10 research outputs found
The Axial Anomaly in D=3+1 Light-Cone QED
We consider -dimensional, Dirac electrons of arbitrary mass,
propagating in the presence of electric and magnetic fields which are both
parallel to the axis. The magnetic field is constant in space and time
whereas the electric field depends arbitrarily upon the light-cone time
parameter . We present an explicit solution to the
Heisenberg equations for the electron field operator in this background. The
electric field results in the creation of electron-positron pairs. We compute
the expectation values of the vector and axial vector currents in the presence
of a state which is free vacuum at . Both current conservation and the
standard result for the axial vector anomaly are verified for the first time
ever in -dimensional light-cone QED. An interesting feature of our
operator solution is the fact that it depends in an essential way upon
operators from the characteristic at , in addition to the usual
dependence upon operators at . This dependence survives even in the
limit of infinite . Ignoring the operators leads to a progressive loss
of unitarity, to the violation of current conservation, to the loss of
renormalizability, and to an incorrect result for the axial vector anomaly.Comment: 31 pages, LaTeX 2 epsilon, no figures, some typoes corrected for
publicatio
Possible Enhancement of High Frequency Gravitational Waves
We study the tensor perturbations in a class of non-local, purely
gravitational models which naturally end inflation in a distinctive phase of
oscillations with slight and short violations of the weak energy condition. We
find the usual generic form for the tensor power spectrum. The presence of the
oscillatory phase leads to an enhancement of gravitational waves with
frequencies somewhat less than 10^{10} Hz.Comment: 27 pages, 11 figures, LaTeX.2
A Nonlocal Metric Formulation of MOND
We study a class of nonlocal, but causal, covariant and conserved field
equations for the metric. Although nonlocal, these equations do not seem to
possess extra graviton solutions in weak field perturbation theory. Indeed, the
equations reduce to those of general relativity when the Ricci scalar vanishes
throughout spacetime. When a static matter source is present we show how these
equations can be adjusted to reproduce Milgrom's Modified Newtonian Dynamics in
the weak field regime, while reducing to general relativity for strong fields.
We compute the angular deflection of light in the weak field regime and
demonstrate that it is the same as for general relativity, resulting in far too
little lensing if no dark matter is present. We also study the field equations
for a general Robertson-Walker geometry. An interesting feature of our
equations is that they become conformally invariant in the MOND limit.Comment: 22 pages, LaTeX 2 epsilon, no figure
corrections to the cosmological dynamics of inflation in the Palatini formulation
We investigate the corrections to the inflationary cosmological dynamics due
to a term in the Palatini formulation which may arise as quantum
corrections to the effective Lagrangian in early universe. We found that the
standard Friedmann equation will not be changed when the scalar field is in the
potential energy dominated era. However, in the kinetic energy dominated era,
the standard Friedmann equation will be modified and in the case of closed and
flat universe, the Modified Friedmann equation will automatically require that
the initial kinetic energy density of the scalar field must be in sub-Planckian
scale.Comment: 11 pages, no figures. Accepted by Class.Quant.Grav.v2:References
adde
Consistent modified gravity: dark energy, acceleration and the absence of cosmic doomsday
We discuss the modified gravity which includes negative and positive powers
of the curvature and which provides the gravitational dark energy. It is shown
that in GR plus the term containing negative power of the curvature the cosmic
speed-up may be achieved, while the effective phantom phase (with less than
-1) follows when such term contains the fractional positive power of the
curvature. The minimal coupling with matter makes the situation more
interesting: even 1/R theory coupled with the usual ideal fliud may describe
the (effective phantom) dark energy. The account of term (consistent
modified gravity) may help to escape of cosmic doomsday.Comment: LaTeX file, 9 pages, based on the talk given by S.D. Odintsov (Int.
Conference Mathematical Methods in Physics, Rio de Janeiro, Augest, 2004), to
appear in CQG, Letter
Palatini formulation of the modified gravity with an additionally squared scalar curvature term
In this paper by deriving the Modified Friedmann equation in the Palatini
formulation of gravity, first we discuss the problem of whether in
Palatini formulation an additional term in Einstein's General Relativity
action can drive an inflation. We show that the Palatini formulation of
gravity cannot lead to the gravity-driven inflation as in the metric formalism.
If considering no zero radiation and matter energy densities, we obtain that
only under rather restrictive assumption about the radiation and matter energy
densities there will be a mild power-law inflation , which is
obviously different from the original vacuum energy-like driven inflation. Then
we demonstrate that in the Palatini formulation of a more generally modified
gravity, i.e., the model that intends to explain both the current
cosmic acceleration and early time inflation, accelerating cosmic expansion
achieved at late Universe evolution times under the model parameters satisfying
.Comment: 14 pages, accepted for publication by CQ
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom