8,001 research outputs found
Moduli, Scalar Charges, and the First Law of Black Hole Thermodynamics
We show that under variation of moduli fields the first law of black
hole thermodynamics becomes , where are the scalar charges. We also show
that the ADM mass is extremized at fixed , , when the moduli
fields take the fixed value which depend only on electric
and magnetic charges. It follows that the least mass of any black hole with
fixed conserved electric and magnetic charges is given by the mass of the
double-extreme black hole with these charges. Our work allows us to interpret
the previously established result that for all extreme black holes the moduli
fields at the horizon take a value depending only
on the electric and magnetic conserved charges: is such
that the scalar charges .Comment: 3 pages, no figures, more detailed versio
Bohm and Einstein-Sasaki Metrics, Black Holes and Cosmological Event Horizons
We study physical applications of the Bohm metrics, which are infinite
sequences of inhomogeneous Einstein metrics on spheres and products of spheres
of dimension 5 <= d <= 9. We prove that all the Bohm metrics on S^3 x S^2 and
S^3 x S^3 have negative eigenvalue modes of the Lichnerowicz operator and by
numerical methods we establish that Bohm metrics on S^5 have negative
eigenvalues too. We argue that all the Bohm metrics will have negative modes.
These results imply that higher-dimensional black-hole spacetimes where the
Bohm metric replaces the usual round sphere metric are classically unstable. We
also show that the stability criterion for Freund-Rubin solutions is the same
as for black-hole stability, and hence such solutions using Bohm metrics will
also be unstable. We consider possible endpoints of the instabilities, and show
that all Einstein-Sasaki manifolds give stable solutions. We show how Wick
rotation of Bohm metrics gives spacetimes that provide counterexamples to a
strict form of the Cosmic Baldness conjecture, but they are still consistent
with the intuition behind the cosmic No-Hair conjectures. We show how the
Lorentzian metrics may be created ``from nothing'' in a no-boundary setting. We
argue that Lorentzian Bohm metrics are unstable to decay to de Sitter
spacetime. We also argue that noncompact versions of the Bohm metrics have
infinitely many negative Lichernowicz modes, and we conjecture a general
relation between Lichnerowicz eigenvalues and non-uniqueness of the Dirichlet
problem for Einstein's equations.Comment: 53 pages, 11 figure
Phases of 4D Scalar-tensor black holes coupled to Born-Infeld nonlinear electrodynamics
Recent results show that when non-linear electrodynamics is considered the
no-scalar-hair theorems in the scalar-tensor theories (STT) of gravity, which
are valid for the cases of neutral black holes and charged black holes in the
Maxwell electrodynamics, can be circumvented. What is even more, in the present
work, we find new non-unique, numerical solutions describing charged black
holes coupled to non-linear electrodynamics in a special class of scalar-tensor
theories. One of the phases has a trivial scalar field and coincides with the
corresponding solution in General Relativity. The other four phases that we
find are characterized by the value of the scalar field charge. The causal
structure and some aspects of the stability of the solutions have also been
studied. For the scalar-tensor theories considered, the black holes have a
single, non-degenerate horizon, i.e., their causal structure resembles that of
the Schwarzschild black hole. The thermodynamic analysis of the stability of
the solutions indicates that a phase transition may occur.Comment: 18 pages, 8 figures, new phases, figures, clarifying remarks and
acknowledgements adde
Cosmic Acceleration from M Theory on Twisted Spaces
In a recent paper [I.P. Neupane and D.L. Wiltshire, Phys. Lett. B 619, 201
(2005).] we have found a new class of accelerating cosmologies arising from a
time--dependent compactification of classical supergravity on product spaces
that include one or more geometric twists along with non-trivial curved
internal spaces. With such effects, a scalar potential can have a local minimum
with positive vacuum energy. The existence of such a minimum generically
predicts a period of accelerated expansion in the four-dimensional
Einstein-conformal frame. Here we extend our knowledge of these cosmological
solutions by presenting new examples and discuss the properties of the
solutions in a more general setting. We also relate the known (asymptotic)
solutions for multi-scalar fields with exponential potentials to the
accelerating solutions arising from simple (or twisted) product spaces for
internal manifolds.Comment: 23 pages, 3 figures; added a summary Table, PRD versio
Isometric Embedding of BPS Branes in Flat Spaces with Two Times
We show how non-near horizon p-brane theories can be obtained from two
embedding constraints in a flat higher dimensional space with 2 time
directions. In particular this includes the construction of D3 branes from a
flat 12-dimensional action, and M2 and M5 branes from 13 dimensions. The
worldvolume actions are determined by constant forms in the higher dimension,
reduced to the usual expressions by Lagrange multipliers. The formulation
affords insight in the global aspects of the spacetime geometries and makes
contact with recent work on two-time physics.Comment: 29 pages, 10 figures, Latex using epsf.sty and here.sty; v2:
reference added and some small correction
Electrodynamics of Black Holes in STU Supergravity
External magnetic fields can probe the composite structure of black holes in
string theory. With this motivation we study magnetised four-charge black holes
in the STU model, a consistent truncation of maximally supersymmetric
supergravity with four types of electromagnetic fields. We employ solution
generating techniques to obtain Melvin backgrounds, and black holes in these
backgrounds. For an initially electrically charged static black hole immersed
in magnetic fields, we calculate the resultant angular momenta and analyse
their global structure. Examples are given for which the ergoregion does not
extend to infinity. We calculate magnetic moments and gyromagnetic ratios via
Larmor's formula. Our results are consistent with earlier special cases. A
scaling limit and associated subtracted geometry in a single surviving magnetic
field is shown to lift to . Magnetizing magnetically charged
black holes give static solutions with conical singularities representing
strings or struts holding the black holes against magnetic forces. In some
cases it is possible to balance these magnetic forces.Comment: 31 page
Nucleating Black Holes via Non-Orientable Instantons
We extend the analysis of black hole pair creation to include non- orientable
instantons. We classify these instantons in terms of their fundamental
symmetries and orientations. Many of these instantons admit the pin structure
which corresponds to the fermions actually observed in nature, and so the
natural objection that these manifolds do not admit spin structure may not be
relevant. Furthermore, we analyse the thermodynamical properties of
non-orientable black holes and find that in the non-extreme case, there are
interesting modifications of the usual formulae for temperature and entropy.Comment: 27 pages LaTeX, minor typos are correcte
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