1,698 research outputs found
Do the constants of nature couple to strong gravitational fields?
Recently, white dwarf stars have found a new use in the fundamental physics
community. Many prospective theories of the fundamental interactions of Nature
allow traditional constants, like the fine structure constant , to vary
in some way. A study by Berengut et al. (2013) used the Fe/Ni V line
measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B,
in an attempt to detect any variation in . It was found that the Fe V
lines indicated an increasing alpha, whereas the Ni V lines indicated a
decreasing alpha. Possible explanations for this could be misidentification of
the lines, inaccurate atomic data, or wavelength dependent distortion in the
spectrum. We examine the first two cases by using a high S/N reference spectrum
from the hot sdO BD+284211 to calibrate the Fe/Ni V atomic data. With
this new data, we re-evaluate the work of Berengut et al. (2013) to derive a
new constraint on the variation of alpha in a gravitational field.Comment: 4 pages, 2 figures: To appear in the proceedings of the "19th
European White Dwarf Workshop" in Montreal, Canada, 201
A Quadratic Spinor Lagrangian for General Relativity
We present a new finite action for Einstein gravity in which the Lagrangian
is quadratic in the covariant derivative of a spinor field. Via a new
spinor-curvature identity, it is related to the standard Einstein-Hilbert
Lagrangian by a total differential term. The corresponding Hamiltonian, like
the one associated with the Witten positive energy proof is fully
four-covariant. It defines quasi-local energy-momentum and can be reduced to
the one in our recent positive energy proof. (Fourth Prize, 1994 Gravity
Research Foundation Essay.)Comment: 5 pages (Plain TeX), NCU-GR-94-QSL
Recommended from our members
Antibiotic Resistance and Typhoid.
Multiple drug (antibiotic) resistance (MDR) has become a major threat to the treatment of typhoid and other infectious diseases. Since the 1970s, this threat has increased in Salmonella enterica serovar Typhi, driven in part by the emergence of successful genetic clades, such as haplotype H58, associated with the MDR phenotype. H58 S. Typhi can express multiple antibiotic resistance determinants while retaining the ability to efficiently transmit and persist within the human population. The recent identification of extensively drug resistant S. Typhi only highlights the dangers of ignoring this threat. Here we discuss the evolution of the S. Typhi MDR phenotype and consider options for management
Transnuclear TRP1-Specific CD8 T Cells with High or Low Affinity TCRs Show Equivalent Antitumor Activity
We have generated, via somatic cell nuclear transfer, two independent lines of transnuclear (TN) mice, using as nuclear donors CD8 T cells, sorted by tetramer staining, that recognize the endogenous melanoma antigen TRP1. These two lines of nominally identical specificity differ greatly in their affinity for antigen (TRP1(high) or TRP1(low)) as inferred from tetramer dissociation and peptide responsiveness. Ex vivo-activated CD8 T cells from either TRP1(high) or TRP1(low) mice show cytolytic activity in 3D tissue culture and in vivo, and slow the progression of subcutaneous B16 melanoma. Although naïve TRP1(low) CD8 T cells do not affect tumor growth, upon activation these cells function indistinguishably from TRP1(high) cells in vivo, limiting tumor cell growth and increasing mouse survival. The anti-tumor effect of both TRP1(high) and TRP1(low) CD8 T cells is enhanced in RAG-deficient hosts. However, tumor outgrowth eventually occurs, likely due to T cell exhaustion. The TRP1 TN mice are an excellent model for examining the functional attributes of T cells conferred by TCR affinity, and they may serve as a platform for screening immunomodulatory cancer therapies
ATF7IP-Mediated Stabilization of the Histone Methyltransferase SETDB1 Is Essential for Heterochromatin Formation by the HUSH Complex.
The histone methyltransferase SETDB1 plays a central role in repressive chromatin processes, but the functional requirement for its binding partner ATF7IP has remained enigmatic. Here, we show that ATF7IP is essential for SETDB1 stability: nuclear SETDB1 protein is degraded by the proteasome upon ablation of ATF7IP. As a result, ATF7IP is critical for repression that requires H3K9 trimethylation by SETDB1, including transgene silencing by the HUSH complex. Furthermore, we show that loss of ATF7IP phenocopies loss of SETDB1 in genome-wide assays. ATF7IP and SETDB1 knockout cells exhibit near-identical defects in the global deposition of H3K9me3, which results in similar dysregulation of the transcriptome. Overall, these data identify a critical functional role for ATF7IP in heterochromatin formation by regulating SETDB1 abundance in the nucleus.This work was supported by the Wellcome Trust through a Principal Research Fellowship to P.J.L. (101835/Z/13/Z) and a Ph.D. studentship to I.A.T. The CIMR is in receipt of a Wellcome Trust strategic award.This is the final version of the article. It first appeared from Elsevier (Cell Press) via https://doi.org/10.1016/j.celrep.2016.09.05
Another positivity proof and gravitational energy localizations
Two locally positive expressions for the gravitational Hamiltonian, one using
4-spinors the other special orthonormal frames, are reviewed. A new quadratic
3-spinor-curvature identity is used to obtain another positive expression for
the Hamiltonian and thereby a localization of gravitational energy and positive
energy proof. These new results provide a link between the other two methods.
Localization and prospects for quasi-localization are discussed.Comment: 14 pages REVTe
Quasi-local energy-momentum and two-surface characterization of the pp-wave spacetimes
In the present paper the determination of the {\it pp}-wave metric form the
geometry of certain spacelike two-surfaces is considered. It has been shown
that the vanishing of the Dougan--Mason quasi-local mass , associated
with the smooth boundary of a spacelike
hypersurface , is equivalent to the statement that the Cauchy
development is of a {\it pp}-wave type geometry with pure
radiation, provided the ingoing null normals are not diverging on and the
dominant energy condition holds on . The metric on
itself, however, has not been determined. Here, assuming that the matter is a
zero-rest-mass-field, it is shown that both the matter field and the {\it
pp}-wave metric of are completely determined by the value of the
zero-rest-mass-field on and the two dimensional Sen--geometry of
provided a convexity condition, slightly stronger than above, holds. Thus the
{\it pp}-waves can be characterized not only by the usual Cauchy data on a {\it
three} dimensional but by data on its {\it two} dimensional boundary
too. In addition, it is shown that the Ludvigsen--Vickers quasi-local
angular momentum of axially symmetric {\it pp}-wave geometries has the familiar
properties known for pure (matter) radiation.Comment: 15 pages, Plain Tex, no figure
Angular momentum and an invariant quasilocal energy in general relativity
Owing to its transformation property under local boosts, the Brown-York
quasilocal energy surface density is the analogue of E in the special
relativity formula: E^2-p^2=m^2. In this paper I will motivate the general
relativistic version of this formula, and thereby arrive at a geometrically
natural definition of an `invariant quasilocal energy', or IQE. In analogy with
the invariant mass m, the IQE is invariant under local boosts of the set of
observers on a given two-surface S in spacetime. A reference energy subtraction
procedure is required, but in contrast to the Brown-York procedure, S is
isometrically embedded into a four-dimensional reference spacetime. This
virtually eliminates the embeddability problem inherent in the use of a
three-dimensional reference space, but introduces a new one: such embeddings
are not unique, leading to an ambiguity in the reference IQE. However, in this
codimension-two setting there are two curvatures associated with S: the
curvatures of its tangent and normal bundles. Taking advantage of this fact, I
will suggest a possible way to resolve the embedding ambiguity, which at the
same time will be seen to incorporate angular momentum into the energy at the
quasilocal level. I will analyze the IQE in the following cases: both the
spatial and future null infinity limits of a large sphere in asymptotically
flat spacetimes; a small sphere shrinking toward a point along either spatial
or null directions; and finally, in asymptotically anti-de Sitter spacetimes.
The last case reveals a striking similarity between the reference IQE and a
certain counterterm energy recently proposed in the context of the conjectured
AdS/CFT correspondence.Comment: 54 pages LaTeX, no figures, includes brief summary of results,
submitted to Physical Review
Energy of Isolated Systems at Retarded Times as the Null Limit of Quasilocal Energy
We define the energy of a perfectly isolated system at a given retarded time
as the suitable null limit of the quasilocal energy . The result coincides
with the Bondi-Sachs mass. Our is the lapse-unity shift-zero boundary value
of the gravitational Hamiltonian appropriate for the partial system
contained within a finite topologically spherical boundary . Moreover, we show that with an arbitrary lapse and zero shift the same
null limit of the Hamiltonian defines a physically meaningful element in the
space dual to supertranslations. This result is specialized to yield an
expression for the full Bondi-Sachs four-momentum in terms of Hamiltonian
values.Comment: REVTEX, 16 pages, 1 figur
- …