THE LEAST-ACTION METHOD, COLD DARK-MATTER, AND OMEGA

Peebles has suggested an interesting technique, called the least-action method, to trace positions of galaxies back in time. This method applied on the Local Group galaxies seems to indicate that we live in an Ohm approximate to 0.1 universe. We have studied a CDM N-body simulation with Ohm = 0.2 and H = 50 km s(-1) Mpc(-1) and compared trajectories traced back by the least-action method with the ones given by the center of mass of the CDM halos. We show that the agreement between these sets of trajectories is at best qualitative. We also show that the line-of-sight peculiar velocities of halos are underestimated. This discrepancy is due to orphans, i.e., CDM particles which do not end up in halos. We vary the value of Ohm in the least-action method until the line-of-sight velocities agree with the CDM ones. The best value for this Ohm underestimates one of the CDM simulations by a factor of 4-5.