties of Dynamical Dark Energy Models

We provide a generic but physically clear discussion of the clustering properties of dark energy models. We explicitly show that in quintessence-type models the dark energy fluctuations, on scales smaller than the Hubble radius, are of the order of the perturbations to the Newtonian gravitational potential, hence necessarily small on cosmological scales. Moreover, comparable fluctuations are associated with different gauge choices. We also demonstrate that the often used homogeneous approximation is unrealistic, and that the so-called dark energy mutation is a trivial artifact of an effective, single fluid description. Finally, we discuss the particular case where the dark energy fluid is coupled to dark matter.

astro-ph↗

arXiv2008

Dynamics of perfect fluid Unified Dark Energy models

In this paper we show that a \emph{one-to-one} correspondence exists between any dark energy model and an equivalent (from a cosmological point of view, in the absence of perturbations) quartessence model in which dark matter and dark energy are described by a single perfect fluid. We further show that if the density fluctuations are small, the evolution of the sound speed squared, $c_s^2$, is fully coupled to the evolution of the scale factor and that the transition from the dark matter to the dark energy dominated epoch is faster (slower) than in a standard $Λ$CDM model if $c_s^2 > 0$ ($c_s^2 < 0$). In particular, we show that the mapping of the simplest quintessence scenario with constant $w_Q \equiv p_Q/ ρ_Q$ into a unified dark energy model requires $c_s^2 < 0$ at the present time (if $w_Q > -1$) contrasting to the Chaplygin gas scenario where one has $c_s^2 > 0$. However, we show that non-linear effects severely complicate the analysis, in particular rendering linear results invalid even on large cosmological scales. Although a detailed analysis of non-linear effects requires solving the full Einstein field equations, some general properties can be understood in simple terms. In particular, we find that in the context of Chaplygin gas models the transition from the dark matter to the dark energy dominated era may be anticipated with respect to linear expectations leading to a background evolution similar to that of standard $Λ$CDM models. On the other hand, in models with $c_s^2 > 0$ the expected transition from the decelerating to the accelerating phase may never happen.

astro-ph↗

arXiv2007

Effects of Inflation on a Cosmic String Loop Population

We study the evolution of simple cosmic string loop solutions in an inflationary universe. We show, for the particular case of circular loops, that periodic solutions do exist in a de Sitter universe, below a critical loop radius $R_c H=1/2$. On the other hand, larger loops freeze in comoving coordinates, and we explicitly show that they can survive more $e$-foldings of inflation than point-like objects. We discuss the implications of these findings for the survival of realistic cosmic string loops during inflation, and for the general characteristics of post-inflationary cosmic string networks. We also consider the analogous solutions for domain walls, in which case the critical radius is $R_c H=2/3$.

hep-ph↗

arXiv2006

Scaling of cosmological domain wall networks with junctions

We describe the results of the largest and most accurate three-dimensional field theory simulations of domain wall networks with junctions. We consider a previously introduced class of models which, in the limit of large number $N$ of coupled scalar fields, approaches the so-called `ideal' model (in terms of its potential to lead to network frustration). We consider values of $N$ between N=2 and N=20. In all cases we find compelling evidence for a gradual approach to scaling, with the quantitative scaling parameters having only a mild dependence on $N$. These results strongly support our no-frustration conjecture.

astro-ph↗

arXiv2006

Reconstructing the dark energy equation of state with varying couplings

We revisit the idea of using varying couplings to probe the nature of dark energy, in particular by reconstructing its equation of state. We show that this method can be far superior to the standard methods (using type Ia supernovae or weak lensing). We also show that the simultaneous use of measurements of the fine-structure constant $α$ and the electron-to-proton mass ratio $μ$ allows a direct probe of grand unification scenarios. We present forecasts for the sensitivity of this method, both for the near future and for the next generation of spectrographs -- for the latter we focus on the planned CODEX instrument for ESO's Extremely Large Telescope (formerly known as OWL). A high-accuracy reconstruction of the equation of state may be possible all the way up to redshift $z\sim4$.

astro-ph↗

arXiv2006

Defect Junctions and Domain Wall Dynamics

We study a number of domain wall forming models where various types of defect junctions can exist. These illustrate some of the mechanisms that will determine the evolution of defect networks with junctions. Understanding these mechanisms is vital for a proper assessment of a number of cosmological scenarios: we will focus on the issue of whether or not cosmological frustrated domain wall networks can exist at all, but our results are also relevant for the dynamics of cosmic (super)strings, where junctions are expected to be ubiquitous. We also define and discuss the properties that would make up the ideal model in terms of hypothetical frustrated wall networks, and provide an explicit construction for such a model. We carry out a number of numerical simulations of the evolution of these networks, analyze and contrast their results, and discuss their implications for our no-frustration conjecture.

hep-ph↗

arXiv2006

Frustrated Expectations: Defect Networks and Dark Energy

We discuss necessary conditions for a network of cosmic domain walls to have a chance of providing the dark energy that might explain the recent acceleration of the universe. We derive a strong bound on the curvature of the walls, which shows that viable candidate networks must be fine-tuned and non-standard. We also discuss various requirements that any stable lattice of frustrated walls must obey. We conjecture that, even though one can build (by hand) lattices that would be stable, no such lattices will ever come out of realistic domain wall forming cosmological phase transitions. We provide some simple numerical simulations that illustrate our results and correct some misconceptions in the published literature, but a detailed numerical analysis is left for a companion paper.

astro-ph↗

arXiv2006

Gravitational Effects of Varying Alpha Strings

We study spatial variations of the fine-structure constant in the presence of static straight cosmic strings in the weak-field approximation in Einstein gravity. We work in the context of a generic Bekenstein-type model and consider a gauge kinetic function linear in the scalar field. We determine an analytical form for the scalar field and the string metric at large distances from the core. We show that the gravitational effects of $α$-varying strings can be seen as a combination of the gravitational effects of global and local strings. We also verify that at large distances to the core the space-time metric is similar to that of a global string. We study the motion of test particles approaching from infinity and show that photons are scattered to infinity while massive particles are trapped in bounded trajectories. We also calculate an overall limit on the magnitude of the variation of $α$ for a GUT string, by considering suitable cosmological constraints coming from the Equivalence Principle.

gr-qc↗

arXiv2005

Evolution of the fine-structure constant in the non-linear regime

We study the evolution of the fine-structure constant, $α$, induced by non-linear density perturbations in the context of the simplest class of quintessence models with a non-minimal coupling to the electromagnetic field, in which the two available free functions (potential and gauge kinetic function) are Taylor-expanded up to linear order. We show that the results obtained using the spherical infall model for an infinite wavelength inhomogeneity are inconsistent with the results of a local linearized gravity study and we argue in favour of the second approach. We also discuss recent claims that the value of $α$ inside virialised regions could be significantly different from the background one on the basis of these findings.

astro-ph↗

arXiv2005

Primordial black hole constraints on non-gaussian inflation models

We determine the abundance of primordial black holes (PBHs) formed in the context of non-gaussian models with primordial density perturbations. We consider models with a renormalized $χ^2$ probability distribution function parametrized by the number, $ν$, of degrees of freedom. We show that if $ν$ is not too large then the PBH abundance will be altered by several orders of magnitude with respect to the standard gaussian result obtained in the $ν\to \infty$ limit. We also study the dependence of the spectral index constraints on the nature of the cosmological perturbations for a power-law primordial power spectrum.

astro-ph↗

arXiv2005

Varying Alpha Monopoles

We study static magnetic monopoles in the context of varying alpha theories and show that there is a group of models for which the t'Hooft-Polyakov solution is still valid. Nevertheless, in general static magnetic monopole solutions in varying alpha theories depart from the classical t'Hooft-Polyakov solution with the electromagnetic energy concentrated inside the core seeding spatial variations of the fine structure constant. We show that Equivalence Principle constraints impose tight limits on the allowed variations of alpha induced by magnetic monopoles which confirms the difficulty to generate significant large-scale spatial variation of the fine structure constant found in previous works. This is true even in the most favorable case where magnetic monopoles are the source for these variations.

hep-ph↗

arXiv2005

One-scale Model for Domain Wall Network Evolution

We introduce a new phenomenological one-scale model for the evolution of domain wall networks, and test it against high-resolution field theory numerical simulations. We argue that previous numerical estimates of wall velocities are inaccurate, and suggest a more accurate method of measurement. We show that the model provides an adequate approximation to the evolution of key parameters characterizing the evolution of the network. We use the model to study possible scaling solutions for domain wall networks, and discuss some of their cosmological consequences.

hep-ph↗

arXiv2005

Understanding Domain Wall Network Evolution

We study the cosmological evolution of domain wall networks in two and three spatial dimensions in the radiation and matter eras using a large number of high-resolution field theory simulations with a large dynamical range. We investigate the dependence of the uncertainty in key parameters characterising the evolution of the network on the size, dynamical range and number of spatial dimensions of the simulations and show that the analytic prediction compares well with the simulation results. We find that there is ample evidence from the simulations of a slow approach of domain wall networks towards a linear scaling solution. However, while at early times the uncertainty in the value of the scaling exponent is small enough for deviations from the scaling solution to be measured, at late times the error bars are much larger and no strong deviations from the scaling solution are found.

hep-th↗

arXiv2005

Cosmic strings in Bekenstein-type models

We study static cosmic string solutions in the context of Bekenstein-type models. We show that there is a class of models of this type for which the classical Nielsen-Olesen vortex is still a valid solution. However, in general static string solutions in Bekenstein-type models strongly depart from the standard Nielsen-Olesen solution with the electromagnetic energy concentrated along the string core seeding spatial variations of the fine structure constant, $α$. We consider models with a generic gauge kinetic function and show that equivalence principle constraints impose tight limits on the allowed variations of $α$ induced by string networks on cosmological scales.

astro-ph↗

arXiv2004

The coincidence problem in linear dark energy models

We show that a solution to the the coincidence problem can be found in the context of a generic class of dark energy models with a scalar field, $ϕ$, with a linear effective potential $V(ϕ)$. We determine the fraction, $f$, of the total lifetime of the universe, $t_U$, which lies within the interval $[t_0-Δt_A,t_0+ Δt_A]$, where $t_0$ is the age of the universe at the present time, $Δt_A \equiv t_0-t_A$ and $t_A$ is the age of the universe when it starts to accelerate. We find that if we require $f$ to be larger than 0.1 (0.01) then $1+ω_{\phi0} \gapp 2 \times 10^{-2}$ ($1 \times 10^{-3}$), where $ω_ϕ \equiv p_ϕ/ρ_ϕ$. These results depend mainly on the linearity of the scalar field potential for $-V(ϕ_0) \lapp V(ϕ) \lapp V(ϕ_0)$ and are weakly dependent on the specific form of $V(ϕ)$ outside this range. We also show that if $ω_{\phi0}$ is close to -1 then $ω_{\phi0}+1 \sim 1.6 ({\tilde ω}_ϕ+1)$, where ${\tilde ω}_ϕ$ is the weighted average value of $ω_ϕ$ in the time interval $[0,t_0]$. We independently confirm current observational constraints on this class of models which give $ω_{\phi0} \lapp -0.6$ and $t_U \gapp 2.4 t_0$ at the $2 σ$ level.

astro-ph↗

arXiv2004

The Cosmological Evolution of Domain Wall Networks

We have studied the cosmological evolution of domain wall networks in two, three and four spatial dimensions using high-resolution field theory simulations. The dynamical range and number of our simulations is larger than in previous works, but does not allow us to exclude previous hints of deviations to the naively expected scale-invariant evolution. These results therefore suggest that the approach of domain wall networks to linear scaling is a much slower process than that of cosmic strings, which has been previously characterized in detail.

hep-ph↗