13 research outputs found
New varying speed of light theories
We review recent work on the possibility of a varying speed of light (VSL).
We start by discussing the physical meaning of a varying , dispelling the
myth that the constancy of is a matter of logical consistency. We then
summarize the main VSL mechanisms proposed so far: hard breaking of Lorentz
invariance; bimetric theories (where the speeds of gravity and light are not
the same); locally Lorentz invariant VSL theories; theories exhibiting a color
dependent speed of light; varying induced by extra dimensions (e.g. in the
brane-world scenario); and field theories where VSL results from vacuum
polarization or CPT violation. We show how VSL scenarios may solve the
cosmological problems usually tackled by inflation, and also how they may
produce a scale-invariant spectrum of Gaussian fluctuations, capable of
explaining the WMAP data. We then review the connection between VSL and
theories of quantum gravity, showing how ``doubly special'' relativity has
emerged as a VSL effective model of quantum space-time, with observational
implications for ultra high energy cosmic rays and gamma ray bursts. Some
recent work on the physics of ``black'' holes and other compact objects in VSL
theories is also described, highlighting phenomena associated with spatial (as
opposed to temporal) variations in . Finally we describe the observational
status of the theory. The evidence is currently slim -- redshift dependence in
the atomic fine structure, anomalies with ultra high energy cosmic rays, and
(to a much lesser extent) the acceleration of the universe and the WMAP data.
The constraints (e.g. those arising from nucleosynthesis or geological bounds)
are tight, but not insurmountable. We conclude with the observational
predictions of the theory, and the prospects for its refutation or vindication.Comment: Final versio
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
Multidimensional cosmological models: cosmological and astrophysical implications and constraints
We investigate four-dimensional effective theories which are obtained by
dimensional reduction of multidimensional cosmological models with factorizable
geometry and consider the interaction between conformal excitations of the
internal space (geometrical moduli excitations) and Abelian gauge fields. It is
assumed that the internal space background can be stabilized by minima of an
effective potential. The conformal excitations over such a background have the
form of massive scalar fields (gravitational excitons) propagating in the
external spacetime. We discuss cosmological and astrophysical implications of
the interaction between gravexcitons and four-dimensional photons as well as
constraints arising on multidimensional models of the type considered in our
paper. In particular, we show that due to the experimental bounds on the
variation of the fine structure constant, gravexcitons should decay before
nucleosynthesis starts. For a successful nucleosynthesis the masses of the
decaying gravexcitons should be m>10^4 GeV. Furthermore, we discuss the
possible contribution of gravexcitons to UHECR. It is shown that, at energies
of about 10^{20}eV, the decay length of gravexcitons with masses m>10^4 GeV is
very small, but that for m <10^2 GeV it becomes much larger than the
Greisen-Zatsepin-Kuzmin cut-off distance. Finally, we investigate the
possibility for gravexciton-photon oscillations in strong magnetic fields of
astrophysical objects. The corresponding estimates indicate that even the high
magnetic field strengths of magnetars are not sufficient for an efficient and
copious production of gravexcitons.Comment: 16 pages, LaTeX2e, minor changes, improved references, to appear in
PR