977 research outputs found

### Supergravity, D-brane Probes and thermal super Yang-Mills: a comparison

A D3-brane probe in the context of AdS/CFT correspondence at finite
temperature is considered. The supergravity predictions for the physical
effective couplings of the world-volume gauge theory of the probe brane are
compared to those calculated in one-loop perturbation theory in the thermal
gauge theory. It is argued that when the Higgs expectation value is much larger
than the temperature, the supergravity result must agree with perturbative
thermal Yang-Mills. This provides a perturbative test of the Maldacena
conjecture. Predictions for the running electric and magnetic effective
couplings, beyond perturbation theory are also obtained. Phenomenological
applications for universe-branes are discussed. In particular mechanisms are
suggested for reducing the induced cosmological constant and naturally
obtaining a varying speed of light and a consequent inflation on the universe
brane.Comment: LaTeX, 23 pages, 4 eps figures; v2: Comparison with
Tseytlin-Yankielowicz ammended. Extra discussion on reduced supersymmetry on
branes added; v3: A few minor changes to improve presentation. Version
published in JHEP10(1999)1

### Brane-bulk energy exchange and cosmological acceleration

The consequences for the brane cosmological evolution of energy exchange
between the brane and the bulk are analyzed. A rich variety of brane
cosmologies is obtained, depending on the precise mechanism of energy transfer,
the equation of state of brane-matter and the spatial topology. An accelerating
era is generically a feature of the solutions.
(Prepared for 36th International Symposium Ahrenshoop on the Theory of
Elementary Particles: Recent Developments in String M Theory and Field Theory,
Wernsdorf, Germany, 26-30 Aug 2003.)Comment: 14 pages, 2 figures, Late

### Duality Symmetries and Topology Change in String Theory

Duality symmetries for strings moving in non-trivial spacetime backgrounds
are analysed. It is shown that for backgrounds generated from WZW and coset CFT
models such duality symmetries are exact to all orders in string perturbation
theory. Their implications for string dynamics in non-trivial/singular
spacetimes are discussed. (Talk given at the EPS 93 Conference, held at
Marseille, July 22-27. To appear in the Proceedings.)Comment: lateX file, 6pp. CERN-TH.7003/9

### 4D gravity on a brane from bulk higher-curvature terms

We study a gravity model where a tensionful codimension-one three-brane is
embedded on a bulk with infinite transverse length. We find that 4D gravity is
induced on the brane already at the classical level if we include
higher-curvature (Gauss-Bonnet) terms in the bulk. Consistency conditions
appear to require a negative brane tension as well as a negative coupling for
the higher-curvature terms.Comment: 10 pages, no figures; a minor change in wording (to appear in MPLA

### Perturbative and Non-Perturbative Partial Supersymmetry Breaking; N=4 -> N=2 -> N=1

We show the existence of a supersymmetry breaking mechanism in string theory,
where N=4 supersymmetry is broken spontaneously to N=2 and N=1 with moduli
dependent gravitino masses. The spectrum of the spontaneously broken theory
with lower supersymmetry is in one-to-one correspondence with the spectrum of
the heterotic N=4 string. The mass splitting of the N=4 spectrum depends on the
compactification moduli as well as the three R-symmetry charges. We also show
that, in string theory, chiral theories can be obtained after spontaneous
breaking of extended supersymmetry. This was impossible at the level of field
theory. In the large moduli limit a restoration of the N=4 supersymmetry is
obtained. As expected the graviphotons and some of the gauge bosons become
massive in N=1 vacua. At some special points of the moduli space some of the
N=4 states with non-zero winding numbers and with spin 0 and {1/2} become
massless chiral superfields of the unbroken N=1 supersymmetry. Such vaccua have
a dual type II description, in which there are magnetically charged states with
spin 0 and {1/2} that become massless. The heterotic-type II duality suggests
some novel non-perturbative transitions on the type II side. Such transitions
do not seem to have a geometric interpretation, since they relate type II
vaccua with symmetric worlsheet structure to assymetric ones. The heteroric
interpretation of such a transition is an ordinary Higgsing of an SU(2) factor.
In the case of N=4 --> N=2, the perturbative N=2 prepotential is determined by
the perturbative N=4 BPS states. This observation let us to suggest a method
which determines the exact non-perturbative prepotential of the effective N=2
supergravity using the shifted spectrum of the non-perturbative BPS states of
the underlying N=4 theory.Comment: 36 pages, Latex. An important part is added. We point out that there
are string theory groundstates with spontaneously broken extended
supersymmetry and chiral spectru

### Mirage Cosmology

A brane universe moving in a curved higher dimensional bulk space is
considered. The motion induces a cosmological evolution on the universe brane
that is indistiguishable from a similar one induced by matter density on the
brane. The phenomenological implications of such an idea are discussed. Various
mirage energy densities are found, corresponding to dilute matter driving the
cosmological expansion, many having superluminal properties $|w|>1$ or
violating the positive energy condition. It is shown that energy density due to
the world-volume fields is nicely incorporated into the picture. It is also
pointed out that the initial singularity problem is naturally resolved in this
context.Comment: JHEP LateX, 20 pages, no figures;v2 references added and a few minor
changes; (v3) Corrected an inconsequential error in eq. 6.2, 6.3. We thank D.
Kutasov for bringing this to our attention (v4) Corrected inconsequential
errors in (4.4)-(4.7) and (5.11). We thank A. Psinas for bringing them to our
attentio

### Gravitational Phenomena from Superstrings in Curved Spacetime

The four-dimensional superstring solutions define at low energy effective
supergravity theories. A class of them extends successfully the validity of the
standard model up to the string scale (${\cal O}(10^{17})~TeV$). We stress the
importance of string corrections which are relevant for low energy (${\cal
O}(1)~TeV$) predictions of gauge and Yukawa couplings as well as the spectrum
of the supersymmetric particles. A class of exact string solutions are also
presented, providing non trivial space-time backgrounds, from which we can draw
some lessons concerning the regions of space-time where the notion of the
effective field theory prescription make sense. We show that the string
gravitational phenomena may induce during the cosmological evolution,
transitions from one effective field theory prescription to a different one
where the geometrical and topological data, as well as the relevant observable
states are drastically different. Talk presented by C. Kounnas in the XXIX
Moriond Meeting, M\'eribel, France.Comment: LateX, 11pp., CERN-TH.7332/9

### U-duality and D-brane Combinatorics

We investigate D-brane instanton contributions to R^4 couplings in any
toroidal compactification of type II theories. Starting from the 11D
supergravity one-loop four-graviton amplitude computed by Green, Gutperle and
Vanhove, we derive the non-perturbative O(e^{-1/\lambda}) corrections to R^4
couplings by a sequence of T-dualities, and interpret them as precise
configurations of bound states of D-branes wrapping cycles of the
compactification torus. Dp-branes explicitely appear as fluxes on
D(p+2)-branes, and as gauge instantons on D(p+4)-branes. Specific rules for
weighting these contributions are obtained, which should carry over to more
general situations. Furthermore, it is shown that U-duality in D<=6 relates
these D-brane configurations to O(e^{-1/\lambda^2}) instantons for which a
geometric interpretation is still lacking.Comment: 17 pages, LaTeX2e, intro. shortened and minor typos corrected, final
versio

### Dynamical Topology Change, Compactification and Waves in a Stringy Early Universe

Exact string solutions are presented, where moduli fields are varying with
time. They provide examples where a dynamical change of the topology of space
is occurring. Some other solutions give cosmological examples where some
dimensions are compactified dynamically or simulate pre-big bang type
scenarios. Some lessons are drawn concerning the region of validity of
effective theories and how they can be glued together, using stringy
information in the region where the geometry and topology are not well defined
from the low energy point of view. Other time dependent solutions are presented
where a hierarchy of scales is absent. Such solutions have dynamics which is
qualitatively different and resemble plane gravitational waves. Talk presented
by E. Kiritsis in the 2\`eme Journ\'ee Cosmologie, Observatoire de Paris, 2-4
June 1994.Comment: LaTeX, 28pp. CERN-TH.7328/9

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