25,838 research outputs found
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
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
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
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
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
Note on Varying Speed of Light Cosmologies
The various requirements on a consistent varying speed of light (`VSL')
theory are surveyed, giving a short check-list of issues that should be
satisfactorily handled by such theories.Comment: 6 pages; to appear in the GRG Journa
Irrotational dust with Div H=0
For irrotational dust the shear tensor is consistently diagonalizable with
its covariant time derivative: , if
and only if the divergence of the magnetic part of the Weyl tensor vanishes:
. We show here that in that case, the consistency of the Ricci
constraints requires that the magnetic part of the Weyl tensor itself vanishes:
.Comment: 19 pages. Latex. Also avaliable at
http://shiva.mth.uct.ac.za/preprints/text/lesame2.te
Non-Critical Liouville String Escapes Constraints on Generic Models of Quantum Gravity
It has recently been pointed out that generic models of quantum gravity must
contend with severe phenomenological constraints imposed by gravitational
Cerenkov radiation, neutrino oscillations and the cosmic microwave background
radiation. We show how the non-critical Liouville-string model of quantum
gravity we have proposed escapes these constraints. It gives energetic
particles subluminal velocities, obviating the danger of gravitational Cerenkov
radiation. The effect on neutrino propagation is naturally flavour-independent,
obviating any impact on oscillation phenomenology. Deviations from the expected
black-body spectrum and the effects of time delays and stochastic fluctuations
in the propagation of cosmic microwave background photons are negligible, as
are their effects on observable spectral lines from high-redshift astrophysical
objects.Comment: 15 pages LaTeX, 2 eps figures include
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