13,671 research outputs found
Who You Gonna Call? Runaway Ghosts, Higher Derivatives and Time-Dependence in EFTs
We briefly review the formulation of effective field theories (EFTs) in
time-dependent situations, with particular attention paid to their domain of
validity. Our main interest is the extent to which solutions of the EFT capture
the dynamics of the full theory. For a simple model we show by explicit
calculation that the low-energy action obtained from a sensible UV completion
need not take the restrictive form required to obtain only second-order field
equations, and we clarify why runaway solutions are nevertheless typically not
a problem for the EFT. Although our results will not be surprising to many, to
our knowledge they are only mentioned tangentially in the EFT literature, which
(with a few exceptions) largely addresses time-independent situations.Comment: 12 page
Massive-Scalar Effective Actions on Anti-de Sitter Spacetime
Closed forms are derived for the effective actions for free, massive spinless
fields in anti-de Sitter spacetimes in arbitrary dimensions. The results have
simple expressions in terms of elementary functions (for odd dimensions) or
multiple Gamma functions (for even dimensions). We use these to argue against
the quantum validity of a recently-proposed duality relating such theories with
differing masses and cosmological constants.Comment: 23 pages, plain TeX, one figur
Self-Tuning at Large (Distances): 4D Description of Runaway Dilaton Capture
We complete here a three-part study (see also arXiv:1506.08095 and
1508.00856) of how codimension-two objects back-react gravitationally with
their environment, with particular interest in situations where the transverse
`bulk' is stabilized by the interplay between gravity and flux-quantization in
a dilaton-Maxwell-Einstein system such as commonly appears in
higher-dimensional supergravity and is used in the Supersymmetric Large Extra
Dimensions (SLED) program. Such systems enjoy a classical flat direction that
can be lifted by interactions with the branes, giving a mass to the would-be
modulus that is smaller than the KK scale. We construct the effective
low-energy 4D description appropriate below the KK scale once the transverse
extra dimensions are integrated out, and show that it reproduces the
predictions of the full UV theory for how the vacuum energy and modulus mass
depend on the properties of the branes and stabilizing fluxes. In particular we
show how this 4D theory learns the news of flux quantization through the
existence of a space-filling four-form potential that descends from the
higher-dimensional Maxwell field. We find a scalar potential consistent with
general constraints, like the runaway dictated by Weinberg's theorem. We show
how scale-breaking brane interactions can give this potential minima for which
the extra-dimensional size, , is exponentially large relative to
underlying physics scales, , with where
can be arranged with a small hierarchy between fundamental
parameters. We identify circumstances where the potential at the minimum can
(but need not) be parametrically suppressed relative to the tensions of the
branes, provide a preliminary discussion of the robustness of these results to
quantum corrections, and discuss the relation between what we find and earlier
papers in the SLED program.Comment: 37 pages + appendice
Inflating in a Trough: Single-Field Effective Theory from Multiple-Field Curved Valleys
We examine the motion of light fields near the bottom of a potential valley
in a multi-dimensional field space. In the case of two fields we identify three
general scales, all of which must be large in order to justify an effective
low-energy approximation involving only the light field, . (Typically
only one of these -- the mass of the heavy field transverse to the trough -- is
used in the literature when justifying the truncation of heavy fields.) We
explicitly compute the resulting effective field theory, which has the form of
a model, with , as a function of these
scales. This gives the leading ways each scale contributes to any low-energy
dynamics, including (but not restricted to) those relevant for cosmology. We
check our results with the special case of a homogeneous roll near the valley
floor, placing into a broader context recent cosmological calculations that
show how the truncation approximation can fail. By casting our results
covariantly in field space, we provide a geometrical criterion for
model-builders to decide whether or not the single-field and/or the truncation
approximation is justified, identify its leading deviations, and to efficiently
extract cosmological predictions.Comment: 28 pages + 3 appendices, references added and typos corrected,
matches published versio
String Inflation After Planck 2013
We briefly summarize the impact of the recent Planck measurements for string
inflationary models, and outline what might be expected to be learned in the
near future from the expected improvement in sensitivity to the primordial
tensor-to-scalar ratio. We comment on whether these models provide sufficient
added value to compensate for their complexity, and ask how they fare in the
face of the new constraints on non-gaussianity and dark radiation. We argue
that as a group the predictions made before Planck agree well with what has
been seen, and draw conclusions from this about what is likely to mean as
sensitivity to primordial gravitational waves improves.Comment: LaTeX, 21 pages plus references; slight modification of the
discussion of inflection point inflation, references added and typos
correcte
The Gravity of Dark Vortices: Effective Field Theory for Branes and Strings Carrying Localized Flux
A Nielsen-Olesen vortex usually sits in an environment that expels the flux
that is confined to the vortex, so flux is not present both inside and outside.
We construct vortices for which this is not true, where the flux carried by the
vortex also permeates the `bulk' far from the vortex. The idea is to mix the
vortex's internal gauge flux with an external flux using off-diagonal kinetic
mixing. Such `dark' vortices could play a phenomenological role in models with
both cosmic strings and a dark gauge sector. When coupled to gravity they also
provide explicit ultra-violet completions for codimension-two brane-localized
flux, which arises in extra-dimensional models when the same flux that
stabilizes extra-dimensional size is also localized on space-filling branes
situated around the extra dimensions. We derive simple formulae for observables
such as defect angle, tension, localized flux and on-vortex curvature when
coupled to gravity, and show how all of these are insensitive to much of the
microscopic details of the solutions, and are instead largely dictated by
low-energy quantities. We derive the required effective description in terms of
a world-sheet brane action, and derive the matching conditions for its
couplings. We consider the case where the dimensions transverse to the bulk
compactify, and determine how the on- and off-vortex curvatures and other bulk
features depend on the vortex properties. We find that the brane-localized flux
does not gravitate, but just renormalizes the tension in a magnetic-field
independent way. The existence of an explicit UV completion puts the effective
description of these models on a more precise footing, verifying that
brane-localized flux can be consistent with sensible UV physics and resolving
some apparent paradoxes that can arise with a naive (but commonly used)
delta-function treatment of the brane's localization within the bulk.Comment: 36 pages + appendices, 7 figure
EFT for Vortices with Dilaton-dependent Localized Flux
We study how codimension-two objects like vortices back-react gravitationally
with their environment in theories (such as 4D or higher-dimensional
supergravity) where the bulk is described by a dilaton-Maxwell-Einstein system.
We do so both in the full theory, for which the vortex is an explicit classical
`fat brane' solution, and in the effective theory of `point branes' appropriate
when the vortices are much smaller than the scales of interest for their
back-reaction (such as the transverse Kaluza-Klein scale). We extend the
standard Nambu-Goto description to include the physics of flux-localization
wherein the ambient flux of the external Maxwell field becomes partially
localized to the vortex, generalizing the results of a companion paper to
include dilaton-dependence for the tension and localized flux. In the effective
theory, such flux-localization is described by the next-to-leading effective
interaction, and the boundary conditions to which it gives rise are known to
play an important role in how (and whether) the vortex causes supersymmetry to
break in the bulk. We track how both tension and localized flux determine the
curvature of the space-filling dimensions. Our calculations provide the tools
required for computing how scale-breaking vortex interactions can stabilize the
extra-dimensional size by lifting the dilaton's flat direction. For small
vortices we derive a simple relation between the near-vortex boundary
conditions of bulk fields as a function of the tension and localized flux in
the vortex action that provides the most efficient means for calculating how
physical vortices mutually interact without requiring a complete construction
of their internal structure. In passing we show why a common procedure for
doing so using a -function can lead to incorrect results. Our
procedures generalize straightforwardly to general co-dimension objects.Comment: 45 pages + appendix, 6 figure
Inflating with Large Effective Fields
We re-examine large scalar fields within effective field theory, in
particular focussing on the issues raised by their use in inflationary models
(as suggested by BICEP2 to obtain primordial tensor modes). We argue that when
the large-field and low-energy regimes coincide the scalar dynamics is most
effectively described in terms of an asymptotic large-field expansion whose
form can be dictated by approximate symmetries, which also help control the
size of quantum corrections. We discuss several possible symmetries that can
achieve this, including pseudo-Goldstone inflatons characterized by a coset
(based on abelian and non-abelian, compact and non-compact symmetries),
as well as symmetries that are intrinsically higher dimensional. Besides the
usual trigonometric potentials of Natural Inflation we also find in this way
simple {\em large-field} power laws (like ) and exponential
potentials, . Both of these can
describe the data well and give slow-roll inflation for large fields without
the need for a precise balancing of terms in the potential. The exponential
potentials achieve large through the limit and so
predict ; consequently gives but not much larger (and so could be ruled out as measurements on
and improve). We examine the naturalness issues for these models and
give simple examples where symmetries protect these forms, using both
pseudo-Goldstone inflatons (with non-abelian non-compact shift symmetries
following familiar techniques from chiral perturbation theory) and
extra-dimensional models.Comment: 21 pages + appendices, 3 figure
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