30,726 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
Consistency of dust solutions with div H=0
One of the necessary covariant conditions for gravitational radiation is the
vanishing of the divergence of the magnetic Weyl tensor H_{ab}, while H_{ab}
itself is nonzero. We complete a recent analysis by showing that in
irrotational dust spacetimes, the condition div H=0 evolves consistently in the
exact nonlinear theory.Comment: 3 pages Revte
A Note on Infinities in Eternal Inflation
In some well-known scenarios of open-universe eternal inflation, developed by
Vilenkin and co-workers, a large number of universes nucleate and thermalize
within the eternally inflating mega-universe. According to the proposal, each
universe nucleates at a point, and therefore the boundary of the nucleated
universe is a space-like surface nearly coincident with the future light cone
emanating from the point of nucleation, all points of which have the same
proper-time. This leads the authors to conclude that at the proper-time t =
t_{nuc} at which any such nucleation occurs, an infinite open universe comes
into existence. We point out that this is due entirely to the supposition of
the nucleation occurring at a single point, which in light of quantum cosmology
seems difficult to support. Even an infinitesimal space-like length at the
moment of nucleation gives a rather different result -- the boundary of the
nucleating universe evolves in proper-time and becomes infinite only in an
infinite time. The alleged infinity is never attained at any finite time.Comment: 13 pages and 6 figure
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
Exact non-equilibrium solutions of the Einstein-Boltzmann equations. II
We find exact solutions of the Einstein-Boltzmann equations with relaxational
collision term in FRW and Bianchi I spacetimes. The kinematic and thermodynamic
properties of the solutions are investigated. We give an exact expression for
the bulk viscous pressure of an FRW distribution that relaxes towards
collision-dominated equilibrium. If the relaxation is toward collision-free
equilibrium, the bulk viscosity vanishes - but there is still entropy
production. The Bianchi I solutions have zero heat flux and bulk viscosity, but
nonzero shear viscosity. The solutions are used to construct a realisation of
the Weyl Curvature Hypothesis.Comment: 16 pages LaTex, CQG documentstyle (ioplppt
Exploring CP Violation in the MSSM
We explore the prospects for observing CP violation in the minimal
supersymmetric extension of the Standard Model (MSSM) with six CP-violating
parameters, three gaugino mass phases and three phases in trilinear soft
supersymmetry-breaking parameters, using the CPsuperH code combined with a
geometric approach to maximize CP-violating observables subject to the
experimental upper bounds on electric dipole moments. We also implement
CP-conserving constraints from Higgs physics, flavour physics and the upper
limits on the cosmological dark matter density and spin-independent scattering.
We study possible values of observables within the constrained MSSM (CMSSM),
the non-universal Higgs model (NUHM), the CPX scenario and a variant of the
phenomenological MSSM (pMSSM). We find values of the CP-violating asymmetry
A_CP in b -> s gamma decay that may be as large as 3%, so future measurements
of A_CP may provide independent information about CP violation in the MSSM. We
find that CP-violating MSSM contributions to the B_s meson mass mixing term
Delta M_Bs are in general below the present upper limit, which is dominated by
theoretical uncertainties. If these could be reduced, Delta M_Bs could also
provide an interesting and complementary constraint on the six CP-violating
MSSM phases, enabling them all to be determined experimentally, in principle.
We also find that CP violation in the h_{2,3} tau+ tau- and h_{2,3} tbar t
couplings can be quite large, and so may offer interesting prospects for future
pp, e+ e-, mu+ mu- and gamma gamma colliders.Comment: 36 pages, 32 figure
Integrability of irrotational silent cosmological models
We revisit the issue of integrability conditions for the irrotational silent
cosmological models. We formulate the problem both in 1+3 covariant and 1+3
orthonormal frame notation, and show there exists a series of constraint
equations that need to be satisfied. These conditions hold identically for
FLRW-linearised silent models, but not in the general exact non-linear case.
Thus there is a linearisation instability, and it is highly unlikely that there
is a large class of silent models. We conjecture that there are no spatially
inhomogeneous solutions with Weyl curvature of Petrov type I, and indicate
further issues that await clarification.Comment: Minor corrections and improvements; 1 new reference; to appear Class.
Quantum Grav.; 16 pages Ioplpp
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