723 research outputs found
A Farey tale for N=4 dyons
We study exponentially suppressed contributions to the degeneracies of
extremal black holes. Within Sen's quantum entropy function framework and
focusing on extremal black holes with an intermediate AdS3 region, we identify
an infinite family of semi-classical AdS2 geometries which can contribute
effects of order exp(S_0/c), where S_0 is the Bekenstein-Hawking-Wald entropy
and c is an integer greater than one. These solutions lift to the extremal
limit of the SL(2,Z) family of BTZ black holes familiar from the "black hole
Farey tail". We test this understanding in N=4 string vacua, where exact dyon
degeneracies are known to be given by Fourier coefficients of Siegel modular
forms. We relate the sum over poles in the Siegel upper half plane to the Farey
tail expansion, and derive a "Farey tale" expansion for the dyon partition
function. Mathematically, this provides a (formal) lift from Hilbert modular
forms to Siegel modular forms with a pole at the diagonal divisor.Comment: 31 page
Boundary String Field Theory, the Boundary State Formalism and D-Brane Tension
Recently a boundary string field theory that had been proposed some time ago,
was used to calculate correctly the ratios of D-brane (both BPS and non-BPS)
tensions. We discuss how this work is related to the boundary state formalism
and open string closed string duality, and argue that the latter clarifies why
the correct tension ratios are obtained in these recent calculations.Comment: References added, typos corrected, minor change
Type 0 Strings in a 2-d Black Hole
We study some aspects of type 0 strings propagating in the two dimensional
black hole geometry, corresponding to the exact SL(2)/U(1) SCFT background.Comment: 33 pages, harvmac. v2: minor clarification adde
Rotating metrics admitting non-perfect fluids in General Relativity
In this paper, by applying Newman-Janis algorithm in spherical symmetric
metrics, a class of embedded rotating solutions of field equations is
presented. These solutions admit non-perfect fluidsComment: LaTex, 39 page
M-theory lift of brane-antibrane systems and localised closed string tachyons
We discuss the lift of certain D6-antiD6-brane systems to M-theory. These are
purely gravitational configurations with a bolt singularity. When reduced along
a trivial circle, and for large bolt radius, the bolt is related to a
non-supersymmetric orbifold type of singularity where some closed string
tachyons are expected in the twisted sectors. This is a kind of open-closed
string duality that relates open string tachyons on one side and localised
tachyons in the other. We consider the evolution of the system of branes from
M-theory point of view. This evolution gives rise to a brane-antibrane
annihilation on the brane side. On the gravity side, the evolution is related
to a reduction of the order of the orbifold and to a contraction of the bolt to
a nut or flat space if the system has non-vanishing or vanishing charge,
respectively. We also consider the inverse process of reducing a
non-supersymmetric orbifold to a D6-brane system. For , the
reduced system is a fractional D6-brane at an orbifold singularity .Comment: 1+22 pages, 5 figures; v2 comments on the M-theory geometry and
further references adde
CMB Imprints of a Pre-Inflationary Climbing Phase
We discuss the implications for cosmic microwave background (CMB)
observables, of a class of pre-inflationary dynamics suggested by string models
where SUSY is broken due to the presence of D-branes and orientifolds
preserving incompatible portions of it. In these models the would-be inflaton
is forced to emerge from the initial singularity climbing up a mild exponential
potential, until it bounces against a steep exponential potential of "brane
SUSY breaking" scenarios, and as a result the ensuing descent gives rise to an
inflationary epoch that begins when the system is still well off its eventual
attractor. If a pre-inflationary climbing phase of this type had occurred
within 6-7 e-folds of the horizon exit for the largest observable wavelengths,
displacement off the attractor and initial-state effects would conspire to
suppress power in the primordial scalar spectrum, enhancing it in the tensor
spectrum and typically superposing oscillations on both. We investigate these
imprints on CMB observables over a range of parameters, examine their
statistical significance, and provide a semi-analytic rationale for our
results. It is tempting to ascribe at least part of the large-angle anomalies
in the CMB to pre-inflationary dynamics of this type.Comment: 38 pages, LaTeX, 11 eps figures, references added, matches version to
appear in JCA
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