2,008 research outputs found
Gravity, antimatter and the Dirac-Milne universe
We review the main arguments against antigravity, a different acceleration of
antimatter relative to matter in a gravitational field, discussing and
challenging Morrison's, Good's and Schiff's arguments. Following Price, we show
that, very surprisingly, the usual expression of the Equivalence Principle is
violated by General Relativity when particles of negative mass are supposed to
exist, which may provide a fundamental explanation of MOND phenomenology,
obviating the need for Dark Matter. Motivated by the observation of repulsive
gravity under the form of Dark Energy, and by the fact that our universe looks
very similar to a coasting (neither decelerating nor accelerating) universe, we
study the Dirac-Milne cosmology, a symmetric matter-antimatter cosmology where
antiparticles have the same gravitational properties as holes in a
semiconductor. Noting the similarities with our universe (age, SN1a luminosity
distance, nucleosynthesis, CMB angular scale), we focus our attention on
structure formation mechanisms, finding strong similarities with our universe.
Additional tests of the Dirac-Milne cosmology are briefly reviewed, and we
finally note that a crucial test of the Dirac-Milne cosmology will be soon
realized at CERN next to the ELENA antiproton decelerator, possibly as early as
fall 2018, with the AEgIS, ALPHA-g and Gbar antihydrogen gravity experiments.Comment: Proceedings of the Low Energy Antiproton Physics Conference (LEAP),
Sorbonne University, Paris, March 12th to 16th, 201
Disproof of the List Hadwiger Conjecture
The List Hadwiger Conjecture asserts that every -minor-free graph is
-choosable. We disprove this conjecture by constructing a
-minor-free graph that is not -choosable for every integer
Spectral Metric Spaces on Extensions of C*-Algebras
We construct spectral triples on C*-algebraic extensions of unital
C*-algebras by stable ideals satisfying a certain Toeplitz type property using
given spectral triples on the quotient and ideal. Our construction behaves well
with respect to summability and produces new spectral quantum metric spaces out
of given ones. Using our construction we find new spectral triples on the
quantum 2- and 3-spheres giving a new perspective on these algebras in
noncommutative geometry.Comment: 29 pages, extensively revised, corrected and improved versio
Independent Sets in Graphs with an Excluded Clique Minor
Let be a graph with vertices, with independence number , and
with with no -minor for some . It is proved that
Circumference and Pathwidth of Highly Connected Graphs
Birmele [J. Graph Theory, 2003] proved that every graph with circumference t
has treewidth at most t-1. Under the additional assumption of 2-connectivity,
such graphs have bounded pathwidth, which is a qualitatively stronger result.
Birmele's theorem was extended by Birmele, Bondy and Reed [Combinatorica, 2007]
who showed that every graph without k disjoint cycles of length at least t has
bounded treewidth (as a function of k and t). Our main result states that,
under the additional assumption of (k + 1)- connectivity, such graphs have
bounded pathwidth. In fact, they have pathwidth O(t^3 + tk^2). Moreover,
examples show that (k + 1)-connectivity is required for bounded pathwidth to
hold. These results suggest the following general question: for which values of
k and graphs H does every k-connected H-minor-free graph have bounded
pathwidth? We discuss this question and provide a few observations.Comment: 11 pages, 4 figure
Propagation of Vortex Electron Wave Functions in a Magnetic Field
The physics of coherent beams of photons carrying axial orbital angular
momentum (OAM) is well understood and such beams, sometimes known as vortex
beams, have found applications in optics and microscopy. Recently electron
beams carrying very large values of axial OAM have been generated. In the
absence of coupling to an external electromagnetic field the propagation of
such vortex electron beams is virtually identical mathematically to that of
vortex photon beams propagating in a medium with a homogeneous index of
refraction. But when coupled to an external electromagnetic field the
propagation of vortex electron beams is distinctly different from photons. Here
we use the exact path integral solution to Schrodingers equation to examine the
time evolution of an electron wave function carrying axial OAM. Interestingly
we find that the nonzero OAM wave function can be obtained from the zero OAM
wave function, in the case considered here, simply by multipling it by an
appropriate time and position dependent prefactor. Hence adding OAM and
propagating can in this case be replaced by first propagating then adding OAM.
Also, the results shown provide an explicit illustration of the fact that the
gyromagnetic ratio for OAM is unity. We also propose a novel version of the
Bohm-Aharonov effect using vortex electron beams.Comment: 14 pages, 2 figures, submitted to Phys Rev
Spin Wave Diffraction Control and Read-out with a Quantum Memory for Light
A scheme for control and read-out of diffracted spins waves to propagating
light fields is presented. Diffraction is obtained via sinusoidally varying
lights shifts and ideal one-to-one mapping to light is realized using a
gradient echo quantum memory. We also show that dynamical control of the
diffracted spin waves spatial orders can be implemented to realize a quantum
pulse sequencer for temporal modes that have high time-bandwidth products. Full
numerical solutions suggest that both co-propagating and couterpropagating
light shift geometries can be used, making the proposal applicable to hot and
cold atomic vapours as well as solid state systems with two-level atoms.Comment: 5 pages, 3 figure
Logarithmic Corrections to Extremal Black Hole Entropy in N = 2, 4 and 8 Supergravity
We compute the logarithmic correction to black hole entropy about
exponentially suppressed saddle points of the Quantum Entropy Function
corresponding to Z(N) orbifolds of the near horizon geometry of the extremal
black hole under study. By carefully accounting for zero mode contributions we
show that the logarithmic contributions for quarter--BPS black holes in N=4
supergravity and one--eighth BPS black holes in N=8 supergravity perfectly
match with the prediction from the microstate counting. We also find that the
logarithmic contribution for half--BPS black holes in N = 2 supergravity
depends non-trivially on the Z(N) orbifold. Our analysis draws heavily on the
results we had previously obtained for heat kernel coefficients on Z(N)
orbifolds of spheres and hyperboloids in arXiv:1311.6286 and we also propose a
generalization of the Plancherel formula to Z(N) orbifolds of hyperboloids to
an expression involving the Harish-Chandra character of SL(2,R), a result which
is of possible mathematical interest.Comment: 40 page
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