Abstract:
The standard LCDM paradigm has been phenomenally successful on many fronts. In particular, all predictions with respect to the overall cosmology and structure on large scales have proven accurate. But on small scales – kilo parsecs and less – the hierarchical model has been neither confirmed nor refuted. For example, dark matter cusps are predicted, but observations do not strongly confirm the predictions. With L, Hui, S. Tremaine, and E. Witten an exploration is under way concerning the possibility that very light axions provide the bulk of the dark matter. This choice, sometimes labeled the Fuzzy Dark Matter (“FDM”) hypothesis, would strongly smooth out small scale structure due to standard quantum effects and in addition would strongly suppress early formation of low mass halos and small galaxies. Thus predictions for “first light” provide strong tests of this cosmological model. In it the peak formation epoch of low mass halos (eg 10^8 Msolar) and the corresponding galaxies is later than in the standard LCDM model.