62 research outputs found
Second-order weak lensing from modified gravity
We explore the sensitivity of weak gravitational lensing to second-order
corrections to the spacetime metric within a cosmological adaptation of the
parameterized post-Newtonian framework. Whereas one might expect nonlinearities
of the gravitational field to introduce non-Gaussianity into the statistics of
the lensing convergence field, we show that such corrections are actually
always small within a broad class of scalar-tensor theories of gravity. We show
this by first computing the weak lensing convergence within our parameterized
framework to second order in the gravitational potential, and then computing
the relevant post-Newtonian parameters for scalar-tensor gravity theories. In
doing so we show that this potential systematic factor is generically
negligible, thus clearing the way for weak lensing to provide a direct tracer
of mass on cosmological scales for a wide class of gravity theories despite
uncertainties in the precise nature of the departures from general relativity.Comment: 13 pages, 1 figure; v2: minor edits to match the PRD accepted versio
Testing dark energy paradigms with weak gravitational lensing
Any theory invoked to explain cosmic acceleration predicts consistency
relations between the expansion history, structure growth, and all related
observables. Currently there exist high-quality measurements of the expansion
history from Type Ia supernovae, the cosmic microwave background temperature
and polarization spectra, and baryon acoustic oscillations. We can use
constraints from these datasets to predict what future probes of structure
growth should observe. We apply this method to predict what range of cosmic
shear power spectra would be expected if we lived in a LambdaCDM universe, with
or without spatial curvature, and what results would be inconsistent and
therefore falsify the model. Though predictions are relaxed if one allows for
an arbitrary quintessence equation of state , we find that any
observation that rules out LambdaCDM due to excess lensing will also rule out
all quintessence models, with or without early dark energy. We further explore
how uncertainties in the nonlinear matter power spectrum, e.g. from approximate
fitting formulas such as Halofit, warm dark matter, or baryons, impact these
limits.Comment: 12 pages, 11 figures, submitted to PR
Luminosity distance in Swiss cheese cosmology with randomized voids. II. Magnification probability distributions
We study the fluctuations in luminosity distances due to gravitational
lensing by large scale (> 35 Mpc) structures, specifically voids and sheets. We
use a simplified "Swiss cheese" model consisting of a \Lambda -CDM
Friedman-Robertson-Walker background in which a number of randomly distributed
non-overlapping spherical regions are replaced by mass compensating comoving
voids, each with a uniform density interior and a thin shell of matter on the
surface. We compute the distribution of magnitude shifts using a variant of the
method of Holz & Wald (1998), which includes the effect of lensing shear. The
standard deviation of this distribution is ~ 0.027 magnitudes and the mean is ~
0.003 magnitudes for voids of radius 35 Mpc, sources at redshift z_s=1.0, with
the voids chosen so that 90% of the mass is on the shell today. The standard
deviation varies from 0.005 to 0.06 magnitudes as we vary the void size, source
redshift, and fraction of mass on the shells today. If the shell walls are
given a finite thickness of ~ 1 Mpc, the standard deviation is reduced to ~
0.013 magnitudes. This standard deviation due to voids is a factor ~ 3 smaller
than that due to galaxy scale structures. We summarize our results in terms of
a fitting formula that is accurate to ~ 20%, and also build a simplified
analytic model that reproduces our results to within ~ 30%. Our model also
allows us to explore the domain of validity of weak lensing theory for voids.
We find that for 35 Mpc voids, corrections to the dispersion due to lens-lens
coupling are of order ~ 4%, and corrections to due shear are ~ 3%. Finally, we
estimate the bias due to source-lens clustering in our model to be negligible
Dark matter and dark energy proposals: maintaining cosmology as a true science?
I consider the relation of explanations for the observed data to testability
in the following contexts: observational and experimental detection of dark
matter; observational and experimental detection of dark energy or a
cosmological constant ; observational or experimental testing of the
multiverse proposal to explain a small non-zero value of ; and
observational testing of the possibility of large scale spatial inhomogeneity
with zero .Comment: 14 pages. Paper for CRAL-IPNL conference "Dark Energy and Dark
Matter", Lyon 200
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