Gauge theories of gravity

Abstract

A novel alternative to Einstein's general theory of relativity is presented, based on the gauge principle. The gravitational Lagrangian has no Einstein--Hilbert term, but is constructed from quadratic invariants of the Riemann--Cartan curvature and torsion, constituting a geometric gauge theory of the Poincaré group. Despite the introduction of Planck and cosmological constant scales through the torsion couplings, the linearised free theory is not only unitary but also power-counting renormalisable. A conformal symmetry in this regime is broken naturally in the nonlinear cosmology, for which constant axial torsion is an attractor state of the background. The Hubble dynamics are then identical to those of Friedmann up to a complete screening of the spatial curvature, and a boundary condition which can dynamically replicate the effects of dark radiation in the early Universe. In a simplified version of the theory, part of the torsion is removed via multipliers without detriment to the known phenomenology. This procedure introduces classical ghosts to the Minkowski background, but produces the Newtonian limit on the axial vector background anticipated in Nature.