392 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
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
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.Comment: 11 RevTeX 4 pages, 6 figures; version to appear in Phys. Rev.
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
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.Comment: 4 LaTeX pages, no figures. To appear in Phys. Lett.
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 and comparable
contributions from the active and passive sources to the CMB. Finally, we
briefly discuss other cosmological consequences of these models.Comment: 11 LaTeX pages with 3 eps 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
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}.Comment: 10 LaTeX pages, 4 eps figures, version to appear in Phys. Lett.
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 .Comment: RevTex4; 7 pages, 4 figures. v2: Improved discussion of constraints
on Chaplygin gas models. Other clarifications added. Phys Rev. D (in press
Impact of string and monopole-type junctions on domain wall dynamics: implications for dark energy
We investigate the potential role of string and monopole-type junctions in
the frustration of domain wall networks using a velocity-dependent one-scale
model for the characteristic velocity, , and the characteristic length, ,
of the network. We show that, except for very special network configurations,
v^2 \lsim (HL)^2 \lsim (\rho_\sigma + \rho_\mu)/\rho_m where is the
Hubble parameter and , and are the average
density of domain walls, strings and monopole-type junctions. We further show
that if domain walls are to provide a significant contribution to the dark
energy without generating exceedingly large CMB temperature fluctuations then,
at the present time, the network must have a characteristic length L_0 \lsim
10 \Omega_{\sigma 0}^{-2/3} {\rm kpc} and a characteristic velocity v_0 \lsim
10^{-5} \Omega_{\sigma 0}^{-2/3} where and is the critical density. In order to satisfy these
constraints with , would have to be at
least 10 orders of magnitude larger than , which would be in
complete disagreement with observations. This result provides very strong
additional support for the conjecture that no natural frustration mechanism,
which could lead to a significant contribution of domain walls to the dark
energy budget, exists.Comment: 4 pages, 1 figur
Scaling of cosmological domain wall networks with junctions
We describe the results of the largest and most accurate three-dimensional
field theory simulations of domain wall networks with junctions. We consider a
previously introduced class of models which, in the limit of large number
of coupled scalar fields, approaches the so-called `ideal' model (in terms of
its potential to lead to network frustration). We consider values of
between N=2 and N=20. In all cases we find compelling evidence for a gradual
approach to scaling, with the quantitative scaling parameters having only a
mild dependence on . These results strongly support our no-frustration
conjecture.Comment: 4 pages, 2 figure
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