305 research outputs found
Decaying Dark Energy in Higher-Dimensional Gravity
We use data from observational cosmology to put constraints on
higher-dimensional extensions of general relativity in which the effective
four-dimensional dark-energy density (or cosmological "constant") decays with
time. In particular we study the implications of this decaying dark energy for
the age of the universe, large-scale structure formation, big-bang
nucleosynthesis and the magnitude-redshift relation for Type Ia supernovae. Two
of these tests (age and the magnitude-redshift relation) place modest lower
limits on the free parameter of the theory, a cosmological length scale L akin
to the de Sitter radius. These limits will improve if experimental
uncertainties on supernova magnitudes can be reduced around z=1.Comment: 11 pages, 5 figures, submitted to A&
Dynamics of a Generalized Cosmological Scalar-Tensor Theory
A generalized scalar-tensor theory is investigated whose cosmological term
depends on both a scalar field and its time derivative. A correspondence with
solutions of five-dimensional Space-Time-Matter theory is noted. Analytic
solutions are found for the scale factor, scalar field and cosmological term.
Models with free parameters of order unity are consistent with recent
observational data and could be relevant to both the dark-matter and
cosmological-"constant" problems.Comment: 13 page
Cosmological Implications of a Non-Separable 5D Solution of the Vacuum Einstein Field Equations
An exact class of solutions of the 5D vacuum Einstein field equations (EFEs)
is obtained. The metric coefficients are found to be non-separable functions of
time and the extra coordinate and the induced metric on = constant
hypersurfaces has the form of a Friedmann-Robertson-Walker cosmology. The 5D
manifold and 3D and 4D submanifolds are in general curved, which distinguishes
this solution from previous ones in the literature. The singularity structure
of the manifold is explored: some models in the class do not exhibit a big
bang, while other exhibit a big bang and a big crunch. For the models with an
initial singularity, the equation of state of the induced matter evolves from
radiation like at early epochs to Milne-like at late times and the big bang
manifests itself as a singular hypersurface in 5D. The projection of comoving
5D null geodesics onto the 4D submanifold is shown to be compatible with
standard 4D comoving trajectories, while the expansion of 5D null congruences
is shown to be in line with conventional notions of the Hubble expansion.Comment: 8 pages, in press in J. Math. Phy
Two-component mixture of charged particles confined in a channel: melting
The melting of a binary system of charged particles confined in a {\it
quasi}-one-dimensional parabolic channel is studied through Monte Carlo
simulations. At zero temperature the particles are ordered in parallel chains.
The melting is anisotropic and different melting temperatures are obtained
according to the spatial direction, and the different types of particles
present in the system. Melting is very different for the single-, two- and
four-chain configurations. A temperature induced structural phase transition is
found between two different four chain ordered states which is absent in the
mono-disperse system. In the mixed regime, where the two types of particles are
only slightly different, melting is almost isotropic and a thermally induced
homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12
figures
Attentional biases for food stimuli in external eaters: Possible mechanism for stress-induced eating?
External eaters reportedly increase snack intake when stressed, which could be due to an attentional shift towards food stimuli. Attentional biases for food stimuli were tested in high and low external eaters in stress and control conditions, using a computerised Stroop. A significant interaction was observed between external eating group and condition for snack word bias. This suggested that low external eaters have a greater bias for snack words when unstressed and that stressed, high external eaters have a greater bias for snack words than stressed, low external eaters, which could contribute to stress-induced snack intake in high external eaters
5-Dimensional Kaluza-Klein Theory with a Source
A free test particle in 5-dimensional Kaluza-Klein spacetime will show its
electricity in the reduced 4-dimensional spacetime when it moves along the
fifth dimension. In the light of this observation, we study the coupling of a
5-dimensional dust field with the Kaluza-Klein gravity. It turns out that the
dust field can curve the 5-dimensional spacetime in such a way that it provides
exactly the source of the electromagnetic field in the 4-dimensional spacetime
after the dimensional reduction.Comment: 8 pages, three references adde
Induced Matter and Particle Motion in Non-Compact Kaluza-Klein Gravity
We examine generalizations of the five-dimensional canonical metric by
including a dependence of the extra coordinate in the four-dimensional metric.
We discuss a more appropriate way to interpret the four-dimensional
energy-momentum tensor induced from the five-dimensional space-time and show it
can lead to quite different physical situations depending on the interpretation
chosen. Furthermore, we show that the assumption of five-dimensional null
trajectories in Kaluza-Klein gravity can correspond to either four-dimensional
massive or null trajectories when the path parameterization is chosen properly.
Retaining the extra-coordinate dependence in the metric, we show the
possibility of a cosmological variation in the rest masses of particles and a
consequent departure from four-dimensional geodesic motion by a geometric
force. In the examples given, we show that at late times it is possible for
particles traveling along 5D null geodesics to be in a frame consistent with
the induced matter scenario.Comment: 29 pages, accepted to GR
Gauge-Dependent Cosmological "Constant"
When the cosmological constant of spacetime is derived from the 5D
induced-matter theory of gravity, we show that a simple gauge transformation
changes it to a variable measure of the vacuum which is infinite at the big
bang and decays to an astrophysically-acceptable value at late epochs. We
outline implications of this for cosmology and galaxy formation.Comment: 14 pages, no figures, expanded version to be published in Class.
Quantum Gra
Inducing the cosmological constant from five-dimensional Weyl space
We investigate the possibility of inducing the cosmological constant from
extra dimensions by embedding our four-dimensional Riemannian space-time into a
five-dimensional Weyl integrable space. Following approach of the induced
matter theory we show that when we go down from five to four dimensions, the
Weyl field may contribute both to the induced energy-tensor as well as to the
cosmological constant, or more generally, it may generate a time-dependent
cosmological parameter. As an application, we construct a simple cosmological
model which has some interesting properties.Comment: 7 page
Can Moduli Fields parametrize the Cosmological Constant?
We study the cosmological evolution of string/M moduli fields T. We use
T-duality to fix the potential and show that the superpotential W is a function
of the duality invariant function j(T) only. If W is given as a finite
polynomial of j then the moduli fields {\it do not} give an accelerating
universe, i.e. they {\it cannot} be used as quintessence. Furthermore, at T >>1
the potential is given by a double exponential potential V \simeq e^{-a
e^{\sqrt{2} T}} leading to a fast decaying behaviour at large times. For moduli
potentials with a finite v.e.v. of T the energy density redshift is model
dependent but if T has a finite mass, m < \infty, then the moduli energy
density redshifts faster or equal to matter. Only if the moduli mass is
infinite can the moduli energy density dominate the universe independently of
the initial conditions.Comment: 13 pages, LaTeX, 3 postscript figure
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