What If w < -1 ?

The cosmological “equation of state parameter”, w, is equal to -1 for a true cosmological constant, and is greater than -1 for quintessence. There is a widespread reluctance to consider the remaining possibility, w < -1, though in fact there is a variety of theoretical and observational evidence in favour of this range. We briefly discuss some of the objections to w < -1 and show that each of them can readily be circumvented. We also briefly consider what string theory has to say about this.