Comments on "Little ado about everything" by A. Lapi et al. and on cosmological back-reaction

TL;DR

The paper critically examines the ηCDM proposal, arguing that attributing cosmic acceleration to back-reaction from standard cold dark matter via a stochastic noise term is not credible. It systematically analyzes the need for a stochastic description, the ad hoc nature of the noise modelling, the misapplication of ensemble averages, and the neglect of robust empirical evidence. The critique emphasizes that deterministic N-body evolution and forward-modeling studies, including both Newtonian and relativistic treatments, show no significant back-reaction altering the expansion history. The work reinforces the importance of physically motivated modelling and observationally anchored validations when challenging ΛCDM, suggesting that ηCDM cannot be considered a viable alternative.

Abstract

In two papers, A. Lapi et al. introduce and discuss what they call the $η$CDM model, a stochastic framework in which they claim that fluctuations in the density field at the scale of tens of Mpc due to structure formation would effectively drive the accelerated expansion of the Universe. They claim that this qualitative behaviour would emerge from the dynamics of standard cold dark matter alone, without introducing any new physics. In this short comment, I argue that such a proposition is implausible. Some of my remarks are relevant more generally to frameworks that try to describe cosmological back-reaction.