arXiv subjects

C. J. A. P. Martins

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

287 records · Page 16Linked to original sources

arXiv2000

Topological defects: fossils of an anisotropic era?

We consider the evolution of domain walls produced during an anisotropic phase in the very early universe, showing that the resulting network can be very anisotropic. If the domain walls are produced during an inflationary era, the network will soon freeze out in comoving coordinates retaining the imprints of the anisotropic regime, even though inflation makes the universe isotropic. Only at late times, when the typical size of the major axis of the domain walls becomes smaller than the Hubble radius, does the network evolve rapidly towards isotropy. Hence, we may hope to see imprints of the anisotropic era if by today the typical size of the major axis of the domain walls is of the order of the Hubble radius, or if the walls re-entered it only very recently. Depending on the mass scale of the domain walls, there is also the possibility that they re-enter at earlier times, but their evolution remains friction-dominated until recently, in which case the signatures of the anisotropic era will be much better preserved. These effects are expected to occur in generic topological defect models.

astro-ph↗

arXiv2000

Looking for a varying $α$ in the Cosmic Microwave Background

We perform a likelihood analysis of the recently released BOOMERanG and MAXIMA data, allowing for the possibility of a time-varying fine-structure constant. We find that in general this data prefers a value of $α$ that was smaller in the past (which is in agreement with measurements of $α$ from quasar observations). However, there are some interesting degeneracies in the problem which imply that strong statements about $α$ can not be made using this method until independent accurate determinations of $Ω_b h^2$ and $H_0$ are available. We also show that a preferred lower value of $α$ comes mainly from the data points around the first Doppler peak, whereas the main effect of the high-$\ell$ data points is to increase the preferred value for $Ω_b h^2$ (while also tightening the constraints on $Ω_0$ and $H_0$). We comment on some implications of our results.

astro-ph↗

arXiv2000

Cosmology with Varying Constants

I motivate and discuss some recent work on theories with varying constants, and consider some possible observational consequences and tests. Particular emphasis is given to models which can (almost) exactly mimic the predictions of standard inflationary models.

astro-ph↗

arXiv2000

Primordial Adiabatic Fluctuations from Cosmic Defects

We point out that in the context of ``two-metric'' theories of gravity there is the possibility that cosmic defects will produce a spectrum of primordial adiabatic density perturbations. This will happen when the speed characterising the defect-producing scalar field is much larger than the speed characterising gravity and all standard model particles. This model will exactly mimic the standard predictions of inflationary models, with the exception of a small non-Gaussian signal which could be detected by future experiments. We briefly discuss defect evolution in these scenarios and analyze their cosmological consequences.

astro-ph↗

arXiv2000

VSL theories and the Doppler peak

We discuss constraints on so-called `varying speed of light theories' coming from the physics of the early universe. Specifically, we consider the position of the first acoustic peak of the CMB angular power spectrum, showing that the recent determination of its position by various CMB experiments, in particular BOOMERanG/NA, can be used to constrain the value of the speed of light $c$ after the epoch of last scattering. Specifically, we find that for a flat universe and a standard density of baryonic matter a variation in $c$ of up to 4% is consistent with the current observations. The position of the Doppler peak is fairly sensitive to changes in $c$, and future observations should dramatically improve this bound. On the other hand, we also find that the maximum baryonic density allowed in VSL theories is about $Ω_B h^2\sim 0.06Ω_0$. We comment on the significance of these results.

astro-ph↗

arXiv2000

Extending the Velocity-dependent One-scale String Evolution Model

We provide a general overview of the velocity-dependent one-scale model for cosmic string evolution and discuss two further extensions to it. We introduce and justify a new ansatz for the momentum parameter $k$, and also incorporate the effect of radiation backreaction. We thus discuss the evolution of the basic large-scale features of cosmic string networks in all relevant cosmological scenarios, concentrating in particular on the `scaling' solutions relevant for each case. In a companion paper, we show, by comparing with numerical simulations, that this model provides an accurate description of the large-scale features of cosmic string networks.

hep-ph↗

arXiv1999

Does a varying speed of light solve the cosmological problems?

We propose a new generalisation of general relativity which incorporates a variation in both the speed of light in vacuum (c) and the gravitational constant (G) and which is both covariant and Lorentz invariant. We solve the generalised Einstein equations for Friedmann universes and show that arbitrary time-variations of c and G never lead to a solution to the flatness, horizon or $Λ$ problems for a theory satisfying the strong energy condition. In order to do so, one needs to construct a theory which does not reduce to the standard one for any choice of time, length and energy units. This can be achieved by breaking a number of invariance principles such as covariance and Lorentz invariance.

astro-ph↗

arXiv1998

Cosmological Consequences of String-forming Open Inflation Models

We present a study of open inflation cosmological scenarios in which cosmic strings form betwen the two inflationary epochs. It is shown that in these models strings are stretched outside the horizon due to the inflationary expansion but must necessarily re-enter the horizon before the epoch of equal matter and radiation densities. We determine the power spectrum of cold dark matter perturbations in these hybrid models, finding good agreement with observations for values of $Γ=Ω_0h\sim0.3$ and comparable contributions from the active and passive sources to the CMB. Finally, we briefly discuss other cosmological consequences of these models.

astro-ph↗

arXiv1998

Limits on Cosmic Chiral Vortons

We study chiral vorton production for Witten-type superconducting string models in the context of a recently developed analytic formalism. We delineate three distinct scenarios: First, a low energy regime (including the electroweak scale) where vortons can be a source of dark matter. Secondly, an intermediate energy regime where the vorton density is too high to be compatible with the standard cosmology (thereby excluding these models). Finally, a high energy regime (including the GUT scale) in which no vortons are expected to form. The vorton density is most sensitive to the order of the string-forming phase transition and relatively insensitive to the current-forming transition. For a second-order string transition, vorton production is cosmologically disastrous for the range $10^{-28}\lsim Gμ\lsim 10^{-10}$ ($10^{5} GeV \lsim T_{c} \lsim 10^{14} GeV$), while for the first-order case we can only exclude $10^{-20}\lsim Gμ\lsim 10^{-14}$ ($10^{9} GeV \lsim T_{c} \lsim 10^{12} GeV$). We provide a fitting formula which summarises our results.

hep-ph↗

arXiv1998

Vorton Formation

In this paper we present the first analytic model for vorton formation. We start by deriving the microscopic string equations of motion in Witten's superconducting model, and show that in the relevant chiral limit these coincide with the ones obtained from the supersonic elastic models of Carter and Peter. We then numerically study a number of solutions of these equations of motion and thereby suggest criteria for deciding whether a given superconducting loop configuration can form a vorton. Finally, using a recently developed model for the evolution of currents in superconducting strings we conjecture, by comparison with these criteria, that string networks formed at the GUT phase transition should produce no vortons. On the other hand, a network formed at the electroweak scale can produce vortons accounting for up to 6% of the critical density. Some consequences of our results are discussed.

hep-ph↗

arXiv1997

Cosmic Strings in an Open Universe: Quantitative Evolution and Observational Consequences

The cosmic string scenario in an open universe is developed -- including the equations of motion, a model of network evolution, the large angular scale CMB temperature anisotropy, and the power spectrum of density fluctuations produced by cosmic strings with dark matter. First we derive the equations of motion for cosmic string in an open FRW space-time and construct a quantitative model of the evolution of the gross features of a cosmic string network. Second, we apply this model of network evolution to estimate the rms CMB temperature anisotropy induced by cosmic strings, obtaining the normalization for the mass per unit length $μ$ as a function of $Ω$. Third, we consider the effects of the network evolution and normalization in an open universe on the large scale structure formation scenarios with either cold or hot dark matter.

astro-ph↗

arXiv1997

Averaged Methods for Vortex-String Evolution

We discuss friction-dominated vortex-string evolution using a new analytic model recently developed by the authors. By treating the average string velocity, as well as the characteristic lengthscale, as dynamical variables, we can provide a quantitative picture of the complete evolution of a vortex-string network. Previously known scaling laws are confirmed, and new quantitative predictions regarding loop production and evolution are made.

cond-mat↗

arXiv1997

Evolution of Superconducting String Currents

We extend the quantititative string evolution model of Martins and Shellard to superconducting strings by introducing a simple toy model for the evolution of the currents. This is based on the dynamics of a `superconducting correlation length'. We derive the relevant evolution equations and discuss the importance of plasma effects. Some consequences of our results are also suggested.

hep-ph↗

arXiv1997

Galactic Magnetic Fields from Superconducting Strings

We use a simple analytic model for the evolution of currents in superconducting strings to estimate the strength of the `seed' magnetic fields generated by these strings. This model is an extension of the evolution model of Martins and Shellard depending on a parameter $f$ which characterizes the importance of equilibration process in the evolution of the currents. For GUT-scale strings, we find that a viable seed magnetic field for the galactic dynamo can be generated if equilibration is weak. On the other hand, electroweak-scale strings originate magnetic fields that are smaller than required.

astro-ph↗

arXiv1997

String Evolution in Open Universes

The velocity-dependent `one-scale' model of Martins and Shellard is used to study the evolution of a cosmic string network (and the corresponding loop population) in open universes. It is shown that in this case there is no linear scaling regime and that even though curvature still dominates the dynamics, at late times strings become the main component of the universe. We also comment on the possible consequences of these results.

astro-ph↗

arXiv1996

Quantitative String Evolution

An analytic model of long string network evolution, recently developed by the authors, is presented in detail, and modified to describe string loop evolution. By treating the average string velocity, as well as the characteristic lengthscale, as dynamical variables, one can include the effects of frictional forces on the evolution of the network. This generalized `one-scale' model provides a quantitative picture of the complete evolution of a string network, including the prediction of previously unknown transient scaling regimes and a detailed analysis of the evolution of the loop population. The evolution of all cosmologically interesting string networks is studied in detail, and possible consequences of our results are discussed.

hep-ph↗

arXiv1995

String Evolution with Friction

We study the effects of friction on the scaling evolution of string networks in condensed matter and cosmological contexts. We derive a generalized `one-scale' model with the string correlation length $L$ and velocity $v$ as dynamical variables. In non-relativistic systems, we obtain a well-known $L\propto t^{1/2}$ law, showing that loop production is important. For electroweak cosmic strings, we show transient damped epoch scaling with $L\propto t^{5/4}$ (or, in the matter era, $L\propto t^{3/2}$). A low initial density implies an earlier period with $L\propto t^{1/2}$. For GUT strings, the approach to linear scaling $L\propto t$ is faster than previously estimated.

hep-ph↗