28,880 research outputs found
Exact solutions in a scalar-tensor model of dark energy
We consider a model of scalar field with non minimal kinetic and Gauss Bonnet
couplings as a source of dark energy. Based on asymptotic limits of the
generalized Friedmann equation, we impose restrictions on the kinetic an
Gauss-Bonnet couplings. This restrictions considerable simplify the equations,
allowing for exact solutions unifying early time matter dominance with
transitions to late time quintessence and phantom phases. The stability of the
solutions in absence of matter has been studied.Comment: 30 pages, 2 figures, to appear in JCA
One Thousand and One Bubbles
We propose a novel strategy that permits the construction of completely
general five-dimensional microstate geometries on a Gibbons-Hawking space. Our
scheme is based on two steps. First, we rewrite the bubble equations as a
system of linear equations that can be easily solved. Second, we conjecture
that the presence or absence of closed timelike curves in the solution can be
detected through the evaluation of an algebraic relation. The construction we
propose is systematic and covers the whole space of parameters, so it can be
applied to find all five-dimensional BPS microstate geometries on a
Gibbons-Hawking base. As a first result of this approach, we find that the
spectrum of scaling solutions becomes much larger when non-Abelian fields are
present. We use our method to describe several smooth horizonless multicenter
solutions with the asymptotic charges of three-charge (Abelian and non-Abelian)
black holes. In particular, we describe solutions with the centers lying on
lines and circles that can be specified with exact precision. We show the power
of our method by explicitly constructing a 50-center solution. Moreover, we use
it to find the first smooth five-dimensional microstate geometries with
arbitrarily small angular momentum.Comment: 33 pages. v2: typos correcte
Toward solving the cosmological constant problem by embedding
The typical scalar field theory has a cosmological constant problem. We
propose a generic mechanism by which this problem is avoided at tree level by
embedding the theory into a larger theory. The metric and the scalar field
coupling constants in the original theory do not need to be fine-tuned, while
the extra scalar field parameters and the metric associated with the extended
theory are fine-tuned dynamically. Hence, no fine-tuning of parameters in the
full Lagrangian is needed for the vacuum energy in the new physical system to
vanish at tree level. The cosmological constant problem can be solved if the
method can be extended to quantum loops.Comment: published versio
Brief comments on Jackiw-Teitelboim gravity coupled to Liouville theory
Jackiw-Teitelboim gravity with non-vanishing cosmological constant coupled to
Liouville theory is considered as a non-critical string on dimensional flat
spacetime. It is discussed how the presence of cosmological constant yields
additional constraints on the parameter space of the theory, even when the
conformal anomaly is independent of the cosmological constant. Such constraints
agree with the necessary conditions for the tachyon field to be a primary
--prelogarithmic-- operator of the worldsheet conformal field theory. Thus, the
linearized tachyon field equation allows to impose the diagonal condition for
the interaction term. We analyze the neutralization of the Liouville mode
induced by the coupling to the Jackiw-Teitelboim Lagrangian. The free field
prescription leads to obtain explicit expressions for three-point correlation
functions for the case of vanishing cosmological constant in terms of a product
of Shapiro-Virasoro integrals. This is a consequence of the mentioned
neutralization effect.Comment: 14 pages, no figures. v2 References added. To be published in
Classical and Quantum Gravity. v3 typos correcte
Monodromy transform and the integral equation method for solving the string gravity and supergravity equations in four and higher dimensions
The monodromy transform and corresponding integral equation method described
here give rise to a general systematic approach for solving integrable
reductions of field equations for gravity coupled bosonic dynamics in string
gravity and supergravity in four and higher dimensions. For different types of
fields in space-times of dimensions with commuting isometries
-- stationary fields with spatial symmetries, interacting waves or partially
inhomogeneous cosmological models, the string gravity equations govern the
dynamics of interacting gravitational, dilaton, antisymmetric tensor and any
number of Abelian vector gauge fields (all depending only on two
coordinates). The equivalent spectral problem constructed earlier allows to
parameterize the infinite-dimensional space of local solutions of these
equations by two pairs of \cal{arbitrary} coordinate-independent holomorphic
- and - matrix functions of a spectral parameter which constitute a complete set
of monodromy data for normalized fundamental solution of this spectral problem.
The "direct" and "inverse" problems of such monodromy transform --- calculating
the monodromy data for any local solution and constructing the field
configurations for any chosen monodromy data always admit unique solutions. We
construct the linear singular integral equations which solve the inverse
problem. For any \emph{rational} and \emph{analytically matched} (i.e.
and
) monodromy data the solution for string
gravity equations can be found explicitly. Simple reductions of the space of
monodromy data leads to the similar constructions for solving of other
integrable symmetry reduced gravity models, e.g. 5D minimal supergravity or
vacuum gravity in dimensions.Comment: RevTex 7 pages, 1 figur
A New Formulation of the Initial Value Problem for Nonlocal Theories
There are a number of reasons to entertain the possibility that locality is
violated on microscopic scales, for example through the presence of an infinite
series of higher derivatives in the fundamental equations of motion. This type
of nonlocality leads to improved UV behaviour, novel cosmological dynamics and
is a generic prediction of string theory. On the other hand, fundamentally
nonlocal models are fraught with complications, including instabilities and
complications in setting up the initial value problem. We study the structure
of the initial value problem in an interesting class of nonlocal models. We
advocate a novel new formulation wherein the Cauchy surface is "smeared out"
over the underlying scale of nonlocality, so that the the usual notion of
initial data at t=0 is replaced with an "initial function" defined over -M^{-1}
\leq t \leq 0 where M is the underlying scale of nonlocality. Focusing on some
specific examples from string theory and cosmology, we show that this
mathematical re-formulation has surprising implications for the well-known
stability problem. For D-brane decay in a linear dilaton background, we are
able to show that the unstable directions in phase space cannot be accessed
starting from a physically sensible initial function. Previous examples of
unstable solutions in this model therefore correspond to unphysical initial
conditions, an observation which is obfuscated in the old formulation of the
initial value problem. We also discuss implication of this approach for
nonlocal cosmological models.Comment: 36 pages, 9 figures. Accepted for publication in Nuclear Physics
Black holes and black strings of N=2, d=5 supergravity in the H-FGK formalism
We study general classes and properties of extremal and non-extremal static
black-hole solutions of N=2, d=5 supergravity coupled to vector multiplets
using the recently proposed H-FGK formalism, which we also extend to static
black strings. We explain how to determine the integration constants and
physical parameters of the black-hole and black-string solutions. We derive
some model-independent statements, including the transformation of non-extremal
flow equations to the form of those for the extremal flow. We apply our methods
to the construction of example solutions (among others a new extremal string
solution of heterotic string theory on K_3 \times S^1). In the cases where we
have calculated it explicitly, the product of areas of the inner and outer
horizon of a non-extremal solution coincides with the square of the
moduli-independent area of the horizon of the extremal solution with the same
charges.Comment: 33 pages. Revised version: references added. No other change
Hagedorn inflation of D-branes
We examine the cosmological effects of the Hagedorn phase in models where the
observable universe is pictured as a D-brane. It is shown that, even in the
absence of a cosmological constant, winding modes cause a negative `pressure'
that can drive brane inflation of various types including both power law and
exponential. We also find regimes in which the cosmology is stable but
oscillating (a bouncing universe) with the Hagedorn phase softening the
singular behavior associated with the collapse.Comment: 44 Pages; JHEP latex; includes 1 postscript figur
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