Quantum evolution across singularities

Attempts to consider evolution across space-time singularities often lead to quantum systems with time-dependent Hamiltonians developing an isolated singularity as a function of time. Examples include matrix theory in certain singular time-dependent backgounds and free quantum fields on the two-dimensional compactified Milne universe. Due to the presence of the singularities in the time dependence, the conventional quantum-mechanical evolution is not well-defined for such systems. We propose a natural way, mathematically analogous to renormalization in conventional quantum field theory, to construct unitary quantum evolution across the singularity. We carry out this procedure explicitly for free fields on the compactified Milne universe and compare our results with the matching conditions considered in earlier work (which were based on the covering Minkowski space).Comment: revised with an emphasis on local counterterm subtraction rather than analyticity; version to be submitted for publicatio

AdS (in)stability: an analytic approach

We briefly review the topic of AdS (in)stability, mainly focusing on a recently introduced analytic approach and its interplay with numerical results.Comment: 14 pages, based in part on an invited talk at the Leuven workshop on The String Theory Universe, September 7-11, 2015; v2: minor clarifications and references adde

The Grey Zone

The ‘grey zone’ is a term coined by the Italian Holocaust survivor Primo Levi in his essay collection The Drowned and the Saved (1989; originally published in Italian in 1986), the last book he completed before his death. In ‘The Grey Zone’, the second chapter and the longest essay in the book, Levi acknowledges the human need to divide the social field into ‘us’ and ‘them’, two clearly distinct and identifiable groups, but points out that such binary thinking is inadequate in the face of the..

Adiabaticity and emergence of classical space-time in time-dependent matrix theories

We discuss the low-curvature regime of time-dependent matrix theories proposed to describe non-perturbative quantum gravity in asymptotically plane-wave space-times. The emergence of near-classical space-time in this limit turns out to be closely linked to the adiabaticity of the matrix theory evolution. Supersymmetry restoration at low curvatures, which is crucial for the usual space-time interpretation of matrix theories, becomes an obvious feature of the adiabatic regime.Comment: 18 pages; v2: disclaimers and references added, version for publicatio

Quantum backreaction in string theory

There are situations in string theory when a finite number of string quanta induce a significant backreaction upon the background and render the perturbation theory infrared-divergent. The simplest example is D0-brane recoil under an impact by closed strings. A more physically interesting case is backreaction on the evolution of a totally compact universe due to closed string gas. Such situations necessitate qualitative amendments to the traditional formulation of string theory in a fixed classical background. In this contribution to the proceedings of the XVII European Workshop on String Theory in Padua, I review solved problems and current investigations in relation to this kind of quantum backreaction effects.Comment: 8 pages, proceedings of XVII European Workshop on String Theor

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

On D-branes in Type 0 String Theory

Using boundary states we derive the presence of (chiral) fermions on the intersection of type 0 D-branes. The corresponding anomalous couplings on the branes are then computed. Furthermore, we discuss systems of branes sitting at $A_n$ singularities. In particular, the massless spectrum on the branes is derived, and a boundary state description is given.Comment: 13 pages, LaTeX2e, 1 figure; v2: presentation made more self-contained, version to be published in Phys. Lett.

Low Tension Strings on a Cosmological Singularity

It has recently been argued that the singularity of the Milne orbifold can be resolved in higher spin theories. In string theory scattering amplitudes, however, the Milne singularity gives rise to ultraviolet divergences that signal uncontrolled backreaction. Since string theory in the low tension limit is expected to be a higher spin theory (although precise proposals only exist in special cases), we investigate what happens to these scattering amplitudes in the low tension limit. We point out that the known problematic ultraviolet divergences disappear in this limit. In addition we systematically identify all divergences of the simplest 2-to-2 string scattering amplitude on the Milne orbifold, and argue that the divergences that survive in the low tension limit have sensible infrared interpretations.Comment: 17 pages, 3 figures. V2: Complete list of IR divergences, more refs, published version. Plus one minor correctio