791 research outputs found
The gravitational wave contribution to CMB anisotropies and the amplitude of mass fluctuations from COBE results
A stochastic background of primordial gravitational waves may substantially
contribute, via the Sachs--Wolfe effect, to the large--scale Cosmic Microwave
Background (CMB) anisotropies recently detected by COBE. This implies a {\it
bias} in any resulting determination of the primordial amplitude of density
fluctuations. We consider the constraints imposed on (``tilted")
power--law fluctuation spectra, taking into account the contribution from both
scalar and tensor waves, as predicted by power--law inflation. The
gravitational--wave contribution to CMB anisotropies generally reduces the
required {\it rms} level of mass fluctuation, thereby increasing the linear
{\it bias parameter}, even in models where the spectral index is close to the
Harrison--Zel'dovich value . This ``gravitational--wave bias" helps to
reconcile the predictions of CDM models with observations on pairwise galaxy
velocity dispersion on small scales.Comment: 11 pages. Two figures available upon reques
The Three--Point Correlation Function of the Cosmic Microwave Background in Inflationary Models
We analyze the temperature three--point correlation function and the skewness
of the Cosmic Microwave Background (CMB), providing general relations in terms
of multipole coefficients. We then focus on applications to large angular scale
anisotropies, such as those measured by the {\em COBE} DMR, calculating the
contribution to these quantities from primordial, inflation generated, scalar
perturbations, via the Sachs--Wolfe effect. Using the techniques of stochastic
inflation we are able to provide a {\it universal} expression for the ensemble
averaged three--point function and for the corresponding skewness, which
accounts for all primordial second--order effects. These general expressions
would moreover apply to any situation where the bispectrum of the primordial
gravitational potential has a {\em hierarchical} form. Our results are then
specialized to a number of relevant models: power--law inflation driven by an
exponential potential, chaotic inflation with a quartic and quadratic potential
and a particular case of hybrid inflation. In all these cases non--Gaussian
effects are small: as an example, the {\em mean} skewness is much smaller than
the cosmic {\em rms} skewness implied by a Gaussian temperature fluctuation
field.Comment: 18 pages; LaTeX; 4 PostScript figures included at the end of the
file; SISSA REF.193/93/A and DFPD 93/A/8
On Relativistic Perturbations of Second and Higher Order
We present the results of a study of the gauge dependence of spacetime
perturbations. In particular, we consider gauge invariance in general, we give
a generating formula for gauge transformations to an arbitrary order n, and
explicit transformation rules at second order.Comment: 6 pages, latex, with special style included, Proceedings of the 12th
Italian Conference on General Relativity and Gravitational Physic
Recent results from the Pierre Auger Observatory
The main results from the Auger Observatory are described. A steepening of
the spectrum is observed at the highest energies, supporting the expectation
that above eV the cosmic ray energies are significantly
degraded by interactions with the CMB photons (the GZK effect). This is further
supported by the correlations observed above eV with the
distribution of nearby active galaxies, which also show the potential of Auger
to start the era of charged particle astronomy. The lack of observation of
photons or neutrinos strongly disfavors top-down models, and these searches may
approach in the long term the sensitivity required to test the fluxes expected
from the secondaries of the very same GZK process. Bounds on the anisotropies
at EeV energies contradict hints from previous experiments that suggested a
large excess from regions near the Galactic centre or the presence of a dipolar
type modulation of the cosmic ray flux.Comment: 6 p., 8 figs., proceedings of the XXIII International Conference on
Neutrino Physics and Astrophysics (Neutrino 08
Relativistic second-order perturbations of the Einstein-de Sitter Universe
We consider the evolution of relativistic perturbations in the Einstein-de
Sitter cosmological model, including second-order effects. The perturbations
are considered in two different settings: the widely used synchronous gauge and
the Poisson (generalized longitudinal) one. Since, in general, perturbations
are gauge dependent, we start by considering gauge transformations at second
order. Next, we give the evolution of perturbations in the synchronous gauge,
taking into account both scalar and tensor modes in the initial conditions.
Using the second-order gauge transformation previously defined, we are then
able to transform these perturbations to the Poisson gauge. The most important
feature of second-order perturbation theory is mode-mixing, which here also
means, for instance, that primordial density perturbations act as a source for
gravitational waves, while primordial gravitational waves give rise to
second-order density fluctuations. Possible applications of our formalism range
from the study of the evolution of perturbations in the mildly non-linear
regime to the analysis of secondary anisotropies of the Cosmic Microwave
Background.Comment: Revised version with minor changes, accepted in The Physical Review
D; LaTeX file using RevTeX macros, 28 page
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