9,624 research outputs found
Tolman wormholes violate the strong energy condition
For an arbitrary Tolman wormhole, unconstrained by symmetry, we shall define
the bounce in terms of a three-dimensional edgeless achronal spacelike
hypersurface of minimal volume. (Zero trace for the extrinsic curvature plus a
"flare-out" condition.) This enables us to severely constrain the geometry of
spacetime at and near the bounce and to derive general theorems regarding
violations of the energy conditions--theorems that do not involve geodesic
averaging but nevertheless apply to situations much more general than the
highly symmetric FRW-based subclass of Tolman wormholes. [For example: even
under the mildest of hypotheses, the strong energy condition (SEC) must be
violated.] Alternatively, one can dispense with the minimal volume condition
and define a generic bounce entirely in terms of the motion of test particles
(future-pointing timelike geodesics), by looking at the expansion of their
timelike geodesic congruences. One re-confirms that the SEC must be violated at
or near the bounce. In contrast, it is easy to arrange for all the other
standard energy conditions to be satisfied.Comment: 8 pages, ReV-TeX 3.
Muon localization site in U(Pt,Pd)3
The angular and temperature (10-250 K) variation of the Knight shift of
single-crystalline U(Pt0.95Pd0.05)3 has been measured in transverse field
(B=0.6 T) mSR experiments. By analysing the temperature variation of the Knight
shift with a modified Curie-Weiss expression the muon localization site in this
hexagonal material is determined at (0,0,0).Comment: 12 pages (including 4 figures); postscript file; Proc. 8th Int. Conf.
on Muon Spin Rotation, Relaxation and Resonance (Aug.30-Sept.3, Les
Diablerets); 2nd version with minor correction
Weak magnetism phenomena in heavy-fermion superconductors: selected SR studies
The behavior of the so-called weak moment antiferromagnetic states, observed
in the heavy-fermion superconductors UPt and URuSi, is discussed in
view of recent SR results obtained as function of control parameters like
chemical substitution and external pressure. In UPt, the Pd substitution
for Pt reveals the dynamical character of the weak moment order. On the other
hand, SR measurements performed on samples in which Th substitutes U
suggest that crystallographic disorder on the magnetic sites deeply affects the
fluctuation timescale. In URuSi, a phase separation between the
so-called hidden order state, present at ambient pressure, and an
antiferromagnetic state, occurring under pressure, is observed. In view of the
pressure-temperature phase diagram obtained by SR, it is deduced that the
respective order parameters have different symmetries.Comment: To appear in: J. Phys.: Cond. Matte
Cosmodynamics: Energy conditions, Hubble bounds, density bounds, time and distance bounds
We refine and extend a programme initiated by one of the current authors
[Science 276 (1997) 88; Phys. Rev. D56 (1997) 7578] advocating the use of the
classical energy conditions of general relativity in a cosmological setting to
place very general bounds on various cosmological parameters. We show how the
energy conditions can be used to bound the Hubble parameter H(z), Omega
parameter Omega(z), density rho(z), distance d(z), and lookback time T(z) as
(relatively) simple functions of the redshift z, present-epoch Hubble parameter
H_0, and present-epoch Omega parameter Omega_0. We compare these results with
related observations in the literature, and confront the bounds with the recent
supernova data.Comment: 21 pages, 2 figure
Wormhole Cosmology and the Horizon Problem
We construct an explicit class of dynamic lorentzian wormholes connecting
Friedmann-Robertson-Walker (FRW) spacetimes. These wormholes can allow two-way
transmission of signals between spatially separated regions of spacetime and
could permit such regions to come into thermal contact. The cosmology of a
network of early Universe wormholes is discussed.Comment: 13 pages, in RevTe
Muon spin rotation study of the topological superconductor SrxBi2Se3
We report transverse-field (TF) muon spin rotation experiments on single
crystals of the topological superconductor SrBiSe with nominal
concentrations and ( K). The TF spectra (
mT), measured after cooling to below in field, did not show any
additional damping of the muon precession signal due to the flux line lattice
within the experimental uncertainty. This puts a lower bound on the magnetic
penetration depth m. However, when we induce disorder in
the vortex lattice by changing the magnetic field below a sizeable
damping rate is obtained for . The data provide microscopic
evidence for a superconducting volume fraction of in the
crystal and thus bulk superconductivity.Comment: 6 pages, includes 4 figure
The causal structure of spacetime is a parameterized Randers geometry
There is a by now well-established isomorphism between stationary
4-dimensional spacetimes and 3-dimensional purely spatial Randers geometries -
these Randers geometries being a particular case of the more general class of
3-dimensional Finsler geometries. We point out that in stably causal
spacetimes, by using the (time-dependent) ADM decomposition, this result can be
extended to general non-stationary spacetimes - the causal structure (conformal
structure) of the full spacetime is completely encoded in a parameterized
(time-dependent) class of Randers spaces, which can then be used to define a
Fermat principle, and also to reconstruct the null cones and causal structure.Comment: 8 page
Wormholes and Child Universes
Evidence to the case that classical gravitation provides the clue to make
sense out of quantum gravity is presented. The key observation is the existence
in classical gravitation of child universe solutions or "almost" solutions,
"almost" because of some singularity problems. The difficulties of these child
universe solutions due to their generic singularity problems will be very
likely be cured by quantum effects, just like for example "almost" instanton
solutions are made relevant in gauge theories with breaking of conformal
invariance. Some well motivated modifcations of General Relativity where these
singularity problems are absent even at the classical level are discussed. High
energy density excitations, responsible for UV divergences in quantum field
theories, including quantum gravity, are likely to be the source of child
universes which carry them out of the original space time. This decoupling
could prevent these high UV excitations from having any influence on physical
amplitudes. Child universe production could therefore be responsible for UV
regularization in quantum field theories which take into account
semiclassically gravitational effects. Child universe production in the last
stages of black hole evaporation, the prediction of absence of tranplanckian
primordial perturbations, connection to the minimum length hypothesis and in
particular the connection to the maximal curvature hypothesis are discussed.
Some discussion of superexcited states in the case these states are Kaluza
Klein excitations is carried out. Finally, the posibility of obtaining "string
like" effects from the wormholes associated with the child universes is
discussed.Comment: Talk presented at the IWARA 2009 Conference, Maresias, Brazil,
October 2009, accepted for publication in the proceedings, World Scientific
format, 8 page
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