16 research outputs found
Linear kinetic Sunyaev-Zel'dovich effect and void models for acceleration
There has been considerable recent interest in cosmological models in which
the current apparent acceleration is due to a very large local underdensity, or
void, instead of some form of dark energy. Here we examine a new proposal to
constrain such models using the linear kinetic Sunyaev-Zel'dovich (kSZ) effect
due to structure within the void. The simplified "Hubble bubble" models
previously studied appeared to predict far more kSZ power than is actually
observed, independently of the details of the initial conditions and evolution
of perturbations in such models. We show that the constraining power of the kSZ
effect is considerably weakened (though still impressive) under a fully
relativistic treatment of the problem, and point out several theoretical
ambiguities and observational shortcomings which further qualify the results.
Nevertheless, we conclude that a very large class of void models is ruled out
by the combination of kSZ and other methods.Comment: 15 pages, 9 figures; minor changes, version published in CQG focus
section "Inhomogeneous Cosmological Models and Averaging in Cosmology
Tilted Physics: A Cosmologically Dipole-Modulated Sky
Physical constants and cosmological parameters could vary with position. On
the largest scales such variations would manifest themselves as gradients
across our Hubble volume, leading to dipole-modulation of the cosmic microwave
anisotropies. This generically leads to a correlation between adjacent
multipoles in the spherical harmonics expansion of the sky, a distinctive
signal which should be searched for in future data sets.Comment: 4 pages, 3 figure
Can we avoid dark energy?
The idea that we live near the centre of a large, nonlinear void has
attracted attention recently as an alternative to dark energy or modified
gravity. We show that an appropriate void profile can fit both the latest
cosmic microwave background and supernova data. However, this requires either a
fine-tuned primordial spectrum or a Hubble rate so low as to rule these models
out. We also show that measurements of the radial baryon acoustic scale can
provide very strong constraints. Our results present a serious challenge to
void models of acceleration.Comment: 5 pages, 4 figures; minor changes; version published in Phys. Rev.
Let
Precision cosmology defeats void models for acceleration
The suggestion that we occupy a privileged position near the centre of a
large, nonlinear, and nearly spherical void has recently attracted much
attention as an alternative to dark energy. Putting aside the philosophical
problems with this scenario, we perform the most complete and up-to-date
comparison with cosmological data. We use supernovae and the full cosmic
microwave background spectrum as the basis of our analysis. We also include
constraints from radial baryonic acoustic oscillations, the local Hubble rate,
age, big bang nucleosynthesis, the Compton y-distortion, and for the first time
include the local amplitude of matter fluctuations, \sigma_8. These all paint a
consistent picture in which voids are in severe tension with the data. In
particular, void models predict a very low local Hubble rate, suffer from an
"old age problem", and predict much less local structure than is observed.Comment: 22 pages, 12 figures; v2 adds models in closed backgrounds;
conclusions strengthened; version accepted to Phys. Rev.
Can decaying modes save void models for acceleration?
The unexpected dimness of Type Ia supernovae (SNe), apparently due to
accelerated expansion driven by some form of dark energy or modified gravity,
has led to attempts to explain the observations using only general relativity
with baryonic and cold dark matter, but by dropping the standard assumption of
homogeneity on Hubble scales. In particular, the SN data can be explained if we
live near the centre of a Hubble-scale void. However, such void models have
been shown to be inconsistent with various observations, assuming the void
consists of a pure growing mode. Here it is shown that models with significant
decaying mode contribution today can be ruled out on the basis of the expected
cosmic microwave background spectral distortion. This essentially closes one of
the very few remaining loopholes in attempts to rule out void models, and
strengthens the evidence for Hubble-scale homogeneity.Comment: 11 pages, 3 figures; discussion expanded, appendix added; version
accepted to Phys. Rev.
The Evolution of the Cosmic Microwave Background
We discuss the time dependence and future of the Cosmic Microwave Background
(CMB) in the context of the standard cosmological model, in which we are now
entering a state of endless accelerated expansion. The mean temperature will
simply decrease until it reaches the effective temperature of the de Sitter
vacuum, while the dipole will oscillate as the Sun orbits the Galaxy. However,
the higher CMB multipoles have a richer phenomenology. The CMB anisotropy power
spectrum will for the most part simply project to smaller scales, as the
comoving distance to last scattering increases, and we derive a scaling
relation that describes this behaviour. However, there will also be a dramatic
increase in the integrated Sachs-Wolfe contribution at low multipoles. We also
discuss the effects of tensor modes and optical depth due to Thomson
scattering. We introduce a correlation function relating the sky maps at two
times and the closely related power spectrum of the difference map. We compute
the evolution both analytically and numerically, and present simulated future
sky maps.Comment: 23 pages, 11 figures; references added; one figure dropped and minor
changes to match published version. For high-resolution versions of figures
and animations, see http://www.astro.ubc.ca/people/scott/future.htm