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

TL;DR

This paper critically examines claims that density fluctuations alone can explain cosmic acceleration, arguing that such a proposition is implausible and discussing broader issues in back-reaction frameworks.

Contribution

It provides a critical perspective on the $ au$CDM model's claim that structure formation drives acceleration without new physics.

Findings

The $ au$CDM model's claims are considered implausible.

Highlights issues in modeling cosmological back-reaction.

Offers a skeptical view on fluctuation-driven acceleration.

Abstract

In two papers, A. Lapi et al. introduce and discuss what they call the $\eta$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.