Reply to `Can infrared gravitons screen $\Lambda$?'

TL;DR

This paper defends the idea that quantum gravitational effects can secularly screen the cosmological constant, addressing criticisms and emphasizing the physical relevance of gauge-invariant and observable quantities.

Contribution

It clarifies misconceptions about renormalization schemes and demonstrates the potential for a stochastic approach to sum secular quantum gravity effects.

Findings

Secular screening of the cosmological constant is still plausible.

Gauge dependence does not invalidate physical predictions.

Scalar models show large secular effects on de Sitter backgrounds.

Abstract

We reply to the recent criticism by Garriga and Tanaka of our proposal that quantum gravitational loop corrections may lead to a secular screening of the effective cosmological constant. Their argument rests upon a renormalization scheme in which the composite operator $(R \sqrt{-g} - 4 \Lambda \sqrt{-g} )_{\rm ren}$ is defined to be the trace of the renormalized field equations. Although this is a peculiar prescription, we show that it {\it does not preclude secular screening}. Moreover, we show that a constant Ricci scalar {\it does not even classically} imply a constant expansion rate. Other important points are: (1) the quantity $R_{\rm ren}$ of Garriga and Tanaka is neither a properly defined composite operator, nor is it constant; (2) gauge dependence does not render a Green's function devoid of physical content; (3) scalar models on a non-dynamical de Sitter background (for which there is no gauge issue) can induce arbitrarily large secular contributions to the stress tensor; (4) the same secular corrections appear in observable quantities in quantum gravity; and (5) the prospects seem good for deriving a simple stochastic formulation of quantum gravity in which the leading secular effects can be summed and for which the expectation values of even complicated, gauge invariant operators can be computed at leading order.