arXiv subjects

P. P. Avelino

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

162 records · Page 9Linked to original sources

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.

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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.

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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.

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arXiv2001

Cosmic strings, loops, and linear growth of matter perturbations

We describe the detailed study and results of high-resolution numerical simulations of string-induced structure formation in open universes and those with a non-zero cosmological constant. The effect from small loops generated from the string network has also been investigated. We provide a semi-analytical model which can reproduce these simulation results. A detailed study of cosmic string network properties regarding structure formation is also given, including the correlation time, the topological analysis of the source spectrum, the correlation between long strings and loops, and the evolution of long-string and loop energy densities. For models with $Γ=Ωh=0.1--0.2 and a cold dark matter background, we show that the linear density fluctuation power spectrum induced by cosmic strings has both an amplitude at $8 h^{-1}$Mpc, $σ_8$, and an overall shape which are consistent within uncertainties with those currently inferred from galaxy surveys. The cosmic string scenario with hot dark matter requires a strongly scale-dependent bias in order to agree with observations.

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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}.

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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.

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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.

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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.

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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.

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arXiv1999

The cluster abundance in cosmic string models for structure formation

We use the present observed number density of large X-ray clusters to constrain the amplitude of matter density perturbations induced by cosmic strings on the scale of $8 h^{-1}$Mpc ($σ_8$), in both open cosmologies and flat models with a non-zero cosmological constant. We find a slightly lower value of $σ_8$ than that obtained in the context of primordial Gaussian fluctuations generated during inflation. This lower normalization of $σ_8$ results from the mild non-Gaussianity on cluster scales, where the one point probability distribution function is well approximated by a $χ^2$ distribution. We use our estimate of $σ_8$ to constrain the string linear energy density $μ$ and show that it is consistent with the COBE normalization.

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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.

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arXiv1998

Cosmic string loops and large-scale structure

We investigate the contribution made by small loops from a cosmic string network as seeds for large-scale structure formation. We show that cosmic string loops are highly correlated with the long-string network on large scales and therefore contribute significantly to the power spectrum of density perturbations if the average loop lifetime is comparable to or above one Hubble time. This effect further improves the large-scale bias problem previously identified in earlier studies of cosmic string models.

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arXiv1998

The growth of linear perturbations in generic defect models for structure formation

We study the growth of linear perturbations induced by a generic causal scaling source as a function of the cosmological parameters $h$, $Ω^m_0$ and $Ω^Λ_0$. We show that for wavenumbers $k \gsim 0.01 h/Mpc$ the spectrum of density and velocity perturbations scale in a similar way to that found in inflationary models with primordial perturbations. We show that this result is independent of the more or less incoherent nature of the source, the small scale power spectrum of the source and of deviations from scaling which naturally occur at late times if $Ω^m_0 \neq 1$.

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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.

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arXiv1998

Non-gaussian Features of Linear Cosmic String Models

We investigate the non-gaussian properties of cosmic-string-seeded linear density perturbations with cold and hot dark matter backgrounds, using high-resolution numerical simulations. We compute the one-point probability density function of the resulting density field, its skewness, kurtosis, and genus curves for different smoothing scales. A semi-analytic model is then invoked to provide a physical interpretation of our results. We conclude that on scales smaller than $1.5{(Ωh^2)}^{-1}$Mpc, perturbations seeded by cosmic strings are very non-gaussian. These scales may still be in a linear or mildly non-linear regime in an open or $Λ$-universe with $Γ=Ωh \lsim 0.2$.

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arXiv1998

Cosmic-String-Seeded Structure Formation

We describe the results of high-resolution numerical simulations of string-induced structure formation in open universes and those with a non-zero cosmological constant. For models with $Γ= Ωh = 0.1 - 0.2$ and a cold dark matter background, we show that the linear density fluctuation power spectrum has both an amplitude at $8 h^{-1} Mpc, σ_8$, and an overall shape which are consistent within uncertainties with those currently inferred from galaxy surveys. The cosmic string scenario with hot dark matter requires a strongly scale-dependent bias in order to agree with observations.

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arXiv1997

Cosmic strings with interacting hot dark matter

We compute the linear power spectrum of cosmic string sedeed fluctuations in the context of neutrinos with a strong self-interaction and show that it is very similar to that obtained in the context of `normal' neutrinos. We compare our results with observational data and show that for any value of the cosmological parameters $h$ and $Ω_0$ the interacting hot dark matter power spectrum requires a large scale dependent biasing parameter.

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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.

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