gevolution: a cosmological N-body code based on General Relativity
  We present a new N-body code, gevolution, for the evolution of large scale
structure in the Universe. Our code is based on a weak field expansion of
General Relativity and calculates all six metric degrees of freedom in Poisson
gauge. N-body particles are evolved by solving the geodesic equation which we
write in terms of a canonical momentum such that it remains valid also for
relativistic particles. We validate the code by considering the Schwarzschild
solution and, in the Newtonian limit, by comparing with the Newtonian N-body
codes Gadget-2 and RAMSES. We then proceed with a simulation of large scale
structure in a Universe with massive neutrinos where we study the gravitational
slip induced by the neutrino shear stress. The code can be extended to include
different kinds of dark energy or modified gravity models and going beyond the
usually adopted quasi-static approximation. Our code is publicly available.
