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C. J. A. P. Martins

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

At least 271 records · Page 15Linked to original sources
arXiv2003

The Role of Baryons in Unified Dark Matter Models

We discuss the importance of including baryons in analyses of unified dark matter scenarios, focusing on toy models involving a generalized Chaplygin gas. We determine observational constraints on this unified dark matter scenario coming from large scale structure, type Ia Supernovae and CMB data showing how this component can bring about a different behaviour from classical $Λ$CDM and thus motivate further studies of this type of models. We also speculate on interesting new features which are likely to be important on non-linear scales in this context.

astro-ph↗
arXiv2002

Alternatives to Quintessence Model Building

We discuss the issue of toy model building for the dark energy component of the universe. Specifically, we consider two generic toy models recently proposed as alternatives to quintessence models, known as Cardassian expansion and the Chaplygin gas. We show that the former is enteriely equivalent to a class of quintessence models. We determine the observational constraints on the latter, coming from recent supernovae results and from the shape of the matter power spectrum. As expected, these restrict the model to a behaviour that closely matches that of a standard cosmological constant $Λ$.

astro-ph↗
arXiv2002

Topological Defects in Contracting Universes

We study the behaviour and consequences of cosmic string networks in contracting universes. They approximately behave during the collapse phase as a radiation fluids. Scaling solutions describing this are derived and tested against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation it generates, can affect the dynamics of the universe both locally and globally, and be an important source of radiation, entropy and inhomogeneity. We discuss possible implications for bouncing and cyclic models.

astro-ph↗
arXiv2002

Extra Dimensions and Varying Alpha

I review recent constraints on and claimed detections of a time-varying fine-structure constant $α$. Our results are consistent with no variation in $α$ from the epoch of recombination to the present day, and restrict any such variation to be less than about 4%. The forthcoming MAP and Planck experiments will be able to break most of the currently existing degeneracies between $α$ and other cosmological parameters, and measure $α$ in the early universe with an accuracy comparable to current claimed recent-universe detections.

hep-ph↗
arXiv2002

Cosmology with Varying Constants

The idea of possible time or space variations of the `fundamental' constants of nature, although not new, is only now beginning to be actively considered by large numbers of researchers in the particle physics, cosmology and astrophysics communities. This revival is mostly due to the claims of possible detection of such variations, in various different contexts and by several groups. Here, I present the current theoretical motivations and expectations for such variations, review the current observational status, and discuss the impact of a possible confirmation of these results in our views of cosmology and physics as a whole.

astro-ph↗
arXiv2002

Measuring $α$ in the Early Universe: CMB Temperature, Large-Scale Structure and Fisher Matrix Analysis

We extend our recent work on the effects of a time-varying fine-structure constant $α$ in the cosmic microwave background, by providing a thorough analysis of the degeneracies between $α$ and the other cosmological parameters, and discussing ways to break these with both existing and/or forthcoming data. In particular, we present the state-of-the-art CMB constraints on $α$, through a combined analysis of the BOOMERanG, MAXIMA and DASI datasets. We also present a novel discussion of the constraints on $α$ coming from large-scale structure observations, focusing in particular on the power spectrum from the 2dF survey. Our results are consistent with no variation in $α$ from the epoch of recombination to the present day, and restrict any such (relative) variation to be less than about 4%. We show that the forthcoming MAP and (particularly) Planck experiments will be able to break most of the currently existing degeneracies between $α$ and other parameters, and measure $α$ to better than percent accuracy.

astro-ph↗
arXiv2002

Varying $α$ in the early universe: CMB, LSS and FMA

I review recent constraints on and claimed detections of a time-varying fine-structure constant $α$, and discuss our own work, particularly on the effects of $α$ in the cosmic microwave background and large-scale structure. Our results are consistent with no variation in $α$ from the epoch of recombination to the present day, and restrict any such variation to be less than about 4%. The forthcoming MAP and Planck experiments will be able to break most of the currently existing degeneracies between $α$ and other parameters, and measure $α$ to better than percent accuracy.

astro-ph↗
arXiv2002

Modified Median Statistics and Type Ia Supernova Data

The median statistic has recently been discussed by Gott \textit{et al.} as a more reliable alternative to the standard $χ^2$ likelihood analysis, in the sense of requiring fewer assumptions about the data and being almost as constraining. We apply this statistic to the currently available combined dataset of 92 distant type Ia supernovae, and also to a mock SNAP-class dataset. We find that the performances of the modified median and $χ^2$ statistics are comparable, particularly in the latter case. We further extend the work of Gott \textit{et al.} by modifying the median statistic to account for the number and size of sequences of consecutive points above or below the median. We also comment on how the performance of the statistic depends on the choice of free parameters that one is estimating.

astro-ph↗
arXiv2001

A Supernova Brane Scan

We consider a `brane-world scenario' recently introduced by Dvali, Gabadadze and Porrati, and subsequently proposed as an alternative to a cosmological constant in explaining the current acceleration of the universe. We show that, contrary to these claims, this proposal is already strongly disfavoured by the available Type Ia Supernovae, Cosmic Microwave Background and cluster data.

astro-ph↗
arXiv2001

On the Evolution of Abelian-Higgs String Networks

We study the evolution of Abelian-Higgs string networks in large-scale numerical simulations in both a static and expanding background. We measure the properties of the network by tracing the motion of the string cores, for the first time estimating the rms velocity of the strings and the invariant string length, that is, the true network energy density. These results are compared with a velocity-dependent one scale model for cosmic string network evolution. This incorporates the contributions of loop production, massive radiation and friction to the energy loss processes that are required for scaling evolution. We use this analysis as a basis for discussing the relative importance of these mechanisms for the evolution of the network. We find that the loop distribution statistics in the simulations are consistent with the long-time scaling of the network being dominated by loop production. Making justifiable extrapolations to cosmological scales, these results appear to be consistent with the standard picture of local string network evolution in which loop production and gravitational radiation are the dominant decay mechanisms.

hep-ph↗
arXiv2001

Can our Universe be inhomogeneous on large sub-horizon scales ?

We show that our Universe may be inhomogeneous on large sub-horizon scales without us being able to realise it. We assume that a network of domain walls permeates the universe dividing it in domains with slightly different vacuum energy densities. We require that the energy scale of the phase transition which produced the domain walls is sufficiently low so that the walls have a negligible effect on structure formation. Nevertheless, the different vacuum densities of different domains will lead to different values of the cosmological parameters $Ω_Λ^0$, $Ω_m^0$ and $h$, in each patch thus affecting the growth of cosmological perturbations at recent times. Hence, if our local patch of the universe (with uniform vacuum density) is big enough -- which is likely to happen given that we should have on average about one domain per horizon volume -- we might not notice these large-scale inhomogeneities. This happens because in order to see a patch with a different vacuum density one may have to look back at a time when the universe was still very homogeneous.

astro-ph↗
arXiv2001

Primordial Gaussian Fluctuations from Cosmic Defects

We extend our recent work on ``two-metric'' theories of gravity by showing how in such models cosmic defects can produce a spectrum of primordial Gaussian density perturbations. This will happen when the speed characterising the decay products of the defect network is much larger than the speed characterising gravity and all standard model particles. This model will exactly mimic all the standard predictions of inflationary models, and the only way of distinguishing the two will be via the detection of the decay products of the network.

astro-ph↗
arXiv2001

CMB constraints on spatial variations of the vacuum energy density

In a recent article, a simple `spherical bubble' toy model for a spatially varying vacuum energy density was introduced, and type Ia supernovae data was used to constrain it. Here we generalize the model to allow for the fact that we may not necessarily be at the centre of a region with a given set of cosmological parameters, and discuss the constraints on these models coming from Cosmic Microwave Background Radiation data. We find tight constraints on possible spatial variations of the vacuum energy density for any significant deviations from the centre of the bubble and we comment on the relevance of our results.

astro-ph↗
arXiv2001

Early-universe constraints on a time-varying fine structure constant

Higher-dimensional theories have the remarkable feature of predicting a time (and hence redshift) dependence of the `fundamental' four dimensional constants on cosmological timescales. In this paper we update the bounds on a possible variation of the fine structure constant alpha at the time of BBN (z =10^10) and CMB (z=10^3). Using the recently-released high-resolution CMB anisotropy data and the latest estimates of primordial abundances of 4He and D, we do not find evidence for a varying alpha at more than one-sigma level at either epoch.

astro-ph↗
arXiv2001

Supernova constraints on spatial variations of the vacuum energy density

We consider a very simple toy model for a spatially varying `cosmological constant', where we are inside a spherical bubble (with a given set of cosmological parameters) that is surrounded by a larger region where these parameters are different. This model includes essential features of more realistic scenarios with a minimum number of parameters. We calculate the luminosity distance in the presence of spatial variations of the vacuum energy density using linear perturbation theory and discuss the use of type Ia supernovae to impose constraints on this type of models. We find that presently available observations are only constraining at very low redshifts, but also provide independent confirmation that the high red-shift supernovae data does prefer a relatively large positive cosmological constant.

astro-ph↗
arXiv2001

Can we predict the fate of the Universe?

We re-analyze the question of the use of cosmological observations to infer the present state and future evolution of our patch of the universe. In particular, we discuss under which conditions one might be able to infer that our patch will enter an inflationary stage, as a prima facie interpretation of the Type Ia supernovae and CMB data would suggest. We then establish a `physical' criterion for the existence of inflation, to be contrasted with the more `mathematical' one recently proposed by Starkman et al. \cite{STV}.

astro-ph↗
arXiv2000

Cosmological consequences of the brane/bulk interaction

We investigate cosmological consequences arising from the interaction between a homogeneous and isotropic brane-universe and the bulk. A Friedmann equation is derived which incorporates both the brane and bulk matter contributions, which are both assumed to be of arbitrary fluid form. In particular, new terms arise which describe the energy flow onto (or away from) the brane, as well as changes of the equation of state in the bulk. We discuss Randall-Sundrum type models as well as dilatonic domain walls and carefully consider the conditions for stabilising the induced gravitational constant. Furthermore, consequences for cosmological perturbations are analysed. We show that, in general, super-horizon amplitudes are not constant.

hep-th↗