106 research outputs found
Reconstruction of the dark matter-vacuum energy interaction
An interaction between the vacuum energy and dark matter is an intriguing
possibility which may offer a way of solving the cosmological constant problem.
Adopting a general prescription for momentum exchange between the two dark
components, we reconstruct , the temporal evolution of the coupling
strength between dark matter and vacuum energy, in a nonparametric Bayesian
approach using combined observational data sets from the cosmic microwave
background, supernovae and large scale structure. An evolving interaction
between the vacuum energy and dark matter removes some of the tensions between
different data sets. However, it is not preferred over CDM in the
Bayesian sense, as improvement in the fit is not sufficient to compensate for
the increase in the volume of the parameter space.Comment: 6 pages, 3 figures. The published versio
Fables of reconstruction: controlling bias in the dark energy equation of state
We develop an efficient, non-parametric Bayesian method for reconstructing
the time evolution of the dark energy equation of state w(z) from observational
data. Of particular importance is the choice of prior, which must be chosen
carefully to minimise variance and bias in the reconstruction. Using a
principal component analysis, we show how a correlated prior can be used to
create a smooth reconstruction and also avoid bias in the mean behaviour of
w(z). We test our method using Wiener reconstructions based on Fisher matrix
projections, and also against more realistic MCMC analyses of simulated data
sets for Planck and a future space-based dark energy mission. While the
accuracy of our reconstruction depends on the smoothness of the assumed w(z),
the relative error for typical dark energy models is <10% out to redshift
z=1.5.Comment: 13 pages, 11 figure
The Complementarity of Redshift-space Distortions and the Integrated Sachs-Wolfe Effect: A 3D Spherical Analysis
Assuming General Relativity is correct on large-scales, Redshift-Space
Distortions (RSDs) and the Integrated Sachs-Wolfe effect (ISW) are both
sensitive to the time derivative of the linear growth function. We investigate
the extent to which these probes provide complementary or redundant information
when they are combined to constrain the evolution of the linear velocity power
spectrum, often quantified by the function , where is the
logarithmic derivative of with respect to . Using a spherical
Fourier-Bessel (SFB) expansion for galaxy number counts and a spherical
harmonic expansion for the CMB anisotropy, we compute the covariance matrices
of the signals for a large galaxy redshift survey combined with a CMB survey
like Planck. The SFB basis allows accurate ISW estimates by avoiding the
plane-parallel approximation, and it retains RSD information that is otherwise
lost when projecting angular clustering onto redshift shells. It also allows
straightforward calculations of covariance with the CMB. We find that the
correlation between the ISW and RSD signals are low since the probes are
sensitive to different modes. For our default surveys, on large scales (k<0.05
\Mpc/h), the ISW can improve constraints on by more than 10%
compared to using RSDs alone. In the future, when precision RSD measurements
are available on smaller scales, the cosmological constraints from ISW
measurements will not be competitive; however, they will remain a useful
consistency test for possible systematic contamination and alternative models
of gravity.Comment: 11 pages, 7 figures, published in MNRA
Examining the evidence for dynamical dark energy
We apply a new non-parametric Bayesian method for reconstructing the
evolution history of the equation-of-state of dark energy, based on
applying a correlated prior for , to a collection of cosmological data.
We combine the latest supernova (SNLS 3-year or Union2.1), cosmic microwave
background, redshift space distortion and the baryonic acoustic oscillation
measurements (including BOSS, WiggleZ and 6dF) and find that the cosmological
constant appears consistent with current data, but that a dynamical dark energy
model which evolves from at higher redshift
is mildly favored. Estimates of the Bayesian evidences show little preference
between the cosmological constant model and the dynamical model for a range of
correlated prior choices. Looking towards future data, we find that the best
fit models for current data could be well distinguished from the CDM
model by observations such as Planck and Euclid-like surveys.Comment: 5 pages, 2 figures, accepted to PR
The Doppler Peaks from Cosmic Texture
We compute the angular power spectrum of temperature anisotropies on the
microwave sky in the cosmic texture theory, with standard recombination
assumed. The spectrum shows `Doppler' peaks analogous to those in scenarios
based on primordial adiabatic fluctuations such as `standard CDM', but at quite
different angular scales. There appear to be excellent prospects for using this
as a discriminant between inflationary and cosmic defect theories.Comment: 14 pages, latex, 3 figures, compressed and uuencoded, replaced
version has minor typographical correction
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