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

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

Quantum evolution across singularities: the case of geometrical resolutions

We continue the study of time-dependent Hamiltonians with an isolated singularity in their time dependence, describing propagation on singular space-times. In previous work, two of us have proposed a "minimal subtraction" prescription for the simplest class of such systems, involving Hamiltonians with only one singular term. On the other hand, Hamiltonians corresponding to geometrical resolutions of space-time tend to involve multiple operator structures (multiple types of dependence on the canonical variables) in an essential way. We consider some of the general properties of such (near-)singular Hamiltonian systems, and further specialize to the case of a free scalar field on a two-parameter generalization of the null-brane space-time. We find that the singular limit of free scalar field evolution exists for a discrete subset of the possible values of the two parameters. The coordinates we introduce reveal a peculiar reflection property of scalar field propagation on the generalized (as well as the original) null-brane. We further present a simple family of pp-wave geometries whose singular limit is a light-like hyperplane (discontinuously) reflecting the positions of particles as they pass through it.Comment: 25 pages, 1 figur

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

Wacker-oxidation of Ethylene over Pillared Layered Material Catalysts

This paper concerns the Wacker oxidation of ethylene by oxygen in the presence of water over supported Pd/VOx catalysts. High surface area porous supports were obtained from layer-structured materials, such as, montmorillonite (MT), laponite (LT) (smectites), and hydrotalcite (layered double hydroxide, LDH) by pillaring. Before introduction of Pd, supports MT and LDH were pillared by vanadia. The laponite was used in titania-pillared form (TiO2-LAP) as support of Pd/VOx active component. Acetaldehyde (AcH), acetic acid (AcOH) and CO2 were the products with yields and selectivities, depending on the reaction conditions and the properties of the applied catalyst. Under comparable conditions the pillared smectite catalysts gave higher AcH yield than the pillared LDH catalyst. UV vis spectroscopic examination suggested that the pillared smectites contained polymeric chains of VO4, whereas only isolated monomeric VO4 species were present in the pillared LDH. The higher catalytic activity in the Wacker oxidation was attributed to the more favorable redox property of the polymeric than of the monomeric vanadia. The V3+ ions in the polymeric species can reduce O2 to O2- ions, whereas the obtained V5+ ions are ready to pass over O to Pd0 to generate PdO whereon the oxidation of the ethylene proceeds

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