1,940 research outputs found
Gauged Galileons From Branes
We show how the coupling of SO(N) gauge fields to galileons arises from a
probe brane construction. The galileons arise from the brane bending modes of a
brane probing a co-dimension N bulk, and the gauge fields arise by turning on
certain off-diagonal components in the zero mode of the bulk metric. By
construction, the equations of motion for both the galileons and gauge fields
remain second order. Covariant gauged galileons are derived as well.Comment: 6 pages. v2: minor changes, version appearing in PL
Stability and superluminality of spherical DBI galileon solutions
The DBI galileons are a generalization of the galileon terms, which extend
the internal galilean symmetry to an internal relativistic symmetry, and can
also be thought of as generalizations of DBI which yield second order field
equations. We show that, when considered as local modifications to gravity,
such as in the Solar system, there exists a region of parameter space in which
spherically symmetric static solutions exist and are stable. However, these
solutions always exhibit superluminality, casting doubt on the existence of a
standard Lorentz invariant UV completion.Comment: 16 pages, 1 figure. Discussions added, version appearing in PR
Soft Theorems For Shift-Symmetric Cosmologies
We derive soft theorems for single-clock cosmologies that enjoy a shift
symmetry. These so-called consistency conditions arise from a combination of a
large diffeomorphism and the internal shift-symmetry and fix the squeezed limit
of all correlators with a soft scalar mode. As an application, we show that our
results reproduce the squeezed bispectrum for Ultra-slow-roll inflation, a
particular shift-symmetric, non-attractor model which is known to violate
Maldacena's consistency relation. Similar results have been previously obtained
by Mooij and Palma using background-wave methods. Our results shed new light on
the infrared structure of single-clock cosmological spacetimes.Comment: 4 pages, v2: citation added, v3: citations added and edited in
accordance with published versio
Partially Massless Fields During Inflation
The representation theory of de Sitter space allows for a category of
partially massless particles which have no flat space analog, but could have
existed during inflation. We study the couplings of these exotic particles to
inflationary perturbations and determine the resulting signatures in
cosmological correlators. When inflationary perturbations interact through the
exchange of these fields, their correlation functions inherit scalings that
cannot be mimicked by extra massive fields. We discuss in detail the squeezed
limit of the tensor-scalar-scalar bispectrum, and show that certain partially
massless fields can violate the tensor consistency relation of single-field
inflation. We also consider the collapsed limit of the scalar trispectrum, and
find that the exchange of partially massless fields enhances its magnitude,
while giving no contribution to the scalar bispectrum. These characteristic
signatures provide clean detection channels for partially massless fields
during inflation.Comment: 48 pages, 5 figures. v2: references added, published versio
Regional Trends in Religion and Politics
As mass communications close the distances over which people routinely interact, there is a question about the increase in social homogenization at the expense of regional identity. In a society covering as much geographic area and encompassing as many cultures as the United States, the question is certainly valid. Traditionally, this diversity has been recognized as a melting pot, an analogy attempting to institutionalize a sort of homogeneous diversity; the melting pot analogy is now giving way to notions of multiculturalism. However, one may wonder if this social diversity can avoid being buried beneath the homogenizing mass media culture. Whether homogenization is good or bad, the subject is worth investigating, if only to learn if such a trend exists in an empirically observable form that can be placed in an objective context accessible to those concerned with regional trends and their implications
Aspects of Galileons
Galileons are a class of scalar field theories which have been found to arise in a disparate variety of contexts and exhibit a host of interesting properties by themselves, both classical and quantum. They obey non-trivial shift symmetries which restrict their self-interactions to be of higher derivative form, yet their equations of motion remain second order so that they are free of ghost instabilities. Further, when used as a force mediator between massive objects, galileons provide a natural realization of the Vainshtein screening mechanism which shuts off the fifth force at distances close to massive sources. As such, they are well suited for cosmology and are naturally incorporated into theories of modified gravity such as the Dvali-Gabadadze-Porrati braneworld model and the de Rham-Gabadadze-Tolley theory of massive gravity. Treated as a quantum field theory, galileons obey a non-trivial non-renormalization theorem which proves that they are not renormalized to any numbers of loops. In this thesis, we explore the properties of galileon theories and their generalizations through a combination of geometric and algebraic means. On the geometry side, we demonstrate that generic galileon theories are naturally thought of as the description of branes moving in higher dimensional spacetimes. On the algebraic side, we show that there exists a precise interpretation in which galileons can be thought of as Goldstone modes which arise when spacetime symmetries are spontaneously broken. In particular, when viewed in this light the galileons are the analogue of the Wess-Zumino-Witten term of the chiral lagrangian and thus represent interactions which are technically special. These methods provide both new technical tools for analyzing galileon-like theories and offer conceptual changes for how these theories can be viewed
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