63 research outputs found
The Reduced Open Membrane Metric
We discuss the reduction of the open membrane metric and determine the
(previously unknown) conformal factor. We also construct SL(2,R) invariant open
string metrics and complex open string coupling constants by reducing the open
membrane metric on a 2-torus. In doing so we also clarify some issues on
manifest SL(2,R) symmetry of the D3-brane. We remark on the consequences of our
results for the recently conjectured existence of decoupled (p,q)
non-commutative open string theories in type IIB string theory.Comment: 20 pages, LaTeX, uses JHEP.cls and JHEP.bst style files, published
JHEP version, note added change
Derivation of the Null Energy Condition
We derive the null energy condition, understood as a constraint on the
Einstein-frame Ricci tensor, from worldsheet string theory. For a closed
bosonic string propagating in a curved geometry, the spacetime interpretation
of the Virasoro constraint condition is precisely the null energy condition, to
leading nontrivial order in the alpha-prime expansion. Thus the deepest origin
of the null energy condition lies in worldsheet diffeomorphism invariance.Comment: 10 pages, journal versio
Chaotic inflation limits for non-minimal models with a Starobinsky attractor
We investigate inflationary attractor points by analyzing non-minimally
coupled single field inflation models in two opposite limits; the `flat' limit
in which the first derivative of the conformal factor is small and the `steep'
limit, in which the first derivative of the conformal factor is large. We
consider a subset of models that yield Starobinsky inflation in the steep
conformal factor, strong coupling, limit and demonstrate that they result in
chaotic inflation in the opposite flat, weak coupling, limit. The suppression
of higher order powers of the inflaton field in the potential is shown to be
related to the flatness condition on the conformal factor. We stress that the
chaotic attractor behaviour in the weak coupling limit is of a different, less
universal, character than the Starobinsky attractor. Agreement with the COBE
normalisation cannot be obtained in both attractor limits at the same time and
in the chaotic attractor limit the scale of inflation depends on the details of
the conformal factor, contrary to the strong coupling Starobinsky attractor.Comment: v2: 4 figures added, refs added, minor textual change
Minimal cut-off vacuum state constraints from CMB bispectrum statistics
In this short note we translate the best available observational bounds on
the CMB bispectrum amplitudes into constraints on a specific scale-invariant
New Physics Hypersurface (NPH) model of vacuum state modifications, as first
proposed by Danielsson, in general models of single-field inflation. As
compared to the power spectrum the bispectrum constraints are less ambiguous
and provide an interesting upper bound on the cut-off scale in general models
of single-field inflation with a small speed of sound. This upper bound is
incompatible with the power spectrum constraint for most of the parameter
domain, leaving very little room for minimal cut-off vacuum state modifications
in general single-field models with a small speed of sound.Comment: 9 pages, 1 figur
Back(reaction) to the Future in the Unruh-de Sitter State
Motivated by black hole physics, we define the Unruh state for a scalar field
in de Sitter space. Like the Bunch-Davies state, the Unruh-de Sitter state
appears thermal to a static observer. However, it breaks some of the symmetries
of de Sitter space. We calculate the expectation value of the energy-momentum
tensor in the Unruh-de Sitter state in two dimensions and find a non-vanishing
flux of outgoing negative energy. Extrapolating the result to four dimensions,
we argue that this backreacts on the initial de Sitter geometry
semi-classically. Notably, we estimate that de Sitter space is destabilized on
a timescale set by the gravitational entropy; analogous to black hole
evaporation, the endpoint of this instability is a singular geometry outside
the regime of effective field theory. Finally, we suggest that the Unruh-de
Sitter state may be a natural initial state for patches of de Sitter space, and
discuss the implications for slow-roll and eternal inflation, and for de Sitter
thermodynamics.Comment: 24 pages, 2 figures. v2: extended discussion about source at the
origin, added references, matches published versio
Observational Tests of Open Strings in Braneworld Scenarios
We consider some consequences of describing the gauge and matter degrees of
freedom in our universe by open strings, as suggested by the braneworld
scenario. We focus on the geometric effects described by the open string metric
and investigate their observational implications. The causal structure of
spacetime on the brane is altered; it is described not by the usual metric
, but instead by the open string metric, that incorporates the
electromagnetic background, . The speed of light is now slower when propagating along
directions transverse to electromagnetic fields or an NS-NS two form, so that
Lorentz invariance is explicitly broken. A generalized equivalence principle
guarantees that the propagation of {\it all} particles, not just photons, (with
the exception of gravitons) is slower in these transverse directions. We
describe a variety of astrophysical and laboratory-based experiments designed
to detect the predicted variations in the causal structure. We show that
current technology cannot probe beyond open string lengths of cm,
corresponding to MeV string scales. We also point out that in a braneworld
scenario, constraints on large scale electromagnetic fields together with a
modest phenomenological bound on the NS-NS two-form naturally lead to a bound
on the scale of canonical noncommutativity that is two orders of magnitude
below the string length. By invoking theoretical constraints on the NS-NS
two-form this bound can be improved to give an extremely strong bound on the
noncommutative scale well below the Planck length, $\sqrt{|\theta|_{max}} <
10^{-35} {\rm cm} \times ({{\rm TeV} \over {\rm string} {\rm scale}}Comment: Minor grammatical corrections and two reference added. 27 pages, 1
eps figure. Submitted to JHE
Extremal Tunneling and Anti-de Sitter Instantons
We rederive and extend the amplitude for charged spherical shells tunneling
through the outer horizon of charged black holes. In particular, we explicitly
confirm that an effective action approach with natural initial conditions for a
spherical shell, including backreaction, reduces to the tunneling integral.
Consequently, we establish a universal expression for the probability of
emission in terms of the change in the horizon entropy. Notably, the result for
the charged black hole also captures the superradiant regime of charged
particle decay at low energies. We then explore an appropriately regulated
extremal and near-horizon limit, relating the tunneling amplitude to a family
of gravitational instantons in the near-horizon Anti-de Sitter geometry,
reducing to the known result for domain walls to leading order in the
probe limit. We comment on the relation to the Weak Gravity Conjecture and the
conjectured instability of (non-supersymmetric) Anti-de Sitter vacua.Comment: 29 pages, 2 figures. v2: fixed typos, matches published versio
Supersymmetric embedding of antibrane polarization
We study the supersymmetry breaking induced by probe anti-D3-branes at the
tip of the Klebanov-Strassler throat geometry. Antibranes inside this geometry
polarize and can be described by an NS5-brane wrapping an . When the
number of antibranes is small compared to the background flux a metastable
state exists that breaks supersymmetry. We present a manifestly supersymmetric
effective model that realizes the polarized metastable state as a solution,
spontaneously breaking the supersymmetry. The supersymmetric model relies
crucially on the inclusion of Kaluza-Klein (matrix) degrees of freedom on the
and two supersymmetric irrelevant deformations of
super-Yang-Mills (SYM), describing a large number of supersymmetric D3-branes
in the IR. We explicitly identify the massless Goldstino and compute the
spectrum of massive fluctuations around the metastable supersymmetry-breaking
minimum, finding a Kaluza-Klein tower with masses warped down from the string
scale. Below the Kaluza-Klein scale the massive tower can be integrated out and
supersymmetry is realized nonlinearly. We comment on the effect of the
Kaluza-Klein modes on the effective description of de Sitter vacua in string
theory and inflationary model building.Comment: v1: 9 pages, v2: added references, changed title slightly and fixed
typo
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