34 research outputs found
Gravitational Collapse of a Rotating Cylindrical Null Shell in the Cosmic String Spacetime
We study the gravitational collapse of a rotating cylindrical null shell with
flat interior and the metric of a spinning cosmic string as the exterior. We
see that there is a critical radius, where the energy density of the shell
vanishes and beyond which it would be negative, thereby signaling that the
matching would be unphysical.Comment: 6 pages, work presented at the 8th Alexander Friedmann International
Seminar on Gravitation and Cosmology, Rio de Janeiro, Brazil, June 201
Thick planar domain wall: its thin wall limit and dynamics
We consider a planar gravitating thick domain wall of the
theory as a spacetime with finite thickness glued to two vacuum spacetimes on
each side of it. Darmois junction conditions written on the boundaries of the
thick wall with the embedding spacetimes reproduce the Israel junction
condition across the wall in the limit of infinitesimal thickness. The thick
planar domain wall located at a fixed position is then transformed to a new
coordinate system in which its dynamics can be formulated. It is shown that the
wall's core expands as if it were a thin wall. The thickness in the new
coordinates is not constant anymore and its time dependence is given.Comment: 11 pages, to appear in IJMP
Matching LTB and FRW spacetimes through a null hypersurface
Matching of a LTB metric representing dust matter to a background FRW
universe across a null hypersurface is studied. In general, an unrestricted
matching is possible only if the background FRW is flat or open. There is in
general no gravitational impulsive wave present on the null hypersurface which
is shear-free and expanding. Special cases of the vanishing pressure or energy
density on the hypersurface is discussed. In the case of vanishing energy
momentum tensor of the null hypersurface, i.e. in the case of a null boundary,
it turns out that all possible definitions of the Hubble parameter on the null
hypersurface, being those of LTB or that of FRW, are equivalent, and that a
flat FRW can only be joined smoothly to a flat LTB.Comment: 9 page
A Note on the Generalized Friedmann Equations for a Thick Brane
Within our thick brane approach previously used to obtain the cosmological
evolution equations on a thick brane embedded in a five-dimensional
Schwarzschild Anti-de Sitter spacetime it is explicitly shown that the
consistency of these equations with the energy conservation equation requires
that, in general, the thickness of the brane evolves in time. This varying
brane thickness entails the possibility that both Newton's gravitational
constant and the effective cosmological constant are time
dependent.Comment: 6 pages,To appear in GR