38 research outputs found
On second-order superhorizon perturbations in multifield inflationary models
We present a method for the study of second-order superhorizon perturbations
in multi field inflationary models with non trivial kinetic terms. We utilise a
change of coordinates in field space to separate isocurvature and adiabatic
perturbations generalizing previous results. We also construct second order
gauge invariant variables related to them. It is found that with an arbitrary
metric in field space the isocurvature perturbation sources the gravitational
potential on long wavelengths even for ``straight'' trajectories. The potential
decouples from the isocurvature perturbations if the background fields'
trajectory is a geodesic in field space. Taking nonlinear effects into account
shows that, in general, the two types of perturbations couple to each other.
This is an outline of a possible procedure to study nonlinear and non-Gaussian
effects during multifield inflation.Comment: 19 pages, 1 figure; Substantial revision from earlier versions.
Published in Classical and Quantum Gravit
How does the electromagnetic field couple to gravity, in particular to metric, nonmetricity, torsion, and curvature?
The coupling of the electromagnetic field to gravity is an age-old problem.
Presently, there is a resurgence of interest in it, mainly for two reasons: (i)
Experimental investigations are under way with ever increasing precision, be it
in the laboratory or by observing outer space. (ii) One desires to test out
alternatives to Einstein's gravitational theory, in particular those of a
gauge-theoretical nature, like Einstein-Cartan theory or metric-affine gravity.
A clean discussion requires a reflection on the foundations of electrodynamics.
If one bases electrodynamics on the conservation laws of electric charge and
magnetic flux, one finds Maxwell's equations expressed in terms of the
excitation H=(D,H) and the field strength F=(E,B) without any intervention of
the metric or the linear connection of spacetime. In other words, there is
still no coupling to gravity. Only the constitutive law H= functional(F)
mediates such a coupling. We discuss the different ways of how metric,
nonmetricity, torsion, and curvature can come into play here. Along the way, we
touch on non-local laws (Mashhoon), non-linear ones (Born-Infeld,
Heisenberg-Euler, Plebanski), linear ones, including the Abelian axion (Ni),
and find a method for deriving the metric from linear electrodynamics (Toupin,
Schoenberg). Finally, we discuss possible non-minimal coupling schemes.Comment: Latex2e, 26 pages. Contribution to "Testing Relativistic Gravity in
Space: Gyroscopes, Clocks, Interferometers ...", Proceedings of the 220th
Heraeus-Seminar, 22 - 27 August 1999 in Bad Honnef, C. Laemmerzahl et al.
(eds.). Springer, Berlin (2000) to be published (Revised version uses
Springer Latex macros; Sec. 6 substantially rewritten; appendices removed;
the list of references updated