2013/04/12 by Tomi S. Koivisto, Tomi Koivisto, Nicola Tamanini · 1 citation
Mathematics · Physics and Astronomy · #Black Holes and Theoretical Physics #Computer science #Cosmology and Gravitation Theories #Geometry #Mathematics #Physics #Relativity and Gravitational Theory #Rotation formalisms in three dimensions #Theoretical physics #gr-qc #hep-th
paper · pdf · doi:10.1103/physrevd.87.104030
9 pages
arxiv created 2013/04/12 · openalex publication_date 2013/05/22 · arxiv updated 2013/05/29 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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=R\ensuremath-\ensuremathκ2T.