61 research outputs found
The Force of Gravity from a Lagrangian containing Inverse Powers of the Ricci Scalar
We determine the gravitational response to a diffuse source, in a locally de
Sitter background, of a class of theories which modify the Einstein-Hilbert
action by adding a term proportional to an inverse power of the Ricci scalar.
We find a linearly growing force which is not phenomenologically acceptable.Comment: 10 pages, LaTeX 2 epsilon, no figures. Minor conceptual change
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
Gravitational waves and lensing of the metric theory proposed by Sobouti
We investigate in detail two physical properties of the metric f(R) theory
developed by Sobouti (2007). We first look for the possibility of producing
gravitational waves that travel at the speed of light. We then check the
possibility of producing extra bending in the lenses produced by the theory. We
do this by using standard weak field approximations to the gravitational field
equations that appear in Sobouti's theory. We show in this article that the
metric theory of gravitation proposed by Sobouti (2007) predicts the existence
of gravitational waves travelling at the speed of light in vacuum. In fact,
this is proved in general terms for all metric theories of gravity which can be
expressed as powers of Ricci's scalar. We also show that an extra additional
lensing as compared to the one predicted by standard general relativity is
produced. These two points are generally considered to be of crucial importance
in the development of relativistic theories of gravity that could provide an
alternative description to the dark matter paradigm.Comment: 10 pages, 2 figures. Added a comment on the recent article by Saffari
(arXiv:0704.3345v1) and small typos as well as general comments in the
introuduction and conclusio
Comparing scalar-tensor gravity and f(R)-gravity in the Newtonian limit
Recently, a strong debate has been pursued about the Newtonian limit (i.e.
small velocity and weak field) of fourth order gravity models. According to
some authors, the Newtonian limit of -gravity is equivalent to the one of
Brans-Dicke gravity with , so that the PPN parameters of these
models turn out to be ill defined. In this paper, we carefully discuss this
point considering that fourth order gravity models are dynamically equivalent
to the O'Hanlon Lagrangian. This is a special case of scalar-tensor gravity
characterized only by self-interaction potential and that, in the Newtonian
limit, this implies a non-standard behavior that cannot be compared with the
usual PPN limit of General Relativity.
The result turns out to be completely different from the one of Brans-Dicke
theory and in particular suggests that it is misleading to consider the PPN
parameters of this theory with in order to characterize the
homologous quantities of -gravity. Finally the solutions at Newtonian
level, obtained in the Jordan frame for a -gravity, reinterpreted as a
scalar-tensor theory, are linked to those in the Einstein frame.Comment: 9 page
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 Note on Energy-Momentum Conservation in Palatini Formulation of L(R) Gravity
By establishing that Palatini formulation of gravity is equivalent to
Brans-Dicke theory, we show that energy-momentum tensor is
covariantly conserved in this type of modified gravity theory.Comment: 7 page
Modified gravity without dark matter
On an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
RelativityComment: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy, minor errors corrected,
references update
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
Matter loops corrected modified gravity in Palatini formulation
Recently, corrections to the standard Einstein-Hilbert action are proposed to
explain the current cosmic acceleration in stead of introducing dark energy. In
the Palatini formulation of those modified gravity models, there is an
important observation due to Arkani-Hamed: matter loops will give rise to a
correction to the modified gravity action proportional to the Ricci scalar of
the metric. In the presence of such term, we show that the current forms of
modified gravity models in Palatini formulation, specifically, the 1/R gravity
and gravity, will have phantoms. Then we study the possible
instabilities due to the presence of phantom fields. We show that the strong
instability in the metric formulation of 1/R gravity indicated by Dolgov and
Kawasaki will not appear and the decay timescales for the phantom fields may be
long enough for the theories to make sense as effective field theory . On the
other hand, if we change the sign of the modification terms to eliminate the
phantoms, some other inconsistencies will arise for the various versions of the
modified gravity models. Finally, we comment on the universal property of the
Palatini formulation of the matter loops corrected modified gravity models and
its implications.Comment: 11 pages, 1 figures, References adde
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