761 research outputs found
Inflationary spacetimes are not past-complete
Many inflating spacetimes are likely to violate the weak energy condition, a
key assumption of singularity theorems. Here we offer a simple kinematical
argument, requiring no energy condition, that a cosmological model which is
inflating -- or just expanding sufficiently fast -- must be incomplete in null
and timelike past directions. Specifically, we obtain a bound on the integral
of the Hubble parameter over a past-directed timelike or null geodesic. Thus
inflationary models require physics other than inflation to describe the past
boundary of the inflating region of spacetime.Comment: We improve the basic argument to apply to a wider class of
spacetimes, use a better title and add a discussion of cyclic models. 4
pages, 1 figure, RevTe
Violations of the Weak Energy Condition in Inflating Spacetimes
We argue that many future-eternal inflating spacetimes are likely to violate
the weak energy condition. It is possible that such spacetimes may not enforce
any of the known averaged conditions either. If this is indeed the case, it may
open the door to constructing non-singular, past-eternal inflating cosmologies.
Simple non-singular models are, however, unsatisfactory, and it is not clear if
satisfactory models can be built that solve the problem of the initial
singularity.Comment: 18 pages, 1 figure (which emerges automatically if you use dvips
Non-Singular Charged Black Hole Solution for Non-Linear Source
A non-singular exact black hole solution in General Relativity is presented.
The source is a non-linear electromagnetic field, which reduces to the Maxwell
theory for weak field. The solution corresponds to a charged black hole with
|q| \leq 2s_c m \approx 0.6 m, having metric, curvature invariants, and
electric field bounded everywhere.Comment: 3 pages, RevTe
Open and Closed Universes, Initial Singularities and Inflation
The existence of initial singularities in expanding universes is proved
without assuming the timelike convergence condition. The assumptions made in
the proof are ones likely to hold both in open universes and in many closed
ones. (It is further argued that at least some of the expanding closed
universes that do not obey a key assumption of the theorem will have initial
singularities on other grounds.) The result is significant for two reasons:
(a)~previous closed-universe singularity theorems have assumed the timelike
convergence condition, and (b)~the timelike convergence condition is known to
be violated in inflationary spacetimes. An immediate consequence of this
theorem is that a recent result on initial singularities in open,
future-eternal, inflating spacetimes may now be extended to include many closed
universes. Also, as a fringe benefit, the time-reverse of the theorem may be
applied to gravitational collapse.Comment: 27 pages, Plain TeX (figures are embedded in the file itself and they
will emerge if it is processed according to the instructions at the top of
the file
Curing singularities: From the big bang to black holes
Singular spacetimes are a natural prediction of Einstein's theory. Most
memorable are the singular centers of black holes and the big bang. However,
dilatonic extensions of Einstein's theory can support nonsingular spacetimes.
The cosmological singularities can be avoided by dilaton driven inflation.
Furthermore, a nonsingular black hole can be constructed in two dimensions.Comment: To appear as a brief report in Phys. Rev.
Neutrino oscillations in a stochastic model for space-time foam
We study decoherence models for flavour oscillations in four-dimensional
stochastically fluctuating space times and discuss briefly the sensitivity of
current neutrino experiments to such models. We pay emphasis on demonstrating
the model dependence of the associated decoherence-induced damping coefficients
in front of the oscillatory terms in the respective transition probabilities
between flavours. Within the context of specific models of foam, involving
point-like D-branes and leading to decoherence-induced damping which is
inversely proportional to the neutrino energies, we also argue that future
limits on the relevant decoherence parameters coming from TeV astrophysical
neutrinos, to be observed in ICE-CUBE, are not far from theoretically expected
values with Planck mass suppression. Ultra high energy neutrinos from Gamma Ray
Bursts at cosmological distances can also exhibit in principle sensitivity to
such effects.Comment: 12 pages RevTex4, no figure
Occurrence of a new mammalian tapeworm Moniezia ovisae
The present communication deals with the occurrence of a new mammalian tapeworm Moniezia (B) ovisae n.sp. from Ovis bharal (L) in Aurangabad (M.S.). It comes closer to all the known species of the genus Moniezia but differs from all the known species of the genus in having the scolex broad anteriorly and narrow towards neck, mature proglottids two times broader than long, testes medium in size, rounded scattered posterior to segment, 155-165 in numbers, vitelline gland post ovarian, cirrus pouch oval, Ovary is compact, bilobed
A report of new mammalian tapeworm Moniezia osmanabadensis from Capra hircus at Osmanabad district (M.S.) India
The present communication deals with the occurrence of a new mammalian tapeworm Moniezia (B) osmanabadensis n.sp. from Capra hircus (L) in Osmanabad district (M.S.) India. It comes closer to all the known species of the genus Moniezia but differs from all the known species of the genus in having the scolex globular, elongated, mature proglottids Craspedote, testes medium in size, rounded scattered posterior to segment, 170-200 in numbers, cirrus pouch oval, ovary compact, bean shaped, vitelline gland post ovarian
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.
On T-Duality in Brane Gas Cosmology
In the context of homogeneous and isotropic superstring cosmology, the
T-duality symmetry of string theory has been used to argue that for a
background space-time described by dilaton gravity with strings as matter
sources, the cosmological evolution of the Universe will be nonsingular. In
this Letter we discuss how T-duality extends to brane gas cosmology, an
approximation in which the background space-time is again described by dilaton
gravity with a gas of branes as a matter source. We conclude that the arguments
for nonsingular cosmological evolution remain valid.Comment: 8 pages, Appendix adde
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