9 research outputs found
The Covariant Entropy Bound, Brane Cosmology, and the Null Energy Condition
In discussions of Bousso's Covariant Entropy Bound, the Null Energy Condition
is always assumed, as a sufficient {\em but not necessary} condition which
helps to ensure that the entropy on any lightsheet shall necessarily be finite.
The spectacular failure of the Strong Energy Condition in cosmology has,
however, led many astrophysicists and cosmologists to consider models of dark
energy which violate {\em all} of the energy conditions, and indeed the current
data do not completely rule out such models. The NEC also has a questionable
status in brane cosmology: it is probably necessary to violate the NEC in the
bulk in order to obtain a "self-tuning" theory of the cosmological constant. In
order to investigate these proposals, we modify the Karch-Randall model by
introducing NEC-violating matter into in such a way that the brane
cosmological constant relaxes to zero. The entropy on lightsheets remains
finite. However, we still find that the spacetime is fundamentally incompatible
with the Covariant Entropy Bound machinery, in the sense that it fails the
Bousso-Randall consistency condition. We argue that holography probably forbids
all {\em cosmological} violations of the NEC, and that holography is in fact
the fundamental physical principle underlying the cosmological version of the
NEC.Comment: 21 pages, 3 figures, version 2:corrected and greatly improved
discussion of the Bousso-Randall consistency check, references added;
version3: more references added, JHEP versio
Fast variability from black-hole binaries
Currently available information on fast variability of the X-ray emission
from accreting collapsed objects constitutes a complex phenomenology which is
difficult to interpret. We review the current observational standpoint for
black-hole binaries and survey models that have been proposed to interpret it.
Despite the complex structure of the accretion flow, key observational
diagnostics have been identified which can provide direct access to the
dynamics of matter motions in the close vicinity of black holes and thus to the
some of fundamental properties of curved spacetimes, where strong-field general
relativistic effects can be observed.Comment: 20 pages, 11 figures. Accepted for publication in Space Science
Reviews. Also to appear in hard cover in the Space Sciences Series of ISSI
"The Physics of Accretion onto Black Holes" (Springer Publisher
Phenomenology of the Lense-Thirring effect in the Solar System
Recent years have seen increasing efforts to directly measure some aspects of
the general relativistic gravitomagnetic interaction in several astronomical
scenarios in the solar system. After briefly overviewing the concept of
gravitomagnetism from a theoretical point of view, we review the performed or
proposed attempts to detect the Lense-Thirring effect affecting the orbital
motions of natural and artificial bodies in the gravitational fields of the
Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of
the impact of several sources of systematic uncertainties of dynamical origin
to realistically elucidate the present and future perspectives in directly
measuring such an elusive relativistic effect.Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in
Astrophysics and Space Science (ApSS). Some uncited references in the text
now correctly quoted. One reference added. A footnote adde