Approximate KMS states for scalar and spinor fields in Friedmann-Robertson-Walker spacetimes

TL;DR

This paper constructs approximate KMS states for scalar and spinor fields in flat Friedmann-Robertson-Walker spacetimes, providing a framework for understanding thermal phenomena in cosmology.

Contribution

It introduces a bulk-to-boundary reconstruction method for spinor fields to create approximate KMS states in cosmological spacetimes.

Findings

States are in thermal equilibrium at specific scale factors.

Exact KMS states are obtained at the boundary for certain cases.

The method applies to both massless and massive fields.

Abstract

We construct and discuss Hadamard states for both scalar and Dirac spinor fields in a large class of spatially flat Friedmann-Robertson-Walker spacetimes characterised by an initial phase either of exponential or of power-law expansion. The states we obtain can be interpreted as being in thermal equilibrium at the time when the scale factor a has a specific value a=a_0. In the case a_0=0, these states fulfil a strict KMS condition on the boundary of the spacetime, which is either a cosmological horizon, or a Big Bang hypersurface. Furthermore, in the conformally invariant case, they are conformal KMS states on the full spacetime. However, they provide a natural notion of an approximate KMS state also in the remaining cases, especially for massive fields. On the technical side, our results are based on a bulk-to-boundary reconstruction technique already successfully applied in the scalar case and here proven to be suitable also for spinor fields. The potential applications of the states we find range over a broad spectrum, but they appear to be suited to discuss in particular thermal phenomena such as the cosmic neutrino background or the quantum state of dark matter.