Testing anthropic predictions for Lambda and the CMB temperature

TL;DR

This paper reevaluates anthropic predictions for the cosmological constant and CMB temperature, considering recollapsing universes and observer formation, and finds that current observations are consistent with these models.

Contribution

It introduces new approximations for structure formation and analyzes the impact of universe recollapse and observer formation assumptions on anthropic predictions.

Findings

Structure forms in recollapsing universes with negative Lambda.

Current universe's Lambda and T are typical in the predicted distribution.

Anthropic arguments on curvature can lead to different conclusions.

Abstract

It has been claimed that the observed magnitude of the vacuum energy density is consistent with the distribution predicted in anthropic models, in which an ensemble of universes is assumed. This calculation is revisited, without making the assumption that the CMB temperature is known, and considering in detail the possibility of a recollapsing universe. New accurate approximations for the growth of perturbations and the mass function of dark haloes are presented. Structure forms readily in the recollapsing phase of a model with negative Lambda, so collapse fraction alone cannot forbid Lambda from being large and negative. A negative Lambda is disfavoured only if we assume that formation of observers can be neglected once the recollapsing universe has heated to T > 8 K. For the case of positive Lambda, however, the current universe does occupy a extremely typical position compared to the predicted distribution on the Lambda-T plane. Contrasting conclusions can be reached if anthropic arguments are applied to the curvature of the universe, and we discuss the falsifiability of this mode of anthropic reasoning.