233 research outputs found
Is it e or is it c? Experimental Tests of Varying Alpha
Is the recent evidence for a time-varying fine structure 'constant'
to be interpreted as a varying , , , or a combination thereof? We
consider the simplest varying electric charge and varying speed of light
theories (VSL) and prove that for the same type of dark matter they predict
opposite senses of variation in over cosmological times. We also show
that unlike varying theories, VSL theories do not predict violations of the
weak equivalence principle (WEP). Varying theories which explain
astronomical inferences of varying predict WEP violations only an
order of magnitude smaller than existing E\"otv\"os experiment limits but could
be decisively tested by STEP. We finally exhibit a set of atomic-clock and
related experiments for which {\it all} (hyperbolic) varying theories
predict non-null results. They provide independent tests of the recent
astronomical evidence
Qualitative Analysis of Universes with Varying Alpha
Assuming a Friedmann universe which evolves with a power-law scale factor,
, we analyse the phase space of the system of equations that describes
a time-varying fine structure 'constant', , in the
Bekenstein-Sandvik-Barrow-Magueijo generalisation of general relativity. We
have classified all the possible behaviours of in ever-expanding
universes with different and find new exact solutions for . We
find the attractors points in the phase space for all . In general, will be a non-decreasing function of time that increases logarithmically in
time during a period when the expansion is dust dominated (), but
becomes constant when . This includes the case of negative-curvature
domination (). also tends rapidly to a constant when the
expansion scale factor increases exponentially. A general set of conditions is
established for to become asymptotically constant at late times in an
expanding universe.Comment: 26 pages, 6 figure
Variable-Speed-of-Light Cosmology and Second Law of Thermodynamics
We examine whether the cosmologies with varying speed of light (VSL) are
compatible with the second law of thermodynamics. We find that the VSL
cosmology with varying fundamental constant is severely constrained by the
second law of thermodynamics, whereas the bimetric cosmological models are less
constrained.Comment: 15 pages, LaTeX, Revised version with minor corrections to appear in
Phys. Rev.
Quantum cosmology with big-brake singularity
We investigate a cosmological model with a big-brake singularity in the
future: while the first time derivative of the scale factor goes to zero, its
second time derivative tends to minus infinity. Although we also discuss the
classical version of the model in some detail, our main interest lies in its
quantization. We formulate the Wheeler-DeWitt equation and derive solutions
describing wave packets. We show that all such solutions vanish in the region
of the classical singularity, a behaviour which we interpret as singularity
avoidance. We then discuss the same situation in loop quantum cosmology. While
this leads to a different factor ordering, the singularity is there avoided,
too.Comment: 24 pages, 7 figures, figures improved, references added, conceptual
clarifications include
Classical and quantum Big Brake cosmology for scalar field and tachyonic models
We study a relation between the cosmological singularities in classical and
quantum theory, comparing the classical and quantum dynamics in some models
possessing the Big Brake singularity - the model based on a scalar field and
two models based on a tachyon-pseudo-tachyon field . It is shown that the
effect of quantum avoidance is absent for the soft singularities of the Big
Brake type while it is present for the Big Bang and Big Crunch singularities.
Thus, there is some kind of a classical - quantum correspondence, because soft
singularities are traversable in classical cosmology, while the strong Big Bang
and Big Crunch singularities are not traversable.Comment: final version, to appear in Phys. Rev.
Brane World Cosmologies with Varying Speed of Light
We study cosmologies in the Randall-Sundrum models, incorporating the
possibility of time-varying speed of light and Newton's constant. The
cosmologies with varying speed of light (VSL) were proposed by Moffat and by
Albrecht and Magueijo as an alternative to inflation for solving the
cosmological problems. We consider the case in which the speed of light varies
with time after the radion or the scale of the extra dimension has been
stabilized. We elaborate on the conditions under which the flatness problem and
the cosmological constant problem can be resolved. We find that the RS models
are more restrictive about possible desirable VSL cosmological models than the
standard general relativity. Particularly, the VSL cosmologies may provide with
a possible mechanism for bringing the quantum corrections to the fine-tuned
brane tensions after the SUSY breaking under control.Comment: 18 pages, LaTeX, revised version with minor corrections to appear in
Phys. Rev.
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