Star formation efficiency is the fraction of available baryons a galaxy actually converts into stars, and in the local universe it is humblingly low: strongly dependent on mass and redshift, peaking around Milky Way-sized halos, and declining on both sides of that peak (Behroozi et al. 2013; Conroy and Wechsler 2009). ΛCDM's galaxy formation models reproduce the broad trends by tuning feedback and gas accretion prescriptions against the data.
The high-redshift universe broke the tuning. JWST keeps finding early systems forming stars at efficiencies approaching 50 percent, three to five times the observed ceiling at any later epoch in any environment (Xiao et al. 2024), and a population whose detailed efficiency scatter at fixed halo mass exceeds what halo-driven rules produce. Within the model this is close to paradox: feedback physics does not know what redshift it is, so the same supernova and stellar wind regulation that caps efficiency near 20 percent locally should have operated at z of 7 to 9, and the energy budget cuts deeper, since a population forming at 50 percent efficiency releases enough supernova energy to unbind its own host halo's baryons several times over. The efficient early universe is not just unexplained but apparently self-contradictory under standard assumptions.
The standing sharpens with each JWST cycle as the efficient-early-system census grows. ALMA gas measurements of the same systems are the discriminating data: they reveal how much fuel these galaxies actually hold, and therefore what efficiency the star formation truly ran at.