66 research outputs found
Markov chain Monte Carlo analysis of Bianchi VII_h models
We have extended the analysis of Jaffe et al. to a complete Markov chain
Monte Carlo (MCMC) study of the Bianchi type models including a
dark energy density, using 1-year and 3-year Wilkinson Microwave Anisotropy
Probe (WMAP) cosmic microwave background (CMB) data. Since we perform the
analysis in a Bayesian framework our entire inference is contained in the
multidimensional posterior distribution from which we can extract marginalised
parameter constraints and the comparative Bayesian evidence. Treating the
left-handed Bianchi CMB anisotropy as a template centred upon the `cold-spot'
in the southern hemisphere, the parameter estimates derived for the total
energy density, `tightness' and vorticity from 3-year data are found to be:
, , with orientation ). This template is preferred by a factor of roughly
unity in log-evidence over a concordance cosmology alone. A Bianchi type
template is supported by the data only if its position on the sky is heavily
restricted. The low total energy density of the preferred template, implies a
geometry that is incompatible with cosmologies inferred from recent CMB
observations. Jaffe et al. found that extending the Bianchi model to include a
term in creates a degeneracy in the plane. We explore this region fully by MCMC and find that the
degenerate likelihood contours do not intersect areas of parameter space that 1
or 3 year WMAP data would prefer at any significance above . Thus we
can confirm that a physical Bianchi model is not responsible for
this signature.Comment: 8 pages, 10 figures, significant update to include more accurate
results and conclusions to match version accepted by MNRA
A Bayesian study of the primordial power spectrum from a novel closed universe model
We constrain the shape of the primordial power spectrum using recent
measurements of the cosmic microwave background (CMB) from the Wilkinson
Microwave Anisotropy Probe (WMAP) 7-year data and other high-resolution CMB
experiments. We also include observations of the matter power spectrum from the
luminous red galaxy (LRG) subset DR7 of the Sloan Digital Sky Survey (SDSS). We
consider two different models of the primordial power spectrum. The first is
the standard nearly scale-invariant spectrum in the form of a generalised
power-law parameterised in terms of the spectral amplitude , the
spectral index and (possibly) the running parameter .
The second spectrum is derived from the Lasenby and Doran (LD) model. The LD
model is based on the restriction of the total conformal time available in a
closed Universe and the predicted primordial power spectrum depends upon just
two parameters. An important feature of the LD spectrum is that it naturally
incorporates an exponential fall-off on large scales, which might provide a
possible explanation for the lower-than-expected power observed at low
multipoles in the CMB. In addition to parameter estimation, we compare both
models using Bayesian model selection. We find there is a significant
preference for the LD model over a simple power-law spectrum for a CMB-only
dataset, and over models with an equal number of parameters for all the
datasets considered.Comment: minor corrections to match accepted version to MNRA
WMAP 3-year primordial power spectrum
We constrain the form of the primordial power spectrum using Wilkinson
Microwave Anisotropy Probe (WMAP) 3-year cosmic microwave background (CMB) data
(+ other high resolution CMB experiments) in addition to complementary
large-scale structure (LSS) data: 2dF, SDSS, Ly-alpha forest and luminous red
galaxy (LRG) data from the SDSS catalogue. We compute the comparative Bayesian
evidence in addition to parameter estimates for a collection of seven models:
(i) a scale invariant Harrison-Zel'dovich (H-Z) spectrum; (ii) a power-law;
(iii) a running spectral index; (iv) a broken spectrum; (v) a power-law with an
abrupt cutoff on large-scales; (vi) a reconstruction of the spectrum in eight
bins in wavenumber; and (vii) a spectrum resulting from a cosmological model
proposed by Lasenby & Doran (L-D). Using a basic dataset of WMAP3 + other CMB +
2dF + SDSS our analysis confirms that a scale-invariant spectrum is disfavoured
by between 0.7 and 1.7 units of log evidence (depending on priors chosen) when
compared with a power-law tilt. Moreover a running spectrum is now
significantly preferred, but only when using the most constraining set of
priors. The addition of Ly-alpha and LRG data independently both suggest much
lower values of the running index than with basic dataset alone and
interestingly the inclusion of Ly-alpha significantly disfavours a running
parameterisation by more than a unit in log evidence. Overall the highest
evidences, over all datasets, were obtained with a power law spectrum
containing a cutoff with a significant log evidence difference of roughly 2
units. The natural tilt and exponential cutoff present in the L-D spectrum is
found to be favoured decisively by a log evidence difference of over 5 units,
but only for a limited study within the best-fit concordance cosmology.Comment: 7 pages, 8 figures, changes and new results to match version accepted
by MNRA
Scale dependence of the primordial spectrum from combining the three-year WMAP, Galaxy Clustering, Supernovae, and Lyman-alpha forests
We probe the scale dependence of the primordial spectrum in the light of the
three-year WMAP (WMAP3) alone and WMAP3 in combination with the other
cosmological observations such as galaxy clustering and Type Ia Supernova
(SNIa). We pay particular attention to the combination with the Lyman
(Ly) forest. Different from the first-year WMAP (WMAP1), WMAP3's
preference on the running of the scalar spectral index on the large scales is
now fairly independent of the low CMB multipoles . A combination with the
galaxy power spectrum from the Sloan Digital Sky Survey (SDSS) prefers a
negative running to larger than 2, regardless the presence of low
CMB () or not. On the other hand if we focus on the
Power Law CDM cosmology with only six parameters (matter density
, baryon density , Hubble Constant , optical
depth , the spectral index, , and the amplitude, , of the
scalar perturbation spectrum) when we drop the low CMB contributions
WMAP3 is consistent with the Harrison-Zel'dovich-Peebles scale-invariant
spectrum ( and no tensor contributions) at . When assuming
a simple power law primordial spectral index or a constant running, in case one
drops the low contributions () WMAP3 is consistent
with the other observations better, such as the inferred value of .
We also find, using a spectral shape with a minimal extension of the running
spectral index model, LUQAS CROFT Ly and SDSS Ly exhibit
somewhat different preference on the spectral shape.Comment: 16 pages, 13 figures Revtex
Slow Roll Reconstruction: Constraints on Inflation from the 3 Year WMAP Dataset
We study the constraints on the inflationary parameter space derived from the
3 year WMAP dataset using ``slow roll reconstruction'', using the SDSS galaxy
power spectrum to gain further leverage where appropriate. This approach
inserts the inflationary slow roll parameters directly into a Monte Carlo
Markov chain estimate of the cosmological parameters, and uses the inflationary
flow hierarchy to compute the parameters' scale-dependence. We work with the
first three parameters (epsilon, eta and xi) and pay close attention to the
possibility that the 3 year WMAP dataset contains evidence for a ``running''
spectral index, which is dominated by the xi term. Mirroring the WMAP team's
analysis we find that the permitted distribution of xi is broad, and centered
away from zero. However, when we require that inflationary parameters yield at
least 30 additional e-folds of inflation after the largest observable scales
leave the horizon, the bounds on xi tighten dramatically. We make use of the
absence of an explicit pivot scale in the slow roll reconstruction formalism to
determine the dependence of the computed parameter distributions on the pivot.
We show that the choice of pivot has a significant effect on the inferred
constraints on the inflationary variables, and the spectral index and running
derived from them. Finally, we argue that the next round of cosmological data
can be expected to place very stringent constraints on the region of parameter
space open to single field models of slow roll inflation.Comment: 26 pages, 11 figures, JHEP format. v2: version accepted by JCAP:
minor clarifications and references added, 1 figure added, v3: 1 reference
adde
The Mock LISA Data Challenges: from Challenge 3 to Challenge 4
The Mock LISA Data Challenges are a program to demonstrate LISA data-analysis
capabilities and to encourage their development. Each round of challenges
consists of one or more datasets containing simulated instrument noise and
gravitational waves from sources of undisclosed parameters. Participants
analyze the datasets and report best-fit solutions for the source parameters.
Here we present the results of the third challenge, issued in Apr 2008, which
demonstrated the positive recovery of signals from chirping Galactic binaries,
from spinning supermassive--black-hole binaries (with optimal SNRs between ~ 10
and 2000), from simultaneous extreme-mass-ratio inspirals (SNRs of 10-50), from
cosmic-string-cusp bursts (SNRs of 10-100), and from a relatively loud
isotropic background with Omega_gw(f) ~ 10^-11, slightly below the LISA
instrument noise.Comment: 12 pages, 2 figures, proceedings of the 8th Edoardo Amaldi Conference
on Gravitational Waves, New York, June 21-26, 200
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
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