arXiv subjects

C. J. A. P. Martins

Publications and source records attributed to C. J. A. P. Martins.

At least 253 records · Page 14Linked to original sources

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↗

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

Fractal Properties and Small-scale Structure of Cosmic String Networks

We present results from a detailed numerical study of the small-scale and loop production properties of cosmic string networks, based on the largest and highest resolution string simulations to date. We investigate the non-trivial fractal properties of cosmic strings, in particular, the fractal dimension and renormalised string mass per unit length, and we also study velocity correlations. We demonstrate important differences between string networks in flat (Minkowski) spacetime and the two very similar expanding cases. For high resolution matter era network simulations, we provide strong evidence that small-scale structure has converged to `scaling' on all dynamical lengthscales, without the need for other radiative damping mechanisms. We also discuss preliminary evidence that the dominant loop production size is also approaching scaling.

astro-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↗

arXiv2004

Scaling Laws for Non-Intercommuting Cosmic String Networks

We study the evolution of non-interacting and entangled cosmic string networks in the context of the velocity-dependent one-scale model. Such networks may be formed in several contexts, including brane inflation. We show that the frozen network solution $L\propto a$, although generic, is only a transient one, and that the asymptotic solution is still $L\propto t$ as in the case of ordinary (intercommuting) strings, although in the present context the universe will usually be string-dominated. Thus the behaviour of two strings when they cross does not seem to affect their scaling laws, but only their densities relative to the background.

hep-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↗

arXiv2004

New Constraints on Varying Alpha

I briefly present some current theoretical motivations for time or space variations of the fundamental constants of nature and review current key observational results. I focus on the fine-structure constant, and particularly on measurements using quasars and the cosmic microwave background. I also compare various observational results to the simplest model-building expectations.

astro-ph↗

arXiv2004

Unified model for vortex-string network evolution

We describe and numerically test the velocity-dependent one-scale (VOS) string evolution model, a simple analytic approach describing a string network with the averaged correlation length and velocity. We show that it accurately reproduces the large-scale behaviour (in particular the scaling laws) of numerical simulations of both Goto-Nambu and field theory string networks. We explicitly demonstrate the relation between the high-energy physics approach and the damped and non-relativistic limits which are relevant for condensed matter physics. We also reproduce experimental results in this context and show that the vortex-string density is significantly reduced by loop production, an effect not included in the usual `coarse-grained' approach.

hep-ph↗

arXiv2004

Linearized Bekenstein Varying Alpha Models

We study the simplest class of Bekenstein-type, varying $α$ models, in which the two available free functions (potential and gauge kinetic function) are Taylor-expanded up to linear order. Any realistic model of this type reduces to a model in this class for a certain time interval around the present day. Nevertheless, we show that no such model is consistent with all existing observational results. We discuss possible implications of these findings, and in particular clarify the ambiguous statement (often found in the literature) that ``the Webb results are inconsistent with Oklo''.

astro-ph↗

arXiv2003

WMAP Constraints on varying $α$ and the Promise of Reionization

We present up-to-date constraints on the value of the fine-structure constant at the epoch of decoupling from the recent observations made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. In the framework of models we considered, a positive (negative) variation of the value of $α$ at decoupling with respect to the present-day value is now bounded to be smaller than 2% (6%) at 95% confidence level. We point out that the existence of an early reionization epoch as suggested by the above measurements will, when more accurate cosmic microwave background polarization data is available, lead to considerably tighter constraints. The so-called `reionization bump', in particular, will be extremely useful for this purpose. We find that constraints on $α$ can in principle only be as small as $0.1 %$ using CMB data alone--tighter constraints will require further (non-CMB) priors.

astro-ph↗

arXiv2003

Measuring $α$ in the Early Universe: CMB Polarization, Reionization and the Fisher Matrix Analysis

We present a detailed analysis of present and future Cosmic Microwave Background constraints of the value of the fine-structure constant, $α$. We carry out a more detailed analysis of the WMAP first-year data, deriving state-of-the-art constraints on $α$ and discussing various other issues, such as the possible hints for the running of the spectral index. We find, at 95% C.L. that $0.95 < α_{\text{dec}} / α_0 < 1.02$. Setting $dn_S /dlnk=0$, yields $0.94< α_{\text{dec}} / α_0 < 1.01$ as previously reported. We find that a lower value of $α/ α_0$ makes a value of $d n_S /dlnk = 0$ more compatible with the data. We also perform a thorough Fisher Matrix Analysis (including both temperature and polarization, as well as $α$ and the optical depth $τ$), in order to estimate how future CMB experiments will be able to constrain $α$ and other cosmological parameters. We find that Planck data alone can constrain $τ$ with a accuracy of the order 4% and that this constraint can be as small as 1.7% for an ideal cosmic variance limited experiment. Constraints on $α$ are of the order 0.3% for Planck and can in principle be as small as 0.1% using CMB data alone - tighter constraints will require further (non-CMB) priors.

astro-ph↗

arXiv2003

New constraints on varying $α$

We present a summary of recent constraints on the value of the fine-structure constant at the epoch of decoupling from the recent observations made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. Within the set of models considered, a variation of the value of $α$ at decoupling with respect to the present-day value is now bounded to be smaller than 2% (6%) at 95% confidence level. We point out that the existence of an early reionization epoch as suggested by the above measurements will, when more accurate cosmic microwave background polarization data is available, lead to considerably tighter constraints. We find that the tightest possible constraint on $α$ is about $0.1 %$ using CMB data alone--tighter constraints will require further (non-CMB) priors.

astro-ph↗

arXiv2003

The $Λ{\rm CDM}$ Limit of the Generalized Chaplygin Gas Scenario

We explicitly demonstrate that, contrary to recent claims, the dynamics of a generalized Chaplygin gas model with an equation of state $p=-C$ (where $C$ is a positive constant) is equivalent to that of a standard $Λ$CDM model to first order in the metric perturbations. We further argue that the analogy between the two models goes well beyond linear theory and conclude that they cannot be distinguished based on gravity alone.

astro-ph↗

arXiv2003

Topological defects: A problem for cyclic universes?

We study the behaviour of cosmic string networks in contracting universes, and discuss some of their possible consequences. We note that there is a fundamental time asymmetry between defect network evolution for an expanding universe and a contracting universe. A string network with negligible loop production and small-scale structure will asymptotically behave during the collapse phase as a radiation fluid. In realistic networks these two effects are important, making this solution only approximate. We derive new scaling solutions describing this effect, and test them against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation background it has generated, can significantly affect the dynamics of the universe both locally and globally. The network can be an important source of radiation, entropy and inhomogeneity. We discuss the possible implications of these findings for bouncing and cyclic cosmological models.

astro-ph↗

arXiv2003

The onset of the non-linear regime in unified dark matter models

We discuss the onset of the non-linear regime in the context of unified dark matter models involving a generalised Chaplygin gas. We show that the transition from dark matter-like to dark energy-like behaviour will never be smooth. In some regions of space the transition will never take place while in others it may happen sooner or later than naively expected. As a result the linear theory used in previous studies may break down late in the matter dominated era even on large cosmological scales. We study the importance of this effect showing that its magnitude depends on the exact form of the equation of state in the low density regime. We expect that our results will be relevant for other unified dark matter scenarios particularly those where the quartessence candidate is a perfect fluid.

astro-ph↗

arXiv2003

String Imprints from a Pre-inflationary Era

We derive the equations governing the dynamics of cosmic strings in a flat anisotropic universe of Bianchi type I and study the evolution of simple cosmic string loop solutions. We show that the anisotropy of the background can have a characteristic effect in the loop motion. We discuss some cosmological consequences of these findings and, by extrapolating our results to cosmic string networks, we comment on their ability to survive an inflationary epoch, and hence be a possible fossil remnant (still visible today) of an anisotropic phase in the very early universe.

astro-ph↗