1,284 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
Scaling laws for weakly interacting cosmic (super)string and p-brane networks
In this paper we find new scaling laws for the evolution of -brane
networks in -dimensional Friedmann-Robertson-Walker universes in the
weakly-interacting limit, giving particular emphasis to the case of cosmic
superstrings () living in a universe with three spatial dimensions (N=3).
In particular, we show that, during the radiation era, the root-mean-square
velocity is and the characteristic length of
non-interacting cosmic string networks scales as ( is
the scale factor), thus leading to string domination even when gravitational
backreaction is taken into account. We demonstrate, however, that a small
non-vanishing constant loop chopping efficiency parameter leads to a
linear scaling solution with constant ( is the Hubble parameter)
and in the radiation era, which may allow for a
cosmologically relevant cosmic string role even in the case of light strings.
We also determine the impact that the radiation-matter transition has on the
dynamics of weakly interacting cosmic superstring networks.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
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.Comment: Minor changes; to appear in Astroparticle Physic
Evolution of domain wall networks: the PRS algorithm
The Press-Ryden-Spergel (PRS) algorithm is a modification to the field theory
equations of motion, parametrized by two parameters ( and ),
implemented in numerical simulations of cosmological domain wall networks, in
order to ensure a fixed comoving resolution. In this paper we explicitly
demonstrate that the PRS algorithm provides the correct domain wall dynamics in
-dimensional Friedmann-Robertson-Walker (FRW) universes if
, fully validating its use in numerical studies of cosmic
domain evolution. We further show that this result is valid for generic thin
featureless domain walls, independently of the Lagrangian of the model.Comment: 4 page
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
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