Inflationary Constraints on the Van der Waals Equation of State

Abstract: In spite of the strong observational evidence suggesting a period of rapid expansion in the early universe, the identity of the inflaton field that drove this expansion remains elusive. Many inflaton candidate particles (both known and hypothesized) have been proposed to explain the early accelerated expansion of the universe. Other explanations for an era of rapid expansion in the early universe have been proposed via modifications of gravity in one form or another. In this paper, we consider the possibility of using a Van der Waals equation of state to describe the early-time accelerated expansion of the universe. We do not attempt to explain why the early universe may be filled with such a fluid, but rather investigate what constraints may be placed on the parameters which describe a Van der Waals fluid that sources an accelerated expansion in the early universe. We consider three different versions of the Van der Waals equation of state and constrain the parameters of each using CMB data. We find that two of the models do not fit constraints from observation. A third model may satisfy observational constraints, but only under very narrow circumstances.