2,054 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
On the structure of the BBGKY hierarchy for a Boltzmann gas
Structure of BBGKY hierarchy for Boltzmann gas and particle distribution
Cosmic microwave background and large-scale structure constraints on a simple quintessential inflation model
We derive constraints on a simple quintessential inflation model, based on a
spontaneously broken Phi^4 theory, imposed by the Wilkinson Microwave
Anisotropy Probe three-year data (WMAP3) and by galaxy clustering results from
the Sloan Digital Sky Survey(SDSS). We find that the scale of symmetry breaking
must be larger than about 3 Planck masses in order for inflation to generate
acceptable values of the scalar spectral index and of the tensor-to-scalar
ratio. We also show that the resulting quintessence equation-of-state can
evolve rapidly at recent times and hence can potentially be distinguished from
a simple cosmological constant in this parameter regime.Comment: 5 pages, 2 figure
Weak Gravitational Lensing by Voids
We consider the prospects for detecting weak gravitational lensing by
underdensities (voids) in the large-scale matter distribution. We derive the
basic expressions for magnification and distortion by spherical voids.
Clustering of the background sources and cosmic variance are the main factors
which limit in principle the detection of lensing by voids. We conclude that
only voids with radii larger than \hm have lensing signal to noise
larger than unity.Comment: 12 pages, 7 figures, uses mn-1_4.sty file, submitted to MNRA
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
Halo Model Analysis of Cluster Statistics
We use the halo model formalism to provide expressions for cluster abundances
and bias, as well as estimates for the correlation matrix between these
observables. Off-diagonal elements due to scatter in the mass tracer scaling
with mass are included, as are observational effects such as biases/scatter in
the data, detection rates (completeness), and false detections (purity). We
apply the formalism to a hypothetical volume limited optical survey where the
cluster mass tracer is chosen to be the number of member galaxies assigned to a
cluster. Such a survey can strongly constrain
(), the power law index where
(), and perhaps even
the Hubble parameter (). We find cluster abundances and
bias not well suited for constraining or the amplitude . We
also find that without bias information and are degenerate,
implying constraints on the former are strongly dependent on priors used for
the latter and vice-versa. The degeneracy stems from an intrinsic scaling
relation of the halo mass function, and hence it should be present regardless
of the mass tracer used in the survey.Comment: 27 pages, 11 figures, references adde
Cosmological constraints on pseudo-Nambu-Goldstone bosons
Particle physics models with pseudo-Nambu-Goldstone bosons (PNGBs) are characterized by two mass scales: a global spontaneous symmetry breaking scale, f, and a soft (explicit) symmetry breaking scale, Lambda. General model insensitive constraints were studied on this 2-D parameter space arising from the cosmological and astrophysical effects of PNGBs. In particular, constraints were studied arising from vacuum misalignment and thermal production of PNGBs, topological defects, and the cosmological effects of PNGB decay products, as well as astrophysical constraints from stellar PNGB emission. Bounds on the Peccei-Quinn axion scale, 10(exp 10) GeV approx. = or less than f sub pq approx. = or less than 10(exp 10) to 10(exp 12) GeV, emerge as a special case, where the soft breaking scale is fixed at Lambda sub QCD approx. = 100 MeV
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