12,511 research outputs found
Non-Gaussianity as a Particle Detector
We study the imprints of massive particles with spin on cosmological
correlators. Using the framework of the effective field theory of inflation, we
classify the couplings of these particles to the Goldstone boson of broken time
translations and the graviton. We show that it is possible to generate
observable non-Gaussianity within the regime of validity of the effective
theory, as long as the masses of the particles are close to the Hubble scale
and their interactions break the approximate conformal symmetry of the
inflationary background. We derive explicit shape functions for the scalar and
tensor bispectra that can serve as templates for future observational searches.Comment: 55 pages, 10 figure
Research priorities for advanced fibrous composites
Priorities for research in advanced laminated fibrous composite materials are presented. Supporting evidence is presented in two bodies, including a general literature survey and a survey of aerospace composite hardware and service experience. Both surveys were undertaken during 1977-1979. Specific results and conclusions indicate that a significant portion of contemporary published research diverges from recommended priorites
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
Desensitizing Inflation from the Planck Scale
A new mechanism to control Planck-scale corrections to the inflationary eta
parameter is proposed. A common approach to the eta problem is to impose a
shift symmetry on the inflaton field. However, this symmetry has to remain
unbroken by Planck-scale effects, which is a rather strong requirement on
possible ultraviolet completions of the theory. In this paper, we show that the
breaking of the shift symmetry by Planck-scale corrections can be
systematically suppressed if the inflaton field interacts with a conformal
sector. The inflaton then receives an anomalous dimension in the conformal
field theory, which leads to sequestering of all dangerous high-energy
corrections. We analyze a number of models where the mechanism can be seen in
action. In our most detailed example we compute the exact anomalous dimensions
via a-maximization and show that the eta problem can be solved using only
weakly-coupled physics.Comment: 34 pages, 3 figures
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A Model of Weighted Network Formation
The paper proposes a game of weighted network formation in which each agent has a limited resource to form links of possibly different intensities with other agents and to use for private purposes. We show that every equilibrium is either “reciprocal” or “non-reciprocal”. In a reciprocal equilibrium, any two agents invest equally in the link between them. In a non-reciprocal equilibrium, agents are partitioned into “concentrated” and “diversified” agents and a concentrated agent is only linked to diversified agents and vice versa. For every link, the concentrated agent invests more in the link than the diversified agent. The unweighted relationship graph of an equilibrium, in which two agents are linked if they both invest positively in each other, uniquely predicts the equilibrium values of each agent's network investment and utility level, as well as the ratio of any two agents' investments in each other. We show that equilibria are not pairwise stable and not efficient due to the positive externalities of investing in a link
The Cosmological Bootstrap: Inflationary Correlators from Symmetries and Singularities
Scattering amplitudes at weak coupling are highly constrained by Lorentz
invariance, locality and unitarity, and depend on model details only through
coupling constants and particle content. In this paper, we develop an
understanding of inflationary correlators which parallels that of flat-space
scattering amplitudes. Specifically, we study slow-roll inflation with weak
couplings to extra massive particles, for which all correlators are controlled
by an approximate conformal symmetry on the boundary of the spacetime. After
classifying all possible contact terms in de Sitter space, we derive an
analytic expression for the four-point function of conformally coupled scalars
mediated by the tree-level exchange of massive scalars. Conformal symmetry
implies that the correlator satisfies a pair of differential equations with
respect to spatial momenta, encoding bulk time evolution in purely boundary
terms. The absence of unphysical singularities completely fixes this
correlator. A spin-raising operator relates it to the correlators associated
with the exchange of particles with spin, while weight-shifting operators map
it to the four-point function of massless scalars. We explain how these de
Sitter four-point functions can be perturbed to obtain inflationary three-point
functions. We reproduce many classic results in the literature and provide a
complete classification of all inflationary three- and four-point functions
arising from weakly broken conformal symmetry. The inflationary bispectrum
associated with the exchange of particles with arbitrary spin is completely
characterized by the soft limit of the simplest scalar-exchange four-point
function of conformally coupled scalars and a series of contact terms. Finally,
we demonstrate that the inflationary correlators contain flat-space scattering
amplitudes via a suitable analytic continuation of the external momenta.Comment: 110 pages, 13 figures, 1 table; V3: minor corrections and references
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Organization of the mitochondrial genomes of whiteflies, aphids, and psyllids (Hemiptera, Sternorrhyncha)
BACKGROUND: With some exceptions, mitochondria within the class Insecta have the same gene content, and generally, a similar gene order allowing the proposal of an ancestral gene order. The principal exceptions are several orders within the Hemipteroid assemblage including the order Thysanoptera, a sister group of the order Hemiptera. Within the Hemiptera, there are available a number of completely sequenced mitochondrial genomes that have a gene order similar to that of the proposed ancestor. None, however, are available from the suborder Sternorryncha that includes whiteflies, psyllids and aphids. RESULTS: We have determined the complete nucleotide sequence of the mitochondrial genomes of six species of whiteflies, one psyllid and one aphid. Two species of whiteflies, one psyllid and one aphid have mitochondrial genomes with a gene order very similar to that of the proposed insect ancestor. The remaining four species of whiteflies had variations in the gene order. In all cases, there was the excision of a DNA fragment encoding for cytochrome oxidase subunit III(COIII)-tRNA(gly)-NADH dehydrogenase subunit 3(ND3)-tRNA(ala)-tRNA(arg)-tRNA(asn )from the ancestral position between genes for ATP synthase subunit 6 and NADH dehydrogenase subunit 5. Based on the position in which all or part of this fragment was inserted, the mitochondria could be subdivided into four different gene arrangement types. PCR amplification spanning from COIII to genes outside the inserted region and sequence determination of the resulting fragments, indicated that different whitefly species could be placed into one of these arrangement types. A phylogenetic analysis of 19 whitefly species based on genes for mitochondrial cytochrome b, NADH dehydrogenase subunit 1, and 16S ribosomal DNA as well as cospeciating endosymbiont 16S and 23S ribosomal DNA indicated a clustering of species that corresponded to the gene arrangement types. CONCLUSIONS: In whiteflies, the region of the mitochondrial genome consisting of genes encoding for COIII-tRNA(gly)-ND3-tRNA(ala)-tRNA(arg)-tRNA(asn )can be transposed from its ancestral position to four different locations on the mitochondrial genome. Related species within clusters established by phylogenetic analysis of host and endosymbiont genes have the same mitochondrial gene arrangement indicating a transposition in the ancestor of these clusters
On the predictive power of Local Scale Invariance
Local Scale Invariance (LSI) is a theory for anisotropic critical phenomena
designed in the spirit of conformal invariance. For a given representation of
its generators it makes non-trivial predictions about the form of universal
scaling functions. In the past decade several representations have been
identified and the corresponding predictions were confirmed for various
anisotropic critical systems. Such tests are usually based on a comparison of
two-point quantities such as autocorrelation and response functions. The
present work highlights a potential problem of the theory in the sense that it
may predict any type of two-point function. More specifically, it is argued
that for a given two-point correlator it is possible to construct a
representation of the generators which exactly reproduces this particular
correlator. This observation calls for a critical examination of the predictive
content of the theory.Comment: 17 pages, 2 eps figure
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