801 research outputs found
PP-waves with torsion and metric-affine gravity
A classical pp-wave is a 4-dimensional Lorentzian spacetime which admits a
nonvanishing parallel spinor field; here the connection is assumed to be
Levi-Civita. We generalise this definition to metric compatible spacetimes with
torsion and describe basic properties of such spacetimes. We use our
generalised pp-waves for constructing new explicit vacuum solutions of
quadratic metric-affine gravity.Comment: 17 pages, LaTeX2
A nonlinear frequency shift caused by asymmetry of the coherent population trapping resonance: a generalization
We investigate the coherent population trapping resonance induced by a
polychromatic optical field with an asymmetric spectrum, i.e., whose sidebands
equidistant from the carrier have unequal powers. A situation is considered
where a modulation is used to provide the so-called in-phase and quadrature
signals in the optical field's transmission, which are used for stabilization
of the local oscillator's frequency in chip-scale atomic clocks. In a general
case, the frequencies of the signals nonlinearly depend on the optical field's
intensity due to the asymmetry of the resonance. In this work, we demonstrate
a) that this effect stems from a multi-peak structure of the resonance; b) a
linear dependence of the frequency on the optical field's intensity at high
modulation values; c) that the regime of the resolved structure has more
advantages than suppression of the frequency shift due to the spectrum
asymmetry
Combinatorial expression for universal Vassiliev link invariant
The most general R-matrix type state sum model for link invariants is
constructed. It contains in itself all R-matrix invariants and is a generating
function for "universal" Vassiliev link invariants. This expression is more
simple than Kontsevich's expression for the same quantity, because it is
defined combinatorially and does not contain any integrals, except for an
expression for "the universal Drinfeld's associator".Comment: 20 page
Link Invariants of Finite Type and Perturbation Theory
The Vassiliev-Gusarov link invariants of finite type are known to be closely
related to perturbation theory for Chern-Simons theory. In order to clarify the
perturbative nature of such link invariants, we introduce an algebra V_infinity
containing elements g_i satisfying the usual braid group relations and elements
a_i satisfying g_i - g_i^{-1} = epsilon a_i, where epsilon is a formal variable
that may be regarded as measuring the failure of g_i^2 to equal 1.
Topologically, the elements a_i signify crossings. We show that a large class
of link invariants of finite type are in one-to-one correspondence with
homogeneous Markov traces on V_infinity. We sketch a possible application of
link invariants of finite type to a manifestly diffeomorphism-invariant
perturbation theory for quantum gravity in the loop representation.Comment: 11 page
Targeted insertion of an anti-CD2 monoclonal antibody transgene into the GGTA1 locus in pigs using FokI-dCas9
Xenotransplantation from pigs has been advocated as a solution to the perennial shortage of donated human organs and tissues. CRISPR/Cas9 has facilitated the silencing of genes in donor pigs that contribute to xenograft rejection. However, the generation of modified pigs using second-generation nucleases with much lower off-target mutation rates than Cas9, such as FokI-dCas9, has not been reported. Furthermore, there have been no reports on the use of CRISPR to knock protective transgenes into detrimental porcine genes. In this study, we used FokI-dCas9 with two guide RNAs to integrate a 7.1 kilobase pair transgene into exon 9 of the GGTA1 gene in porcine fetal fibroblasts. The modified cells lacked expression of the αGal xenoantigen, and secreted an anti-CD2 monoclonal antibody encoded by the transgene. PCR and sequencing revealed precise integration of the transgene into one allele of GGTA1, and a small deletion in the second allele. The cells were used for somatic cell nuclear transfer to generate healthy male knock-in piglets, which did not express αGal and which contained anti-CD2 in their serum. We have therefore developed a versatile high-fidelity system for knocking transgenes into the pig genome for xenotransplantation purposes.Mark B. Nottle, Evelyn J. Salvaris, Nella Fisicaro, Stephen McIlfatrick, Ivan Vassiliev, Wayne J. Hawthorne, Philip J. O’Connell, Jamie L. Brady, Andrew M. Lew and Peter J. Cowa
Canonical quantum gravity in the Vassiliev invariants arena: I. Kinematical structure
We generalize the idea of Vassiliev invariants to the spin network context,
with the aim of using these invariants as a kinematical arena for a canonical
quantization of gravity. This paper presents a detailed construction of these
invariants (both ambient and regular isotopic) requiring a significant
elaboration based on the use of Chern-Simons perturbation theory which extends
the work of Kauffman, Martin and Witten to four-valent networks. We show that
this space of knot invariants has the crucial property -from the point of view
of the quantization of gravity- of being loop differentiable in the sense of
distributions. This allows the definition of diffeomorphism and Hamiltonian
constraints. We show that the invariants are annihilated by the diffeomorphism
constraint. In a companion paper we elaborate on the definition of a
Hamiltonian constraint, discuss the constraint algebra, and show that the
construction leads to a consistent theory of canonical quantum gravity.Comment: 21 Pages, RevTex, many figures included with psfi
Dark Matter Dynamics and Indirect Detection
Non-baryonic, or "dark," matter is believed to be a major component of the
total mass budget of the universe. We review the candidates for particle dark
matter and discuss the prospects for direct detection (via interaction of dark
matter particles with laboratory detectors) and indirect detection (via
observations of the products of dark matter self-annihilations), focusing in
particular on the Galactic center, which is among the most promising targets
for indirect detection studies. The gravitational potential at the Galactic
center is dominated by stars and by the supermassive black hole, and the dark
matter distribution is expected to evolve on sub-parsec scales due to
interaction with these components. We discuss the dominant interaction
mechanisms and show how they can be used to rule out certain extreme models for
the dark matter distribution, thus increasing the information that can be
gleaned from indirect detection searches.Comment: 17 pages. Invited review for Modern Physics Letters
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