24,276 research outputs found
Friedmann cosmology on codimension 2 brane with time dependent tension
A solution of codimension 2 brane is found for which 4 dimensional Friedmann
cosmology is recovered on the brane with time dependent tension, in the
Einstein frame. The effective parameter of equation of state on the
brane can be quintessence like, de Sitter like or phantom like, depending on
integration constants of the solution.Comment: 6 pages, 4 figure
Impact of radiative corrections on sterile neutrino scenarios
In sterile neutrino scenarios, radiative corrections induce mass splittings
proportional to the top Yukawa coupling, in contrast to the three active
neutrino case where the induced splittings are proportional to the tau Yukawa
coupling. In view of this, we have analyzed the stability of the four-neutrino
schemes favored by oscillation experiments, consisting in two pairs of nearly
degenerate neutrinos separated by the LSND gap. Requiring compatibility with
the measurements of the abundances of primordial elements produced in Big Bang
Nucleosynthesis, we find that when the heaviest pair corresponds to the solar
neutrinos (mainly an admixture of nu_e - nu_s) the natural mass splitting is
3-5 orders of magnitude larger than the observed one, discrediting the scenario
from a theoretical point of view. On the contrary, the scheme where the
heaviest pair corresponds to the atmospheric neutrinos (mainly an admixture of
nu_mu - nu_tau) is safe from radiative corrections due to the small sterile
component of these mass eigenstates.Comment: 14 pages, LaTeX, 2 figures. Discussion enlarged, references added and
typos correcte
Dark energy, MOND and sub-millimeter tests of gravity
We consider modifications of General Relativity obtained by adding the
logarithm of some curvature invariants to the Einstein-Hilbert action. These
non-linear actions can explain the late-time acceleration of the universe
giving an expansion history that differs from that of a pure cosmological
constant. We show that they also modify the Newtonian potential below a fixed
acceleration scale given by the late-time Hubble constant times the speed of
light. This is exactly what is required in MOND, a phenomenological
modification of the Newtonian potential that is capable of explaining galactic
rotation curves without the need to introduce dark matter. We show that this
kind of modification also predicts short distance deviations of Newton's law at
the sub-mm scale and an anomalous shift in the precession of the Moon's orbit
around the Earth, both effects of a size that is less than an order of
magnitude below current bounds.Comment: 6 pages, to appear in proceedings of the XLIrst Rencontres de Morion
Long distance modifications of gravity in four dimensions
We discuss some general characteristics of modifications of the 4D
Einstein-Hilbert action that become important for low space-time curvatures. In
particular we focus on the chameleon-like behaviour of the massive
gravitational degrees of freedom. Generically there is at least one extra
scalar that is light on cosmic scales, but for certain models it becomes heavy
close to any mass source.Comment: 4 pages, contribution to the proceedings of the Rencontres de
Moriond: Contents and Structures of the Universe, March 18-25, 2006, La
Thuil
Footballs, Conical Singularities and the Liouville Equation
We generalize the football shaped extra dimensions scenario to an arbitrary
number of branes. The problem is related to the solution of the Liouville
equation with singularities and explicit solutions are presented for the case
of three branes. The tensions of the branes do not need to be tuned with each
other but only satisfy mild global constraints.Comment: 15 pages, Refs. added, minor changes. Typo in eq. 4.3 corrected.
Version to be published in PR
Consistency Condition of Spherically Symmetric Solutions in Gravity
In this work we study the spherical symmetric solutions of gravity in
the metric formalism. We show that for a generic gravity, the spherical
symmetric solution is consistent with the modified gravity equations except in
the case of imposing an extra condition for the metric.Comment: 4 pages, accepted in MPL
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