Star Formation Rate Density

The cosmic star formation rate density — the total rate of stellar mass production per unit comoving volume as a function of redshift — rises from z=0 to a peak around z~2–3, then declines steeply at higher redshift. ΛCDM reproduces the broad shape of this curve through the interplay of gas cooling, feedback, and the hierarchical assembly of halos, but the peak amplitude, the sharpness of the decline above z~3, and the relatively flat low-redshift tail each have residual tensions with observations. The absolute calibration of the star formation rate density also depends on assumptions about the initial mass function and dust correction that can shift results by factors of order two. Successive Collision Theory provides a physically motivated origin for the entire shape of this curve. The post-collision plasma thermalization produces an initial burst of star formation as the plasma cools and the first density enhancements at collision nodes begin forming stars. This burst defines the high-redshift rise of the star formation rate density.

The peak at z~2–3 corresponds in SCT to the epoch when the most massive collision-node structures — the proto-cluster cores and dense filament intersections — were simultaneously undergoing their peak gas infall rate and converting infalling gas into stars most efficiently. Above z~3, star formation was concentrated in the most overdense collision nodes but was also partially suppressed by the extreme radiation field from the collision thermalization era and by the high gas velocity dispersions inherited from the angular momentum of the collision. Below z~2, the star formation rate density declines as the dense gas reservoir at collision nodes is consumed and the globally declining tensor mesh reduces the rate at which halo potentials can cool and confine gas. The quenching of star formation in massive galaxies — which drives the sharp decline at low redshift — is accelerated by feedback from the massive early black holes that grew at collision nodes, linking the star formation rate density decline to the early SMBH assembly that SCT naturally produces through pre-existing matter.