88 research outputs found
A new duality transformation for fourth-order gravity
We prove that for non-linear L = L(R), the Lagrangians L and \hat L give
conformally equivalent fourth-order field equations being dual to each other.
The proof represents a new application of the fact that the operator
is conformally invariant.Comment: 11 pages, LaTeX, no figures. Gen. Relat. Grav. in prin
The Universality of Einstein Equations
It is shown that for a wide class of analytic Lagrangians which depend only
on the scalar curvature of a metric and a connection, the application of the
so--called ``Palatini formalism'', i.e., treating the metric and the connection
as independent variables, leads to ``universal'' equations. If the dimension
of space--time is greater than two these universal equations are Einstein
equations for a generic Lagrangian and are suitably replaced by other universal
equations at bifurcation points. We show that bifurcations take place in
particular for conformally invariant Lagrangians and prove
that their solutions are conformally equivalent to solutions of Einstein
equations. For 2--dimensional space--time we find instead that the universal
equation is always the equation of constant scalar curvature; the connection in
this case is a Weyl connection, containing the Levi--Civita connection of the
metric and an additional vectorfield ensuing from conformal invariance. As an
example, we investigate in detail some polynomial Lagrangians and discuss their
bifurcations.Comment: 15 pages, LaTeX, (Extended Version), TO-JLL-P1/9
Topological Defects in Gravitational Theories with Non Linear Lagrangians
The gravitational field of monopoles, cosmic strings and domain walls is
studied in the quadratic gravitational theory with , and is compared with the result in Einstein's theory. The metric
aquires modifications which correspond to a short range `Newtonian' potential
for gauge cosmic strings, gauge monopoles and domain walls and to a long range
one for global monopoles and global cosmic strings. In this theory the
corrections turn out to be attractive for all the defects. We explain, however,
that the sign of these corrections in general depends on the particular higher
order derivative theory and topological defect under consideration. The
possible relevance of our results to the study of the evolution of topological
defects in the early universe is pointed out.Comment: LaTeX (uses revrex macros), 13 page
Accelerating Universes in String Theory via Field Redefinition
We study cosmological solutions in the effective heterotic string theory with
-correction terms in string frame. It is pointed out that the
effective theory has an ambiguity via field redefinition and we analyze
generalized effective theories due to this ambiguity. We restrict our analysis
to the effective theories which give equations of motion of second order in the
derivatives, just as "Galileon" field theory. This class of effective actions
contains two free coupling constants. We find de Sitter solutions as well as
the power-law expanding universes in our four-dimensional Einstein frame. The
accelerated expanding universes are always the attractors in the present
dynamical system.Comment: 22 pages, 3 figures, some additional formulae adde
On Physical Equivalence between Nonlinear Gravity Theories
We argue that in a nonlinear gravity theory, which according to well-known
results is dynamically equivalent to a self-gravitating scalar field in General
Relativity, the true physical variables are exactly those which describe the
equivalent general-relativistic model (these variables are known as Einstein
frame). Whenever such variables cannot be defined, there are strong indications
that the original theory is unphysical. We explicitly show how to map, in the
presence of matter, the Jordan frame to the Einstein one and backwards. We
study energetics for asymptotically flat solutions. This is based on the
second-order dynamics obtained, without changing the metric, by the use of a
Helmholtz Lagrangian. We prove for a large class of these Lagrangians that the
ADM energy is positive for solutions close to flat space. The proof of this
Positive Energy Theorem relies on the existence of the Einstein frame, since in
the (Helmholtz--)Jordan frame the Dominant Energy Condition does not hold and
the field variables are unrelated to the total energy of the system.Comment: 37 pp., TO-JLL-P 3/93 Dec 199
Equivalence of black hole thermodynamics between a generalized theory of gravity and the Einstein theory
We analyze black hole thermodynamics in a generalized theory of gravity whose
Lagrangian is an arbitrary function of the metric, the Ricci tensor and a
scalar field. We can convert the theory into the Einstein frame via a
"Legendre" transformation or a conformal transformation. We calculate
thermodynamical variables both in the original frame and in the Einstein frame,
following the Iyer--Wald definition which satisfies the first law of
thermodynamics. We show that all thermodynamical variables defined in the
original frame are the same as those in the Einstein frame, if the spacetimes
in both frames are asymptotically flat, regular and possess event horizons with
non-zero temperatures. This result may be useful to study whether the second
law is still valid in the generalized theory of gravity.Comment: 14 pages, no figure
New constraints on multi-field inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a
multi-field model where a second scalar field is nonminimally coupled to
the scalar curvature ). When is positive, the usual
inflationary prediction for large-scale anisotropies is hardly altered while
the fluctuation in sub-Hubble modes can be amplified during preheating
for large . For negative values of , however, long-wave modes of the
fluctuation exhibit exponential increase during inflation, leading to
the strong enhancement of super-Hubble metric perturbations even when
is less than unity. This is because the effective mass becomes negative
during inflation. We constrain the strength of and the initial by
the amplitude of produced density perturbations. One way to avoid nonadiabatic
growth of super-Hubble curvature perturbations is to stabilize the mass
through a coupling to the inflaton. Preheating may thus be necessary in these
models to protect the stability of the inflationary phase.Comment: 20 pages, 8 figures, submitted to Physical Review
Resonant particle production with non-minimally coupled scalar fields in preheating after inflation
We investigate a resonant particle production of a scalar field
coupled non-minimally to a spacetime curvature () as well as
to an inflaton field (). In the case of , effect assists -resonance in certain parameter regimes.
However, for , -resonance is not enhanced by
effect because of suppression effect as well as a back reaction effect.
If , the maximal fluctuation of produced -particle is
GeV for , which is larger than the minimally coupled case with .Comment: 33pages, 12figures. to appear in Physical Review
Rapid Evaluation in Whole Blood Culture of Regimens for XDR-TB Containing PNU-100480 (Sutezolid), TMC207, PA-824, SQ109, and Pyrazinamide
There presently is no rapid method to assess the bactericidal activity of new regimens for tuberculosis. This study examined PNU-100480, TMC207, PA-824, SQ109, and pyrazinamide, singly and in various combinations, against intracellular M. tuberculosis, using whole blood culture (WBA). The addition of 1,25-dihydroxy vitamin D facilitated detection of the activity of TMC207 in the 3-day cultures. Pyrazinamide failed to show significant activity against a PZA-resistant strain (M. bovis BCG), and was not further considered. Low, mid, and high therapeutic concentrations of each remaining drug were tested individually and in a paired checkerboard fashion. Observed bactericidal activity was compared to that predicted by the sum of the effects of individual drugs. Combinations of PNU-100480, TMC207, and SQ109 were fully additive, whereas those including PA-824 were less than additive or antagonistic. The cumulative activities of 2, 3, and 4 drug combinations were predicted based on the observed concentration-activity relationship, published pharmacokinetic data, and, for PNU-100480, published WBA data after oral dosing. The most active regimens, including PNU-100480, TMC207, and SQ109, were predicted to have cumulative activity comparable to standard TB therapy. Further testing of regimens including these compounds is warranted. Measurement of whole blood bactericidal activity can accelerate the development of novel TB regimens
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