1,969 research outputs found
A model universe with variable dimension: Expansion as decrumpling
We propose a model universe, in which the dimension of the space is a
continuous variable, which can take any real positive number. The dynamics
leads to a model in which the universe has no singularity. The difference
between our model and the standard Friedman-Robertson-Walker models become
effective for times much before the presently accepted age of the universe.Comment: 12 pages, emTeX version 3.0, no figure
Holographic Thermodynamic on the Brane in Topological Reissner-Nordstr\"om de Sitter Space
We consider the brane universe in the bulk background of the topological
Reissner-Nordstr\"om de Sitter black holes. We show that the thermodynamic
quantities (including entropy) of the dual CFT take usual special forms
expressed in terms of Hubble parameter and its time derivative at the moment,
when the brane crosses the black hole horizon or the cosmological horizon. We
obtain the generalized Cardy-Verlinde formula for the CFT with an charge and
cosmological constant, for any values of the curvature parameter in the
Friedmann equations.Comment: 8 page
Equivalence of Darmois-Israel and Distributional-Methods for Thin Shells in General Relativity
A distributional method to solve the Einstein's field equations for thin
shells is formulated. The familiar field equations and jump conditions of
Darmois-Israel formalism are derived. A carefull analysis of the Bianchi
identities shows that, for cases under consideration, they make sense as
distributions and lead to jump conditions of Darmois-Israel formalism.Comment: 17 pages Latex, no figures, to be published in Journ. Math. Phy
Quantum Tests of the Foundations of General Relativity
The role of the equivalence principle in the context of non-relativistic
quantum mechanics and matter wave interferometry, especially atom beam
interferometry, will be discussed. A generalised form of the weak equivalence
principle which is capable of covering quantum phenomena too, will be proposed.
It is shown that this generalised equivalence principle is valid for matter
wave interferometry and for the dynamics of expectation values. In addition,
the use of this equivalence principle makes it possible to determine the
structure of the interaction of quantum systems with gravitational and inertial
fields. It is also shown that the path of the mean value of the position
operator in the case of gravitational interaction does fulfill this generalised
equivalence principle.Comment: Classical and Quantum Gravity 15, 13 (1998
E-Learning for Teachers and Trainers : Innovative Practices, Skills and Competences
Reproduction is authorised provided the source is acknowledged.Final Published versio
Tests of relativity using a microwave resonator
The frequencies of a cryogenic sapphire oscillator and a hydrogen maser are
compared to set new constraints on a possible violation of Lorentz invariance.
We determine the variation of the oscillator frequency as a function of its
orientation (Michelson-Morley test) and of its velocity (Kennedy-Thorndike
test) with respect to a preferred frame candidate. We constrain the
corresponding parameters of the Mansouri and Sexl test theory to and which is equivalent to the best previous result for the
former and represents a 30 fold improvement for the latter.Comment: 8 pages, 2 figures, submitted to Physical Review Letters (October 3,
2002
Matching LTB and FRW spacetimes through a null hypersurface
Matching of a LTB metric representing dust matter to a background FRW
universe across a null hypersurface is studied. In general, an unrestricted
matching is possible only if the background FRW is flat or open. There is in
general no gravitational impulsive wave present on the null hypersurface which
is shear-free and expanding. Special cases of the vanishing pressure or energy
density on the hypersurface is discussed. In the case of vanishing energy
momentum tensor of the null hypersurface, i.e. in the case of a null boundary,
it turns out that all possible definitions of the Hubble parameter on the null
hypersurface, being those of LTB or that of FRW, are equivalent, and that a
flat FRW can only be joined smoothly to a flat LTB.Comment: 9 page
Limits on the Time Evolution of Space Dimensions from Newton's Constant
Limits are imposed upon the possible rate of change of extra spatial
dimensions in a decrumpling model Universe with time variable spatial
dimensions (TVSD) by considering the time variation of (1+3)-dimensional
Newton's constant. Previous studies on the time variation of (1+3)-dimensional
Newton's constant in TVSD theory had not been included the effects of the
volume of the extra dimensions and the effects of the surface area of the unit
sphere in D-space dimensions. Our main result is that the absolute value of the
present rate of change of spatial dimensions to be less than about
10^{-14}yr^{-1}. Our results would appear to provide a prima facie case for
ruling the TVSD model out. We show that based on observational bounds on the
present-day variation of Newton's constant, one would have to conclude that the
spatial dimension of the Universe when the Universe was at the Planck scale to
be less than or equal to 3.09. If the dimension of space when the Universe was
at the Planck scale is constrained to be fractional and very close to 3, then
the whole edifice of TVSD model loses credibility.Comment: 22 pages, accepted for publication in Int.J.Mod.Phys.
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