Abstract:

I discuss two different speculations about the acceleration of the universe, both involving a modification of gravity on large scales in the universe. In the first part I will discuss the 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. However, such a component leads to dramatic instabilities. On the other hand, 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.