26 research outputs found
p-branes on the waves
We present a large family of simple, explicit ten-dimensional supergravity
solutions describing extended extremal supersymmetric Ramond-Ramond p-branes
embedded into time-dependent dilaton-gravity plane waves of an arbitrary
(isotropic) profile, with the brane world-volume aligned parallel to the
propagation direction of the wave. Generalizations to the non-extremal case are
not analyzed explicitly, but can be pursued as indicated.Comment: 11 pages; v.2 minor notation changes, minor typos corrected
(published version
Light-like Big Bang singularities in string and matrix theories
Important open questions in cosmology require a better understanding of the
Big Bang singularity. In string and matrix theories, light-like analogues of
cosmological singularities (singular plane wave backgrounds) turn out to be
particularly tractable. We give a status report on the current understanding of
such light-like Big Bang models, presenting both solved and open problems.Comment: 20 pages, invited review for Class. Quant. Grav; v3: section 2.3
shortened, discussion on DLCQ added in section 3.1, published versio
Nonlinear instability and solitons in a self-gravitating fluid
We study a spherical, self-gravitating fluid model, which finds applications
in cosmic structure formation. We argue that since the system features
nonlinearity and gravity-induced dispersion, the emergence of solitons becomes
possible. We thus employ a multiscale expansion method to study, in the weakly
nonlinear regime, the evolution of small-amplitude perturbations around the
equilibrium state. This way, we derive a spherical nonlinear Schr{\"o}dinger
(NLS) equation that governs the envelope of the perturbations. The effective
NLS description allows us to predict a "nonlinear instability" (occurring in
the nonlinear regime of the system), namely, the modulational instability
which, in turn, may give rise to spherical soliton states. The latter feature a
very slow (polynomial) curvature-induced decay in time. The soliton profiles
may be used to describe the shape of dark matter halos at the rims of the
galaxies
Can free strings propagate across plane wave singularities?
We study free string propagation in families of plane wave geometries
developing strong scale-invariant singularities in certain limits. We relate
the singular limit of the evolution for all excited string modes to that of the
center-of-mass motion (the latter existing for discrete values of the overall
plane wave profile normalization). Requiring that the entire excitation energy
of the string should be finite turns out to be quite restrictive and
essentially excludes consistent propagation across the singularity, unless
dimensionful scales are introduced at the singular locus (in an otherwise
scale-invariant space-time).Comment: 24 pages; v.2: published version, minor clarifications adde
A smooth bouncing cosmology with scale invariant spectrum
We present a bouncing cosmology which evolves from the contracting to the
expanding phase in a smooth way, without developing instabilities or
pathologies and remaining in the regime of validity of 4d effective field
theory. A nearly scale invariant spectrum of perturbations is generated during
the contracting phase by an isocurvature scalar with a negative exponential
potential and then converted to adiabatic. The model predicts a slightly blue
spectrum, n_S >~ 1, no observable gravitational waves and a high (but model
dependent) level of non-Gaussianities with local shape. The model represents an
explicit and predictive alternative to inflation, although, at present, it is
clearly less compelling.Comment: 20 pages, 1 fig. v2: references added, JCAP published versio
Aspects of Plane Wave (Matrix) String Dynamics
We analyse two issues that arise in the context of (matrix) string theories
in plane wave backgrounds, namely (1) the use of Brinkmann- versus
Rosen-variables in the quantum theory for general plane waves (which we settle
conclusively in favour of Brinkmann variables), and (2) the regularisation of
the quantum dynamics for a certain class of singular plane waves (discussing
the benefits and limitations of regularisations of the plane-wave metric
itself).Comment: 29 page