790 research outputs found
`c' is the speed of light, isn't it?
Theories proposing a varying speed of light have recently been widely
promoted under the claim that they offer an alternative way of solving the
standard cosmological problems. Recent observational hints that the fine
structure constant may have varied during over cosmological scales also has
given impetus to these models. In theoretical physics the speed of light, ,
is hidden in almost all equations but with different facets that we try to
distinguish. Together with a reminder on scalar-tensor theories of gravity,
this sheds some light on these proposed varying speed of light theories.Comment: 14 pages, Late
The blackness of the cosmic microwave background spectrum as a probe of the distance-duality relation
A violation of the reciprocity relation, which induces a violation of the
distance duality relation, reflects itself in a change in the normalisation of
the cosmic microwave spectrum in such a way that its spectrum is grey. We show
that existing observational constraints imply that the reciprocity relation
cannot be violated by more than 0.01% between decoupling and today. We compare
this effect to other sources of violation of the distance duality relations
which induce spectral distortion of the cosmic microwave background spectrum.Comment: 5 pages, 2 figures, references added, minor typos correcte
Fundamental constants and tests of general relativity - Theoretical and cosmological considerations
The tests of the constancy of the fundamental constants are tests of the
local position invariance and thus of the equivalence principle. We summarize
the various constraints that have been obtained and then describe the
connection between varying constants and extensions of general relativity. To
finish, we discuss the link with cosmology, and more particularly with the
acceleration of the Universe. We take the opportunity to summarize various
possibilities to test general relativity (but also the Copernican principle) on
cosmological scales.Comment: Proceedings of the workshop ``The nature of gravity, confronting
theory and experiment in space'', ISSI, Bern, october 200
Quantum Corrections to the Cosmological Evolution of Conformally Coupled Fields
Because the source term for the equations of motion of a conformally coupled
scalar field, such as the dilaton, is given by the trace of the matter energy
momentum tensor, it is commonly assumed to vanish during the radiation
dominated epoch in the early universe. As a consequence, such fields are
generally frozen in the early universe. Here we compute the finite temperature
radiative correction to the source term and discuss its consequences on the
evolution of such fields in the early universe. We discuss in particular, the
case of scalar tensor theories of gravity which have general relativity as an
attractor solution. We show that, in some cases, the universe can experience an
early phase of contraction, followed by a non-singular bounce, and standard
expansion. This can have interesting consequences for the abundance of thermal
relics; for instance, it can provide a solution to the gravitino problem. We
conclude by discussing the possible consequences of the quantum corrections to
the evolution of the dilaton.Comment: 24 pages, 7 figure
Constraints on mode couplings and modulation of the CMB with WMAP data
We investigate a possible asymmetry in the statistical properties of the
cosmic microwave background temperature field and to do so we construct an
estimator aiming at detecting a dipolar modulation. Such a modulation is found
to induce correlations between multipoles with . Applying this
estimator, to the V and W bands of the WMAP data, we found a significant
detection in the V band. We argue however that foregrounds and in particular
point sources are the origin of this signal.Comment: 14 pages, 14 figure
Distinguishing Marks of Simply-connected Universes
A statistical quantity suitable for distinguishing simply-connected
Robertson-Walker (RW) universes is introduced, and its explicit expressions for
the three possible classes of simply-connected RW universes with an uniform
distribution of matter are determined. Graphs of the distinguishing mark for
each class of RW universes are presented and analyzed.There sprout from our
results an improvement on the procedure to extract the topological signature of
multiply-connected RW universes, and a refined understanding of that
topological signature of these universes studied in previous works.Comment: 13 pages, 4 figures, LaTeX2e. To appear in Int. J. Mod. Phys. D
(2000
Gyromagnetic Factors and Atomic Clock Constraints on the Variation of Fundamental Constants
We consider the effect of the coupled variations of fundamental constants on
the nucleon magnetic moment. The nucleon g-factor enters into the
interpretation of the measurements of variations in the fine-structure
constant, alpha, in both the laboratory (through atomic clock measurements) and
in astrophysical systems (e.g. through measurements of the 21 cm transitions).
A null result can be translated into a limit on the variation of a set of
fundamental constants, that is usually reduced to alpha. However, in specific
models, particularly unification models, changes in alpha are always
accompanied by corresponding changes in other fundamental quantities such as
the QCD scale, Lambda_QCD. This work tracks the changes in the nucleon
g-factors induced from changes in Lambda_QCD and the light quark masses. In
principle, these coupled variations can improve the bounds on the variation of
alpha by an order of magnitude from existing atomic clock and astrophysical
measurements. Unfortunately, the calculation of the dependence of g-factors on
fundamental parameters is notoriously model-dependent.Comment: 35 pages, 3 figures. Discussions of the effects of the polarization
of the non-valence nucleons, spin-spin interaction and nuclear radius on the
nuclear g-factor are added. References added. Matches published versio
Weak lensing B-modes on all scales as a probe of local isotropy
This article derives a multipolar hierarchy for the propagation of the
weak-lensing shear and convergence in a general spacetime. The origin of
B-modes, in particular on large angular scales, is related to the local
isotropy of space. Known results assuming a Friedmann-Lema\^itre background are
naturally recovered. The example of a Bianchi I spacetime illustrates our
formalism and its implications for future observations are stressed.Comment: 10 pages, 2 figures. Replaced to match published versio
CMB temperature anisotropy at large scales induced by a causal primordial magnetic field
We present an analytical derivation of the Sachs Wolfe effect sourced by a
primordial magnetic field. In order to consistently specify the initial
conditions, we assume that the magnetic field is generated by a causal process,
namely a first order phase transition in the early universe. As for the
topological defects case, we apply the general relativistic junction conditions
to match the perturbation variables before and after the phase transition which
generates the magnetic field, in such a way that the total energy momentum
tensor is conserved across the transition and Einstein's equations are
satisfied. We further solve the evolution equations for the metric and fluid
perturbations at large scales analytically including neutrinos, and derive the
magnetic Sachs Wolfe effect. We find that the relevant contribution to the
magnetic Sachs Wolfe effect comes from the metric perturbations at
next-to-leading order in the large scale limit. The leading order term is in
fact strongly suppressed due to the presence of free-streaming neutrinos. We
derive the neutrino compensation effect dynamically and confirm that the
magnetic Sachs Wolfe spectrum from a causal magnetic field behaves as
l(l+1)C_l^B \propto l^2 as found in the latest numerical analyses.Comment: 31 pages, 2 figures, minor changes, matches published versio
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