Quantum effects can render w<-1 on cosmological scales

TL;DR

This paper revises the calculation of quantum effects on a scalar field in de Sitter space, showing that such effects can violate the weak energy condition on cosmological scales, though the effect is very small.

Contribution

It introduces a modified propagator enabling dimensional regularization in the stress-energy tensor computation, refining previous results and exploring implications for initial state corrections.

Findings

Quantum effects can violate the weak energy condition on cosmological scales.

The revised propagator allows for consistent regularization in non-four-dimensional settings.

The effect size remains too small for observational detection.

Abstract

We report on a revision of our previous computation of the renormalized expectation value of the stress-energy tensor of a massless, minimally coupled scalar with a quartic self-interaction on a locally de Sitter background. This model is important because it demonstrates that quantum effects can lead to violations of the weak energy condition on cosmological scales - on average, not just in fluctuations - although the effect in this particular model is far too small to be observed. The revision consists of modifying the propagator so that dimensional regularization can be used when the dimension of the renormalized theory is not four. Although the finite part of the stress-energy tensor does not change (in D=4) from our previous result, the counterterms do. We also speculate that a certain, finite and separately conserved part of the stress tensor can be subsumed into a natural correction of the initial state from free Bunch-Davies vacuum.