Inflationary Cosmology, Diffeomorphism Group of the Line and Virasoro Coadjoint Orbits

Abstract

The cosmological field equations sourced by a self-interacting scalar field are dynamically equivalent to a closed system of equations obtained by applying the moment method to non-linear Schr\"odinger equations possessing an underlying non-relativistic conformal $SL(2,\mathbb{R})$ symmetry. We consider the one-dimensional, quintic Schr\"odinger equation relevant to strongly repulsive, dilute Bose gases. The action of the diffeomorphism group on the space of Schr\"odinger operators generates an harmonic trapping potential that can be identified with the kinetic energy of the cosmological scalar field. Inflationary cosmologies are represented by points on the orbit of the de Sitter solution, which is the quotient manifold ${\rm Diff}(\mathbb{R})/SL(2,\mathbb{R})$. Key roles are played by the Schwarzian derivative of the diffeomorphism and the Ermakov-Pinney equation. The underlying $SL(2,\mathbb{R})$ symmetry results in a first integral constraint which ensures energy-momentum conservation. When the analysis is restricted to the universal cover group of diffeomorphisms on the circle, the generation of a rolling scalar field can be understood in terms of the Virasoro coadjoint action. The corresponding symplectic two-form and Hamiltonian generator of the coadjoint orbit are determined by the scalar field kinetic energy.