1,074 research outputs found
Cosmic Numbers: A Physical Classification for Cosmological Models
We introduce the notion of the cosmic numbers of a cosmological model, and
discuss how they can be used to naturally classify models according to their
ability to solve some of the problems of the standard cosmological model.Comment: 3 pages, no figures. v2: Two references added, cosmetic changes.
Version to appear in Phys. Rev. D (Brief reports
Domain wall network evolution in (N+1)-dimensional FRW universes
We develop a velocity-dependent one-scale model for the evolution of domain
wall networks in flat expanding or collapsing homogeneous and isotropic
universes with an arbitrary number of spatial dimensions, finding the
corresponding scaling laws in frictionless and friction dominated regimes. We
also determine the allowed range of values of the curvature parameter and the
expansion exponent for which a linear scaling solution is possible in the
frictionless regime.Comment: 5 pages, 2 figure
Anthropic versus cosmological solutions to the coincidence problem
In this paper we investigate possible solutions to the coincidence problem in
flat phantom dark energy models with a constant dark energy equation of state
and quintessence models with a linear scalar field potential. These models are
representative of a broader class of cosmological scenarios in which the
universe has a finite lifetime. We show that, in the absence of anthropic
constraints, including a prior probability for the models inversely
proportional to the total lifetime of the universe excludes models very close
to the model. This relates a cosmological solution to the
coincidence problem with a dynamical dark energy component having an equation
of state parameter not too close to -1 at the present time. We further show,
that anthropic constraints, if they are sufficiently stringent, may solve the
coincidence problem without the need for dynamical dark energy.Comment: 7 pages, 7 figure
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.Comment: 5 pages, 3 figure
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.Comment: Further cosmetic changes; to appear in The Astrophysical Journal,
v56
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.Comment: 5 pages, 5 figure
Matter power spectrum for the generalized Chaplygin gas model: The relativistic case
The generalized Chaplygin gas (GCG) model is the prototype of a unified model
of dark energy (DE) and dark matter (DM). It is characterized by
equation-of-state (EoS) parameters and . We use a statistical
analysis of the 2dFGRS data to constrain these parameters. In particular, we
find that very small (close to zero) and very large values () of
the equation-of-state parameter are preferred. To test the validity of
this type of unification of the dark sector we admit the existence of a
separate DM component in addition to the Chaplygin gas and calculate the
probability distribution for the fractional contributions of both components to
the total energy density. This analysis favors a model for which the Universe
is nearly entirely made up of the separate DM component with an almost
negligible Chaplygin gas part. This confirms the results of a previous
Newtonian analysis.Comment: Latex file, 8 pages, 15 figures in eps forma
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