Ghosts in pure and hybrid formalisms of gravity theories: A unified analysis

Tomi S. Koivisto and Nicola Tamanini
Phys. Rev. D 87, 104030 – Published 22 May 2013

Abstract

In the first-order formalism of gravitational theories, the spacetime connection is considered as an independent variable to vary together with the metric. However, the metric still generates its Levi-Civita connection that turns out to determine the geodesics of matter. Recently, “hybrid” gravity theories have been introduced by constructing actions involving both the independent Palatini connection and the metric Levi-Civita connection. In this study a method is developed to analyze the field content of such theories, in particular to determine whether the propagating degrees of freedom are ghosts or tachyons. New types of second-, fourth- and sixth-order derivative gravity theories are investigated and the so-called f(X) theories are singled out as a viable class of “hybrid” extensions of general relativity. These are the theories in which the corrections to Einstein’s theory are written in terms of the deviations from the usual trace equation X=Rκ2T.

  • Received 23 April 2013

DOI:https://doi.org/10.1103/PhysRevD.87.104030

© 2013 American Physical Society

Authors & Affiliations

Tomi S. Koivisto*

  • Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo, Norway

Nicola Tamanini

  • Department of Mathematics, University College London, Gower Street, London WC1E 6BT, United Kingdom

  • *tomi.koivisto@fys.uio.no
  • n.tamanini.11@ucl.ac.uk

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Issue

Vol. 87, Iss. 10 — 15 May 2013

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