1,080 research outputs found
A reassessment of the evidence of the Integrated Sachs-Wolfe effect through the WMAP-NVSS correlation
We reassess the estimate of the cross-correlation of the spatial distribution
of the NRAO VLA Sky Survey (NVSS) radio sources with that of Cosmic Microwave
Background (CMB) anisotropies from the Wilkinson Microwave Anisotropy Probe
(WMAP). This re-analysis is motivated by the fact that most previous studies
adopted a redshift distribution of NVSS sources inconsistent with recent data.
We find that the constraints on the bias-weighted redshift distribution,
b(z)xN(z), of NVSS sources, set by the observed angular correlation function,
w(theta), strongly mitigate the effect of the choice of N(z). If such
constraints are met, even highly discrepant redshift distributions yield
NVSS-WMAP cross-correlation functions consistent with each other within
statistical errors. The models favoured by recent data imply a bias factor,
b(z), decreasing with increasing z, rather than constant, as assumed by most
previous analyses. As a consequence, the function b(z)xN(z) has more weight at
z<1, i.e. in the redshift range yielding the maximum contribution to the ISW in
a standard LambdaCDM cosmology. On the whole, the NVSS turns out to be better
suited for ISW studies than generally believed, even in the absence of an
observational determination of the redshift distribution. The NVSS-WMAP
cross-correlation function is found to be fully consistent with the prediction
of the standard LambdaCDM cosmology.Comment: 6 pages, 2 figures, submitted to MNRA
General Relativistic Dynamics of Irrotational Dust: Cosmological Implications
The non--linear dynamics of cosmological perturbations of an irrotational
collisionless fluid is analyzed within General Relativity. Relativistic and
Newtonian solutions are compared, stressing the different role of boundary
conditions in the two theories. Cosmological implications of relativistic
effects, already present at second order in perturbation theory, are studied
and the dynamical role of the magnetic part of the Weyl tensor is elucidated.Comment: 12 pages , DFPD 93/A/6
Stochastic Inflation and the Lower Multipoles in the CMB Anisotropies
We generalize the treatment of inflationary perturbations to deal with the
non-Markovian colored noise emerging from any realistic approach to stochastic
inflation. We provide a calculation of the power-spectrum of the
gauge-invariant comoving curvature perturbation to first order in the slow-roll
parameters. Properly accounting for the constraint that our local patch of the
Universe is homogeneous on scales just above the present Hubble radius, we find
a blue tilt of the power-spectrum on the largest observable scales, in
agreement with the WMAP data which show an unexpected suppression of the low
multipoles of the CMB anisotropy. Our explanation of the anomalous behaviour of
the lower multipoles of the CMB anisotropies does not invoke any ad-hoc
introduction of new physical ingredients in the theory.Comment: 9 pages, 2 figure
Non-linear Power Spectrum including Massive Neutrinos: the Time-RG Flow Approach
Future large scale structure observations are expected to be sensitive to
small neutrino masses, of the order of 0.05 eV or more. However, forecasts are
based on the assumption that by the time at which these datasets will be
available, the non-linear spectrum in presence of neutrino mass will be
predicted with an accuracy at least equal to the neutrino mass effect itself,
i.e. about 3%. Motivated by these considerations, we present the computation of
the non-linear power spectrum of LambdaCDM models in the presence of massive
neutrinos using the Renormalization Group time-flow approach, which amounts to
a resummation of perturbative corrections to the matter power spectrum to all
orders. We compare our results with those obtained with other methods, i.e.
linear theory, one-loop perturbation theory and N-body simulations and show
that the time-RG method improves the one-loop method in fitting the N-body
data, especially in determining the suppression of the matter power spectrum
when neutrino are massive with respect to the linear power spectrum.Comment: 8 pages, 11 figure
On the Physical Significance of Infra-red Corrections to Inflationary Observables
Inflationary observables, like the power spectrum, computed at one- and
higher-order loop level seem to be plagued by large infra-red corrections. In
this short note, we point out that these large infra-red corrections appear
only in quantities which are not directly observable. This is in agreement with
general expectations concerning infra-red effects.Comment: 11 pages; LateX file; 5 figures. Some coefficients in Eq.(A6)
corrected; References adde
Signatures of Primordial Non-Gaussianity in the Large-Scale Structure of the Universe
We discuss how primordial (e.g. inflationary) non-Gaussianity in the
cosmological perturbations is left imprinted in the Large-Scale Structure of
the universe. Our findings show that the information on the primordial
non-Gaussianity set on super-Hubble scales flows into Post-Newtonian terms,
leaving an observable imprint in the Large-Scale Structure. Future
high-precision measurements of the statistics of the dark matter density and
peculiar velocity fields will allow to pin down the primordial non-Gaussianity,
thus representing a tool complementary to studies of the Cosmic Microwave
Background anisotropies.Comment: 8 pages, LaTeX file; Revised to match the final version accepted for
publication on JCAP (some comments and one figure added
Strip blowing from a wedge at hypersonic speeds
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76052/1/AIAA-12372-854.pd
Constraining Warm Dark Matter candidates including sterile neutrinos and light gravitinos with WMAP and the Lyman-alpha forest
The matter power spectrum at comoving scales of (1-40) h^{-1} Mpc is very
sensitive to the presence of Warm Dark Matter (WDM) particles with large free
streaming lengths. We present constraints on the mass of WDM particles from a
combined analysis of the matter power spectrum inferred from the large samples
of high resolution high signal-to-noise Lyman-alpha forest data of Kim et al.
(2004) and Croft et al. (2002) and the cosmic microwave background data of
WMAP. We obtain a lower limit of m_wdm > 550 eV (2 sigma for early decoupled
thermal relics and m_wdm > 2.0 keV (2 sigma) for sterile neutrinos. We also
investigate the case where in addition to cold dark matter a light thermal
gravitino with fixed effective temperature contributes significantly to the
matter density. In that case the gravitino density is proportional to its mass,
and we find an upper limit m_{3/2} < 16 eV (2 sigma). This translates into a
bound on the scale of supersymmetry breaking, Lambda_{susy} < 260 TeV, for
models of supersymmetric gauge mediation in which the gravitino is the lightest
supersymmetric particle.Comment: 10 pages, 5 figures, 1 table. PRD in pres
Signatures of Primordial non-Gaussianities in the Matter Power-Spectrum and Bispectrum: the Time-RG Approach
We apply the time-renormalization group approach to study the effect of
primordial non-Gaussianities in the non-linear evolution of cosmological dark
matter density perturbations. This method improves the standard perturbation
approach by solving renormalization group-like equations governing the dynamics
of gravitational instability. The primordial bispectra constructed from the
dark matter density contrast and the velocity fields represent initial
conditions for the renormalization group flow. We consider local, equilateral
and folded shapes for the initial non-Gaussianity and analyze as well the case
in which the non-linear parameter f_{NL} parametrizing the strength of the
non-Gaussianity depends on the momenta in Fourier space through a power-law
relation, the so-called running non-Gaussianity. For the local model of
non-Gaussianity we compare our findings for the power-spectrum with those of
recent N-body simulations and find that they accurately fit the N-body data up
to wave-numbers k \sim 0.25 h/Mpc at z=0. We also present predictions for the
(reduced) matter bispectra for the various shapes of non-Gaussianity.Comment: 27 pages, 12 figures. Results and discussion for a particular case
added. One figure and one reference added. Matches with the version accepted
for publication in the JCAP
Perturbations of spacetime: gauge transformations and gauge invariance at second order and beyond
We consider in detail the problem of gauge dependence that exists in
relativistic perturbation theory, going beyond the linear approximation and
treating second and higher order perturbations. We first derive some
mathematical results concerning the Taylor expansion of tensor fields under the
action of one-parameter families (not necessarily groups) of diffeomorphisms.
Second, we define gauge invariance to an arbitrary order . Finally, we give
a generating formula for the gauge transformation to an arbitrary order and
explicit rules to second and third order. This formalism can be used in any
field of applied general relativity, such as cosmological and black hole
perturbations, as well as in other spacetime theories. As a specific example,
we consider here second order perturbations in cosmology, assuming a flat
Robertson-Walker background, giving explicit second order transformations
between the synchronous and the Poisson (generalized longitudinal) gauges.Comment: slightly revised version, accepted for publication in Classical and
Quantum Gravity. 27 pages including 4 figures, latex using 2 CQG style files:
ioplppt.sty, iopl10.st
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