Initial state effects on the cosmic microwave background and   trans-planckian physics
  There exist a one complex parameter family of de Sitter invariant vacua, known as alpha vacua. In the context of slow roll inflation, we show that all but the Bunch-Davies vacuum generates unacceptable production of high energy particles at the end of inflation. As a simple model for the effects of trans-planckian physics, we go on to consider non-de Sitter invariant vacua obtained by patching modes in the Bunch-Davies vacuum above some momentum scale M_c, with modes in an alpha vacuum below M_c. Choosing M_c near the Planck scale M_pl, we find acceptable levels of hard particle production, and corrections to the cosmic microwave perturbations at the level of H M_pl/M_c^2, where H is the Hubble parameter during inflation. More general initial states of this type with H<< M_c << M_pl can give corrections to the spectrum of cosmic microwave background perturbations at order 1. The parameter characterizing the alpha-vacuum during inflation is a new cosmological observable. 
Kevin Goldstein and David A. Lowe