812 research outputs found
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
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
String Thermodynamics in D-Brane Backgrounds
We discuss the thermal properties of string gases propagating in various
D-brane backgrounds in the weak-coupling limit, and at temperatures close to
the Hagedorn temperature. We determine, in the canonical ensemble, whether the
Hagedorn temperature is limiting or non-limiting. This depends on the
dimensionality of the D-brane, and the size of the compact dimensions. We find
that in many cases the non-limiting behaviour manifest in the canonical
ensemble is modified to a limiting behaviour in the microcanonical ensemble and
show that, when there are different systems in thermal contact, the energy
flows into open strings on the `limiting' D-branes of largest dimensionality.
Such energy densities may eventually exceed the D-brane intrinsic tension. We
discuss possible implications of this for the survival of Dp-branes with large
values of p in an early cosmological Hagedorn regime. We also discuss the
general phase diagram of the interacting theory, as implied by the holographic
and black-hole/string correspondence principles.Comment: 50 pages, LaTeX, 4 eps figures. Added discussion of random walk
picture. Corrected technical error in the treatment of ND strings (notice
some formulas are rewritten). Conclusions unchange
Some Thermodynamical Aspects of String Theory
Thermodynamical aspects of string theory are reviewed and discussed.Comment: 22 Pages plain latex; based on contributions to Golfand Memorial
Volume and Englertfest by E.Rabinovic
Amyloid Imaging in Aging and Dementia: Testing the Amyloid Hypothesis In Vivo
Amyloid imaging represents a major advance in neuroscience, enabling the detection and quantification of pathologic protein aggregations in the brain. In this review we survey current amyloid imaging techniques, focusing on positron emission tomography (PET) with ^{11}carbon-labelled Pittsburgh Compound-B (11C-PIB), the most extensively studied and best validated tracer. PIB binds specifically to fibrillar beta-amyloid (Aβ) deposits, and is a sensitive marker for Aβ pathology in cognitively normal older individuals and patients with mild cognitive impairment (MCI) and Alzheimer’s disease (AD). PIB-PET provides us with a powerful tool to examine in vivo the relationship between amyloid deposition, clinical symptoms, and structural and functional brain changes in the continuum between normal aging and AD. Amyloid imaging studies support a model in which amyloid deposition is an early event on the path to dementia, beginning insidiously in cognitively normal individuals, and accompanied by subtle cognitive decline and functional and structural brain changes suggestive of incipient AD. As patients progress to dementia, clinical decline and neurodegeneration accelerate and proceed independently of amyloid accumulation. In the future, amyloid imaging is likely to supplement clinical evaluation in selecting patients for anti-amyloid therapies, while MRI and FDG-PET may be more appropriate markers of clinical progression
- …