12 research outputs found
Black holes and wormholes subject to conformal mappings
Solutions of the field equations of theories of gravity which admit distinct
conformal frame representations can look very different in these frames. We
show that Brans class IV solutions describe wormholes in the Jordan frame (in a
certain parameter range) but correspond to horizonless geometries in the
Einstein frame. The reasons for such a change of behaviour under conformal
mappings are elucidated in general, using Brans IV solutions as an example.Comment: 7 pages, 2 figure
Are quantization rules for horizon areas universal?
Doubts have been expressed on the universality of holographic/string-inspired
quantization rules for the horizon areas of stationary black holes or the
products of their radii, already in simple 4-dimensional general relativity.
Realistic black holes are not stationary but time-dependent. Using two examples
of 4D general-relativistic spacetimes containing dynamical black holes for at
least part of the time, it is shown that the quantization rules (even counting
virtual horizons) cannot hold, except possibly at isolated instants of time,
and do not seem to be universal.Comment: One example and one figure added, two figures improved, bibliography
expanded and updated. Matches the version accepted for publication in Phys.
Rev.
Are stealth scalar fields stable?
Non-gravitating (stealth) scalar fields associated with Minkowski space in
scalar-tensor gravity are examined. Analytical solutions for both non-minimally
coupled scalar field theory and for Brans-Dicke gravity are studied and their
stability with respect to tensor perturbations is assessed using a covariant
and gauge-invariant formalism developed for alternative gravity. For
Brans-Dicke solutions, the stability with respect to homogeneous perturbations
is also studied. There are regions of parameter space corresponding to
stability and other regions corresponding to instability.Comment: 10 pages, 1 table, no figures, to appear in Phys. Rev,
Making sense of the bizarre behaviour of horizons in the McVittie spacetime
The bizarre behaviour of the apparent (black hole and cosmological) horizons
of the McVittie spacetime is discussed using, as an analogy, the
Schwarzschild-de Sitter-Kottler spacetime (which is a special case of McVittie
anyway). For a dust-dominated "background" universe, a black hole cannot exist
at early times because its (apparent) horizon would be larger than the
cosmological(apparent) horizon. A phantom-dominated "background" universe
causes this situation, and the horizon behaviour, to be time-reversed.Comment: 8 pages, 3 figure