Friedmann-Robertson-Walker models do not require zero active mass

TL;DR

The paper refutes the claim that Friedmann-Robertson-Walker models require zero active mass, clarifying misconceptions about the $R_h=ct$ cosmological model and its underlying assumptions.

Contribution

It demonstrates that the zero active mass condition is not a necessary requirement of FRW spacetime symmetries, correcting previous misconceptions.

Findings

The zero active mass condition is not mandated by FRW symmetries.

Melia's claim about the necessity of this condition is based on flawed logic.

The $R_h=ct$ model's assumptions are clarified and corrected.

Abstract

The $R_h = ct$ cosmological model has received considerable attention in recent years owing to claims that it is favoured over the standard $\Lambda$ cold dark matter ($\Lambda$CDM) model by most observational data. A key feature of the $R_h = ct$ model is that the zero active mass condition $\rho + 3p = 0$ holds at all epochs. Most recently, Melia has claimed that this condition is a requirement of the symmetries of the Friedmann-Robertson-Walker (FRW) spacetime. We demonstrate that this claim is false and results from a flaw in the logic of Melia's argument.