146 research outputs found

### The Geometry of The Entropic Principle and the Shape of the Universe

Ooguri, Vafa, and Verlinde have outlined an approach to two-dimensional
accelerating string cosmology which is based on topological string theory, the
ultimate objective being to develop a string-theoretic understanding of
"creating the Universe from nothing". The key technical idea here is to assign
*two different* Lorentzian spacetimes to a certain Euclidean space. Here we
give a simple framework which allows this to be done in a systematic way. This
framework allows us to extend the construction to higher dimensions. We find
then that the general shape of the spatial sections of the newly created
Universe is constrained by the OVV formalism: the sections have to be flat and
compact.Comment: 24 pages, 4 eps figures, improved exposition of Euclidean/Lorentzian
smoothin

### A Holographic Bound on Cosmic Magnetic Fields

Magnetic fields large enough to be observable are ubiquitous in astrophysics,
even at extremely large length scales. This has led to the suggestion that such
fields are seeded at very early (inflationary) times, and subsequently
amplified by various processes involving, for example, dynamo effects. Many
such mechanisms give rise to extremely large magnetic fields at the end of
inflationary reheating, and therefore also during the quark-gluon plasma epoch
of the early universe. Such plasmas have a well-known holographic description
in terms of a thermal asymptotically AdS black hole. We show that holography
imposes an upper bound on the intensity of magnetic fields ($\approx \; 3.6
\times 10^{18}\;\; \text{gauss}$ at the hadronization temperature) in these
circumstances; this is above, but not far above, the values expected in some
models of cosmic magnetogenesis.Comment: 16 pages, 2 figures, explicit numerical value given for the bound,
improved discussion of implications for superadiabatic amplification, version
to appear in Nucl Phys

### How Does the Quark-Gluon Plasma Know the Collision Energy?

Heavy ion collisions at the LHC facility generate a Quark-Gluon Plasma (QGP)
which, for central collisions, has a higher energy density and temperature than
the plasma generated in central collisions at the RHIC. But sufficiently
peripheral LHC collisions give rise to plasmas which have the \emph{same}
energy density and temperature as the "central" RHIC plasmas. One might assume
that the two versions of the QGP would have very similar properties (for
example, with regard to jet quenching), but recent investigations have
suggested that \emph{they do not}: the plasma "knows" that the overall
collision energy is different in the two cases. We argue, using a gauge-gravity
analysis, that the strong magnetic fields arising in one case (peripheral
collisions), but not the other, may be relevant here. If the residual magnetic
field in peripheral LHC plasmas is of the order of at least $eB\,\approx
\,5\,m^2_{\pi}$, then the model predicts modifications of the relevant
quenching parameter which approach those recently reported.Comment: 16 pages, one figure; version to appear in Nuclear Physics

### Angular Momentum in QGP Holography

The quark chemical potential is one of the fundamental parameters describing
the Quark-Gluon Plasma produced by sufficiently energetic heavy-ion collisions.
It is not large at the extremely high temperatures probed by the LHC, but it
plays a key role in discussions of the beam energy scan programmes at the RHIC
and other facilities. On the other hand, collisions at such energies typically
(that is, in peripheral collisions) give rise to very high values of the
angular momentum density. Here we explain that holographic estimates of the
quark chemical potential of a rotating sample of plasma can be very
considerably improved by taking the angular momentum into account.Comment: 22 pages, 2 figures, version to appear in Nuclear Physics

### Stringy Instability of Topologically Non-Trivial Ads Black Holes and of desitter S-Brane Spacetimes

Seiberg and Witten have discussed a specifically "stringy" kind of
instability which arises in connection with "large" branes in asymptotically
AdS spacetimes. It is easy to see that this instability actually arises in most
five-dimensional asymptotically AdS black hole string spacetimes with
non-trivial horizon topologies. We point out that this is a more serious
problem than it may at first seem, for it cannot be resolved even by taking
into account the effect of the branes on the geometry of spacetime. [It is
ultimately due to the {\em topology} of spacetime, not its geometry.] Next,
assuming the validity of some kind of dS/CFT correspondence, we argue that
asymptotically deSitter versions of the Hull-Strominger-Gutperle S-brane
spacetimes are also unstable in this "topological" sense, at least in the case
where the R-symmetries are preserved. We conjecture that this is due to the
unrestrained creation of "late" branes, the spacelike analogue of large branes,
at very late cosmological times.Comment: References added, NPB versio

### Orbifold Physics and de Sitter Spacetime

It is generally believed the way to resolve the black hole information
paradox in string theory is to embed the black hole in anti-deSitter spacetime
-- without of course claiming that Schwarzschild-AdS is a realistic spacetime.
Here we propose that, similarly, the best way to study topologically
non-trivial versions of de Sitter spacetime from a stringy point of view is to
embed them in an anti-de Sitter orbifold bulk, again without claiming that this
is literally how de Sitter arises in string theory. Our results indicate that
string theory may rule out the more complex spacetime topologies which are
compatible with local de Sitter geometry, while still allowing the simplest
versions.Comment: Introduction re-written to explain our attitude to the use of Anti-de
Sitter spacetime. 33 pages, 5 diagrams, Nuclear Physics B versio

### AdS/CFT For Non-Boundary Manifolds

In its Euclidean formulation, the AdS/CFT correspondence begins as a study of
Yang-Mills conformal field theories on the sphere, S^4. It has been
successfully extended, however, to S^1 X S^3 and to the torus T^4. It is
natural to hope that it can be made to work for any manifold on which it is
possible to define a stable Yang-Mills conformal field theory. We consider a
possible classification of such manifolds, and show how to deal with the most
obvious objection : the existence of manifolds which cannot be represented as
boundaries. We confirm Witten's suggestion that this can be done with the help
of a brane in the bulk.Comment: 21 pages, 1 eps figure (1000x500), remarks on p-brane stress-tensor
clarifie

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