Modern cosmological observations indicate that the expansion of the universe is accelerating. This is typically described in terms of the equation of state parameter of a hypothetical new component of the cosmic energy budget, presumed to be driving the acceleration. Observations then provide bounds on this parameter.
In this talk I will discuss theoretical limits on the values of this parameter. In the first part I will discuss the (dire) implications of inferring from the data that the equation of state parameter is less than -1. This may happen if cosmic acceleration is driven by an energy component that violates the energy conditions of general relativity. I will show how such a component leads to dramatic instabilities. I will also mention the interesting possibility that deviations from general relativity may lead to an inferred w<-1 even for more conventional dark energy sources. However, in that case, the theory must be extremely finely tuned to lead to an appreciable effect.
In the second part of the talk I will discuss the possibility that cosmic acceleration is due entirely to new gravitational physics, without the need for dark energy, and with the equation of state parameter having no physical interpretation.