831 research outputs found
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
Brane-World Motion in Compact Dimensions
The topology of extra dimensions can break global Lorentz invariance,singling
out a globally preferred frame even in flat spacetime. Through experiments that
probe global topology, an observer can determine her state of motion with
respect to the preferred frame. This scenario is realized if we live on a brane
universe moving through a flat space with compact extra dimensions. We identify
three experimental effects due to the motion of our universe that one could
potentially detect using gravitational probes. One of these relates to the
peculiar properties of the twin paradox in multiply-connected spacetimes.
Another relies on the fact that the Kaluza-Klein modes of any bulk field are
sensitive to boundary conditions. A third concerns the modification to the
Newtonian potential on a moving brane. Remarkably, we find that even small
extra dimensions are detectable by brane observers if the brane is moving
sufficiently fast
`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
On the stability and causality of scalar-vector theories
Various extensions of standard inflationary models have been proposed
recently by adding vector fields. Because they are generally motivated by
large-scale anomalies, and the possibility of statistical anisotropy of
primordial fluctuations, such models require to introduce non-standard
couplings between vector fields on the one hand, and either gravity or scalar
fields on the other hand. In this article, we study models involving a vector
field coupled to a scalar field. We derive restrictive necessary conditions for
these models to be both stable (Hamiltonian bounded by below) and causal
(hyperbolic equations of motion).Comment: 20 pages, references added, v2 matches published version in JCA
Probing dark energy beyond with CODEX
Precision measurements of nature's fundamental couplings and a first
measurement of the cosmological redshift drift are two of the key targets for
future high-resolution ultra-stable spectrographs such as CODEX. Being able to
do both gives CODEX a unique advantage, allowing it to probe dynamical dark
energy models (by measuring the behavior of their equation of state) deep in
the matter era and thereby testing classes of models that would otherwise be
difficult to distinguish from the standard CDM paradigm. We illustrate
this point with two simple case studies.Comment: 4 pages, 4 figures; submitted to Phys. Rev.
The variation of fundamental constants and the role of A=5 and A=8 nuclei on primordial nucleosynthesis
We investigate the effect of a variation of fundamental constants on
primordial element production in big bang nucleosynthesis (BBN). We focus on
the effect of a possible change in the nucleon-nucleon interaction on nuclear
reaction rates involving the A=5 (Li-5 and He-5) and A=8 (Be-8) unstable nuclei
and complement earlier work on its effect on the binding energy of deuterium.
The reaction rates for He3(d,p)He4 and H3(d,n)He4 are dominated by the
properties of broad analog resonances in He-5 and Li-5 compound nuclei
respectively. While the triple alpha process is normally not effective in BBN,
its rate is very sensitive to the position of the "Hoyle state" and could in
principle be drastically affected if Be-8 were stable during BBN. The nuclear
properties (resonance energies in He-5 and Li-5 nuclei, and the binding
energies of Be-8 and D) are all computed in a consistent way using a
microscopic cluster model. The n(p,gamma)d, He3(d,p)He4 and H3(d,n)He4 and
triple-alpha reaction rates are subsequently calculated as a function of the
nucleon-nucleon interaction that can be related to the fundamental constants.
We found that the effect of the variation of constants on the He3(d,p)He4 and
H3(d,n)He4 and triple-alpha reaction rates is not sufficient to induce a
significant effect on BBN, even if Be-8 was stable. In particular, no
significant production of carbon by the triple alpha reaction is found when
compared to standard BBN. We also update our previous analysis on the effect of
a variation of constants on the n(p,gamma)d reaction rate.Comment: 14 pages, 12 figure
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
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