Into the MAG-verse or: Cosmology of the Complete Quadratic Metric-Affine Gravity

TL;DR

This paper explores the cosmological implications of a complete quadratic metric-affine gravity theory, revealing how torsion and nonmetricity invariants influence universe evolution and naturally produce exotic matter phenomena.

Contribution

It introduces a comprehensive quadratic metric-affine gravity model with 17 invariants and analyzes its cosmological solutions under homogeneous and isotropic conditions.

Findings

Rich phenomenology from torsion and nonmetricity effects

Emergence of exotic matter like cosmic strings and domain walls

Multiple solutions with unique cosmological features

Abstract

We study the cosmology of the complete quadratic (in torsion and nonmetricity) metric-affine gravity. Namely, we add to the scalar-curvature gravitational Lagrangian, the 17 independent quadratic (parity-even and parity-odd) torsion and nonmetricity invariants. Sticking to a homogeneous and isotropic Friedmann-Robertson-Walker spacetime and assuming a perfect hyperfluid source, we explore the new effects that torsion and nonmetricity bring into play. It is shown that the inclusion of these invariants offers rich phenomenology. In particular, some well-known examples of exotic matter like cosmic strings, domain walls, stiff matter, etc., emerge quite naturally as manifestations of the fluid's intrinsic structure (hypermomentum). By studying the extended Friedmann equations in the complete quadratic theory and isolating the various parts of the hypermomentum, we find a plethora of solutions with interesting features.