Is the dark energy equation of state parameter singular?

TL;DR

The paper demonstrates that a perfect fluid dark energy model with a zero energy density at a specific redshift necessarily exhibits a pole in its equation of state parameter, impacting how we should infer dark energy properties from observations.

Contribution

It establishes a theoretical link between zero energy density points and poles in the dark energy equation of state, highlighting implications for observational reconstructions.

Findings

A zero in energy density implies a pole in w(z) at that point.

The converse, a pole in w(z), does not necessarily mean zero energy density.

Reconstruction of dark energy should focus on density, not w(z).

Abstract

A dark energy with a negative energy density in the past can simultaneously address various cosmological tensions, and if it is to be positive today to drive the observed acceleration of the universe, we show that it should have a pole in its equation of state parameter. More precisely, in a spatially uniform universe, a perfect fluid (submitting to the usual continuity equation of local energy conservation) whose energy density $\rho(z)$ vanishes at an isolated zero $z=z_p$, necessarily has a pole in its equation of state parameter $w(z)$ at $z_p$, and, $w(z)$ diverges to positive infinity in the limit $z\to z_p^+$ and it diverges to negative infinity in the limit $z\to z_p^-$ -- we assume that $z_p$ is not an accumulation point for poles of $w(z)$. However, the converse statement that this kind of a pole of $w(z)$ corresponds to a vanishing energy density at that point is not true as we show by a counterexample. An immediate implication of this result is that one should be hesitant to observationally reconstruct the equation of state parameter of the dark energy directly, and rather infer it from a directly reconstructed dark energy density.