27 research outputs found
Observational constraints on self-accelerating cosmology
The DGP brane-world model provides a simple alternative to the standard LCDM
cosmology, with the same number of parameters. There is no dark energy - the
late universe self-accelerates due to an infrared modification of gravity. We
compute the joint constraints on the DGP model from supernovae, the cosmic
microwave background shift parameter, and the baryon oscillation peak in the
SDSS luminous red galaxy sample. Flat DGP models are within the joint 2 sigma
contour, but the LCDM model provides a significantly better fit to the data.
These tests are based on the background dynamics of the DGP model, and we
comment on further tests that involve structure formation.Comment: matches published version - DGP is within joint 2 sigma contour using
WMAP3 (1 sigma with WMAP1
Fingerprinting Dark Energy III: distinctive marks of viscosity
The characterisation of dark energy is one of the primary goals in cosmology
especially now that many new experiments are being planned with the aim of
reaching a high sensitivity on cosmological parameters. It is known that if we
move away from the simple cosmological constant model then we need to consider
perturbations in the dark energy fluid. This means that dark energy has two
extra degrees of freedom: the sound speed \cs and the anisotropic stress
. If dark energy is inhomogenous at the scales of interest then the
gravitational potentials are modified and the evolution of the dark matter
perturbations is also directly affected. In this paper we add an anisotropic
component to the dark energy perturbations. Following the idea introduced in
\cite{Sapone:2009mb}, we solve analytically the equations of perturbations in
the dark sector, finding simple and accurate approximated solutions. We also
find that the evolution of the density perturbations is governed by an
effective sound speed which depends on both the sound speed and the anisotropic
stress parameter. We then use these solutions to look at the impact of the dark
energy perturbations on the matter power spectrum and on the Integrated
Sachs-Wolfe effect in the Cosmic Microwave Background.Comment: 14 pages, 8 figures, to be submitted to PR
Observational constraints on phantom-like braneworld cosmologies
We investigate a simple braneworld model in which the universe contains only
cold dark matter and a cosmological constant, but the effective dark energy is
phantom-like because of extra-dimensional gravity effects. Modified gravity
screens the cosmological constant Lambda, allowing for a larger Lambda. In
practice, observations do not favour any significant screening. We use
supernova data, the cosmic microwave background shift parameter, and the baryon
oscillation peak in the galaxy distribution to constrain the model. We find the
mean value of Omega_m with 68% confidence limits, and an upper limit on
Omega_Lambda at the 68% confidence level. The best-fit model is very close to a
standard LCDM model, but the LCDM model provides a better fit since it has one
less parameter.Comment: small improvements; matches the version to appear in Phys Rev
Adiabatic initial conditions for perturbations in interacting dark energy models
We present a new systematic analysis of the early radiation era solution in
an interacting dark energy model to find the adiabatic initial conditions for
the Boltzmann integration. In a model where the interaction is proportional to
the dark matter density, adiabatic initial conditions and viable cosmologies
are possible if the early-time dark energy equation of state parameter is . We find that when adiabaticity between cold dark matter, baryons,
neutrinos and photons is demanded, the dark energy component satisfies
automatically the adiabaticity condition. As supernovae Ia or baryon acoustic
oscillation data require the recent-time equation of state parameter to be more
negative, we consider a time-varying equation of state in our model. In a
companion paper [arXiv:0907.4987] we apply the initial conditions derived here,
and perform a full Monte Carlo Markov Chain likelihood analysis of this model.Comment: 12 pages. V2: Minor changes, references added, conclusions extended;
Accepted by MNRA
Can dark energy viscosity be detected with the Euclid survey?
Recent work has demonstrated that it is important to constrain the dynamics of cosmological perturbations, in addition to the evolution of the background, if we want to distinguish among different models of the dark sector. Especially the anisotropic stress of the (possibly effective) dark energy fluid has been shown to be an important discriminator between modified gravity and dark energy models. In this paper we use approximate analytical solutions of the perturbation equations in the presence of viscosity to study how the anisotropic stress affects the weak lensing and galaxy power spectrum. We then forecast how sensitive the photometric and spectroscopic Euclid surveys will be to both the speed of sound and the viscosity of our effective dark energy fluid when using weak lensing tomography and the galaxy power spectrum. We find that Euclid alone can only constrain models with a very small speed of sound and viscosity, while it will need the help of other observables in order to give interesting constraints on models with a sound speed close to one. This conclusion is also supported by the expected Bayes factor between modelsD. S. acknowledges support from the JAEDoc program with Grant No. JAEDoc074 and the Spanish MICINN under Project No. AYA2009-13936-C06-06. D. S. also acknowledges financial support from the Madrid Regional Government (CAM) under the program HEPHACOS P-ESP-00346, Consolider-Ingenio 2010 PAU (CSD2007-00060), as well as the European Union Marie Curie Network ‘‘UniverseNet’’ under Contract No. MRTN-CT-2006-035863. E. M. was supported by the Spanish MICINNs Juan de la Cierva programme (Grant No. JCI-2010-08112), by CICYT through Project No. FPA-2009 09017, by the Community of Madrid through the project HEPHACOS (Grant No. S2009/ESP- 1473) under Grant No. P-ESP-00346 and by the European Union FP7 ITN INVISIBLES (Marie Curie Actions, PITNGA-2011-289442). M. K. acknowledges funding by the Swiss NS
Measuring deviations from a cosmological constant: a field-space parameterization
Most parameterizations of the dark energy equation of state do not reflect
realistic underlying physical models. Here, we develop a relatively simple
description of dark energy based on the dynamics of a scalar field which is
exact in the limit that the equation of state approaches a cosmological
constant, assuming some degree of smoothness of the potential. By introducing
just two parameters defined in the configuration space of the field we are able
to reproduce a wide class of quintessence models. We examine the observational
constraints on these models as compared to linear evolution models, and show
how priors in the field space translate into priors on observational
parameters.Comment: 5 pages, 6 figures. Final versio