A note on observational signatures in superluminal unified dark matter
  models
  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.
