2,508 research outputs found
pp Wave Big Bangs: Matrix Strings and Shrinking Fuzzy Spheres
We find pp wave solutions in string theory with null-like linear dilatons.
These provide toy models of big bang cosmologies. We formulate Matrix String
Theory in these backgrounds. Near the big bang ``singularity'', the string
theory becomes strongly coupled but the Yang-Mills description of the matrix
string is weakly coupled. The presence of a second length scale allows us to
focus on a specific class of non-abelian configurations, viz. fuzzy cylinders,
for a suitable regime of parameters. We show that, for a class of pp waves,
fuzzy cylinders which start out big at early times dynamically shrink into
usual strings at sufficiently late times.Comment: 29 pages, ReVTeX and AMSLaTeX. 4 Figures. v2: Typo corrected and
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Modern Michelson-Morley experiment using cryogenic optical resonators
We report on a new test of Lorentz invariance performed by comparing the
resonance frequencies of two orthogonal cryogenic optical resonators subject to
Earth's rotation over 1 year. For a possible anisotropy of the speed of light
c, we obtain 2.6 +/- 1.7 parts in 10^15. Within the Robertson-Mansouri-Sexl
test theory, this implies an isotropy violation parameter beta - delta - 1/2 of
-2.2 +/- 1.5 parts in 10^9, about three times lower than the best previous
result. Within the general extension of the standard model of particle physics,
we extract limits on 7 parameters at accuracies down to a part in 10^15,
improving the best previous result by about two orders of magnitude
Cosmologies with Null Singularities and their Gauge Theory Duals
We investigate backgrounds of Type IIB string theory with null singularities
and their duals proposed in hep-th/0602107. The dual theory is a deformed N=4
Yang-Mills theory in 3+1 dimensions with couplings dependent on a light-like
direction. We concentrate on backgrounds which become AdS_5 x S^5 at early and
late times and where the string coupling is bounded, vanishing at the
singularity. Our main conclusion is that in these cases the dual gauge theory
is nonsingular. We show this by arguing that there exists a complete set of
gauge invariant observables in the dual gauge theory whose correlation
functions are nonsingular at all times. The two-point correlator for some
operators calculated in the gauge theory does not agree with the result from
the bulk supergravity solution. However, the bulk calculation is invalid near
the singularity where corrections to the supergravity approximation become
important. We also obtain pp-waves which are suitable Penrose limits of this
general class of solutions, and construct the Matrix Membrane theory which
describes these pp-wave backgrounds.Comment: 43 pages REVTeX and AMSLaTeX. v2: references adde
Relativity tests by complementary rotating Michelson-Morley experiments
We report Relativity tests based on data from two simultaneous
Michelson-Morley experiments, spanning a period of more than one year. Both
were actively rotated on turntables. One (in Berlin, Germany) uses optical
Fabry-Perot resonators made of fused silica; the other (in Perth, Australia)
uses microwave whispering-gallery sapphire resonators. Within the standard
model extension, we obtain simultaneous limits on Lorentz violation for
electrons (5 coefficients) and photons (8) at levels down to ,
improved by factors between 3 and 50 compared to previous work.Comment: 5 pages revtex, 2 figure
Arago (1810): the first experimental result against the ether
95 years before Special Relativity was born, Arago attempted to detect the
absolute motion of the Earth by measuring the deflection of starlight passing
through a prism fixed to the Earth. The null result of this experiment gave
rise to the Fresnel's hypothesis of an ether partly dragged by a moving
substance. In the context of Einstein's Relativity, the sole frame which is
privileged in Arago's experiment is the proper frame of the prism, and the null
result only says that Snell's law is valid in that frame. We revisit the
history of this premature first evidence against the ether theory and calculate
the Fresnel's dragging coefficient by applying the Huygens' construction in the
frame of the prism. We expose the dissimilar treatment received by the ray and
the wave front as an unavoidable consequence of the classical notions of space
and time.Comment: 16 pages. To appear in European Journal of Physic
Bounds on Lorentz and CPT Violation from the Earth-Ionosphere Cavity
Electromagnetic resonant cavities form the basis of many tests of Lorentz
invariance involving photons. The effects of some forms of Lorentz violation
scale with cavity size. We investigate possible signals of violations in the
naturally occurring resonances formed in the Earth-ionosphere cavity.
Comparison with observed resonances places the first terrestrial constraints on
coefficients associated with dimension-three Lorentz-violating operators at the
level of 10^{-20} GeV.Comment: 8 pages REVTe
Superconformal Quantum Mechanics of Small Black Holes
Recently, Gaiotto, Strominger and Yin have proposed a holographic dual
description for the near-horizon physics of certain N=2 black holes in terms of
the superconformal quantum mechanics on D0-branes in the attractor geometry. We
provide further evidence for their proposal by applying it to the case of
`small' black holes which have vanishing horizon area in the leading
supergravity approximation. We consider 2-charge black holes in type IIA on
, where can be either or , made up out of
D0-branes and D4-branes wrapping . We construct the corresponding
superconformal quantum mechanics and show that the asymptotic growth of chiral
primaries exactly matches with the known entropy of these black holes. The
state-counting problem reduces to counting lowest Landau levels on and
Dolbeault cohomology classes on .Comment: Latex, 16 pages; v2: minor corrections, references added, published
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Dimensional Reduction without Extra Continuous Dimensions
We describe a novel approach to dimensional reduction in classical field
theory. Inspired by ideas from noncommutative geometry, we introduce extended
algebras of differential forms over space-time, generalized exterior
derivatives and generalized connections associated with the "geometry" of
space-times with discrete extra dimensions. We apply our formalism to theories
of gauge- and gravitational fields and find natural geometrical origins for an
axion- and a dilaton field, as well as a Higgs field.Comment: 23 page
Improved Constraints on Isotropic Shift and Anisotropies of the Speed of Light using Rotating Cryogenic Sapphire Oscillators
We demonstrate that Michelson-Morley tests, which detect direction-dependent
anisotropies in the speed of light, can also be used to place limits upon
isotropic deviations of the vacuum speed of light from , as described by the
photon sector Standard Model Extension (SME) parameter . A
shift in the speed of light that is isotropic in one inertial frame implies
anisotropic shifts in others. Using observer Lorentz covariance, we derive the
time-dependent variations in the relative resonance frequencies of a pair of
electromagnetic resonators that would be generated by such a shift in the rest
frame of the Sun. A new analysis of a recent experimental test of relativity
using this result constrains with a precision of
. This represents the first constraint on
by a Michelson-Morley experiment and the first analysis
of a single experiment to simultaneously set limits on all nine
non-birefringent terms in the photon sector of the SME
Electrodynamics with Lorentz-violating operators of arbitrary dimension
The behavior of photons in the presence of Lorentz and CPT violation is
studied. Allowing for operators of arbitrary mass dimension, we classify all
gauge-invariant Lorentz- and CPT-violating terms in the quadratic Lagrange
density associated with the effective photon propagator. The covariant
dispersion relation is obtained, and conditions for birefringence are
discussed. We provide a complete characterization of the coefficients for
Lorentz violation for all mass dimensions via a decomposition using
spin-weighted spherical harmonics. The resulting nine independent sets of
spherical coefficients control birefringence, dispersion, and anisotropy. We
discuss the restriction of the general theory to various special models,
including among others the minimal Standard-Model Extension, the isotropic
limit, the case of vacuum propagation, the nonbirefringent limit, and the
vacuum-orthogonal model. The transformation of the spherical coefficients for
Lorentz violation between the laboratory frame and the standard Sun-centered
frame is provided. We apply the results to various astrophysical observations
and laboratory experiments. Astrophysical searches of relevance include studies
of birefringence and of dispersion. We use polarimetric and dispersive data
from gamma-ray bursts to set constraints on coefficients for Lorentz violation
involving operators of dimensions four through nine, and we describe the mixing
of polarizations induced by Lorentz and CPT violation in the cosmic-microwave
background. Laboratory searches of interest include cavity experiments. We
present the theory for searches with cavities, derive the experiment-dependent
factors for coefficients in the vacuum-orthogonal model, and predict the
corresponding frequency shift for a circular-cylindrical cavity.Comment: 58 pages two-column REVTeX, accepted in Physical Review
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