Metallicity Floor

The lowest-metallicity stars and galaxies in the universe — metal-poor dwarf spheroidals, extremely metal-poor halo stars, and low-surface-brightness galaxies — show a metallicity floor: a minimum metal abundance below which essentially no stellar systems are found. This floor, at roughly 10^{-4} to 10^{-3} of solar metallicity in the most extreme cases, is significantly above the prediction for purely primordial (metal-free) gas and implies that all currently observed stellar systems formed from gas that had already been pre-enriched to some minimum level. In ΛCDM, this floor is attributed to Population III star enrichment — the first generation of massive, metal-free stars that explode as supernovae and scatter metals into the surrounding interstellar and intergalactic medium before the observed stellar populations formed. However, producing the right enrichment level at the right spatial coverage requires Population III stars to have enriched the gas uniformly to the observed floor level before any of the surviving metal-poor stars formed — a coordination problem that is difficult to achieve in a hierarchical assembly picture.

Successive Collision Theory solves the metallicity floor problem at its origin through pre-existing matter. The spacetime pockets that collided to produce our observable universe had been processing gas through stellar nucleosynthesis for an unbounded prior cosmic history. The interstellar and intergalactic medium within these pockets therefore already contained a baseline level of metals synthesized in prior stellar generations. When the collision thermalized the overlap volume, this pre-existing metal enrichment was mixed into the new plasma along with the thermalized gas, establishing a minimum metallicity in the post-collision medium that set the floor for all subsequent star formation. Even the most metal-poor environments observed — the outer halos of galaxies, dwarf spheroidals, and the intergalactic medium — formed from gas that started with this pre-existing metal floor rather than from truly primordial hydrogen-helium gas. The metallicity floor is therefore a direct observational signature of the eternal pre-collision universe from which our spacetime pocket originated, encoding a record of prior cosmic chemistry that predates the Big Bang event itself.