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P. P. Avelino

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

At least 145 records · Page 8Linked to original sources
arXiv2004

Scaling defect decay and the reionization history of the Universe

We consider a model for the reionization history of the Universe in which a significant fraction of the observed optical depth is a result of direct reionization by the decay products of a scaling cosmic defect network. We show that such network can make a significant contribution to the reionization history of the Universe even if its energy density is very small (the defect energy density has to be greater than about $10^{-11}$ of the background density). We compute the Cosmic Microwave Background temperature, polarization and temperature-polarization cross power spectrum and show that a contribution to the observed optical depth due to the decay products of a scaling defect network may help to reconcile a high optical depth with a low redshift of complete reionization suggested by quasar data. However, if the energy density of defects is approximately a constant fraction of the background density then these models do not explain the large scale bump in the temperature-polarization cross power spectrum observed by WMAP.

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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''.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 $Λ$.

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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.

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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.

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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.

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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.

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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.

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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.

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