A note on observational signatures in superluminal unified dark matter models

TL;DR

This paper investigates superluminal Chaplygin gas models as unified dark matter and energy candidates, revealing that such models produce excessive integrated Sachs-Wolfe effects incompatible with CMB observations.

Contribution

It provides the first detailed analysis of the ISW effect in superluminal Chaplygin gas models, highlighting their observational inconsistencies.

Findings

Superluminal models cause large ISW effect enhancement.

These models are inconsistent with current CMB measurements.

The study clarifies causality issues in superluminal dark matter models.

Abstract

We explore the possibility that the dark matter and dark energy are mimicked by a single fluid or by a single $k$-essence-like scalar field. The so called Chaplygin gas unified dark matter models can reproduce the observed matter power spectrum by adding a baryon component. It has been argued that the evolution of the baryon fluctuations is particularly favoured for the "superluminal" case where the sound speed of the Chaplygin gas exceeds the speed of light at late times, as well as for the models with the negligibly small sound speed. In this note we compute the integrated Sachs-Wolfe signal in the Chaplygin gas models, focusing on the superluminal case which has not been investigated before because of the premature understanding of causality. It is shown that the superluminal model leads to large enhancement of the integrated Sachs-Wolfe effect, which is inconsistent with the CMB measurements.