5,551 research outputs found
Observational Constraints on Silent Quartessence
We derive new constraints set by SNIa experiments (`gold' data sample of
Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of
the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan
Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the
quartessence Chaplygin model. We consider both adiabatic perturbations and
intrinsic non-adiabatic perturbations such that the effective sound speed
vanishes (Silent Chaplygin). We show that for the adiabatic case, only models
with equation of state parameter are allowed: this
means that the allowed models are very close to \LambdaCDM. In the Silent case,
however, the results are consistent with observations in a much broader range,
-0.3<\alpha<0.7.Comment: 7 pages, 12 figures, to be submitted to JCA
General CMB and Primordial Trispectrum Estimation
We present trispectrum estimation methods which can be applied to general
non-separable primordial and CMB trispectra. We present a general optimal
estimator for the connected part of the trispectrum, for which we derive a
quadratic term to incorporate the effects of inhomogeneous noise and masking.
We describe a general algorithm for creating simulated maps with given
arbitrary (and independent) power spectra, bispectra and trispectra. We propose
a universal definition of the trispectrum parameter , so that the
integrated bispectrum on the observational domain can be consistently compared
between theoretical models. We define a shape function for the primordial
trispectrum, together with a shape correlator and a useful parametrisation for
visualizing the trispectrum. We derive separable analytic CMB solutions in the
large-angle limit for constant and local models. We present separable mode
decompositions which can be used to describe any primordial or CMB bispectra on
their respective wavenumber or multipole domains. By extracting coefficients of
these separable basis functions from an observational map, we are able to
present an efficient estimator for any given theoretical model with a
nonseparable trispectrum. The estimator has two manifestations, comparing the
theoretical and observed coefficients at either primordial or late times. These
mode decomposition methods are numerically tractable with order
operations for the CMB estimator and approximately order for the general
primordial estimator (reducing to order in both cases for a special class
of models). We also demonstrate how the trispectrum can be reconstructed from
observational maps using these methods.Comment: 38 pages, 9 figures. In v2 Figures 4-7 are altered slightly and some
extra references are included in the bibliography. v3 matches version
submitted to journal. Includes discussion of special case
Dark Energy and Dark Matter
It is a puzzle why the densities of dark matter and dark energy are nearly
equal today when they scale so differently during the expansion of the
universe. This conundrum may be solved if there is a coupling between the two
dark sectors. In this paper we assume that dark matter is made of cold relics
with masses depending exponentially on the scalar field associated to dark
energy. Since the dynamics of the system is dominated by an attractor solution,
the dark matter particle mass is forced to change with time as to ensure that
the ratio between the energy densities of dark matter and dark energy become a
constant at late times and one readily realizes that the present-day dark
matter abundance is not very sensitive to its value when dark matter particles
decouple from the thermal bath. We show that the dependence of the present
abundance of cold dark matter on the parameters of the model differs
drastically from the familiar results where no connection between dark energy
and dark matter is present. In particular, we analyze the case in which the
cold dark matter particle is the lightest supersymmetric particle.Comment: 4 pages latex, 2 figure
An entirely analytical cosmological model
The purpose of the present study is to show that in a particular cosmological
model, with an affine equation of state, one can obtain, besides the background
given by the scale factor, Hubble and deceleration parameters, a representation
in terms of scalar fields and, more important, explicit mathematical
expressions for the density contrast and the power spectrum. Although the model
so obtained is not realistic, it reproduces features observed in some previous
numerical studies and, therefore, it may be useful in the testing of numerical
codes and as a pedagogical tool.Comment: 4 pages (revtex4), 4 figure
Scaling solutions in general non-minimal coupling theories
A class of generalized non-minimal coupling theories is investigated, in
search of scaling attractors able to provide an accelerated expansion at the
present time. Solutions are found in the strong coupling regime and when the
coupling function and the potential verify a simple relation. In such cases,
which include power law and exponential functions, the dynamics is independent
of the exact form of the coupling and the potential. The constraint from the
time variability of , however, limits the fraction of energy in the scalar
field to less than 4% of the total energy density, and excludes accelerated
solutions at the present.Comment: 10 pages, 3 figures, accepted for publication in Phys. Rev.
Gauss-Bonnet lagrangian G ln G and cosmological exact solutions
For the lagrangian L = G ln G where G is the Gauss-Bonnet curvature scalar we
deduce the field equation and solve it in closed form for 3-flat Friedman
models using a statefinder parametrization. Further we show, that among all
lagrangians F(G) this L is the only one not having the form G^r with a real
constant r but possessing a scale-invariant field equation. This turns out to
be one of its analogies to f(R)-theories in 2-dimensional space-time. In the
appendix, we systematically list several formulas for the decomposition of the
Riemann tensor in arbitrary dimensions n, which are applied in the main
deduction for n=4.Comment: 18 pages, amended version, accepted by Phys. Rev.
Dark Matter and Dark Energy
I briefly review our current understanding of dark matter and dark energy.
The first part of this paper focusses on issues pertaining to dark matter
including observational evidence for its existence, current constraints and the
`abundance of substructure' and `cuspy core' issues which arise in CDM. I also
briefly describe MOND. The second part of this review focusses on dark energy.
In this part I discuss the significance of the cosmological constant problem
which leads to a predicted value of the cosmological constant which is almost
times larger than the observed value \la/8\pi G \simeq
10^{-47}GeV. Setting \la to this small value ensures that the
acceleration of the universe is a fairly recent phenomenon giving rise to the
`cosmic coincidence' conundrum according to which we live during a special
epoch when the density in matter and \la are almost equal. Anthropic
arguments are briefly discussed but more emphasis is placed upon dynamical dark
energy models in which the equation of state is time dependent. These include
Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model
independent methods to determine the cosmic equation of state and the
Statefinder diagnostic are also discussed. The Statefinder has the attractive
property \atridot/a H^3 = 1 for LCDM, which is helpful for differentiating
between LCDM and rival dark energy models. The review ends with a brief
discussion of the fate of the universe in dark energy models.Comment: 40 pages, 11 figures, Lectures presented at the Second Aegean Summer
School on the Early Universe, Syros, Greece, September 2003, New References
added Final version to appear in the Proceeding
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