74 research outputs found
Evidence for extra radiation? Profile likelihood versus Bayesian posterior
A number of recent analyses of cosmological data have reported hints for the
presence of extra radiation beyond the standard model expectation. In order to
test the robustness of these claims under different methods of constructing
parameter constraints, we perform a Bayesian posterior-based and a likelihood
profile-based analysis of current data. We confirm the presence of a slight
discrepancy between posterior- and profile-based constraints, with the
marginalised posterior preferring higher values of the effective number of
neutrino species N_eff. This can be traced back to a volume effect occurring
during the marginalisation process, and we demonstrate that the effect is
related to the fact that cosmic microwave background (CMB) data constrain N_eff
only indirectly via the redshift of matter-radiation equality. Once present CMB
data are combined with external information about, e.g., the Hubble parameter,
the difference between the methods becomes small compared to the uncertainty of
N_eff. We conclude that the preference of precision cosmological data for
excess radiation is "real" and not an artifact of a specific choice of
credible/confidence interval construction.Comment: 10 pages, 4 figures; v2: discussion section expanded and references
added, version accepted for publication by JCA
Detection of relic gravitational waves in the CMB: Prospects for CMBPol mission
Detection of relic gravitational waves, through their imprint in the cosmic
microwave background radiation, is one of the most important tasks for the
planned CMBPol mission. In the simplest viable theoretical models the
gravitational wave background is characterized by two parameters, the
tensor-to-scalar ratio and the tensor spectral index . In this paper,
we analyze the potential joint constraints on these two parameters, and
, using the potential observations of the CMBPol mission, which is
expected to detect the relic gravitational waves if . The
influence of the contaminations, including cosmic weak lensing, various
foreground emissions, and systematical errors, is discussed.Comment: 26 pages, 19 figures, 4 tables; JCAP in pres
Cosmic string parameter constraints and model analysis using small scale Cosmic Microwave Background data
We present a significant update of the constraints on the Abelian Higgs
cosmic string tension by cosmic microwave background (CMB) data, enabled both
by the use of new high-resolution CMB data from suborbital experiments as well
as the latest results of the WMAP satellite, and by improved predictions for
the impact of Abelian Higgs cosmic strings on the CMB power spectra. The new
cosmic string spectra (presented in a previous work) were improved especially
for small angular scales, through the use of larger Abelian Higgs string
simulations and careful extrapolation. If Abelian Higgs strings are present
then we find improved bounds on their contribution to the CMB anisotropies,
f10< 0.095, and on their tension, G\mu< 0.57 x 10^-6, both at 95% confidence
level using WMAP7 data; and f10 < 0.048 and G\mu < 0.42 x 10^-6 using all the
CMB data. We also find that using all the CMB data, a scale invariant initial
perturbation spectrum, ns=1, is now disfavoured at 2.4\sigma\ even if strings
are present. A Bayesian model selection analysis no longer indicates a
preference for strings.Comment: 8 pages, 3 figures; Minor corrections, matches published versio
Causality and Primordial Tensor Modes
We introduce the real space correlation function of -mode polarization of
the cosmic microwave background (CMB) as a probe of superhorizon tensor
perturbations created by inflation. By causality, any non-inflationary
mechanism for gravitational wave production after reheating, like global phase
transitions or cosmic strings, must have vanishing correlations for angular
separations greater than the angle subtended by the particle horizon at
recombination, i.e. . Since ordinary -modes are
defined non-locally in terms of the Stokes parameters and and therefore
don't have to respect causality, special care is taken to define `causal
-modes' for the analysis. We compute the real space -mode
correlation function for inflation and discuss its detectability on
superhorizon scales where it provides an unambiguous test of inflationary
gravitational waves. The correct identification of inflationary tensor modes is
crucial since it relates directly to the energy scale of inflation. Wrongly
associating tensor modes from causal seeds with inflation would imply an
incorrect inference of the energy scale of inflation. We find that the
superhorizon -mode signal is above cosmic variance for the angular
range and is therefore in principle detectable. In
practice, the signal will be challenging to measure since it requires
accurately resolving the recombination peak of the -mode power spectrum.
However, a future CMB satellite (CMBPol), with noise level K-arcmin and sufficient resolution to efficiently correct for
lensing-induced -modes, should be able to detect the signal at more than
3 if the tensor-to-scalar ratio isn't smaller than .Comment: 19 pages, 7 figure
Primordial Black Holes, Eternal Inflation, and the Inflationary Parameter Space after WMAP5
We consider constraints on inflation driven by a single, minimally coupled
scalar field in the light of the WMAP5 dataset, as well as ACBAR and the
SuperNova Legacy Survey. We use the Slow Roll Reconstruction algorithm to
derive optimal constraints on the inflationary parameter space. The scale
dependence in the slope of the scalar spectrum permitted by WMAP5 is large
enough to lead to viable models where the small scale perturbations have a
substantial amplitude when extrapolated to the end of inflation. We find that
excluding parameter values which would cause the overproduction of primordial
black holes or even the onset of eternal inflation leads to potentially
significant constraints on the slow roll parameters. Finally, we present a more
sophisticated approach to including priors based on the total duration of
inflation, and discuss the resulting restrictions on the inflationary parameter
space.Comment: v2: version published in JCAP. Minor clarifications and references
adde
Reconstruction of the primordial power spectrum from CMB data
Measuring the deviation from scale invariance of the primordial power
spectrum is a critical test of inflation. In this paper we reconstruct the
shape of the primordial power spectrum of curvature perturbations from the
cosmic microwave background data, including the 7-year Wilkinson Microwave
Anisotropy Probe data and the Atacama Cosmology Telescope 148 GHz data, by
using a binning method of a cubic spline interpolation in log-log space. We
find that the power-law spectrum is preferred by the data and that the
Harrison-Zel'dovich spectrum is disfavored at 95% confidence level. These
conclusions hold with and without allowing for tensor modes, however the
simpler model without tensors is preferred by the data. We do not find evidence
for a feature in the primordial power spectrum - in full agreement with generic
predictions from cosmological inflation.Comment: 9 pages, 2 figures, 3 tables, JCAP style, published versio
How light can the lightest neutralino be?
In this talk we summarize previous work on mass bounds of a light neutralino
in the Minimal Supersymmetric Standard Model. We show that without the GUT
relation between the gaugino mass parameters M_1 and M_2, the mass of the
lightest neutralino is essentially unconstrained by collider bounds and
precision observables. We conclude by considering also the astrophysics and
cosmology of a light neutralino.Comment: 6 pages, 3 figures, to appear in the proceedings of the 16th
International Symposium on Particles, Strings and Cosmology (PASCOS2010),
Valencia (Spain), July 19th - 23rd, 201
Running Spectral Index from Inflation with Modulations
We argue that a large negative running spectral index, if confirmed, might
suggest that there are abundant structures in the inflaton potential, which
result in a fairly large (both positive and negative) running of the spectral
index at all scales. It is shown that the center value of the running spectral
index suggested by the recent CMB data can be easily explained by an inflaton
potential with superimposed periodic oscillations. In contrast to cases with
constant running, the perturbation spectrum is enhanced at small scales, due to
the repeated modulations. We mention that such features at small scales may be
seen by 21 cm observations in the future.Comment: 7 pages, 6 figures, v2: published in JCA
Model selection applied to reconstruction of the Primordial Power Spectrum
The preferred shape for the primordial spectrum of curvature perturbations is
determined by performing a Bayesian model selection analysis of cosmological
observations. We first reconstruct the spectrum modelled as piecewise linear in
\log k between nodes in k-space whose amplitudes and positions are allowed to
vary. The number of nodes together with their positions are chosen by the
Bayesian evidence, so that we can both determine the complexity supported by
the data and locate any features present in the spectrum. In addition to the
node-based reconstruction, we consider a set of parameterised models for the
primordial spectrum: the standard power-law parameterisation, the spectrum
produced from the Lasenby & Doran (LD) model and a simple variant
parameterisation. By comparing the Bayesian evidence for different classes of
spectra, we find the power-law parameterisation is significantly disfavoured by
current cosmological observations, which show a preference for the LD model.Comment: Minor changes to match version accepted by JCA
Cosmological constraints on neutrino plus axion hot dark matter: Update after WMAP-5
We update our previous constraints on two-component hot dark matter (axions
and neutrinos), including the recent WMAP 5-year data release. Marginalising
over sum m_nu provides m_a < 1.02 eV (95% C.L.) for the axion mass. In the
absence of axions we find sum m_nu < 0.63 eV (95% C.L.).Comment: 4 pages, 1 figure, uses iopart.cls; v2 matches published versio
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