Distinguished quantum states in a class of cosmological spacetimes and their Hadamard property

TL;DR

This paper extends previous work to show that a preferred quantum state in open FRW cosmological spacetimes has the Hadamard property, ensuring finite backreaction and suitability for quantum field renormalization in early Universe models.

Contribution

It proves that the bulk-to-boundary reconstructed quantum state in open FRW spacetimes is of Hadamard form, facilitating quantum field analysis in cosmological settings.

Findings

The preferred state in open FRW models is of Hadamard form.

The state ensures finite backreaction on the metric.

It provides a suitable starting point for renormalization in early Universe models.

Abstract

In a recent paper, we proved that a large class of spacetimes, not necessarily homogeneous or isotropous and relevant at a cosmological level, possesses a preferred codimension one submanifold, i.e., the past cosmological horizon, on which it is possible to encode the information of a scalar field theory living in the bulk. Such bulk-to-boundary reconstruction procedure entails the identification of a preferred quasifree algebraic state for the bulk theory, enjoying remarkable properties concerning invariance under isometries (if any) of the bulk and energy positivity, and reducing to well-known vacua in standard situations. In this paper, specialising to open FRW models, we extend previously obtained results and we prove that the preferred state is of Hadamard form, hence the backreaction on the metric is finite and the state can be used as a starting point for renormalisation procedures. That state could play a distinguished role in the discussion of the evolution of scalar fluctuations of the metric, an analysis often performed in the development of any model describing the dynamic of an early Universe which undergoes an inflationary phase of rapid expansion in the past.