Dear London Cosmologists,
There will be a cosmology seminar at Imperial College this Friday 3
November at 12 noon in the Huxley Building Room 503.
All would be very welcome.
Speaker : Gianluca Calcagni
Title : Ghost conditions for Gauss-Bonnet cosmologies (hep-th/0604201)
Abstract:
We investigate the stability against inhomogeneous perturbations and
the appearance of ghost modes in Gauss-Bonnet gravitational theories
with a non-minimally coupled scalar field, which can be regarded as
either the dilaton or a compactification modulus in the context of
string theory.
Through cosmological linear perturbations we extract four no-ghost
and two sub-luminal constraint equations, written in terms of
background quantities, which must be satisfied for consistency. We
also argue that, for a general action with quadratic Riemann
invariants, homogeneous and inhomogeneous perturbations are, in
general, inequivalent, and that attractors in the phase space can
have ghosts. Single-field models as candidates for dark energy are
explored numerically and severe bounds on the parameter space of
initial conditions are placed.
A number of cases proposed in the literature are tested and most of
them are found to be unstable or observationally unviable.
Joe Zuntz
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