34,495 research outputs found
Growth of covariant perturbations in the contracting phase of a bouncing universe
In this paper we examine the validity of the linear perturbation theory near
a bounce in the covariant analysis. Some linearity parameters are defined to
set up conditions for a linear theory. Linear evolution of density perturbation
and gravitational waves have been computed previously. We have calculated the
vector and scalar induced parts of the shear tensor. For radiationlike and
dustlike single fluid dominated collapsing Friedmann-Lemaitre-Robertson-Walker
background it is shown that the linearity conditions are not satisfied near a
bounce.Comment: 9 pages, final 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
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
Linearisation instability of gravity waves?
Gravity waves in irrotational dust spacetimes are characterised by nonzero
magnetic Weyl tensor . In the linearised theory, the divergence of
is set to zero. Recently Lesame et al. [Phys. Rev. D {\bf 53}, 738
(1996)] presented an argument to show that, in the exact nonlinear theory, forces , thus implying a linearisation instability for gravity
waves interacting with matter. However a sign error in the equations
invalidates their conclusion. Bianchi type V spacetimes are shown to include
examples with . An improved covariant formalism is used to
show that in a generic irrotational dust spacetime, the covariant constraint
equations are preserved under evolution. It is shown elsewhere that \mbox{div}
H=0 does not generate further conditions.Comment: 8 pages Revtex; to appear Phys. Rev.
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
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
Advanced general aviation comparative engine/airframe integration study
The NASA Advanced Aviation Comparative Engine/Airframe Integration Study was initiated to help determine which of four promising concepts for new general aviation engines for the 1990's should be considered for further research funding. The engine concepts included rotary, diesel, spark ignition, and turboprop powerplants; a conventional state-of-the-art piston engine was used as a baseline for the comparison. Computer simulations of the performance of single and twin engine pressurized aircraft designs were used to determine how the various characteristics of each engine interacted in the design process. Comparisons were made of how each engine performed relative to the others when integrated into an airframe and required to fly a transportation mission
Improved techniques for thermomechanical testing in support of deformation modeling
The feasibility of generating precise thermomechanical deformation data to support constitutive model development was investigated. Here, the requirement is for experimental data that is free from anomalies caused by less than ideal equipment and procedures. A series of exploratory tests conducted on Hastelloy X showed that generally accepted techniques for strain controlled tests were lacking in at least three areas. Specifically, problems were encountered with specimen stability, thermal strain compensation, and temperature/mechanical strain phasing. The source of these difficulties was identified and improved thermomechanical testing techniques to correct them were developed. These goals were achieved by developing improved procedures for measuring and controlling thermal gradients and by designing a specimen specifically for thermomechanical testing. In addition, innovative control strategies were developed to correctly proportion and phase the thermal and mechanical components of strain. Subsequently, the improved techniques were used to generate deformation data for Hastelloy X over the temperature range, 200 to 1000 C
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