149 research outputs found
The Embedding of Superstring Backgrounds in Einstein Gravity
A theorem of differential geometry is employed to locally embed a wide class
of superstring backgrounds that admit a covariantly constant null Killing
vector field in eleven-dimensional, Ricci-flat spaces. Included in this class
are exact type IIB superstring backgrounds with non-trivial Ramond-Ramond
fields and a class of supersymmetric string waves. The embedding spaces
represent exact solutions to eleven-dimensional, vacuum Einstein gravity. A
solution of eleven-dimensional supergravity is also embedded in a
twelve--dimensional, Ricci-flat space.Comment: 14 pages, Latex sourc
Primordial non-gaussianities in single field inflation
We calculate the three-point function for primordial scalar fluctuations in a
single field inflationary scenario where the scalar field Lagrangian is a
completely general function of the field and its first derivative. We obtain an
explicit expression for the three-point correlation function in a
self-consistent approximation scheme where the expansion rate varies slowly,
analogous to the slow-roll limit in standard, single-field inflation. The
three-point function can be written in terms of the familiar slow-roll
parameters and three new parameters which measure the non-trivial kinetic
structure of the scalar field, the departure of the sound speed from the speed
of light, and the rate of change of the sound speed.Comment: 26 pages, uses iopart.cls. Updated to match version published in JCA
Dualities and Hidden Supersymmetry in String Quantum Cosmology
A supersymmetric approach to string quantum cosmology based on the
non-compact, global duality symmetries of the effective action is developed. An
N=2 supersymmetric action is derived whose bosonic component is the
Neveu-Schwarz/Neveu-Schwarz sector of the --dimensional effective action
compactified on a --torus. A representation for the supercharges is found
and the form of the zero-and one-fermion quantum states is determined. The
purely bosonic component of the wavefunction is unique and manifestly invariant
under the symmetry of the action. The formalism applies to a wide class of
non-linear sigma-models.Comment: 18 pages, plain Latex, no figure
Primordial non-gaussianities from multiple-field inflation
We calculate the three-point correlation function evaluated at horizon
crossing for a set of interacting scalar fields coupled to gravity during
inflation. This provides the initial condition for the three-point function of
the curvature perturbation in the Sasaki--Stewart \delta N formulation. We find
that the effect is small, of the order of a slow-roll parameter, and that the
non-gaussianity can be determined on large scales once the unperturbed
background evolution is known. As an example of the use of our formalism, we
calculate the primordial non-gaussianity arising in a model of assisted
inflation.Comment: 24 pages, JCAP LaTeX style; replaced with version accepted by JCAP.
Some corrections to Sections 2 and 5, conclusions unchange
Primordial Non-Gaussianity and Gravitational Waves: Observational Tests of Brane Inflation in String Theory
We study brane inflation scenarios in a warped throat geometry and show that
there exists a consistency condition between the non-Gaussianity of the
curvature perturbation and the amplitude and scale-dependence of the primordial
gravitational waves. This condition is independent of the warping of the throat
and the form of the inflaton potential. We find that such a relation could be
tested by a future CMB polarization experiment if the Planck satellite is able
to detect both a gravitational wave background and a non-Gaussian statistic. In
models where the observable stage of inflation occurs when the brane is in the
tip region of the throat, we derive a further consistency condition involving
the scalar spectral index, the tensor-scalar ratio and the curvature
perturbation bispectrum. We show that when such a relation is combined with the
WMAP3 results, it leads to a model-independent bound on the gravitational wave
amplitude given by 0.001 < r < 0.01. This corresponds to the range of
sensitivity of the next generation of CMB polarization experiments.Comment: 7 pages, uses RevTeX4. v2, replaced with version accepted by Phys.
Rev.
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