170 research outputs found
Non-extremal black holes from the generalised r-map
We review the timelike dimensional reduction of a class of five-dimensional
theories that generalises 5D, N = 2 supergravity coupled to vector multiplets.
As an application we construct instanton solutions to the four-dimensional
Euclidean theory, and investigate the criteria for solutions to lift to static
non-extremal black holes in five dimensions.
We focus specifically on two classes of models: STU-like models, and models
with a block diagonal target space metric. For STU-like models the second order
equations of motion of the four-dimensional theory can be solved explicitly,
and we obtain the general solution. For block diagonal models we find a
restricted class of solutions, where the number of independent scalar fields
depends on the number of blocks. When lifting these solutions to five
dimensions we show, by explicit calculation, that one obtains static
non-extremal black holes with scalar fields that take finite values on the
horizon only if the number of integration constants reduces by exactly half.Comment: 22 pages. Based on talk by OV at "Black Objects in Supergravity
School" (BOSS2011), INFN, Frascati, Italy, 9-13 May, 201
The Kahler Cone as Cosmic Censor
M-theory effects prevent five-dimensional domain-wall and black-hole
solutions from developing curvature singularities. While so far this analysis
was performed for particular models, we now present a model-independent proof
that these solutions do not have naked singularities as long as the Kahler
moduli take values inside the extended Kahler cone. As a by-product we obtain
information on the regularity of the Kahler-cone metric at boundaries of the
Kahler cone and derive relations between the geometry of moduli space and
space-time.Comment: 21 pages, 1 figure. Improved discussion of the relation between
Kahler moduli and five-dimensional scalars. No changes in the conclusion
Higher Derivative Extension of 6D Chiral Gauged Supergravity
Six-dimensional (1,0) supersymmetric gauged Einstein-Maxwell supergravity is
extended by the inclusion of a supersymmetric Riemann tensor squared invariant.
Both the original model as well as the Riemann tensor squared invariant are
formulated off-shell and consequently the total action is off-shell invariant
without modification of the supersymmetry transformation rules. In this
formulation, superconformal techniques, in which the dilaton Weyl multiplet
plays a crucial role, are used. It is found that the gauging of the U(1)
R-symmetry in the presence of the higher-order derivative terms does not modify
the positive exponential in the dilaton potential. Moreover, the supersymmetric
Minkowski(4) x S^2 compactification of the original model, without the
higher-order derivatives, is remarkably left intact. It is shown that the model
also admits non-supersymmetric vacuum solutions that are direct product spaces
involving de Sitter spacetimes and negative curvature internal spaces.Comment: 32 pages; typos corrected, footnote in conclusions section adde
Unified N=2 Maxwell-Einstein and Yang-Mills-Einstein Supergravity Theories in Four Dimensions
We study unified N=2 Maxwell-Einstein supergravity theories (MESGTs) and
unified Yang-Mills Einstein supergravity theories (YMESGTs) in four dimensions.
As their defining property, these theories admit the action of a global or
local symmetry group that is (i) simple, and (ii) acts irreducibly on all the
vector fields of the theory, including the ``graviphoton''. Restricting
ourselves to the theories that originate from five dimensions via dimensional
reduction, we find that the generic Jordan family of MESGTs with the scalar
manifolds [SU(1,1)/U(1)] X [SO(2,n)/SO(2)X SO(n)] are all unified in four
dimensions with the unifying global symmetry group SO(2,n). Of these theories
only one can be gauged so as to obtain a unified YMESGT with the gauge group
SO(2,1). Three of the four magical supergravity theories defined by simple
Euclidean Jordan algebras of degree 3 are unified MESGTs in four dimensions.
Two of these can furthermore be gauged so as to obtain 4D unified YMESGTs with
gauge groups SO(3,2) and SO(6,2), respectively. The generic non-Jordan family
and the theories whose scalar manifolds are homogeneous but not symmetric do
not lead to unified MESGTs in four dimensions. The three infinite families of
unified five-dimensional MESGTs defined by simple Lorentzian Jordan algebras,
whose scalar manifolds are non-homogeneous, do not lead directly to unified
MESGTs in four dimensions under dimensional reduction. However, since their
manifolds are non-homogeneous we are not able to completely rule out the
existence of symplectic sections in which these theories become unified in four
dimensions.Comment: 47 pages; latex fil
Extremal single-charge small black holes: Entropy function analysis
We study stretched horizons of the type AdS_2 x S^8 for certain spherically
symmetric extremal small black holes in type IIA carrying only D0-brane charge
making use of Sen's entropy function formalism for higher derivative gravity. A
scaling argument is given to show that the entropy of this class of black holes
for large charge behaves as \sqrt{|q|} where q is the electric charge. The
leading order result arises from IIA string loop corrections. We find that for
solutions to exist the force on a probe D0-brane has to vanish and we prove
that this feature persists to all higher derivative orders. We comment on the
nature of the extremum of these solutions and on the sub-leading corrections to
the entropy. The entropy of other small black holes related by dualities to our
case is also discussed.Comment: 19 pages, v2:typos corrected and references adde
Phase Space Analysis of Quintessence Cosmologies with a Double Exponential Potential
We use phase space methods to investigate closed, flat, and open
Friedmann-Robertson-Walker cosmologies with a scalar potential given by the sum
of two exponential terms. The form of the potential is motivated by the
dimensional reduction of M-theory with non-trivial four-form flux on a
maximally symmetric internal space. To describe the asymptotic features of
run-away solutions we introduce the concept of a `quasi fixed point.' We give
the complete classification of solutions according to their late-time behavior
(accelerating, decelerating, crunch) and the number of periods of accelerated
expansion.Comment: 46 pages, 5 figures; v2: minor changes, references added; v3: title
changed, refined classification of solutions, 3 references added, version
which appeared in JCA
On the Thermodynamic Geometry of BTZ Black Holes
We investigate the Ruppeiner geometry of the thermodynamic state space of a
general class of BTZ black holes. It is shown that the thermodynamic geometry
is flat for both the rotating BTZ and the BTZ Chern Simons black holes in the
canonical ensemble. We further investigate the inclusion of thermal
fluctuations to the canonical entropy of the BTZ Chern Simons black holes and
show that the leading logartithmic correction due to Carlip is reproduced. We
establish that the inclusion of thermal fluctuations induces a non zero scalar
curvature to the thermodynamic geometry.Comment: 1+17 pages, LaTeX, 4 eps figure
String Gas Cosmology
We present a critical review and summary of String Gas Cosmology. We include
a pedagogical derivation of the effective action starting from string theory,
emphasizing the necessary approximations that must be invoked. Working in the
effective theory, we demonstrate that at late-times it is not possible to
stabilize the extra dimensions by a gas of massive string winding modes. We
then consider additional string gases that contain so-called enhanced symmetry
states. These string gases are very heavy initially, but drive the moduli to
locations that minimize the energy and pressure of the gas. We consider both
classical and quantum gas dynamics, where in the former the validity of the
theory is questionable and some fine-tuning is required, but in the latter we
find a consistent and promising stabilization mechanism that is valid at
late-times. In addition, we find that string gases provide a framework to
explore dark matter, presenting alternatives to CDM as recently
considered by Gubser and Peebles. We also discuss quantum trapping with string
gases as a method for including dynamics on the string landscape.Comment: 55 pages, 1 figure, minor corrections, version to appear in Reviews
of Modern Physic
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