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The back reaction of gravitational perturbations in a homogeneous background is determined by an effective energy-momentum tensor quadratic in the perturbations. We show that this nonlinear feedback effect is important in the case of long wavelength scalar perturbations in inflationary universe models. We also show how to solve an old problem concerning the gauge dependence of the effective energy-momentum tensor of perturbations.
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After discussing the various issues regarding and requirements on pure quantum gravitational observables in homogeneous-isotropic conditions, we construct a composite operator observable satisfying most of them. We also expand it to first order in the loop counting parameter and suggest it as a physical quantifier of gravitational back-reaction in an initially inflating cosmology.
We find the leading-order effect of gravitational back-reaction on cosmic strings for points near kinks and cusps. Near a kink, the effect diverges as the inverse cube root of the distance to the kink, and acts in a direction transverse to the worldsheet. Over time the kink is rounded off, but only regions fairly close to the kink are significantly affected. Near cusps, the effect diverges inverse linearly with the distance to the cusp, and acts against the direction of the cusp motion. This results in a fractional loss of string energy that diverges logarithmically with the distance of closest approach to the cusp.
The problem of the gravitational radiation damping of neutron star fundamental ($f$) mode oscillations has received considerable attention. Many studies have looked at the stability of such oscillations in rapidly rotating stars, calculating the growth/decay rate of the mode amplitude. In this paper, we look at the relatively neglected problem of the radiation reaction on the spin of the star. We specialise greatly to the so-called Kelvin modes: the modes of oscillation of (initially) non-rotating incompressible stars. We find the unexpected result that the excitation of a mode of angular momentum $delta J$ on an initially non-rotating star ends up radiating an angular momentum $2 delta J$ to infinity, leaving the star itself with a bulk angular momentum of $-delta J$. This result is interesting in itself, and also will have implications for the angular momentum budgets of spinning down neutron stars, should such modes be excited.
We use dimensional regularization in pure quantum gravity on de Sitter background to evaluate the one loop expectation value of an invariant operator which gives the local expansion rate. We show that the renormalization of this nonlocal composite operator can be accomplished using the counterterms of a simple local theory of gravity plus matter, at least at one loop order. This renormalization completely absorbs the one loop correction, which accords with the prediction that the lowest secular back-reaction should be a 2-loop effect.
We analyze the effects of the back reaction due to a conformal field theory (CFT) on a black hole spacetime with negative cosmological constant. We study the geometry numerically obtained by taking into account the energy momentum tensor of CFT approximated by a radiation fluid. We find a sequence of configurations without a horizon in thermal equilibrium (CFT stars), followed by a sequence of configurations with a horizon. We discuss the thermodynamic properties of the system and how back reaction effects alter the space-time structure. We also provide an interpretation of the above sequence of solutions in terms of the AdS/CFT correspondence. The dual five-dimensional description is given by the Karch-Randall model, in which a sequence of five-dimensional floating black holes followed by a sequence of brane localized black holes correspond to the above solutions.
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