27,203 research outputs found
Constraints on Inflationary Solutions in the Presence of Shear and Bulk Viscosity
Inflationary models and their claim to solve many of the outstanding problems
in cosmology have been the subject of a great deal of debate over the last few
years. A major sticking point has been the lack of both good observational and
theoretical arguments to single out one particular model out of the many that
solve these problems. Here we examine the degree of restrictiveness on the
dynamical relationship between the cosmological scale factor and the inflation
driving self-interaction potential of a minimally coupled scalar field, imposed
by the condition that the scalar field is required to be real during a
classical regime (the reality condition). We systema\-tically look at the
effects of this constraint on many of the inflationary models found in the
literature within the FLRW framework, and also look at what happens when
physically motivated perturbations such as shear and bulk viscosity are
introduced. We find that in many cases, either the models are totally excluded
or the reality condition gives rise to constraints on the scale factor and on
the various parameters of the model.Comment: 21 pages, LaTe
Dynamics of Inflationary Universes with Positive Spatial Curvature
If the spatial curvature of the universe is positive, then the curvature term
will always dominate at early enough times in a slow-rolling inflationary
epoch. This enhances inflationary effects and hence puts limits on the possible
number of e-foldings that can have occurred, independently of what happened
before inflation began and in particular without regard for what may have
happened in the Planck era. We use a simple multi-stage model to examine this
limit as a function of the present density parameter and the epoch
when inflation ends.Comment: 9 Pages RevTex4. Revised and update
Local freedom in the gravitational field
In a cosmological context, the electric and magnetic parts of the Weyl
tensor, E_{ab} and H_{ab}, represent the locally free curvature - i.e. they are
not pointwise determined by the matter fields. By performing a complete
covariant decomposition of the derivatives of E_{ab} and H_{ab}, we show that
the parts of the derivative of the curvature which are locally free (i.e. not
pointwise determined by the matter via the Bianchi identities) are exactly the
symmetrised trace-free spatial derivatives of E_{ab} and H_{ab} together with
their spatial curls. These parts of the derivatives are shown to be crucial for
the existence of gravitational waves.Comment: New results on gravitational waves included; new references added;
revised version (IOP style) to appear Class. Quantum Gra
Lensing and caustic effects on cosmological distances
We consider the changes which occur in cosmological distances due to the
combined effects of some null geodesics passing through low-density regions
while others pass through lensing-induced caustics. This combination of effects
increases observed areas corresponding to a given solid angle even when
averaged over large angular scales, through the additive effect of increases on
all scales, but particularly on micro-angular scales; however angular sizes
will not be significantly effected on large angular scales (when caustics
occur, area distances and angular-diameter distances no longer coincide). We
compare our results with other works on lensing, which claim there is no such
effect, and explain why the effect will indeed occur in the (realistic)
situation where caustics due to lensing are significant. Whether or not the
effect is significant for number counts depends on the associated angular
scales and on the distribution of inhomogeneities in the universe. It could
also possibly affect the spectrum of CBR anisotropies on small angular scales,
indeed caustics can induce a non-Gaussian signature into the CMB at small
scales and lead to stronger mixing of anisotropies than occurs in weak lensing.Comment: 28 pages, 6 ps figures, eps
Hubble's law and faster than light expansion speeds
Naively applying Hubble's law to a sufficiently distant object gives a
receding velocity larger than the speed of light. By discussing a very similar
situation in special relativity, we argue that Hubble's law is meaningful only
for nearby objects with non-relativistic receding speeds. To support this
claim, we note that in a curved spacetime manifold it is not possible to
directly compare tangent vectors at different points, and thus there is no
natural definition of relative velocity between two spatially separated objects
in cosmology. We clarify the geometrical meaning of the Hubble's receding speed
v by showing that in a Friedmann-Robertson-Walker spacetime if the
four-velocity vector of a comoving object is parallel-transported along the
straight line in flat comoving coordinates to the position of a second comoving
object, then v/c actually becomes the rapidity of the local Lorentz
transformation, which maps the fixed four-velocity vector to the transported
one.Comment: 5 pages, 2 figures, to appear in Am. J. Phy
Scale-Invariant Curvature Fluctuations from an Extended Semiclassical Gravity
We present an extension of the semiclassical Einstein equations which couples
n-point correlation functions of a stochastic Einstein tensor to the n-point
functions of the quantum stress-energy tensor. We apply this extension to
calculate the quantum fluctuations during an inflationary period, where we take
as a model a massive conformally coupled scalar field on a perturbed de Sitter
space and describe how a renormalization independent, almost-scale-invariant
power spectrum of the scalar metric perturbation is produced. Furthermore, we
discuss how this model yields a natural basis for the calculation of
non-Gaussianities of the considered metric fluctuations.Comment: 16 pages, 2 figures; final versio
Vorticity production and survival in viscous and magnetized cosmologies
We study the role of viscosity and the effects of a magnetic field on a
rotating, self-gravitating fluid, using Newtonian theory and adopting the ideal
magnetohydrodynamic approximation. Our results confirm that viscosity can
generate vorticity in inhomogeneous environments, while the magnetic tension
can produce vorticity even in the absence of fluid pressure and density
gradients. Linearizing our equations around an Einstein-de Sitter cosmology, we
find that viscosity adds to the diluting effect of the universal expansion.
Typically, however, the dissipative viscous effects are confined to relatively
small scales. We also identify the characteristic length bellow which the
viscous dissipation is strong and beyond which viscosity is essentially
negligible. In contrast, magnetism seems to favor cosmic rotation. The magnetic
presence is found to slow down the standard decay-rate of linear vortices, thus
leading to universes with more residual rotation than generally anticipated.Comment: Minor changes. References added and updated. Published versio
Electroweak supersymmetric effects on high energy unpolarized and polarized single top production at LHC
We consider various processes of single top production at LHC in the
theoretical framework of the MSSM and examine the role of the supersymmetric
electroweak one-loop corrections in a special moderately light SUSY scenario,
in an initial parton-pair c.m. high energy range where a logarithmic asymptotic
expansion of Sudakov type can be used. We show that the electroweak virtual
effects are systematically large, definitely beyond the relative ten percent
size, particularly for a final pair where a special enhancement is
present. We show then in a qualitative way the kind of precision tests of the
model that would be obtainable from accurate measurements of the energy
distributions of the various cross sections and of the top polarization
asymmetries.Comment: 30 pages, 9 figure
Comment on `Smooth and Discontinuous Signature Type Change in General Relativity'
Kossowski and Kriele derived boundary conditions on the metric at a surface
of signature change. We point out that their derivation is based not only on
certain smoothness assumptions but also on a postulated form of the Einstein
field equations. Since there is no canonical form of the field equations at a
change of signature, their conclusions are not inescapable. We show here that a
weaker formulation is possible, in which less restrictive smoothness
assumptions are made, and (a slightly different form of) the Einstein field
equations are satisfied. In particular, in this formulation it is possible to
have a bounded energy-momentum tensor at a change of signature without
satisfying their condition that the extrinsic curvature vanish.Comment: Plain TeX, 6 pages; Comment on Kossowski and Kriele: Class. Quantum
Grav. 10, 2363 (1993); Reply by Kriele: Gen. Rel. Grav. 28, 1409-1413 (1996
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