10 research outputs found
Local cosmic strings in Brans-Dicke theory with a cosmological constant
It is known that Vilenkin's phenomenological equation of state for static
straight cosmic strings is inconsistent with Brans-Dicke theory. We will prove
that, in the presence of a cosmological constant, this equation of state is
consistent with Brans-Dicke theory. The general solution of the full nonlinear
field equations, representing the interior of a cosmic string with a
cosmological constant is also presented.Comment: 5 pages, Revte
Gravitational hedgehog, stringy hedgehog and stringy sphere
We investigate the solutions of Einstein equations such that a hedgehog
solution is matched to different exterior or interior solutions via a spherical
shell. In the case where both the exterior and the interior regions are
hedgehog solutions or one of them is flat, the resulting spherical shell
becomes a stringy shell. We also consider more general matchings and see that
in this case the shell deviates from its stringy character.Comment: 11 page
Modified Einstein-Gauss-Bonnet gravity: Riemann-Cartan and pseudo-Riemannian cases
WOS: 000382366800003A modified Einstein-Gauss-Bonnet gravity in four dimensions where the quadratic Gauss-Bonnet term is coupled to a scalar field is considered. The field equations of the model are obtained by variational methods by making use of the constrained first-order formalism covering both pseudo-Riemannian and non-Riemannian cases. In the pseudo-Riemannian case, the Lagrange multiplier forms, which impose the vanishing torsion constraint, are eliminated in favor of the remaining fields and the resulting metric field equations are expressed in terms of the double dual curvature 2-form. In the non-Riemannian case with torsion, the field equations are expressed in terms of the pseudo-Riemannian quantities by a perturbative scheme valid for a weak coupling constant. It is shown that, for both cases, the model admits a maximally symmetric de Sitter solution with non-trivial scalar field. Minimal coupling of a Dirac spinor to the Gauss-Bonnet modified gravity is also discussed briefly