A comparison of perturbations in fluid and scalar field models of dark energy

TL;DR

This paper compares fluid and scalar field models of dark energy, showing that while they behave similarly on small scales, differences emerge at larger scales affecting matter perturbations.

Contribution

It demonstrates the conditions under which a fluid model can effectively emulate a scalar field dark energy model and identifies scale-dependent limitations.

Findings

Fluid models match scalar fields on sub-Hubble scales.

At large scales, fluid and scalar models diverge in perturbation behavior.

Large-scale matter power spectrum is suppressed compared to $ ext{Lambda}$CDM.

Abstract

We compare perturbations in a fluid model of dark energy with those in a scalar field. As compared to the $\Lambda$CDM model, large scale matter power spectrum is suppressed in fluid model as well as in a generic quintessence dark energy model.To check the efficacy of fluid description of dark energy in emulating a scalar field, we consider a potential which gives the same background evolution as a fluid with a constant equation of state. We show that for sub-Hubble scales, a fluid model effectively emulates a scalar field model. At larger scales, where dark energy perturbations may play a significant role, the fluid analogy breaks down and the evolution of matter density contrast depends on individual scalar field models.