1,238 research outputs found
The initial conditions of the universe: how much isocurvature is allowed?
We investigate the constraints imposed by the current data on correlated
mixtures of adiabatic and non-adiabatic primordial perturbations. We discover
subtle flat directions in parameter space that tolerate large (~60%)
contributions of non-adiabatic fluctuations. In particular, larger values of
the baryon density and a spectral tilt are allowed. The cancellations in the
degenerate directions are explored and the role of priors elucidated.Comment: 4 pages, 4 figures. Submitted to PR
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
Constraints on isocurvature models from the WMAP first-year data
We investigate the constraints imposed by the first-year WMAP CMB data
extended to higher multipole by data from ACBAR, BOOMERANG, CBI and the VSA and
by the LSS data from the 2dF galaxy redshift survey on the possible amplitude
of primordial isocurvature modes. A flat universe with CDM and Lambda is
assumed, and the baryon, CDM (CI), and neutrino density (NID) and velocity
(NIV) isocurvature modes are considered. Constraints on the allowed
isocurvature contributions are established from the data for various
combinations of the adiabatic mode and one, two, and three isocurvature modes,
with intermode cross-correlations allowed. Since baryon and CDM isocurvature
are observationally virtually indistinguishable, these modes are not considered
separately. We find that when just a single isocurvature mode is added, the
present data allows an isocurvature fraction as large as 13+-6, 7+-4, and 13+-7
percent for adiabatic plus the CI, NID, and NIV modes, respectively. When two
isocurvature modes plus the adiabatic mode and cross-correlations are allowed,
these percentages rise to 47+-16, 34+-12, and 44+-12 for the combinations
CI+NID, CI+NIV, and NID+NIV, respectively. Finally, when all three isocurvature
modes and cross-correlations are allowed, the admissible isocurvature fraction
rises to 57+-9 per cent. The sensitivity of the results to the choice of prior
probability distribution is examined.Comment: 20 pages, 24 figures. Submitted to PR
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
Primordial Nucleosynthesis: an updated comparison of observational light nuclei abundances with theoretical predictions
An up to date review of Standard Big Bang Nucleosynthesis predictions vs the
astrophysical estimates of light nuclei abundances is here presented. In
particular the analysis reports the expected ranges for baryon fraction and
effective number of neutrinos as obtained by BBN only.Comment: 5 pages, 4 figures, to appear in the proceedings of NOW 200
Hybrid natural inflation from non Abelian discrete symmetry
A spontaneously broken global discrete symmetry may have pseudo Goldstone
modes associated with the spontaneous breaking of the approximate continuous
symmetry of the low dimension terms in the Lagrangian. These provide natural
candidates for an inflaton that can generate slow roll inflation. We show that,
in the case of a non Abelian discrete symmetry, the pseudo Goldstone modes
readily couple to further scalar fields in a manner that the end of inflation
is determined by these additional scalar fields, generating hybrid inflation.
We give a simple parameterisation of the inflationary potential in this case,
determine the inflationary parameters resulting, and show that phenomenological
successful inflation is possible while keeping the scale of symmetry breaking
sub-Plankian. Unlike natural inflation the inflation scale can be very low. We
construct two simple hybrid inflation models, one non supersymmetric and one
supersymmetric. In the latter case no parameters need be chosen anomalously
small.Comment: Accepted for publication in Phys. Lett.
Measuring the geometry of the Universe in the presence of isocurvature modes
The Cosmic Microwave Background (CMB) anisotropy constrains the geometry of
the Universe because the positions of the acoustic peaks of the angular power
spectrum depend strongly on the curvature of underlying three-dimensional
space. In this Letter we exploit current observations to determine the spatial
geometry of the Universe in the presence of isocurvature modes. Previous
analyses have always assumed that the cosmological perturbations were initially
adiabatic. A priori one might expect that allowing additional isocurvature
modes would substantially degrade the constraints on the curvature of the
Universe. We find, however, that if one considers additional data sets, the
geometry remains well constrained. When the most general isocurvature
perturbation is allowed, the CMB alone can only poorly constrain the geometry
to Omega_0=1.6+-0.3. Including large-scale structure (LSS) data one obtains
Omega_0=1.07+-0.03, and Omega_0=1.06+-0.02 when supplemented by the Hubble
Space Telescope (HST) Key Project determination of H_0 and SNIa data.Comment: 4 pages, 5 figs. Submitted to PR
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Angular Power Spectra
We present the temperature and polarization angular power spectra of the
cosmic microwave background (CMB) derived from the first 5 years of WMAP data.
The 5-year temperature (TT) spectrum is cosmic variance limited up to multipole
l=530, and individual l-modes have S/N>1 for l<920. The best fitting
six-parameter LambdaCDM model has a reduced chi^2 for l=33-1000 of
chi^2/nu=1.06, with a probability to exceed of 9.3%. There is now significantly
improved data near the third peak which leads to improved cosmological
constraints. The temperature-polarization correlation (TE) is seen with high
significance. After accounting for foreground emission, the low-l reionization
feature in the EE power spectrum is preferred by \Delta\chi^2=19.6 for optical
depth tau=0.089 by the EE data alone, and is now largely cosmic variance
limited for l=2-6. There is no evidence for cosmic signal in the BB, TB, or EB
spectra after accounting for foreground emission. We find that, when averaged
over l=2-6, l(l+1)C^{BB}_l/2\pi < 0.15 uK^2 (95% CL).Comment: 29 pages, 13 figures, accepted by ApJ
Fast and reliable MCMC for cosmological parameter estimation
Markov Chain Monte Carlo (MCMC) techniques are now widely used for
cosmological parameter estimation. Chains are generated to sample the posterior
probability distribution obtained following the Bayesian approach. An important
issue is how to optimize the efficiency of such sampling and how to diagnose
whether a finite-length chain has adequately sampled the underlying posterior
probability distribution. We show how the power spectrum of a single such
finite chain may be used as a convergence diagnostic by means of a fitting
function, and discuss strategies for optimizing the distribution for the
proposed steps. The methods developed are applied to current CMB and LSS data
interpreted using both a pure adiabatic cosmological model and a mixed
adiabatic/isocurvature cosmological model including possible correlations
between modes. For the latter application, because of the increased
dimensionality and the presence of degeneracies, the need for tuning MCMC
methods for maximum efficiency becomes particularly acute.Comment: 12 pages, 17 figures. Submitted to MNRA
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