168 research outputs found
Future Supernovae observations as a probe of dark energy
We study the potential impact of improved future supernovae data on our
understanding of the dark energy problem. We carefully examine the relative
utility of different fitting functions that can be used to parameterize the
dark energy models, and provide concrete reasons why a particular choice (based
on a parameterization of the equation of state) is better in almost all cases.
We discuss the details of a representative sample of dark energy models and
show how future supernova observations could distinguish among these. As a
specific example, we consider the proposed ``SNAP'' satellite which is planned
to observe around 2000 supernovae. We show how a SNAP-class data set taken
alone would be a powerful discriminator among a family of models that would be
approximated by a constant equation of state for the most recent epoch of
cosmic expansion. We show how this family includes most of the dark energy
models proposed so far. We then show how an independent measurement of
can allow SNAP to probe the evolution of the equation of state
as well, allowing further discrimination among a larger class of proposed dark
energy models. We study the impact of the satellite design parameters on this
method to distinguish the models and compare SNAP to alternative measurements.
We establish that if we exploit the full precision of SNAP it provides a very
powerful probe.Comment: 29 pages, 22 figures; replaced to match version accepted for
publication in PRD, section V shortend and merged into section VI; brief
discussion on non-flat cosmologies adde
Optimizing the yield of Sunyaev-Zel'dovich cluster surveys
We consider the optimum depth of a cluster survey selected using the
Sunyaev-Zel'dovich effect. By using simple models for the evolution of the
cluster mass function and detailed modeling for a variety of observational
techniques, we show that the optimum survey yield is achieved when the average
size of the clusters selected is close to the size of the telescope beam. For a
total power measurement, we compute the optimum noise threshold per beam as a
function of the beam size and then discuss how our results can be used in more
general situations. As a by-product we gain some insight into what is the most
advantageous instrumental set-up. In the case of beam switching observations
one is not severely limited if one manages to set the noise threshold close to
the point which corresponds to the optimum yield. By defining a particular
reference configuration, we show how our results can be applied to
interferometer observations. Considering a variety of alternative scenarios, we
discuss how robust our conclusions are to modifications in the cluster model
and cosmological parameters. The precise optimum is particularly sensitive to
the amplitude of fluctuations and the profile of the gas in the cluster.Comment: 16 pages, 18 figure
Constraining Dark Energy with X-ray Galaxy Clusters, Supernovae and the Cosmic Microwave Background
We present new constraints on the evolution of dark energy from an analysis
of Cosmic Microwave Background, supernova and X-ray galaxy cluster data. Our
analysis employs a minimum of priors and exploits the complementary nature of
these data sets. We examine a series of dark energy models with up to three
free parameters: the current dark energy equation of state w_0, the early time
equation of state w_et and the scale factor at transition, a_t. From a combined
analysis of all three data sets, assuming a constant equation of state and that
the Universe is flat, we measure w_0=-1.05+0.10-0.12. Including w_et as a free
parameter and allowing a_t to vary over the range 0.5<a_t<0.95 where the data
sets have discriminating power, we measure w_0=-1.27+0.33-0.39 and
w_et=-0.66+0.44-0.62. We find no significant evidence for evolution in the dark
energy equation of state parameter with redshift. Marginal hints of evolution
in the supernovae data become less significant when the cluster constraints are
also included in the analysis. The complementary nature of the data sets leads
to a tight constraint on the mean matter density, Omega_m and alleviates a
number of other parameter degeneracies, including that between the scalar
spectral index n_s, the physical baryon density Omega_bh^2 and the optical
depth tau. This complementary nature also allows us to examine models in which
we drop the prior on the curvature. For non-flat models with a constant
equation of state, we measure w_0=-1.09+0.12-0.15 and Omega_de=0.70+-0.03. Our
analysis includes spatial perturbations in the dark energy fluid, assuming a
sound speed c_s^2 =1. For our most general dark energy model, not including
such perturbations would lead to spurious constraints on w_et which would be
tighter by approximately a factor two with the current data. (abridged)Comment: 11 pages, 13 figures, 2 tables. Accepted for publication in MNRAS.
Two new figures added: Fig.9 shows the effects of including dark energy
perturbations and Fig.10 compares X-ray cluster data with 2dF dat
Cluster Probes of Dark Energy Clustering
Cluster abundances are oddly insensitive to canonical early dark energy.
Early dark energy with sound speed equal to the speed of light cannot be
distinguished from a quintessence model with the equivalent expansion history
for but negligible early dark energy density, despite the different early
growth rate. However, cold early dark energy, with a sound speed much smaller
than the speed of light, can give a detectable signature. Combining cluster
abundances with cosmic microwave background power spectra can determine the
early dark energy fraction to 0.3 % and distinguish a true sound speed of 0.1
from 1 at 99 % confidence. We project constraints on early dark energy from the
Euclid cluster survey, as well as the Dark Energy Survey, using both current
and projected Planck CMB data, and assess the impact of cluster mass
systematics. We also quantify the importance of dark energy perturbations, and
the role of sound speed during a crossing of
Constraining dark sector perturbations II: ISW and CMB lensing tomography
Any Dark Energy (DE) or Modified Gravity (MG) model that deviates from a
cosmological constant requires a consistent treatment of its perturbations,
which can be described in terms of an effective entropy perturbation and an
anisotropic stress. We have considered a recently proposed generic
parameterisation of DE/MG perturbations and compared it to data from the Planck
satellite and six galaxy catalogues, including temperature-galaxy (Tg), CMB
lensing-galaxy and galaxy-galaxy (gg) correlations. Combining these observables
of structure formation with tests of the background expansion allows us to
investigate the properties of DE/MG both at the background and the perturbative
level. Our constraints on DE/MG are mostly in agreement with the cosmological
constant paradigm, while we also find that the constraint on the equation of
state w (assumed to be constant) depends on the model assumed for the
perturbation evolution. We obtain (95% CL; CMB+gg+Tg)
in the entropy perturbation scenario; in the anisotropic stress case the result
is . Including the lensing correlations shifts the
results towards higher values of w. If we include a prior on the expansion
history from recent Baryon Acoustic Oscillations (BAO) measurements, we find
that the constraints tighten closely around , making it impossible to
measure any DE/MG perturbation evolution parameters. If, however, upcoming
observations from surveys like DES, Euclid or LSST show indications for a
deviation from a cosmological constant, our formalism will be a useful tool
towards model selection in the dark sector.Comment: 25 pages, 8 figures; minor update for consistency with version
accepted by JCAP (13/01/2015
Feature importance for machine learning redshifts applied to SDSS galaxies
We present an analysis of importance feature selection applied to photometric
redshift estimation using the machine learning architecture Decision Trees with
the ensemble learning routine Adaboost (hereafter RDF). We select a list of 85
easily measured (or derived) photometric quantities (or `features') and
spectroscopic redshifts for almost two million galaxies from the Sloan Digital
Sky Survey Data Release 10. After identifying which features have the most
predictive power, we use standard artificial Neural Networks (aNN) to show that
the addition of these features, in combination with the standard magnitudes and
colours, improves the machine learning redshift estimate by 18% and decreases
the catastrophic outlier rate by 32%. We further compare the redshift estimate
using RDF with those from two different aNNs, and with photometric redshifts
available from the SDSS. We find that the RDF requires orders of magnitude less
computation time than the aNNs to obtain a machine learning redshift while
reducing both the catastrophic outlier rate by up to 43%, and the redshift
error by up to 25%. When compared to the SDSS photometric redshifts, the RDF
machine learning redshifts both decreases the standard deviation of residuals
scaled by 1/(1+z) by 36% from 0.066 to 0.041, and decreases the fraction of
catastrophic outliers by 57% from 2.32% to 0.99%.Comment: 10 pages, 4 figures, updated to match version accepted in MNRA
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