11 research outputs found
On a class of stable, traversable Lorentzian wormholes in classical general relativity
It is known that Lorentzian wormholes must be threaded by matter that
violates the null energy condition. We phenomenologically characterize such
exotic matter by a general class of microscopic scalar field Lagrangians and
formulate the necessary conditions that the existence of Lorentzian wormholes
imposes on them. Under rather general assumptions, these conditions turn out to
be strongly restrictive. The most simple Lagrangian that satisfies all of them
describes a minimally coupled massless scalar field with a reversed sign
kinetic term. Exact, non-singular, spherically symmetric solutions of
Einstein's equations sourced by such a field indeed describe traversable
wormhole geometries. These wormholes are characterized by two parameters: their
mass and charge. Among them, the zero mass ones are particularly simple,
allowing us to analytically prove their stability under arbitrary space-time
dependent perturbations. We extend our arguments to non-zero mass solutions and
conclude that at least a non-zero measure set of these solutions is stable.Comment: 23 pages, 4 figures, uses RevTeX4. v2: Changes to accommodate added
references. Statement about masses of the wormhole correcte
Spinors, Inflation, and Non-Singular Cyclic Cosmologies
We consider toy cosmological models in which a classical, homogeneous, spinor
field provides a dominant or sub-dominant contribution to the energy-momentum
tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a
field were to exist, appropriate choices of the spinor self-interaction would
generate a rich variety of behaviors, quite different from their widely studied
scalar field counterparts. We first discuss solutions that incorporate a stage
of cosmic inflation and estimate the primordial spectrum of density
perturbations seeded during such a stage. Inflation driven by a spinor field
turns out to be unappealing as it leads to a blue spectrum of perturbations and
requires considerable fine-tuning of parameters. We next find that, for simple,
quartic spinor self-interactions, non-singular cyclic cosmologies exist with
reasonable parameter choices. These solutions might eventually be incorporated
into a successful past- and future-eternal cosmological model free of
singularities. In an Appendix, we discuss the classical treatment of spinors
and argue that certain quantum systems might be approximated in terms of such
fields.Comment: 12 two-column pages, 3 figures; uses RevTeX
Solutions to the cosmological constant problems
We critically review several recent approaches to solving the two
cosmological constant problems. The "old" problem is the discrepancy between
the observed value of and the large values suggested by particle
physics models. The second problem is the "time coincidence" between the epoch
of galaxy formation and the epoch of -domination t_\L. It is
conceivable that the "old" problem can be resolved by fundamental physics
alone, but we argue that in order to explain the "time coincidence" we must
account for anthropic selection effects. Our main focus here is on the
discrete- models in which can change through nucleation of
branes. We consider the cosmology of this type of models in the context of
inflation and discuss the observational constraints on the model parameters.
The issue of multiple brane nucleation raised by Feng {\it et. al.} is
discussed in some detail. We also review continuous-\L models in which the
role of the cosmological constant is played by a slowly varying potential of a
scalar field. We find that both continuous and discrete models can in principle
solve both cosmological constant problems, although the required values of the
parameters do not appear very natural. M-theory-motivated brane models, in
which the brane tension is determined by the brane coupling to the four-form
field, do not seem to be viable, except perhaps in a very tight corner of the
parameter space. Finally, we point out that the time coincidence can also be
explained in models where is fixed, but the primordial density
contrast is treated as a random variable.Comment: 30 pages, 3 figures, two notes adde
Predictions and Observations in Theories with Varying Couplings
We consider a toy universe containing conventional matter and an additional
real scalar field, and discuss how the requirements of gauge and diffeomorphism
invariance essentially single out a particular set of theories which might
describe such a world at low energies. In these theories, fermion masses and
g-factors, as well as the electromagnetic coupling turn to be scalar field
dependent; fermion charges and the gravitational coupling might be assumed to
be constant. We then proceed to study the impact of a time variation of the
scalar field on measurements of atomic spectra at high redshifts. Light
propagation is not affected by a sufficiently slow change of the fine structure
constant, but changes of the latter as well as variations of fermion masses and
g-factors do affect the observed atomic spectra. Finally, we prove the
independence of these predictions on the chosen conformal frame, in a further
attempt to address differing views about the subject expressed in the
literature.Comment: 19 pages, no figures; uses RevTeX
Diagnostic for Dilaton Dark Energy
diagnostic can differentiate between different models of dark energy
without the accurate current value of matter density. We apply this geometric
diagnostic to dilaton dark energy(DDE) model and differentiate DDE model from
LCDM. We also investigate the influence of coupled parameter on the
evolutive behavior of with respect to redshift . According to the
numerical result of , we get the current value of equation of state
=-0.952 which fits the WMAP5+BAO+SN very well.Comment: 6 pages and 6 figures
Anisotropic Inflation and the Origin of Four Large Dimensions
In the context of (4+d)-dimensional general relativity, we propose an
inflationary scenario wherein 3 spatial dimensions grow large, while d extra
dimensions remain small. Our model requires that a self-interacting d-form
acquire a vacuum expectation value along the extra dimensions. This causes 3
spatial dimensions to inflate, whilst keeping the size of the extra dimensions
nearly constant. We do not require an additional stabilization mechanism for
the radion, as stable solutions exist for flat, and for negatively curved
compact extra dimensions. From a four-dimensional perspective, the radion does
not couple to the inflaton; and, the small amplitude of the CMB temperature
anisotropies arises from an exponential suppression of fluctuations, due to the
higher-dimensional origin of the inflaton. The mechanism triggering the end of
inflation is responsible, both, for heating the universe, and for avoiding
violations of the equivalence principle due to coupling between the radion and
matter.Comment: 24 pages, 2 figures; uses RevTeX4. v2: Minor changes and added
references. v3: Improved discussion of slow-rol
Could dark energy be vector-like?
In this paper I explore whether a vector field can be the origin of the
present stage of cosmic acceleration. In order to avoid violations of isotropy,
the vector has be part of a ``cosmic triad'', that is, a set of three identical
vectors pointing in mutually orthogonal spatial directions. A triad is indeed
able to drive a stage of late accelerated expansion in the universe, and there
exist tracking attractors that render cosmic evolution insensitive to initial
conditions. However, as in most other models, the onset of cosmic acceleration
is determined by a parameter that has to be tuned to reproduce current
observations. The triad equation of state can be sufficiently close to minus
one today, and for tachyonic models it might be even less than that. I briefly
analyze linear cosmological perturbation theory in the presence of a triad. It
turns out that the existence of non-vanishing spatial vectors invalidates the
decomposition theorem, i.e. scalar, vector and tensor perturbations do not
decouple from each other. In a simplified case it is possible to analytically
study the stability of the triad along the different cosmological attractors.
The triad is classically stable during inflation, radiation and matter
domination, but it is unstable during (late-time) cosmic acceleration. I argue
that this instability is not likely to have a significant impact at present.Comment: 28 pages, 6 figures. Uses RevTeX4. v2: Discussion about relation to
phantoms added and additional references cite
Localized D-dimensional global k-defects
We explicitly demonstrate the existence of static global defect solutions of
arbitrary dimensionality whose energy does not diverge at spatial infinity, by
considering maximally symmetric solutions described by an action with
non-standard kinetic terms in a D+1 dimensional Minkowski space-time. We
analytically determine the defect profile both at small and large distances
from the defect centre. We verify the stability of such solutions and discuss
possible implications of our findings, in particular for dark matter and charge
fractionalization in graphene.Comment: 6 pages, published versio
Regular black holes and black universes
We give a comparative description of different types of regular static,
spherically symmetric black holes (BHs) and discuss in more detail their
particular type, which we suggest to call black universes. The latter have a
Schwarzschild-like causal structure, but inside the horizon there is an
expanding Kantowski-Sachs universe and a de Sitter infinity instead of a
singularity. Thus a hypothetic BH explorer gets a chance to survive. Solutions
of this kind are naturally obtained if one considers static, spherically
symmetric distributions of various (but not all) kinds of phantom matter whose
existence is favoured by cosmological observations. It also looks possible that
our Universe has originated from phantom-dominated collapse in another universe
and underwent isotropization after crossing the horizon. An explicit example of
a black-universe solution with positive Schwarzschild mass is discussed.Comment: 13 pages, 1 figure. 6 referenses and some discussion added, misprints
correcte
Cosmological implications of interacting polytropic gas dark energy model in non-flat universe
The polytropic gas model is investigated as an interacting dark energy
scenario. The cosmological implications of the model including the evolution of
EoS parameter , energy density and deceleration
parameter are investigated. We show that, depending on the parameter of
model, the interacting polytropic gas can behave as a quintessence or phantom
dark energy. In this model, the phantom divide is crossed from below to up. The
evolution of in the context of polytropic gas dark energy model represents
the decelerated phase at the early time and accelerated phase later. The
singularity of this model is also discussed. Eventually, we establish the
correspondence between interacting polytropic gas model with tachyon, K-essence
and dilaton scalar fields. The potential and the dynamics of these scalar field
models are reconstructed according to the evolution of interacting polytropic
gas.Comment: 19 pages, 3 figures, accepted by ijt