1,935 research outputs found
Lensing and high-z supernova surveys
Gravitational lensing causes the distribution of observed brightnesses of
standard candles at a given redshift to be highly non-gaussian. The
distribution is strongly, and asymmetrically, peaked at a value less than the
expected value in a homogeneous Robertson-Walker universe. Therefore, given any
small sample of observations in an inhomogeneous universe, the most likely
observed luminosity is at flux values less than the Robertson-Walker value.
This paper explores the impact of this systematic error due to lensing upon
surveys predicated on measuring standard candle brightnesses. We re-analyze
recent results from the high-z supernova team (Riess et al. 1998), both when
most of the matter in the universe is in the form of compact objects
(represented by the empty-beam expression, corresponding to the maximal case of
lensing), and when the matter is continuously distributed in galaxies. We find
that the best-fit model remains unchanged (at Omega_m=0, Omega_Lambda=0.45),
but the confidence contours change size and shape, becoming larger (and thus
allowing a broader range of parameter space) and dropping towards higher values
of matter density, Omega_m (or correspondingly, lower values of the
cosmological constant, Omega_Lambda). These effects are slight when the matter
is continuously distributed. However, the effects become considerably more
important if most of the matter is in compact objects. For example, neglecting
lensing, the Omega_m=0.5, Omega_Lambda=0.5 model is more than 2 sigma away from
the best fit. In the empty-beam analysis, this cosmology is at 1 sigma.Comment: 11 pages, 3 ps figures. uses aaspp4.sty. accepted to ApJ Letters.
includes analysis of lensing due to matter continuously distributed in
galaxie
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
Model-Independent Constraints on Dark Energy Density from Flux-averaging Analysis of Type Ia Supernova Data
We reconstruct the dark energy density as a free function from
current type Ia supernova (SN Ia) data (Tonry et al. 2003; Barris et al. 2003;
Knop et al. 2003), together with the Cosmic Microwave Background (CMB) shift
parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure
(LSS) growth factor from 2dF galaxy survey data. We parametrize as
a continuous function, given by interpolating its amplitudes at equally spaced
values in the redshift range covered by SN Ia data, and a constant at
larger (where is only weakly constrained by CMB data). We
assume a flat universe, and use the Markov Chain Monte Carlo (MCMC) technique
in our analysis. We find that the dark energy density is constant
for 0 \la z \la 0.5 and increases with redshift for 0.5 \la z \la 1 at
68.3% confidence level, but is consistent with a constant at 95% confidence
level. For comparison, we also give constraints on a constant equation of state
for the dark energy.
Flux-averaging of SN Ia data is required to yield cosmological parameter
constraints that are free of the bias induced by weak gravitational lensing
\citep{Wang00b}. We set up a consistent framework for flux-averaging analysis
of SN Ia data, based on \cite{Wang00b}. We find that flux-averaging of SN Ia
data leads to slightly lower and smaller time-variation in
. This suggests that a significant increase in the number of SNe Ia
from deep SN surveys on a dedicated telescope \citep{Wang00a} is needed to
place a robust constraint on the time-dependence of the dark energy density.Comment: Slightly revised in presentation, ApJ accepted. One color figure
shows rho_X(z) reconstructed from dat
A tracker solution to the cold dark matter cosmic coincidence problem
Recently, we introduced the notion of "tracker fields," a form of
quintessence which has an attractor-like solution. Using this concept, we
showed how to construct models in which the ratio of quintessence to matter
densities today is independent of initial conditions. Here we apply the same
idea to the standard cold dark matter component in cases where it is composed
of oscillating fields. Combining these ideas, we can construct a model in which
quintessence, cold dark matter, and ordinary matter all contribute comparable
amounts to the total energy density today irrespective of initial conditions.Comment: 8 pages, 2 eps figures, use epsfig.sty, accepted for publication in
Physics Letters
Future supernovae data and quintessence models
The possibility to unambiguously determine the equation-of-state of the
cosmic dark energy with existing and future supernovae data is investigated. We
consider four evolution laws for this equation-of-state corresponding to four
quintessential models, i.e. i) a cosmological constant, ii) a general
barotropic fluid, iii) a perfect fluid with a linear equation-of-state and iv)
a more physical model based on a pseudo-Nambu-Goldstone boson field. We
explicitly show the degeneracies present not only within each model but also
between the different models : they are caused by the multi-integral relation
between the equation-of-state of dark energy and the luminosity distance.
Present supernova observations are analysed using a standard method
and the minimal values obtained for each model are compared. We
confirm the difficulty to discriminate between these models using present SNeIa
data only. By means of simulations, we then show that future SNAP observations
will not remove all the degeneracies. For example, wrong estimations of
with a good value of could be found if the right
cosmological model is not used to fit the data. We finally give some
probabilities to obtain unambiguous results, free from degeneracies. In
particular, the probability to confuse a cosmological constant with a true
barotropic fluid with an equation-of-state different from -1 is shown to be 95%
at a level.Comment: 12 pages. This improved version has been accepted for publication in
M.N.R.A.
Localizing coalescing massive black hole binaries with gravitational waves
Massive black hole binary coalescences are prime targets for space-based
gravitational wave (GW) observatories such as {\it LISA}. GW measurements can
localize the position of a coalescing binary on the sky to an ellipse with a
major axis of a few tens of arcminutes to a few degrees, depending on source
redshift, and a minor axis which is times smaller. Neglecting weak
gravitational lensing, the GWs would also determine the source's luminosity
distance to better than percent accuracy for close sources, degrading to
several percent for more distant sources. Weak lensing cannot, in fact, be
neglected and is expected to limit the accuracy with which distances can be
fixed to errors no less than a few percent. Assuming a well-measured cosmology,
the source's redshift could be inferred with similar accuracy. GWs alone can
thus pinpoint a binary to a three-dimensional ``pixel'' which can help guide
searches for the hosts of these events. We examine the time evolution of this
pixel, studying it at merger and at several intervals before merger. One day
before merger, the major axis of the error ellipse is typically larger than its
final value by a factor of . The minor axis is larger by a factor
of , and, neglecting lensing, the error in the luminosity distance is
larger by a factor of . This large change over a short period of
time is due to spin-induced precession, which is strongest in the final days
before merger. The evolution is slower as we go back further in time. For , we find that GWs will localize a coalescing binary to within $\sim 10\
\mathrm{deg}^2$ as early as a month prior to merger and determine distance (and
hence redshift) to several percent.Comment: 30 pages, 10 figures, 5 tables. Version published in Ap
Bias and high-order galaxy correlation functions in the APM Galaxy Survey
On large scales, the higher order moments of the mass distribution,
S_J=\xibar_J/\xibar_2^{J-1}, e.g., the skewness and kurtosis , can
be predicted using non-linear perturbation theory. Comparison of these
predictions with moments of the observed galaxy distribution probes the bias
between galaxies and mass. Applying this method to models with initially
Gaussian fluctuations and power spectra similar to that of galaxies in
the APM survey, we find that the predicted higher order moments are in
good agreement with those directly inferred from the APM survey {\it in the
absence of bias}. We use this result to place limits on the linear and
non-linear bias parameters. Models in which the extra power observed on large
scales (with respect to standard CDM) is produced by scale-dependent bias match
the APM higher order amplitudes only if non-linear bias (rather than non-linear
gravity) generates the observed higher order moments. When normalized to COBE
DMR, these models are significantly ruled out by the observations. The
cold plus hot dark matter model normalized to COBE can reproduce the APM higher
order correlations if one introduces non-linear bias terms, while the
low-density CDM model with a cosmological constant does not require any bias to
fit the large-scale amplitudes.Comment: 8 pages, 2 figures included, uuencoded postscript file (100 kB),
Fermilab-Pub-94/207-
The Hubble constant and dark energy
The Hubble Constant measured from the anisotropy in the cosmic microwave
background (CMB) is shown to be independent of small changes from the standard
model of the redshift dependence of dark energy. Modifications of the Friedmann
equation to include phantom power (w < -1), textures (w = -2/3) and curvature
are considered, and constraints on these dark energy contributors from
supernova observations are derived. Modified values for the density of matter
inferred from cosmic density perturbations and from the CMB under these
circumstances are also estimated, as exemplified by 2df and SDSS.Comment: PASP accepted; PASP September 201
Future Type Ia Supernova Data as Tests of Dark Energy from Modified Friedmann Equations
In the Cardassian model, dark energy density arises from modifications to the
Friedmann equation, which becomes H^2 = g(\rhom), where g(\rhom) is a new
function of the energy density. The universe is flat, matter dominated, and
accelerating. The distance redshift relation predictions of generalized
Cardassian models can be very different from generic quintessence models, and
can be differentiated with data from upcoming pencil beam surveys of Type Ia
Supernovae such as SNAP. We have found the interesting result that, once
is known to 10% accuracy, SNAP will be able to determine the sign of
the time dependence of the dark energy density. Knowledge of this sign (which
is related to the weak energy condition) will provide a first discrimination
between various cosmological models that fit the current observational data
(cosmological constant, quintessence, Cardassian expansion). Further, we have
performed Monte Carlo simulations to illustrate how well one can reproduce the
form of the dark energy density with SNAP.
To be concrete we study a class of two parameter (,) generalized
Cardassian models that includes the original Cardassian model (parametrized by
only) as a special case. Examples are given of MP Cardassian models that
fit current supernovae and CMB data, and prospects for differentiating between
MP Cardassian and other models in future data are discussed. We also note that
some Cardassian models can satisfy the weak energy condition even with a
dark energy component that has an effective equation of state .Comment: revised version accepted by Ap
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