We discuss plane wave backgrounds of string theory and their relation to Gödel-like universes. This involves a twisted compactification along the direction of propagation of the wave, which induces closed timelike curves. We show, however, that no such curves are geodesic. The particle geodesics and the preferred holographic screens we find are qualitatively different from those in the Gödel-like universes. Of the two types of preferred screen, only one is suited to dimensional reduction and/or T-duality, and this provides a "holographic protection" of chronology. The other type of screen, relevant to an observer localized in all directions, is constructed both for the compact and non-compact plane waves, a result of possible independent interest. We comment on the consistency of field theory in such spaces, in which there are closed timelike (and null) curves but no closed timelike (or null) geodesics.The four-dimensional Gödel universe [1] is a topologically trivial homogeneous space with non-zero rotation. This gives rise to some unusual properties: not only do all observers see themselves as the centre of rotation, but there exist closed timelike curves (CTCs) through every point. As an example of a space with CTCs for all times, it is unclear as to what extent Hawking's chronology protection conjecture [2], concerning the impossibility of forming a CTC in nature, is applicable.For this reason, the discussion of the physics of chronology protection (see, e.g., [3] for a recent review), has mostly avoided the Gödel universe. However, the discovery [4] of supersymmetric Gödel-like solutions to five-dimensional minimal supergravity, and to its eleven-dimensional M-theoretic lift, has forced the issue, at least within the string theory community. Surprisingly, these solutions to string and M-theory turn out [5,6] to be related to another supersymmetric space of much recent interest, namely the plane wave [7,8,9], and this is the relation of interest to us here. We should also note that supergravity solutions describing supersymmetric deformations of the extreme five-dimensional Reissner-Nordstrom black hole have been studied in this context. Both of the deformations discussed in [10] -one corresponding to the rotating black hole of [11], the other to a black hole in a Gödel-like universe -have CTCs, the precise nature of which has been examined in some detail [12,13,10,14,15]. Other non-supersymmetric solutions of interest, describing black holes in a Gödel-like universe, have recently been discussed in [16].It turns out that these Gödel-like universes (GLUs) are related to compactified plane waves (CPWs) in two different ways. The eleven-dimensional CPW, dimensionally reduced on an everywhere spacelike circle, yields a ten-dimensional GLU [6]. On the other hand, a CPW in type II string theory is T-dual to a GLU times a transverse circle [5]. Depending on the specific plane wave we start with, these procedures give rise to a large variety of GLUs. In addition to the R-R fluxes typically supporting the CPW solutio...
We examine the issue of renormalizability of asymtotically free field theories on non-commutative spaces. As an example, we solve the non-commutative O(N) invariant Gross-Neveu model at large N. On commutative space this is a renormalizable model with non-trivial interactions. On the noncommutative space, if we take the translation invariant ground state, we find that the model is nonrenormalizable. Removing the ultraviolet cutoff yields a trivial non-interacting theory.
We calculate, using holographic duality, the thermal two-point function in finite temperature little string theory. The analysis of those correlators reveals possible instabilities of the thermal ensemble, as in previous discussions of the thermodynamics of little string theory. We comment on the dependence of the instability on the spatial volume of the system.
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