1,143 research outputs found
Reply Comment: Comparison of Approaches to Classical Signature Change
We contrast the two approaches to ``classical" signature change used by
Hayward with the one used by us (Hellaby and Dray). There is (as yet) no
rigorous derivation of appropriate distributional field equations. Hayward's
distributional approach is based on a postulated modified form of the field
equations. We make an alternative postulate. We point out an important
difference between two possible philosophies of signature change --- ours is
strictly classical, while Hayward's Lagrangian approach adopts what amounts to
an imaginary proper ``time" on one side of the signature change, as is
explicitly done in quantum cosmology. We also explain why we chose to use the
Darmois-Israel type junction conditions, rather than the Lichnerowicz type
junction conditions favoured by Hayward. We show that the difference in results
is entirely explained by the difference in philosophy (imaginary versus real
Euclidean ``time"), and not by the difference in approach to junction
conditions (Lichnerowicz with specific coordinates versus Darmois with general
coordinates).Comment: 10 pages, latex, no figures. Replying to - "Comment on `Failure of
Standard Conservation Laws at a Classical Change of Signature'", S.A.
Hayward, Phys. Rev. D52, 7331-7332 (1995) (gr-qc/9606045
Fate of the first traversible wormhole: black-hole collapse or inflationary expansion
We study numerically the stability of Morris & Thorne's first traversible
wormhole, shown previously by Ellis to be a solution for a massless ghost
Klein-Gordon field. Our code uses a dual-null formulation for spherically
symmetric space-time integration, and the numerical range covers both universes
connected by the wormhole. We observe that the wormhole is unstable against
Gaussian pulses in either exotic or normal massless Klein-Gordon fields. The
wormhole throat suffers a bifurcation of horizons and either explodes to form
an inflationary universe or collapses to a black hole, if the total input
energy is respectively negative or positive. As the perturbations become small
in total energy, there is evidence for critical solutions with a certain
black-hole mass or Hubble constant. The collapse time is related to the initial
energy with an apparently universal critical exponent. For normal matter, such
as a traveller traversing the wormhole, collapse to a black hole always
results. However, carefully balanced additional ghost radiation can maintain
the wormhole for a limited time. The black-hole formation from a traversible
wormhole confirms the recently proposed duality between them. The inflationary
case provides a mechanism for inflating, to macroscopic size, a Planck-sized
wormhole formed in space-time foam.Comment: 10 pages, RevTeX4, 11 figures, epsf.st
The E5 protein of BPV-4 interacts with the heavy chain of MHC class I and irreversibly retains the MHC complex in the Golgi apparatus
BPV-4 E5 inhibits transcription of the bovine MHC class I heavy chain (HC) gene, increases degradation of HC and downregulates surface expression of MHC class I by retaining the complex in the Golgi apparatus (GA). Here we report that transcription inhibition can be alleviated by interferon treatment and the degradation of HC can be reversed by treatment with inhibitors of proteasomes and lysosomes. However, the inhibition of transport of MHC class I to the cell surface is irreversible. We show that E5 is capable of physically interacting with HC. Together with the inhibition of the vacuolar ATPase (due to the interaction between E5 and 16k subunit c), the interaction between E5 and HC is likely to be responsible for retention of MHC class I in the GA. C-terminus deletion mutants of E5 are incapable of either downregulating surface MHC class I or interacting with HC, establishing that the C-terminus domain of E5 is important in the inhibition of MHC class I
Neutralino Dark Matter in a Class of Unified Theories
The cosmological significance of the neutralino sector is studied for a class
of models in which electroweak symmetry breaking is seeded by a gauge singlet.
Extensive use is made of the renormalisation group equations to significantly
reduce the parameter space, by deriving analytic expressions for all the
supersymmetry-breaking couplings in terms of the universal gaugino mass
, the universal scalar mass and the coupling . The
composition of the LSP is determined exactly below the W mass, no
approximations are made for sfermion masses, and all particle exchanges are
considered in calculating the annihilation cross-section; the relic abundance
is then obtained by an analytic approximation. We find that in these models,
stable neutralinos may make a significant contribution to the dark matter in
the universe.Comment: 24 Pages, OUTP-92-10
The E5 oncoprotein of BPV-4 does not interfere with the biosynthetic pathway of non-classical MHC class I
The major histocompatibility complex (MHC) class I region in mammals contains both classical and non-classical MHC class I genes. Classical MHC class I molecules present antigenic peptides to cytotoxic T lymphocytes, whereas non-classical MHC class I molecules have a variety of functions. Both classical and non-classical MHC molecules interact with natural killer cell receptors and may under some circumstances prevent cell death by natural killer cytotoxicity. The E5 oncoprotein of BPV-4 down-regulates the expression of classical MHC class I on the cell surface and retains the complex in the Golgi apparatus. The inhibition of classical MHC class I to the cell surface results from both the impaired acidification of the Golgi, due to the interaction of E5 with subunit c of the H+ V-ATPase, and to the physical binding of E5 to the heavy chain of MHC class I. Despite the profound effect of E5 on classical MHC class I, E5 does not retain a non-classical MHC class I in the Golgi, does not inhibit its transport to the cell surface and does not bind its heavy chain. We conclude that, as is the case for HPV-16 E5, BPV-4 E5 does not down-regulate certain non-classical MHC class I, potentially providing a mechanism for the escape of the infected cell from attack by both cytotoxic T lymphocytes and NK cells
Failure of Standard Conservation Laws at a Classical Change of Signature
The Divergence Theorem as usually stated cannot be applied across a change of
signature unless it is re-expressed to allow for a finite source term on the
signature change surface. Consequently all conservation laws must also be
`modified', and therefore insistence on conservation of matter across such a
surface cannot be physically justified. The Darmois junction conditions
normally ensure conservation of matter via Israel's identities for the jump in
the energy-momentum density, but not when the signature changes. Modified
identities are derived for this jump when a signature change occurs, and the
resulting surface effects in the conservation laws are calculated. In general,
physical vector fields experience a jump in at least one component, and a
source term may therefore appear in the corresponding conservation law. Thus
current is also not conserved. These surface effects are a consequence of the
change in the character of physical law. The only way to recover standard
conservation laws is to impose restrictions that no realistic cosmological
model can satisfy.Comment: 15pp, figures available on request from Charles Hellaby at
[email protected]
Enhanced antiproton production in Pb(160 AGeV)+Pb reactions: evidence for quark gluon matter?
The centrality dependence of the antiproton per participant ratio is studied
in Pb(160 AGeV)+Pb reactions. Antiproton production in collisions of heavy
nuclei at the CERN/SPS seems considerably enhanced as compared to conventional
hadronic physics, given by the antiproton production rates in and
antiproton annihilation in reactions. This enhancement is consistent
with the observation of strong in-medium effects in other hadronic observables
and may be an indication of partial restoration of chiral symmetry
The next-to-leading QCD approximation to the Ellis-Jaffe sum rule
The \alpha_s^2 correction to the Ellis-Jaffe sum rule for the structure
function g_1 of polarized deep inelastic lepton-nucleon scattering is
calculated.Comment: 9 pages, latex, CERN-TH.7208/9
Causality and the speed of sound
A usual causal requirement on a viable theory of matter is that the speed of
sound be at most the speed of light. In view of various recent papers querying
this limit, the question is revisited here. We point to various issues
confronting theories that violate the usual constraint.Comment: v2: additional discussion on models that appear to have superluminal
signal speeds; version to appear in GR
Destabilising Divergences in the NMSSM
The problem of destabilising divergences is discussed for singlet extensions
of the MSSM. It is shown that models which possess either gauged- symmetry
or target space duality at the Planck scale are able to circumvent this problem
whilst avoiding cosmological domain walls.Comment: 19 pages, 5 figures requiring axodraw.sty, plain LaTeX, to appear in
Nucl.Phys.
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