17,704 research outputs found
Non-Gaussianities from isocurvature modes
This contribution discusses isocurvature modes, in particular the
non-Gaussianities of local type generated by these modes. Since the
isocurvature transfer functions differ from the adiabatic one, the coexistence
of a primordial isocurvature mode with the usual adiabatic mode leads to a rich
structure of the angular bispectrum, which can be decomposed into six
elementary bispectra. Future analysis of the CMB data will enable to measure
their relative weights, or at least constrain them. Non-Gaussianity thus
provides a new window on isocurvature modes. This is particularly relevant for
some scenarios, such as those presented here, which generate isocurvature modes
whose contribution in the power spectrum is suppressed, as required by present
data, but whose contribution in the non-Gaussianities could be dominant and
measurable.Comment: 8 pages, 2 figures; to appear in the Proceedings of COSGRAV-2012
(International Conference on Modern Perspectives of Cosmology and
Gravitation), Indian Statistical Institute, Kolkata, India, February 7-11,
201
Hawking radiation for Dirac spinors on the RP^3 geon
We analyse the Hawking(-Unruh) effect for a massive Dirac spinor on the Z_2
quotient of Kruskal spacetime known as the RP^3 geon. There are two distinct
Hartle-Hawking-like vacua, depending on the choice of the spin structure, and
suitable measurements in the static region (which on its own has only one spin
structure) distinguish these two vacua. However, both vacua appear thermal in
the usual Hawking temperature to certain types of restricted operators,
including operators with support in the asymptotic future (or past). Similar
results hold in a family of topologically analogous flat spacetimes, where we
show the two vacua to be distinguished also by the shear stresses in the
zero-mass limit. As a by-product, we display the explicit Bogolubov
transformation between the Rindler-basis and the Minkowski-basis for massive
Dirac fermions in four-dimensional Minkowski spacetime.Comment: 23 pages, no figures, uses amsmath, amssymb, latexsym packages,
replaced with version published in Phys.Rev.
Cosmological evolution in a brane-universe
The idea of extra-dimensions has recently gone through a renewal with the
hypothesis, suggested by recent developments in string theory, that ordinary
matter is confined to a sub-space, called brane, embedded in a higher
dimensional spacetime. I summarize here some consequences in cosmology of this
type of models. The most remarkable aspect is that the Friedmann laws, which
govern the expansion of the Universe, are modified. An important direction of
research is the study of cosmological perturbations and the possible signature
of extra-dimensions in cosmological observations.Comment: 2 pages, "mprocl.sty" with LaTeX 2.09; contribution to the 9th Marcel
Grossmann Meeting (MG9), Rome, July 200
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