43 research outputs found
Optimal dataset combining in f_nl constraints from large scale structure in an idealised case
We consider the problem of optimal weighting of tracers of structure for the
purpose of constraining the non-Gaussianity parameter f_NL. We work within the
Fisher matrix formalism expanded around fiducial model with f_NL=0 and make
several simplifying assumptions. By slicing a general sample into infinitely
many samples with different biases, we derive the analytic expression for the
relevant Fisher matrix element. We next consider weighting schemes that
construct two effective samples from a single sample of tracers with a
continuously varying bias. We show that a particularly simple ansatz for
weighting functions can recover all information about f_NL in the initial
sample that is recoverable using a given bias observable and that simple
division into two equal samples is considerably suboptimal when sampling of
modes is good, but only marginally suboptimal in the limit where Poisson errors
dominate.Comment: 6 pages, 5 figures; v2: comment on weighting for PS determination,
fixed a couple of typos; v3: revised, matches version accepted by JCA
Standard and non-standard primordial neutrinos
The standard cosmological model predicts the existence of a cosmic neutrino
background with a present density of about 110 cm^{-3} per flavour, which
affects big-bang nucleosynthesis, cosmic microwave background anisotropies, and
the evolution of large scale structures. We report on a precision calculation
of the cosmic neutrino background properties including the modification
introduced by neutrino oscillations. The role of a possible
neutrino-antineutrino asymmetry and the impact of non-standard
neutrino-electron interactions on the relic neutrinos are also briefly
discussed.Comment: 4 pages, no figures. Contribution to the proceedings of SNOW 2006,
Stockholm, May 2-6, 2006. Typos corrected, updated reference
The Cosmic Neutrino Background and the Age of the Universe
We discuss the cosmological degeneracy between the age of the Universe, the
Hubble parameter and the effective number of relativistic particles N_eff. We
show that independent determinations of the Hubble parameter H(z) as those
recently provided by Simon,Verde, Jimenez (2006), combined with other
cosmological data sets can provide the most stringent constraint on N_eff,
yielding N_eff=3.7 (-1.2) (+1.1) at 95% confidence level. A neutrino background
is detected with high significance: N_eff >1.8 at better than 99% confidence
level. Constraints on the age of the universe in the framework of an extra
background of relativistic particles are improved by a factor 3.Comment: JCAP, in pres
The Price of WMAP Inflation in Supergravity
The three-year data from WMAP are in stunning agreement with the simplest
possible quadratic potential for chaotic inflation, as well as with new or
symmetry-breaking inflation. We investigate the possibilities for incorporating
these potentials within supergravity, particularly of the no-scale type that is
motivated by string theory. Models with inflation driven by the matter sector
may be constructed in no-scale supergravity, if the moduli are assumed to be
stabilised by some higher-scale dynamics and at the expense of some
fine-tuning. We discuss specific scenarios for stabilising the moduli via
either D- or F-terms in the effective potential, and survey possible
inflationary models in the presence of D-term stabilisation.Comment: 15 pages, 6 figures, plain Late
Hilltop Non-Gaussianity
We study non-Gaussianity induced by a pseudo Nambu-Goldstone boson with a
cosine-type scalar potential. We focus on how the non-Gaussianity is affected
when the pseudo Nambu-Goldstone boson rolls down from near the top of the
scalar potential where the deviation from a quadratic potential is large. We
find that the resultant non-Gaussianity is similar to that obtained in the
quadratic potential, if the pseudo Nambu-Goldstone boson accounts for the
curvature perturbation; the non-Gaussianity is enhanced, otherwise.Comment: 12 pages, 3 figures; References added; to appear in JCA
Cosmology of neutrinos and extra light particles after WMAP3
We study how present data probe standard and non-standard properties of
neutrinos and the possible existence of new light particles, freely-streaming
or interacting, among themselves or with neutrinos. Our results include: sum
m_nu < 0.40 eV at 99.9% C.L.; that extra massless particles have abundance
Delta N_nu = 2 pm 1 if freely-streaming and Delta N_nu = 0 pm 1.3 if
interacting; that 3 interacting neutrinos are disfavored at about 4 sigma. We
investigate the robustness of our results by fitting to different sub-sets of
data. We developed our own cosmological computational tools, somewhat different
from the standard ones.Comment: 18 pages, 8 figures. Added in v2: an explicit comparison of our code
with CAMB, some clarifications on the statistical analysis and some
references. Matches version published in JCA
Crossing the phantom divide without phantom matter
A class of braneworld models can lead to phantom-like acceleration of the
late universe, but without the need for any phantom matter. In the simplest
models, the universe contains only cold dark matter and a cosmological
constant. We generalize these models by introducing a quintessence field. The
new feature in our models is that quintessence leads to a crossing of the
phantom divide, . This is a purely gravitational effect, and there is no
phantom instability. Furthermore, the Hubble parameter is always decreasing,
and there is no big rip singularity in the future.Comment: 5 pages, 5 figures, revtex
Non-Gaussianity from isocurvature perturbations
We develop a formalism to study non-Gaussianity in both curvature and
isocurvature perturbations. It is shown that non-Gaussianity in the
isocurvature perturbation between dark matter and photons leaves distinct
signatures in the CMB temperature fluctuations, which may be confirmed in
future experiments, or possibly, even in the currently available observational
data. As an explicit example, we consider the QCD axion and show that it can
actually induce sizable non-Gaussianity for the inflationary scale, H_{inf} =
O(10^9 - 10^{11})GeV.Comment: 24 pages, 6 figures; references added; version to appear in JCA
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
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