Numerical action reconstruction of the dynamical history of dark matter haloes in N-body simulations

We test the ability of the numerical action method (NAM) to recover the individual orbit histories of mass tracers in an expanding universe in a region of radius 26Mpc/h, given the masses and redshift-space coordinates at the present epoch. The mass tracers are represented by dark matter haloes identified in a high resolution N-body simulation of the standard LCDM cosmology. Since previous tests of NAM at this scale have traced the underlying distribution of dark matter particles rather than extended haloes, our study offers an assessment of the accuracy of NAM in a scenario which more closely approximates the complex dynamics of actual galaxy haloes. We show that NAM can recover present-day halo distances with typical errors of less than 3 per cent, compared to 5 per cent errors assuming Hubble flow distances. The total halo mass and the linear bias were both found to be constained at the 50 per cent level. The accuracy of individual orbit reconstructions was limited by the inability of NAM, in some instances, to correctly model the positions of haloes at early times solely on the basis of the redshifts, angular positions, and masses of the haloes at the present epoch. Improvements in the quality of NAM reconstructions may be possible using the present-day three-dimensional halo velocities and distances to further constrain the dynamics. This velocity data is expected to become available for nearby galaxies in the coming generations of observations by SIM and GAIA.Comment: 12 pages, 9 figures. submitted to MNRA

Phenomenology of the Invisible Universe

Cosmology is operating now on a well established and tightly constraining empirical basis. The relativistic LambdaCDM hot big bang theory is consistent with all the present tests; it has become the benchmark. But the many open issues in this subject make it reasonable to expect that a more accurate cosmology will have more interesting physics in the invisible sector of the universe, and maybe also in the visible part.Comment: Proceedings of 'Invisible Universe International Conference', Paris, June 29- July 3, 200

Discovery of the Hot Big Bang: What happened in 1948

The idea that the universe is filled with the thermal radiation now termed the Cosmic Microwave Background was first discussed in eleven publications in the year 1948. These papers offer a detailed example of the process of development of a new and now very productive line of research, and of the confusion that can attend new ideas. The confusion in this case left a common misunderstanding of the considerations that motivated the idea of the sea of radiation.Comment: A 24-page preprint; version 2 has additions, revisions and corrections to version