638 research outputs found
Constraining the Properties of Dark Energy
The presence of dark energy in the Universe is inferred directly from the
accelerated expansion of the Universe, and indirectly, from measurements of
cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3
of the critical density, is very smoothly distributed, and has large negative
pressure. Its nature is very much unknown. Most of its discernible consequences
follow from its effect on evolution of the expansion rate of the Universe,
which in turn affects the growth of density perturbations and the age of the
Universe, and can be probed by the classical kinematic cosmological tests.
Absent a compelling theoretical model (or even a class of models), we describe
dark energy by an effective equation of state w=p_X/rho_X which is allowed to
vary with time. We describe and compare different approaches for determining
w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies
and clusters, and CMB anisotropy, focusing particular attention on the use of a
sample of several thousand type Ia supernova with redshifts z < 1.7, as might
be gathered by the proposed SNAP satellite. Among other things, we derive
optimal strategies for constraining cosmological parameters using type Ia
supernovae. While in the near term CMB anisotropy will provide the first
measurements of w, supernovae and number counts appear to have the most
potential to probe dark energy.Comment: 6 pages, 3 figures; proceedings of 20th Texas Symposium on Relavistic
Astrophysic
Constraints on the Inner Cluster Mass Profile and the Power Spectrum Normalization from Strong Lensing Statistics
Strong gravitational lensing is a useful probe of both the intrinsic
properties of the lenses and the cosmological parameters of the universe. The
large number of model parameters and small sample of observed lens systems,
however, have made it difficult to obtain useful constraints on more than a few
parameters from lensing statistics. Here we examine how the recent WMAP
measurements help improve the constraining power of statistics from the radio
lensing survey JVAS/CLASS. We find that the absence of theta>3'' lenses in
CLASS places an upper bound of beta<1.25 (1.60) at 68% (95%) CL on the inner
density profile, rho \propto r^{-beta}, of cluster-sized halos. Furthermore,
the favored power spectrum normalization is sigma_8 >= 0.7 (95% CL). We discuss
two possibilities for stronger future constraints: a positive detection of at
least one large-separation system, and next-generation radio surveys such as
LOFAR.Comment: Scatter in concentration included; virial mass used consistently; new
Fig 3. Final version published in ApJ
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
Probing Dark Energy Dynamics from Current and Future Cosmological Observations
We report the constraints on the dark energy equation-of-state w(z) using the
latest 'Constitution' SNe sample combined with the WMAP5 and SDSS data. Based
on the localized principal component analysis and the model selection criteria,
we find that the LCDM model is generally consistent with the current data, yet
there exists weak hint of the possible dynamics of dark energy. In particular,
a model predicting w(z)-1 at z\in[0.5,0.75),
which means that w(z) crosses -1 in the range of z\in[0.25,0.75), is mildly
favored at 95% confidence level. Given the best fit model for current data as a
fiducial model, we make future forecast from the joint data sets of JDEM,
Planck and LSST, and we find that the future surveys can reduce the error bars
on the w bins by roughly a factor of 10 for a 5-w-bin model.Comment: Accepted by PRD; minor changes from v
Hubble parameter reconstruction from a principal component analysis: minimizing the bias
A model-independent reconstruction of the cosmic expansion rate is essential
to a robust analysis of cosmological observations. Our goal is to demonstrate
that current data are able to provide reasonable constraints on the behavior of
the Hubble parameter with redshift, independently of any cosmological model or
underlying gravity theory. Using type Ia supernova data, we show that it is
possible to analytically calculate the Fisher matrix components in a Hubble
parameter analysis without assumptions about the energy content of the
Universe. We used a principal component analysis to reconstruct the Hubble
parameter as a linear combination of the Fisher matrix eigenvectors (principal
components). To suppress the bias introduced by the high redshift behavior of
the components, we considered the value of the Hubble parameter at high
redshift as a free parameter. We first tested our procedure using a mock sample
of type Ia supernova observations, we then applied it to the real data compiled
by the Sloan Digital Sky Survey (SDSS) group. In the mock sample analysis, we
demonstrate that it is possible to drastically suppress the bias introduced by
the high redshift behavior of the principal components. Applying our procedure
to the real data, we show that it allows us to determine the behavior of the
Hubble parameter with reasonable uncertainty, without introducing any ad-hoc
parameterizations. Beyond that, our reconstruction agrees with completely
independent measurements of the Hubble parameter obtained from red-envelope
galaxies.Comment: Modified to match journal versio
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
Weak Lensing as a Calibrator of the Cluster Mass-Temperature Relation
The abundance of clusters at the present epoch and weak gravitational lensing
shear both constrain roughly the same combination of the power spectrum
normalization sigma_8 and matter energy density Omega_M. The cluster constraint
further depends on the normalization of the mass-temperature relation.
Therefore, combining the weak lensing and cluster abundance data can be used to
accurately calibrate the mass-temperature relation. We discuss this approach
and illustrate it using data from recent surveys.Comment: Matches the version in ApJL. Equation 4 corrected. Improvements in
the analysis move the cluster contours in Fig1 slightly upwards. No changes
in the conclusion
Is the Universe Inflating? Dark Energy and the Future of the Universe
We consider the fate of the observable universe in the light of the discovery
of a dark energy component to the cosmic energy budget. We extend results for a
cosmological constant to a general dark energy component and examine the
constraints on phenomena that may prevent the eternal acceleration of our patch
of the universe. We find that the period of accelerated cosmic expansion has
not lasted long enough for observations to confirm that we are undergoing
inflation; such an observation will be possible when the dark energy density
has risen to between 90% and 95% of the critical. The best we can do is make
cosmological observations in order to verify the continued presence of dark
energy to some high redshift. Having done that, the only possibility that could
spoil the conclusion that we are inflating would be the existence of a
disturbance (the surface of a true vacuum bubble, for example) that is moving
toward us with sufficiently high velocity, but is too far away to be currently
observable. Such a disturbance would have to move toward us with speed greater
than about 0.8c in order to spoil the late-time inflation of our patch of the
universe and yet avoid being detectable.Comment: 7 pages, 7 figure
Gamma-ray burst contributions to constraining the evolution of dark energy
We explore the gamma-ray bursts' (GRBs') contributions in constraining the
dark energy equation of state (EOS) at high () and at middle
redshifts () and estimate how many GRBs are needed to get
substantial constraints at high redshifts. We estimate the constraints with
mock GRBs and mock type Ia supernovae (SNe Ia) for comparisons. When
constraining the dark energy EOS in a certain redshift range, we allow the dark
energy EOS parameter to vary only in that redshift bin and fix EOS parameters
elsewhere to -1. We find that it is difficult to constrain the dark energy EOS
beyond the redshifts of SNe Ia with GRBs unless some new luminosity relations
for GRBs with smaller scatters are discovered. However, at middle redshifts,
GRBs have comparable contributions with SNe Ia in constraining the dark energy
EOS.Comment: 3 pages, 5 figures. Published in Astronomy and Astrophysics.
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