  Negative absolute temperatures (NAT) are an exotic thermodynamical
consequence of quantum physics which has been known since the 1950's (having
been achieved in the lab on a number of occasions). Recently, the work of Braun
et al (2013) has rekindled interest in negative temperatures and hinted at a
possibility of using NAT systems in the lab as dark energy analogues. This
paper goes one step further, looking into the cosmological consequences of the
existence of a NAT component in the Universe. NAT-dominated expanding Universes
experience a borderline phantom expansion ($w<-1$) with no Big Rip, and their
contracting counterparts are forced to bounce after the energy density becomes
sufficiently large. Both scenarios might be used to solve horizon and flatness
problems analogously to standard inflation and bouncing cosmologies. We discuss
the difficulties in obtaining and ending a NAT-dominated epoch, and possible
ways of obtaining density perturbations with an acceptable spectrum.
