16,586 research outputs found
Dark Radiation Emerging After Big Bang Nucleosynthesis?
We show how recent data from observations of the cosmic microwave background
may suggest the presence of additional radiation density which appeared after
big bang nucleosynthesis. We propose a general scheme by which this radiation
could be produced from the decay of non-relativistic matter, we place
constraints on the properties of such matter, and we give specific examples of
scenarios in which this general scheme may be realized.Comment: v3: 5 pages, 1 figure. References added, typos corrected, notation
changed throughout. v2: 5 pages, 1 figure. Reformatted, references added,
acknowledgments updated, effect of radiation on CMB clarified. v1: 11 pages,
1 figur
The Cardy-Verlinde equation in a spherical symmetric gravitational collapse
The Cardy-Verlinde formula is analyzed in the contest of the gravitational
collapse. Starting from the holographic principle, we show how the equations
for a homogeneous and isotropic gravitational collapse describe the formation
of the black hole entropy. Some comments on the role of the entangled entropy
and the connection with the c-theorem are made
Cosmological Density Perturbations with a Scale-Dependent Newton's G
We explore possible cosmological consequences of a running Newton's constant
, as suggested by the non-trivial ultraviolet fixed point
scenario in the quantum field-theoretic treatment of Einstein gravity with a
cosmological constant term. In particular we focus here on what possible
effects the scale-dependent coupling might have on large scale cosmological
density perturbations. Starting from a set of manifestly covariant effective
field equations derived earlier, we systematically develop the linear theory of
density perturbations for a non-relativistic, pressure-less fluid. The result
is a modified equation for the matter density contrast, which can be solved and
thus provides an estimate for the growth index parameter in the
presence of a running . We complete our analysis by comparing the fully
relativistic treatment with the corresponding results for the non-relativistic
(Newtonian) case, the latter also with a weakly scale dependent .Comment: 54 pages, 4 figure
Improved Action Functionals in Non-Perturbative Quantum Gravity
Models of gravity with variable G and Lambda have acquired greater relevance
after the recent evidence in favour of the Einstein theory being
non-perturbatively renormalizable in the Weinberg sense. The present paper
builds a modified Arnowitt-Deser-Misner (ADM) action functional for such models
which leads to a power-law growth of the scale factor for pure gravity and for
a massless phi**4 theory in a Universe with Robertson-Walker symmetry, in
agreement with the recently developed fixed-point cosmology. Interestingly, the
renormalization-group flow at the fixed point is found to be compatible with a
Lagrangian description of the running quantities G and Lambda.Comment: Latex file. Record without file already exists on SLAC-SPIRES, and
hence that record and the one for the present arxiv submission should become
one record onl
Vacuum energy and Universe in special relativity
The problem of cosmological constant and vacuum energy is usually thought of
as the subject of general relativity. However, the vacuum energy is important
for the Universe even in the absence of gravity, i.e. in the case when the
Newton constant G is exactly zero, G=0. We discuss the response of the vacuum
energy to the perturbations of the quantum vacuum in special relativity, and
find that as in general relativity the vacuum energy density is on the order of
the energy density of matter. In general relativity, the dependence of the
vacuum energy on the equation of state of matter does not contain G, and thus
is valid in the limit when G tends to zero. However, the result obtained for
the vacuum energy in the world without gravity, i.e. when G=0 exactly, is
different.Comment: LaTeX file, 7 pages, no figures, to appear in JETP Letters, reference
is adde
Aspects of Nucleon Chiral Perturbation Theory
I review recent progress made in the calculation of nucleon properties in the
framework of heavy baryon CHPT. Topics include: Compton scattering,
scattering, the anatomy of a low-energy constant and the induced pseudoscalar
form factor.Comment: plain TeX (macro included), 12pp, lecture delivered at the workshop
on "Chiral Dynamics: Theory and Experiments", MIT, July 25-29, 199
The spin-statistics connection in classical field theory
The spin-statistics connection is obtained for a simple formulation of a
classical field theory containing even and odd Grassmann variables. To that
end, the construction of irreducible canonical realizations of the rotation
group corresponding to general causal fields is reviewed. The connection is
obtained by imposing local commutativity on the fields and exploiting the
parity operation to exchange spatial coordinates in the scalar product of
classical field evaluated at one spatial location with the same field evaluated
at a distinct location. The spin-statistics connection for irreducible
canonical realizations of the Poincar\'{e} group of spin is obtained in the
form: Classical fields and their conjugate momenta satisfy fundamental
field-theoretic Poisson bracket relations for 2 even, and fundamental
Poisson antibracket relations for 2 oddComment: 27 pages. Typos and sign error corrected; minor revisions to tex
On the Gauged Non-compact Spin System
We examine classical and quantum aspects of the planar non-compact spin
system coupled with Chern-Simons gauge field in the presence of background
charge. We first define our classical spin system as non- relativistic
non-linear sigma model in which the order parameter spin takes value in the
non-compact manifold . Although the naive model does not
allow any finite energy self dual solitons, it is shown that the gauged system
admits static Bogomol'nyi solitons with finite energy whose rotationally
symmetric soliton solutions are analyzed in detail. We also discuss the large
spin limit in which the self-dual equation reduces to the well-known gauged
non- linear Schr\"odinger model or Abelian Higgs model, depending on the choice
of the background charge term. Then, we perform quantization of the model. We
find that the spin algebra satisfies anomalous commutation relations, and the
system is a field theoretic realization of the anyons.Comment: 21 pages, Latex, 3 figures include
Vapor Deposited Tungsten for Application as a Thermionic Emitter Material
Purity and resistance to grain growth of vapor deposited tungsten tubing for use as thermionic emitte
- …