10 research outputs found
Loitering universe models in light of the CMB
Spatially flat loitering universe models have recently been shown to arise in
the context of brane world scenarios. Such models allow more time for structure
formation to take place at high redshifts, easing, e.g., the tension between
the observed and predicted evolution of the quasar population with redshift.
While having the desirable effect of boosting the growth of structures, we show
that in such models the position of the first peak in the power spectrum of the
cosmic microwave background anisotropies severely constrains the amount of
loitering at high redshifts.Comment: 4 pages, 3 figures. Included discussion of the linear growth factor.
Matches version accepted for publication in PR
The Integrated Sachs-Wolfe effect as a probe of non-standard cosmological evolution
The Integrated Sachs-Wolfe effect is studied in non-standard cosmologies. By
considering flat universes with a non-fluctuating dark energy component, it is
shown how the quadrupole power can be suppressed by atypical evolution of the
scale factor. For example, a brief period of non-standard evolution at a high
redshift can suppress the quadrupole significantly. The effect on the overall
normalization of the CMB power spectrum is also discussed. Non-standard
cosmologies can affect the overall normalization significantly and enhance the
primordial fluctuations. The possibility of constraining such non-standard
models with CMB and independent measures of , is considered.Comment: 18 pages, 8 figure
The present universe in the Einstein frame, metric-affine R+1/R gravity
We study the present, flat isotropic universe in 1/R-modified gravity. We use
the Palatini (metric-affine) variational principle and the Einstein
(metric-compatible connected) conformal frame. We show that the energy density
scaling deviates from the usual scaling for nonrelativistic matter, and the
largest deviation occurs in the present epoch. We find that the current
deceleration parameter derived from the apparent matter density parameter is
consistent with observations. There is also a small overlap between the
predicted and observed values for the redshift derivative of the deceleration
parameter. The predicted redshift of the deceleration-to-acceleration
transition agrees with that in the \Lambda-CDM model but it is larger than the
value estimated from SNIa observations.Comment: 11 pages; published versio
Bayesian analysis of Friedmannless cosmologies
Assuming only a homogeneous and isotropic universe and using both the 'Gold'
Supernova Type Ia sample of Riess et al. and the results from the Supernova
Legacy Survey, we calculate the Bayesian evidence of a range of different
parameterizations of the deceleration parameter. We consider both spatially
flat and curved models. Our results show that although there is strong evidence
in the data for an accelerating universe, there is little evidence that the
deceleration parameter varies with redshift.Comment: 7 pages, 3 figure
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
Cosmological constraints on f(R) gravity theories within the Palatini approach
We investigate f(R) theories of gravity within the Palatini approach and show
how one can determine the expansion history, H(a), for an arbitrary choice of
f(R). As an example, we consider cosmological constraints on such theories
arising from the supernova type Ia, large scale structure formation and cosmic
microwave background observations. We find that best fit to the data is a
non-null leading order correction to the Einstein gravity, but the current data
exhibits no significant preference over the concordance LCDM model. Our results
show that the often considered 1/R models are not compatible with the data. The
results demonstrate that the background expansion alone can act as a good
discriminator between modified gravity models when multiple data sets are used.Comment: 9 pages (A&A), 7 figures. Minor changes to text plus added some
references. Accepted for publication in A&
On using the cosmic microwave background shift parameter in tests of models of dark energy
Context.The so-called shift parameter is related to the position of the first acoustic
peak in the power spectrum of the temperature anisotropies of the cosmic microwave background (CMB). It is an often used quantity in simple tests of dark energy models. However, the
shift parameter is not directly measurable from the cosmic microwave background, and its
value is usually derived from the data assuming a spatially flat cosmology with dark matter
and a cosmological constant.
Aims.To evaluate the effectiveness of the shift parameter as a constraint on dark energy models.
Methods.We discuss the potential pitfalls in using the shift parameter
as a test of non-standard dark energy models.
Results.By comparing to full CMB fits, we show
that combining the shift parameter with the position of the first acoustic peak in
the CMB power spectrum improves the accuracy of the test considerably