1,295 research outputs found
An indirect limit on the amplitude of primordial Gravitational Wave Background from CMB-Galaxy Cross Correlation
While large scale cosmic microwave background (CMB) anisotropies involve a
combination of the scalar and tensor fluctuations, the scalar amplitude can be
independently determined through the CMB-galaxy cross-correlation. Using
recently measured cross-correlation amplitudes, arising from the
cross-correlation between galaxies and the Integrated Sachs Wolfe effect in CMB
anisotropies, we obtain a constraint r < 0.5 at 68% confidence level on the
tensor-to-scalar fluctuation amplitude ratio. The data also allow us to exclude
gravity waves at a level of a few percent, relative to the density field, in a
low - Lambda dominated universe(Omega_Lambda~0.5). In future, joining
cross-correlation ISW measurements, which captures cosmological parameter
information, with independent determinations of the matter density and CMB
anisotropy power spectrum, may constrain the tensor-to-scalar ratio to a level
above 0.05. This value is the ultimate limit on tensor-to-scalar ratio from
temperature anisotropy maps when all other cosmological parameters except for
the tensor amplitude are known and the combination with CMB-galaxy correlation
allows this limit to be reached easily by accounting for degeneracies in
certain cosmological parameters.Comment: 5 Pages, 1 Figure, revised discussion on cosmic variance limits on
the tensor-to-scalar ratio from CMB, matches PRD accepted versio
Constraining Variations in the Fine Structure Constant in the presence of Early Dark Energy
We discuss present and future cosmological constraints on variations of the
fine structure constant induced by an early dark energy component
having the simplest allowed (linear) coupling to electromagnetism. We find that
current cosmological data show no variation of the fine structure constant at
recombination respect to the present-day value, with / =
0.975 \pm 0.020 at 95 % c.l., constraining the energy density in early dark
energy to < 0.060 at 95 % c.l.. Moreover, we consider constraints on
the parameter quantifying the strength of the coupling by the scalar field. We
find that current cosmological constraints on the coupling are about 20 times
weaker than those obtainable locally (which come from Equivalence Principle
tests). However forthcoming or future missions, such as Planck Surveyor and
CMBPol, can match and possibly even surpass the sensitivity of current local
tests.Comment: 5 pages, 3 figure
New Constraints on variations of the fine structure constant from CMB anisotropies
We demonstrate that recent measurements of Cosmic Microwave Background
temperature and polarization anisotropy made by the ACBAR, QUAD and BICEP
experiments substantially improve the cosmological constraints on possible
variations of the fine structure constant in the early universe. This data,
combined with the five year observations from the WMAP mission yield the
constraint alpha/alpha_0 = 0.987 \pm 0.012 at 68% c.l.. The inclusion of the
new HST constraints on the Hubble constant further increases the accuracy to
alpha/alpha_0 = 1.001 \pm 0.007 at 68% c.l., bringing possible deviations from
the current value below the 1% level and improving previous constraints by a
factor 3.Comment: 3 pages, 2 figure
The Fine Structure Constant and the CMB Damping Scale
The recent measurements of the Cosmic Microwave Background anisotropies at
arcminute angular scales performed by the ACT and SPT experiments are probing
the damping regime of CMB fluctuations. The analysis of these datasets
unexpectedly suggests that the effective number of relativistic degrees of
freedom is larger than the standard value of Neff = 3.04, and inconsistent with
it at more than two standard deviations. In this paper we study the role of a
mechanism that could affect the shape of the CMB angular fluctuations at those
scales, namely a change in the recombination process through variations in the
fine structure constant. We show that the new CMB data significantly improve
the previous constraints on variations of {\alpha}, with {\alpha}/{\alpha}0 =
0.984 \pm 0.005, i.e. hinting also to a more than two standard deviation from
the current, local, value {\alpha}0. A significant degeneracy is present
between {\alpha} and Neff, and when variations in the latter are allowed the
constraints on {\alpha} are relaxed and again consistent with the standard
value. Deviations of either parameter from their standard values would imply
the presence of new, currently unknown physics.Comment: 4 pages, 1 figur
Bayesian Evidence for a Cosmological Constant using new High-Redshift Supernovae Data
We carry out a Bayesian model selection analysis of different dark energy
parametrizations using the recent luminosity distance data of high redshift
supernovae from Riess et al. 2007 and from the new ESSENCE Supernova Survey.
Including complementary cosmological datasets, we found substantial evidence
() against a time-varying dark energy equation of state
parameter, and against phantom dark energy models. We find a small preference
for a standard cosmological constant over accelerating non-phantom models where
w is constant, but allowed to vary in the range -1 to -0.33.Comment: 8 Pages, 8 Figures, Discussion on VEGAS extended, References added,
typos correcte
Measuring in the Early Universe: CMB Temperature, Large-Scale Structure and Fisher Matrix Analysis
We extend our recent work on the effects of a time-varying fine-structure
constant in the cosmic microwave background, by providing a thorough
analysis of the degeneracies between and the other cosmological
parameters, and discussing ways to break these with both existing and/or
forthcoming data. In particular, we present the state-of-the-art CMB
constraints on , through a combined analysis of the BOOMERanG, MAXIMA
and DASI datasets. We also present a novel discussion of the constraints on
coming from large-scale structure observations, focusing in particular
on the power spectrum from the 2dF survey. Our results are consistent with no
variation in from the epoch of recombination to the present day, and
restrict any such (relative) variation to be less than about 4%. We show that
the forthcoming MAP and (particularly) Planck experiments will be able to break
most of the currently existing degeneracies between and other
parameters, and measure to better than percent accuracy.Comment: 11 pages in RevTex4 format. Low-quality figures to comply with arXiv
restrictions (better ones available from the authors). v2: Updated Oklo
discussion, plus other cosmetic changes. Version to appear in Phys Rev
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