5 research outputs found
Using galaxy pairs as cosmological tracers
The Alcock-Paczynski (AP) effect uses the fact that, when analyzed with the
correct geometry, we should observe structure that is statistically isotropic
in the Universe. For structure undergoing cosmological expansion with the
background, this constrains the product of the Hubble parameter and the angular
diameter distance. However, the expansion of the Universe is inhomogeneous and
local curvature depends on density. We argue that this distorts the AP effect
on small scales. After analyzing the dynamics of galaxy pairs in the Millennium
simulation, we find an interplay between peculiar velocities, galaxy properties
and local density that affects how pairs trace cosmological expansion. We find
that only low mass, isolated galaxy pairs trace the average expansion with a
minimum "correction" for peculiar velocities. Other pairs require larger, more
cosmology and redshift dependent peculiar velocity corrections and, in the
small-separation limit of being bound in a collapsed system, do not carry
cosmological information.Comment: 15 pages, 14 figures, 1 tabl
Anti-lensing: the bright side of voids
More than half of the volume of our Universe is occupied by cosmic voids. The lensing magni ca-
tion e ect from those under-dense regions is generally thought to give a small dimming contribution:
objects on the far side of a void are supposed to be observed as slightly smaller than if the void were
not there, which together with conservation of surface brightness implies net reduction in photons
received. This is predicted by the usual weak lensing integral of the density contrast along the line
of sight. We show that this standard e ect is swamped at low redshifts by a relativistic Doppler
term that is typically neglected. Contrary to the usual expectation, objects on the far side of a void
are brighter than they would be otherwise. Thus the local dynamics of matter in and near the void
is crucial and is only captured by the full relativistic lensing convergence. There are also signi cant
nonlinear corrections to the relativistic linear theory, which we show actually under-predicts the
e ect. We use exact solutions to estimate that these can be more than 20% for deep voids. This
remains an important source of systematic errors for weak lensing density reconstruction in galaxy
surveys and for supernovae observations, and may be the cause of the reported extra scatter of eld
supernovae located on the edge of voids compared to those in clusters.Web of Scienc
Redshift and distances in a {\Lambda}CDM cosmology with non-linear inhomogeneities
Motivated by the dawn of precision cosmology and the wealth of forthcoming
high precision and volume galaxy surveys, in this paper we study the effects of
inhomogeneities on light propagation in a flat \Lambda CDM background. To this
end we use exact solutions of Einstein's equations (Meures & Bruni 2011) where,
starting from small fluctuations, inhomogeneities arise from a standard growing
mode and become non-linear. While the matter distribution in these models is
necessarily idealised, there is still enough freedom to assume an arbitrary
initial density profile along the line of sight. We can therefore model
over-densities and voids of various sizes and distributions, e.g. single
harmonic sinusoidal modes, coupled modes, and more general distributions in a
\Lambda CDM background. Our models allow for an exact treatment of the light
propagation problem, so that the results are unaffected by approximations and
unambiguous. Along lines of sight with density inhomogeneities which average
out on scales less than the Hubble radius, we find the distance redshift
relation to diverge negligibly from the Friedmann-Lemaitre-Robertson-Walker
(FLRW) result. On the contrary, if we observe along lines of sight which do not
have the same average density as the background, we find large deviations from
the FLRW distance redshift relation. Hence, a possibly large systematic might
be introduced into the analysis of cosmological observations, e.g. supernovae,
if we observe along lines of sight which are typically more or less dense than
the average density of the Universe. In turn, this could lead to wrong
parameter estimation: even if the Cosmological Principle is valid, the
identification of the true FLRW background in an inhomogeneous universe maybe
more difficult than usually assumed.Comment: 15 pages, 12 figures, published in MNRAS. Corrected typos,
re-formatted figures, added references and slightly changed notation (r->z
Exact nonlinear inhomogeneities in CDM cosmology
At a time when galaxy surveys and other observations are reaching
unprecedented sky coverage and precision it seems timely to investigate the
effects of general relativistic nonlinear dynamics on the growth of structures
and on observations. Analytic inhomogeneous cosmological models are an
indispensable way of investigating and understanding these effects in a
simplified context.
In this paper, we develop exact inhomogeneous solutions of general relativity
with pressureless matter (dust, describing cold dark matter) and cosmological
constant , which can be used to model an arbitrary initial matter
distribution along one line of sight. In particular, we consider the second
class Szekeres models with and split their dynamics into a flat
CDM background and exact nonlinear inhomogeneities, obtaining several
new results. One single metric function describes the deviation from the
background. We show that , the time dependent part of , satisfies the
familiar linear differential equation for , the first-order density
perturbation of dust, with the usual growing and decaying modes. In the limit
of small perturbations, as expected, and the growth of
inhomogeneities links up exactly with standard perturbation theory. We provide
analytic expressions for the exact nonlinear and the growth factor in
our models. For the case of over-densities, we find that, depending on the
initial conditions, the growing mode may or may not lead to a pancake
singularity, analogous to a Zel'dovich pancake. This is in contrast with the
pure Einstein-de-Sitter background where, at any given point in
comoving (Lagrangian) coordinates pancakes will always occur.Comment: 20 pages, 4 figures, some changes made in the text, 4 references
added and Eqs. (46) and (C3) correcte