15,338 research outputs found
Growth of covariant perturbations in the contracting phase of a bouncing universe
In this paper we examine the validity of the linear perturbation theory near
a bounce in the covariant analysis. Some linearity parameters are defined to
set up conditions for a linear theory. Linear evolution of density perturbation
and gravitational waves have been computed previously. We have calculated the
vector and scalar induced parts of the shear tensor. For radiationlike and
dustlike single fluid dominated collapsing Friedmann-Lemaitre-Robertson-Walker
background it is shown that the linearity conditions are not satisfied near a
bounce.Comment: 9 pages, final versio
Vapor Deposited Tungsten for Application as a Thermionic Emitter Material
Purity and resistance to grain growth of vapor deposited tungsten tubing for use as thermionic emitte
Low Energy Constants from High Energy Theorems
New constraints on resonance saturation in chiral perturbation theory are
investigated. These constraints arise because each consistent saturation scheme
must map to a representation of the full QCD chiral symmetry group. The
low-energy constants of chiral perturbation theory are then related by a set of
mixing angles. It is shown that vector meson dominance is a consequence of the
fact that nature has chosen the lowest-dimensional nontrivial chiral
representation. It is further shown that chiral symmetry places an upper bound
on the mass of the lightest scalar in the hadron spectrum.Comment: 11 pages TeX and mtexsis.te
Comment on "Does Gluons Carry Half of the Nucleon Momentum?" by X. S. Chen et. al. (PRL103, 062001 (2009))
The authors claim to have found a "proper", "gauge-invariant" definition of a
charged-particle's momentum in gauge theory, which is more "superior" than the
textbook version. I show that their result arises from a misunderstanding of
gauge symmetry by generalizing the Coulomb gauge result indiscriminately and is
not physical
The spin-statistics connection in classical field theory
The spin-statistics connection is obtained for a simple formulation of a
classical field theory containing even and odd Grassmann variables. To that
end, the construction of irreducible canonical realizations of the rotation
group corresponding to general causal fields is reviewed. The connection is
obtained by imposing local commutativity on the fields and exploiting the
parity operation to exchange spatial coordinates in the scalar product of
classical field evaluated at one spatial location with the same field evaluated
at a distinct location. The spin-statistics connection for irreducible
canonical realizations of the Poincar\'{e} group of spin is obtained in the
form: Classical fields and their conjugate momenta satisfy fundamental
field-theoretic Poisson bracket relations for 2 even, and fundamental
Poisson antibracket relations for 2 oddComment: 27 pages. Typos and sign error corrected; minor revisions to tex
A Magellanic Origin for the Warp of the Galaxy
We show that a Magellanic Cloud origin for the warp of the Milky Way can
explain most quantitative features of the outer HI layer recently identified by
Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg
(1998) that produces distortions in the dark matter halo, and we calculate the
combined effect of these dark-halo distortions and the direct tidal forcing by
the Magellanic Clouds on the disk warp in the linear regime. The interaction of
the dark matter halo with the disk and resonances between the orbit of the
Clouds and the disk account for the large amplitudes observed for the vertical
m=0,1,2 harmonics. The observations lead to six constraints on warp forcing
mechanisms and our model reasonably approximates all six. The disk is shown to
be very dynamic, constantly changing its shape as the Clouds proceed along
their orbit. We discuss the challenges to MOND placed by the observations.Comment: 4 pages, 3 figures, submitted to ApJ Letters. Additional graphics, 3d
visualizations and movies available at
http://www.astro.umass.edu/~weinberg/lm
Glassy Phase of Optimal Quantum Control
We study the problem of preparing a quantum many-body system from an initial
to a target state by optimizing the fidelity over the family of bang-bang
protocols. We present compelling numerical evidence for a universal
spin-glass-like transition controlled by the protocol time duration. The glassy
critical point is marked by a proliferation of protocols with close-to-optimal
fidelity and with a true optimum that appears exponentially difficult to
locate. Using a machine learning (ML) inspired framework based on the manifold
learning algorithm t-SNE, we are able to visualize the geometry of the
high-dimensional control landscape in an effective low-dimensional
representation. Across the transition, the control landscape features an
exponential number of clusters separated by extensive barriers, which bears a
strong resemblance with replica symmetry breaking in spin glasses and random
satisfiability problems. We further show that the quantum control landscape
maps onto a disorder-free classical Ising model with frustrated nonlocal,
multibody interactions. Our work highlights an intricate but unexpected
connection between optimal quantum control and spin glass physics, and shows
how tools from ML can be used to visualize and understand glassy optimization
landscapes.Comment: Modified figures in appendix and main text (color schemes). Corrected
references. Added figures in SI and pseudo-cod
Non-Gaussian Correlations Outside the Horizon
It is shown that under essentially all conditions, the non-linear classical
equations governing gravitation and matter in cosmology have a solution in
which far outside the horizon in a suitable gauge the reduced spatial metric
(the spatial metric divided by the square of the Robertson--Walker scale factor
) is time-independent, though with an arbitrary dependence on co-moving
coordinates, and all perturbations to the other metric components and to all
matter variables vanish, to leading order in . The corrections are of
order , and are explicitly given for the reduced metric in a multifield
model with a general potential. Further, this is the solution that describes
the metric and matter produced by single-field inflation. These results justify
the use of observed non-Gaussian correlations (or their absence) as a test of
theories of single-field inflation, despite our ignorance of the constituents
of the universe while fluctuations are outside the horizon after inflation, as
long as graphs with loops can be neglected.Comment: 25 pages. This version clarifies the scale transformation used in
Section II and the gauge transformation used in Section III, and corrects
some typos, including new typos introduced in version
Parity and the Spin-Statistics Connection
The spin-statistics connection is obtained in a simple and elementary way for
general causal fields by using the parity operation to exchange spatial
coordinates in the scalar product of a locally commuting field operator,
evaluated at position x, with the same field operator evaluated at -x, at equal
times.Comment: 6 page
Synchronization of extended systems from internal coherence
A condition for the synchronizability of a pair of PDE systems, coupled
through a finite set of variables, is commonly the existence of internal
synchronization or internal coherence in each system separately. The condition
was previously illustrated in a forced-dissipative system, and is here extended
to Hamiltonian systems, using an example from particle physics. Full
synchronization is precluded by Liouville's theorem. A form of synchronization
weaker than "measure synchronization" is manifest as the positional coincidence
of coherent oscillations ("breathers" or "oscillons") in a pair of coupled
scalar field models in an expanding universe with a nonlinear potential, and
does not occur with a variant of the model that does not exhibit oscillons.Comment: version accepted for publication in PRE (paragraph beginning at the
bottom of pg. 5 has been rewritten to suggest unifying principle for
synchronizability, applying to both forced-dissipative and Hamiltonian
systems; other minor changes
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