762 research outputs found
String Theory
This is a rendering of a review talk on the state of String Theory, given at
the EPS-2003 Conference, intended for a wide audience of experimental and
theoretical physicists. It emphasizes general ideas rather than technical
aspects.Comment: 11 pages. Contribution to the EPS2003 conferenc
On the Nature of the Hagedorn Transition in NCOS Systems
We extend the study of the nature of the Hagedorn transition in NCOS systems
in various dimensions. The canonical analysis results in a microscopic
ionization picture of a bound state system in which the Hagedorn transition is
postponed till irrelevancy. A microcanonical analysis leads to a limiting
Hagedorn behaviour dominated by highly excited, long open strings. The study of
the full phase diagram of the NCOS system using the AdS/CFT correspondence
suggests that the microscopic ionization picture is the correct one. We discuss
some refinements of the ionization mechanism for NCOS systems, including
the formation of a temperature-dependent barrier for the process. Some possible
consequences of this behaviour, including a potential puzzle for , are
discussed. Phase diagrams of a regularized form of NCOS systems are introduced
and do accomodate a phase of long open strings which disappears in the strict
NCOS limit.Comment: 37 pages, 3 Postscript figure
Long time scales and eternal black holes
We discuss the various scales determining the temporal behaviour of
correlation functions in the presence of eternal black holes. We point out the
origins of the failure of the semiclassical gravity approximation to respect a
unitarity-based bound suggested by Maldacena. We find that the presence of a
subleading (in the large-N approximation involved) master field does restore
the compliance with one bound but additional configurations are needed to
explain the more detailed expected time dependence of the Poincare recurrences
and their magnitude.Comment: 10 pages, 6 figures. Presented at Johns Hopkins 2003 and Ahrenshoop
2003 workshop
Remarks on Black Hole Instabilities and Closed String Tachyons
Physical arguments stemming from the theory of black-hole thermodynamics are
used to put constraints on the dynamics of closed-string tachyon condensation
in Scherk--Schwarz compactifications. A geometrical interpretation of the
tachyon condensation involves an effective capping of a noncontractible cycle,
thus removing the very topology that supports the tachyons. A semiclassical
regime is identified in which the matching between the tachyon condensation and
the black-hole instability flow is possible. We formulate a generalized
correspondence principle and illustrate it in several different circumstances:
an Euclidean interpretation of the transition from strings to black holes
across the Hagedorn temperature and instabilities in the brane-antibrane
system.Comment: harvmac, 20 pp, 4 eps figures. Contribution to Jacob Bekenstein's
Festschrif
Noncommutative Field Theory and the Dynamics of Quantum Hall Fluids
We study the spectrum of density fluctuations of Fractional Hall Fluids in
the context of the noncommutative hidrodynamical model of Susskind. We show
that, within the weak-field expansion, the leading correction to the
noncommutative Chern--Simons Lagrangian (a Maxwell term in the effective
action,) destroys the incompressibility of the Hall fluid due to strong UV/IR
effects at one loop. We speculate on possible relations of this instability
with the transition to the Wigner crystal, and conclude that calculations
within the weak-field expansion must be carried out with an explicit
ultraviolet cutoff at the noncommutativity scale. We point out that the
noncommutative dipoles exactly match the spatial structure of the
Halperin--Kallin quasiexcitons. Therefore, we propose that the noncommutative
formalism must describe accurately the spectrum at very large momenta, provided
no weak-field approximations are made. We further conjecture that the
noncommutative open Wilson lines are `vertex operators' for the quasiexcitons.Comment: 20 pages, harvma
Touring the Hagedorn Ridge
We review aspects of the Hagedorn regime in critical string theories, from
basic facts about the ideal gas approximation to the proposal of a global
picture inspired by general ideas of holography. It was suggested that the
condensation of thermal winding modes triggers a first-order phase transition.
We propose, by an Euclidean analogue of the string/black hole correspondence
principle, that the transition is actually related to a topology change in
spacetime. Similar phase transitions induced by unstable winding modes can be
studied in toy models. There, using T-duality of supersymmetric cycles, one can
identify a topology change of the Gregory--Laflamme type, which we associate
with large-N phase transitions of Yang--Mills theories on tori. This essay is
dedicated to the memory of Ian Kogan.Comment: 29 pages, 18 figures, contribution to I.I. Kogan memorial volume,
references adde
Higgs Inflation as a Mirage
We discuss a simple unitarization of Higgs inflation that is genuinely weakly
coupled up to Planckian energies. A large non-minimal coupling between the
Higgs and the Ricci curvature is induced dynamically at intermediate energies,
as a simple ratio of mass scales. Despite not being dominated by the Higgs
field, inflationary dynamics simulates the `Higgs inflation' one would get by
blind extrapolation of the low-energy effective Lagrangian, at least
qualitatively. Hence, Higgs inflation arises as an approximate `mirage' picture
of the true dynamics. We further speculate on the generality of this phenomenon
and show that, if Higgs-inflation arises as an effective description, the
details of the UV completion are necessary to extract robust quantitative
predictions.Comment: 21 pages, 2 figure
Remarks on Time-Space Noncommutative Field Theories
We propose a physical interpretation of the perturbative breakdown of
unitarity in time-like noncommutative field theories in terms of production of
tachyonic particles. These particles may be viewed as a remnant of a continuous
spectrum of undecoupled closed-string modes. In this way, we give a unified
view of the string-theoretical and the field-theoretical no-go arguments
against time-like noncommutative theories. We also perform a quantitative study
of various locality and causality properties of noncommutative field theories
at the quantum level.Comment: 19 pages, LaTe
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