Understand
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.