Isochronous Spacetimes
  The possibility has been recently demonstrated to manufacture (nonrelativistic, Hamiltonian) many-body problems which feature an isochronous time evolution with an arbitrarily assigned period $T$ yet mimic with good approximation, or even exactly, any given many-body problem (within a quite large class, encompassing most of nonrelativistic physics) over times $\tilde{T}$ which may also be arbitrarily large (but of course such that $\tilde{T}<T$). In this paper we review and further explore the possibility to extend this finding to a general relativity context, so that it becomes relevant for cosmology. 
Fabio Briscese and Francesco Calogero
gr-qc math-ph math.MP