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We study phase structures of quantum field theories in fuzzy geometries. Several examples of fuzzy geometries as well as QFT's on such geometries are considered. They are fuzzy spheres and beyond as well as noncommutative deformations of BTZ blackholes. Analysis is done analytically and through simulations. Several features like novel stripe phases as well as spontaneous symmetry breaking avoiding Colemen, Mermin, Wagner theorem are brought out. Also we establish that these phases are stable due to topological obstructions., Comment: 19 pages,13 figures,Proceedings of the Corfu Summer Institute 2011 School and Workshops on Elementary Particle Physics and Gravity

Noncommutative deformations of the BTZ blackholes are described by noncommutative cylinders. We study the scalar fields in this background. The spectrum is studied analytically and through numerical simulations we establish the existence of novel `stripe phases'. These are different from stripes on Moyal spaces and stable due to topological obstruction., Comment: 18 pages, 4 figures, minor changes in the text

We derive an explicit expression for an associative *-product on fuzzy complex projective spaces. This generalises previous results for the fuzzy 2-sphere and gives a discrete non-commutative algebra of functions on fuzzy complex projective spaces, represented by matrix multiplication. The matrices are restricted to ones whose dimension is that of the totally symmetric representations of SU(N). In the limit of infinite dimensional matrices we recover the commutative algebra of functions on ordinary projective space. Derivatives on the fuzzy projective space are also expressed as matrix commutators., Comment: 20 pages, typeset in plain TeX

We study the ``renormalization group action'' induced by cycles of cosmic expansion and contraction, within the context of a family of stochastic dynamical laws for causal sets derived earlier. We find a line of fixed points corresponding to the dynamics of transitive percolation, and we prove that there exist no other fixed points and no cycles of length two or more. We also identify an extensive ``basin of attraction'' of the fixed points but find that it does not exhaust the full parameter space. Nevertheless, we conjecture that every trajectory is drawn toward the fixed point set in a suitably weakened sense., Comment: 22 pages, 1 firgure, submitted to Phys. Rev. D

Previously matrix model actions for ``fuzzy'' fields have been proposed using non-commutative geometry. They retained ``topological'' properties extremely well, being capable of describing instantons, $\theta$--states, the chiral anomaly, and even chiral fermions with no ``doubling''. Here, we demonstrate that the standard scalar and spinor actions on a $d$--dimensional manifold are recovered from such actions in the limit of large matrices if their normalizations are correctly scaled as the limit is taken., Comment: 14 pages LaTeX

The dynamical evolution of superconducting cosmic strings is much more complicated than that of simple Goto-Nambu strings. For this reason, there are only a few known analytical solutions and no numerical ones. The goal of this paper is to present numerical solutions for the dynamics of planar superconducting cosmic string loops. In most cases, a purely dynamical approach turns out to be insufficient to describe correctly the evolution of a loop due mainly to the appearance of shocks when spacelike currents are present and kinks for timelike currents, leading to yet unaccounted for quantum effects. The consequences of the quantum effects are mostly unknown at this time because the problem requires a dynamical field theory treatment. It is however likely that ultimately the result will be massive radiation in the form of charge carriers from the string., Comment: 13 pages including 16 figures, submitted to Phys. Rev. D. Major changes: the title changed, new figures were added, new physics was found, the conclusions have changed. Overall, the article has doubled in size

Monopole-antimonopole pairs connected by strings can be formed as topological defects in a sequence of cosmological phase transitions. Such hybrid defects typically decay early in the history of the universe but can still generate an observable background of gravitational waves. We study the spectrum of gravitational radiation from these objects both analytically and numerically, concentrating on the simplest case of an oscillating pair connected by a straight string., Comment: 18 pages, RevTex and 2 postscript figures. Submitted to Phys. Rev. D

The dynamical stability of cosmic rings, or vortons, is investigated for the particular equation of state given by the Witten bosonic model. It is found that there exists a finite range of the state parameter for which the vorton states are actually stable against dynamical perturbations. Inclusion of the electromagnetic self action into the equation of state slightly shrinks the stability region but otherwise yields no qualitative difference. If the Witten bosonic model represents a good approximation for more realistic string models, then the cosmological vorton excess problem can only be solved by assuming either that strings are formed at low energy scales or that some quantum instability may develop at a sufficient rate., Comment: 11 pages, LaTeX-ReVTeX (v.3), 2 figures available upon request, DAMTP R-94/19