[Phys-seminars] 2017-11-06 Particles and Fields Seminar

lublinm lublinm at bgu.ac.il
Tue Oct 31 10:28:25 IST 2017

Particles and Fields Seminar

 DATE: 06-11-17

 TIME: 14:00

 PLACE: Physics building (#54) room 207

Causality Violation in Lovelock Theories

Yotam Sherf, BGU

Higher-derivative gravity theories, such as Lovelock theories, generalize Einstein\'s general relativity (GR). Modifications to GR are expected when curvatures are near Planckian and appear in string theory or supergravity. But can such theories describe gravity on length scales much larger than the cutoff length scale? Here we find causality constraints on Lovelock theories that arise from the requirement that the equations of motion (EOM) of perturbations be hyperbolic. We find a general expression for the ``effective metric\" in field space when Lovelock theories are perturbed around some background solution. In particular, we calculate explicitly the effective metric for a general Lovelock theory perturbed around cosmological Friedman-Robertson-Walker backgrounds and for some specific cases when perturbed around Schwarzschild-like solutions. For the EOM to be hyperbolic, the effective metric needs to be Lorentzian. We find that, unlike for GR, the effective metric is gene
 rically not Lorentzian when the Lovelock modifications are significant. So, we conclude that Lovelock theories can only be considered as perturbative extension of GR and not as truly modified theories of gravity. We compare our results to those in the literature and find that they are consistent with the results of previous studies.

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