93 research outputs found
Strings in flat space and pp waves from Super Yang Mills
We explain how the string spectrum in flat space and pp-waves arises from the
large limit, at fixed , of U(N) super Yang Mills.
We reproduce the spectrum by summing a subset of the planar Feynman diagrams.
We give a heuristic argument for why we can neglect other diagrams. We also
discuss some other aspects of pp-waves and we present a matrix model associated
to the DLCQ description of the maximally supersymmetric eleven dimensional
pp-waves.Comment: 36 pages, 5 figures. v3: minor typos corrected, references adde
Stringy Instantons and Cascading Quivers
D-brane instantons can perturb the quantum field theories on space-time
filling D-branes by interesting operators. In some cases, these D-brane
instantons are novel "stringy" effects (not interpretable directly as instanton
effects in the low-energy quantum field theory), while in others the D-brane
instantons can be directly interpreted as field theory effects. In this note,
we describe a situation where both perspectives are available, by studying
stringy instantons in quivers which arise at simple Calabi-Yau singularities.
We show that a stringy instanton which wraps an unoccupied node of the quiver,
and gives rise to a non-perturbative mass in the space-time field theory, can
be reinterpreted as a conventional gauge theory effect by going up in an
appropriate renormalization group cascade. Interestingly, in the cascade, the
contribution of the stringy instanton does not come from gauge theory
instantons but from strong coupling dynamics.Comment: 17 pages, 6 figures, harvma
Compactification on negatively curved manifolds
We show that string/M theory compactifications to maximally symmetric
space-times using manifolds whose scalar curvature is everywhere negative, must
have significant warping, large stringy corrections, or both.Comment: 18 pages, JHEP3.cl
Gravity waves and the LHC: Towards high-scale inflation with low-energy SUSY
It has been argued that rather generic features of string-inspired
inflationary theories with low-energy supersymmetry (SUSY) make it difficult to
achieve inflation with a Hubble scale H > m_{3/2}, where m_{3/2} is the
gravitino mass in the SUSY-breaking vacuum state. We present a class of
string-inspired supergravity realizations of chaotic inflation where a simple,
dynamical mechanism yields hierarchically small scales of post-inflationary
supersymmetry breaking. Within these toy models we can easily achieve small
ratios between m_{3/2} and the Hubble scale of inflation. This is possible
because the expectation value of the superpotential relaxes from large to
small values during the course of inflation. However, our toy models do not
provide a reasonable fit to cosmological data if one sets the SUSY-breaking
scale to m_{3/2} < TeV. Our work is a small step towards relieving the apparent
tension between high-scale inflation and low-scale supersymmetry breaking in
string compactifications.Comment: 21+1 pages, 5 figures, LaTeX, v2: added references, v3: very minor
changes, version to appear in JHE
The Cosmological Constant Problem and Inflation in the String Landscape
An earlier paper points out that a quantum treatment of the string landscape
is necessary. It suggests that the wavefunction of the universe is mobile in
the landscape until the universe reaches a meta-stable site with its
cosmological constant smaller than the critical value ,
where is estimated to be exponentially small compared to the Planck
scale. Since this site has an exponentially long lifetime, it may well be
today's universe. We investigate specific scenarios based on this quantum
diffusion property of the cosmic landscape and find a plausible scenario for
the early universe. In the last fast tunneling to the
() site in this scenario, all energies are stored in the nucleation
bubble walls, which are released to radiation only after bubble collisions and
thermalization. So the site is chosen even if plus
radiation is larger than , as long as the radiation does not
destabilize the vacuum. A consequence is that inflation must happen
before this last fast tunneling, so the inflationary scenario that emerges
naturally is extended brane inflation, where the brane motion includes a
combination of rolling, fast tunnelings, slow-roll, hopping and percolation in
the landscape. We point out that, in the brane world, radiation during
nucleosynthesis are mostly on the standard model branes (brane radiation, as
opposed to radiation in the bulk). This distinction may lead to interesting
dynamics. We consider this paper as a road map for future investigations.Comment: 34 pages, 4 figures; minor improvements and references added; more
discussions on CDL tunneling adde
Testing String Theory with CMB
Future detection/non-detection of tensor modes from inflation in CMB
observations presents a unique way to test certain features of string theory.
Current limit on the ratio of tensor to scalar perturbations, r=T/S, is r <
0.3, future detection may take place for r > 10^{-2}-10^{-3}. At present all
known string theory inflation models predict tensor modes well below the level
of detection. Therefore a possible experimental discovery of tensor modes may
present a challenge to string cosmology.
The strongest bound on r in string inflation follows from the observation
that in most of the models based on the KKLT construction, the value of the
Hubble constant H during inflation must be smaller than the gravitino mass. For
the gravitino mass in the usual range, m_{3/2} < O(1) TeV, this leads to an
extremely strong bound r < 10^{-24}. A discovery of tensor perturbations with r
> 10^{-3} would imply that the gravitinos in this class of models are
superheavy, m_{3/2} > 10^{13} GeV. This would have important implications for
particle phenomenology based on string theory.Comment: 13 pages, 2 figure
Stringy Instantons and Quiver Gauge Theories
We explore contributions to the 4D effective superpotential which arise from
Euclidean D3 branes (``instantons'') that intersect space-filling D-branes.
These effects can perturb the effective field theory on the space-filling
branes by nontrivial operators composed of charged matter fields, changing the
vacuum structure in a qualitative way in some examples. Our considerations are
exemplified throughout by a careful study of a fractional brane configuration
on a del Pezzo surface.Comment: 30 pages, 4 figures; v2: reference added; v3: confusing minor error
in axion charges fixed (thanks to D. Green for pointing it out
Multiple universes, cosmic coincidences, and other dark matters
Even when completely and consistently formulated, a fundamental theory of
physics and cosmological boundary conditions may not give unambiguous and
unique predictions for the universe we observe; indeed inflation, string/M
theory, and quantum cosmology all arguably suggest that we can observe only one
member of an ensemble with diverse properties. How, then, can such theories be
tested? It has been variously asserted that in a future measurement we should
observe the a priori most probable set of predicted properties (the
``bottom-up'' approach), or the most probable set compatible with all current
observations (the ``top-down'' approach), or the most probable set consistent
with the existence of observers (the ``anthropic'' approach). These inhabit a
spectrum of levels of conditionalization and can lead to qualitatively
different predictions. For example, in a context in which the densities of
various species of dark matter vary among members of an ensemble of otherwise
similar regions, from the top-down or anthropic viewpoints -- but not the
bottom-up -- it would be natural for us to observe multiple types of dark
matter with similar contributions to the observed dark matter density. In the
anthropic approach it is also possible in principle to strengthen this argument
and the limit the number of likely dark matter sub-components. In both cases
the argument may be extendible to dark energy or primordial density
perturbations. This implies that the anthropic approach to cosmology,
introduced in part to explain "coincidences" between unrelated constituents of
our universe, predicts that more, as-yet-unobserved coincidences should come to
light.Comment: 18 JCAP-style pages, accepted by JCAP. Revised version adds
references and some clarification
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