Tracing galaxy orbits back in time

To test the assumption that galaxy clustering grew by gravity, we need a way to predict the motions. The method presented here chooses trial orbits to minimize the action given the observed positions and assuming galaxies at high redshift have negligible peculiar velocities. Results are presented for the orbits of Local Group members along with the Maffei and Sculptor systems treated as point particles. The solution are checked by using them to generate initial conditions at high redshift for a conventional N-body integration forward in time; the results retrace the solution quite well. The predicted velocities generally resemble the observations, though there are discrepancies. It is not clear whether this is a result of the lack of uniqueness of the solutions, or the effect of perturbations of galaxies not included, or a problem for the simple gravitational instability picture.