Variational principle and 1-point functions in 3-dimensional flat space   Einstein gravity
  We provide a well-defined variational principle for 3-dimensional flat space Einstein gravity by adding one half of the Gibbons-Hawking-York boundary term to the bulk action. We check the 0-point function, recovering consistency with thermodynamics of flat space cosmologies. We then apply our result to calculate the 1-point functions in flat space Einstein gravity for the vacuum and all flat space cosmologies. The results are compatible with the ones for the zero mode charges obtained by canonical analysis. 
Stephane Detournay, Daniel Grumiller, Friedrich Scholler and Joan   Simon
hep-th