The Odd Meanderings of the IMF Across Cosmic Time
  It is difficult to reconcile the observed evolution of the star formation
rate versus stellar mass (SFR-M*) relation with expectations from current
hierarchical galaxy formation models. The observed SFR-M* relation shows a
rapid rise in SFR(M*) from z=0-2, and then a surprisingly lack of amplitude
evolution out to z~6+. Hierarchical models of galaxy formation match this trend
qualitatively but not quantitatively, with a maximum discrepancy of ~x3 in SFR
at z~2. One explanation, albeit radical, is that the IMF becomes modestly
weighted towards massive stars out to z~2, and then evolves back towards its
present-day form by z~4 or so. We observe that this redshift trend mimics that
of the cosmic fraction of obscured star formation, perhaps hinting at a
physical connection. Such IMF evolution would concurrently go towards
explaining persistent discrepancies between integrated measures of star
formation and present-day stellar mass or cosmic colors.
