2,172 research outputs found
Constraining Dark Energy with Clusters: Complementarity with Other Probes
The Figure of Merit Science Working Group (FoMSWG) recently forecast the
constraints on dark energy that will be achieved prior to the Joint Dark Energy
Mission (JDEM) by ground-based experiments that exploit baryon acoustic
oscillations, type Ia supernovae, and weak gravitational lensing. We show that
cluster counts from on-going and near-future surveys should provide robust,
complementary dark energy constraints. In particular, we find that optimally
combined optical and Sunyaev-Zel'dovich effect cluster surveys should improve
the Dark Energy Task Force (DETF) figure of merit for pre-JDEM projects by a
factor of two even without prior knowledge of the nuisance parameters in the
cluster mass-observable relation. Comparable improvements are achieved in the
forecast precision of parameters specifying the principal component description
of the dark energy equation of state parameter as well as in the growth index
gamma. These results indicate that cluster counts can play an important
complementary role in constraining dark energy and modified gravity even if the
associated systematic errors are not strongly controlled.Comment: 6 pages, 3 figures, accepted to Phys. Rev. D. Discussion section
adde
High-resolution temporal constraints on the dynamics of dark energy
We use the recent type Ia supernova, cosmic microwave background and
large-scale structure data to shed light on the temporal evolution of the dark
energy equation of state out to redshift one. We constrain the most
flexible parametrization of dark energy to date, and include the dark energy
perturbations consistently throughout. Interpreting our results via the
principal component analysis, we find no significant evidence for dynamical
dark energy: the cosmological constant model is consistent with data everywhere
between redshift zero and one at 95% C.L.Comment: 5 pages, 2 figures Version for PRD (Rapid Communications
Cosmology from supernova magnification maps
High-z Type Ia supernovae are expected to be gravitationally lensed by the
foreground distribution of large-scale structure. The resulting magnification
of supernovae is statistically measurable, and the angular correlation of the
magnification pattern directly probes the integrated mass density along the
line of sight. Measurements of cosmic magnification of supernovae therefore
complements galaxy shear measurements in providing a direct measure of
clustering of the dark matter. As the number of supernovae is typically much
smaller than the number of sheared galaxies, the two-point correlation function
of lensed Type Ia supernovae suffers from significantly increased shot noise.
Neverthless, we find that the magnification map of a large sample of
supernovae, such as that expected from next generation dedicated searches, will
be easily measurable and provide an important cosmological tool. For example, a
search over 20 sq. deg. over five years leading to a sample of ~ 10,000
supernovae would measure the angular power spectrum of cosmic magnification
with a cumulative signal-to-noise ratio of ~20. This detection can be further
improved once the supernova distance measurements are cross-correlated with
measurements of the foreground galaxy distribution. The magnification maps made
using supernovae can be used for important cross-checks with traditional
lensing shear statistics obtained in the same fields, as well as help to
control systematics. We discuss two applications of supernova magnification
maps: the breaking of the mass-sheet degeneracy when estimating masses of
shear-detected clusters, and constraining the second-order corrections to weak
lensing observables.Comment: 4 pages, 2 figures, ApJL submitted; "Signal" discussed here is the
extra covariance in astro-ph/050958
Problems with Pencils: Lensing Covariance of Supernova Distance Measurements
While luminosity distances from Type Ia supernovae (SNe) provide a powerful
probe of cosmological parameters, the accuracy with which these distances can
be measured is limited by cosmic magnification due to gravitational lensing by
the intervening large-scale structure. Spatial clustering of foreground mass
fluctuations leads to correlated errors in distance estimates from SNe. By
including the full covariance matrix of supernova distance measurements, we
show that a future survey covering more than a few square degrees on the sky,
and assuming a total of ~2000 SNe, will be largely unaffected by covariance
noise. ``Pencil beam'' surveys with small fields of view, however, will be
prone to the lensing covariance, leading to potentially significant
degradations in cosmological parameter estimates. For a survey with 30 arcmin
mean separation between SNe, lensing covariance leads to a ~45% increase in the
expected errors in dark energy parameters compared to fully neglecting lensing,
and a ~20% increase compared to including just the lensing variance. Given that
the lensing covariance is cosmology dependent and cannot be mapped out
sufficiently accurately with direct weak lensing observations, surveys with
small mean SN separation must incorporate the effects of lensing covariance,
including its dependence on the cosmological parameters.Comment: 4 pages, 2 figures, PRL submitted; "Noise" discussed here is the
"signal" in astro-ph/050957
Interference of Fock states in a single measurement
We study analytically the structure of an arbitrary order correlation
function for a pair of Fock states and prove without any approximations that in
a single measurement of particle positions interference effects must occur as
experimentally observed with Bose-Einstein condensates. We also show that the
noise level present in the statistics is slightly lower than for a respective
measurement of phase states.Comment: 4 page
Parameterization of Dark-Energy Properties: a Principal-Component Approach
Considerable work has been devoted to the question of how to best
parameterize the properties of dark energy, in particular its equation of state
w. We argue that, in the absence of a compelling model for dark energy, the
parameterizations of functions about which we have no prior knowledge, such as
w(z), should be determined by the data rather than by our ingrained beliefs or
familiar series expansions. We find the complete basis of orthonormal
eigenfunctions in which the principal components (weights of w(z)) that are
determined most accurately are separated from those determined most poorly.
Furthermore, we show that keeping a few of the best-measured modes can be an
effective way of obtaining information about w(z).Comment: Unfeasibility of a truly model-independent reconstruction of w at z>1
illustrated. f(z) left out, and w(z) discussed in more detail. Matches the
PRL versio
Dynamical behavior of generic quintessence potentials: constraints on key dark energy observables
We perform a comprehensive study of a class of dark energy models - scalar
field models where the effective potential can be described by a polynomial
series - exploring their dynamical behavior using the method of flow equations
that has previously been applied to inflationary models. Using supernova,
baryon oscillation, CMB and Hubble constant data, and an implicit theoretical
prior imposed by the scalar field dynamics, we find that the LCDM model
provides an excellent fit to the data. Constraints on the generic scalar field
potential parameters are presented, along with the reconstructed w(z) histories
consistent with the data and the theoretical prior. We propose and pursue
computationally feasible algorithms to obtain estimates of the principal
components of the equation of state, as well as parameters w_0 and w_a.
Further, we use the Monte Carlo Markov Chain machinery to simulate future data
based on the Joint Dark Energy Mission, Planck and baryon acoustic oscillation
surveys and find that the inverse area figure of merit improves nearly by an
order of magnitude. Therefore, most scalar field models that are currently
consistent with data can be potentially ruled out by future experiments. We
also comment on the classification of dark energy models into "thawing'" and
"freezing" in light of the more diverse evolution histories allowed by this
general class of potentials.Comment: 22 pages and 12 figures, minor clarifications and a new Figure (#9)
added in v3, matches the published PRD version. Chains and high-res figures
are available at
http://kicp.uchicago.edu/~dhuterer/DE_FLOWROLL/de_flowroll.htm
Gravitational Lensing as a Probe of Quintessence
A large number of cosmological studies now suggest that roughly two-thirds of
the critical energy density of the Universe exists in a component with negative
pressure. If the equation of state of such an energy component varies with
time, it should in principle be possible to identify such a variation using
cosmological probes over a wide range in redshift. Proper detection of any time
variation, however, requires cosmological probes beyond the currently studied
range in redshift of 0.1 to 1. We extend our analysis to gravitational
lensing statistics at high redshift and suggest that a reliable sample of
lensed sources, out to a redshift of 5, can be used to constrain the
variation of the equation of state, provided that both the redshift
distribution of lensed sources and the selection function involved with the
lensed source discovery process are known. An exciting opportunity to catalog
an adequate sample of lensed sources (quasars) to probe quintessence is now
available with the ongoing Sloan Digital Sky Survey. Writing , we study the expected accuracy to which the equation of state
today and its rate of change can simultaneously be
constrained. Such a determination can rule out some missing-energy candidates,
such as classes of quintessence models or a cosmological constant.Comment: Accepted for publication in ApJ Letters (4 pages, including 4
figures
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