4,177 research outputs found
Dark energy: the absolute electric potential of the universe
Is there an absolute cosmic electric potential?. The recent discovery of the
accelerated expansion of the universe could be indicating that this is
certainly the case. In this essay we show that the consistency of the covariant
and gauge invariant theory of electromagnetism is truly questionable when
considered on cosmological scales. Out of the four components of the
electromagnetic field, Maxwell's theory only contains two physical degrees of
freedom. However, in the presence of gravity, one of the "unphysical" states
cannot be consistently eliminated, thus becoming real. This third polarization
state is completely decoupled from charged matter, but can be excited
gravitationally thus breaking gauge invariance. On large scales the new state
can be seen as a homogeneous cosmic electric potential, whose energy density
behaves as a cosmological constant.Comment: 9 pages, 2 figures. Essay selected for "Honorable Mention" in the
2009 Awards for Essays on Gravitation (Gravity Research Foundation
Cosmological magnetic fields from inflation in extended electromagnetism
In this work we consider an extended electromagnetic theory in which the
scalar state which is usually eliminated by means of the Lorenz condition is
allowed to propagate. This state has been shown to generate a small
cosmological constant in the context of standard inflationary cosmology. Here
we show that the usual Lorenz gauge-breaking term now plays the role of an
effective electromagnetic current. Such a current is generated during inflation
from quantum fluctuations and gives rise to a stochastic effective charge
density distribution. Due to the high electric conductivity of the cosmic
plasma after inflation, the electric charge density generates currents which
give rise to both vorticity and magnetic fields on sub-Hubble scales. Present
upper limits on vorticity coming from temperature anisotropies of the CMB are
translated into lower limits on the present value of cosmic magnetic fields. We
find that, for a nearly scale invariant vorticity spectrum, magnetic fields
G are typically generated with coherence lengths
ranging from sub-galactic scales up to the present Hubble radius. Those fields
could act as seeds for a galactic dynamo or even account for observations just
by collapse and differential rotation of the protogalactic cloud.Comment: 5 pages, 2 figures. Final version to appear in Phys. Rev.
Electromagnetic nature of dark energy
Out of the four components of the electromagnetic field, Maxwell's theory
only contains two physical degrees of freedom. However, in an expanding
universe, consistently eliminating one of the "unphysical" states in the
covariant (Gupta-Bleuler) formalism turns out to be difficult to realize. In
this work we explore the possibility of quantization without subsidiary
conditions. This implies that the theory would contain a third physical state.
The presence of such a new (temporal) electromagnetic mode on cosmological
scales is shown to generate an effective cosmological constant which can
account for the accelerated expansion of the universe. This new polarization
state is completely decoupled from charged matter, but can be excited
gravitationally. In fact, primordial electromagnetic quantum fluctuations
produced during electroweak scale inflation could naturally explain the
presence of this mode and also the measured value of the cosmological constant.
The theory is compatible with all the local gravity tests, it is free from
classical or quantum instabilities and reduces to standard QED in the flat
space-time limit. Thus we see that, not only the true nature of dark energy can
be established without resorting to new physics, but also the value of the
cosmological constant finds a natural explanation in the context of standard
inflationary cosmology. Possible signals, discriminating this model from LCDM,
are also discussed.Comment: 10 pages, 2 figures. Contribution to the Proceedings of Invisible
Universe International Conference, UNESCO, Paris, June 29-July 3, 200
Pressure-induced phase transitions in AgClO4
AgClO4 has been studied under compression by x-ray diffraction and density
functional theory calculations. Experimental evidence of a structural phase
transition from the tetragonal structure of AgClO4 to an orthorhombic
barite-type structure has been found at 5.1 GPa. The transition is supported by
total-energy calculations. In addition, a second transition to a monoclinic
structure is theoretically proposed to take place beyond 17 GPa. The equation
of state of the different phases is reported as well as the calculated
Raman-active phonons and their pressure evolution. Finally, we provide a
description of all the structures of AgClO4 and discuss their relationships.
The structures are also compared with those of AgCl in order to explain the
structural sequence determined for AgClO4.Comment: 38 pages, 11 figures, 4 table
Advanced nozzle and engine components test facility
A test facility for conducting scaled advanced nozzle and engine component research is described. The CE-22 test facility, located in the Engine Research Building of the NASA Lewis Research Center, contains many systems for the economical testing of advanced scale-model nozzles and engine components. The combustion air and altitude exhaust systems are described. Combustion air can be supplied to a model up to 40 psig for primary air flow, and 40, 125, and 450 psig for secondary air flow. Altitude exhaust can be simulated up to 48,000 ft, or the exhaust can be atmospheric. Descriptions of the multiaxis thrust stand, a color schlieren flow visualization system used for qualitative flow analysis, a labyrinth flow measurement system, a data acquisition system, and auxiliary systems are discussed. Model recommended design information and temperature and pressure instrumentation recommendations are included
Using the Physical Activity Kit (PAK) to Get People Moving.
Presented at: Administration for Children and Families Tribal/Native American Grantee Conference; June 6-8; Alexandria, VA.https://digitalrepository.unm.edu/prc-posters-presentations/1061/thumbnail.jp
Sparse bounds for pseudodifferential operators
We prove sparse bounds for pseudodifferential operators associated to H\"ormander symbol classes. Our sparse bounds are sharp up to the endpoint and rely on a single scale analysis. As a consequence, we deduce a range of weighted estimates for pseudodifferential operators. The results naturally apply to the context of oscillatory Fourier multipliers, with applications to dispersive equations and oscillatory convolution kernels
Bilinear identities involving the -plane transform and Fourier extension operators
We prove certain bilinear estimates for Fourier extension operators associated to spheres and hyperboloids under the action of the -plane transform. As the estimates are -based, they follow from bilinear identities: in particular, these are the analogues of a known identity for paraboloids, and may be seen as higher-dimensional versions of the classical -bilinear identity for Fourier extension operators associated to curves in
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