Star formation in the universe did not proceed at a steady pace. After the first stars it rose quickly, reached a peak some 3–4 billion years after the Big Bang (10–11 Gya, what cosmologists call redshift z ≈ 1.5–2.5), and then began a slow decline. Today the cosmic rate of star formation is roughly ten times below that peak. The peak itself is called 'cosmic noon.'

Cosmic noon matters for more than just star counts. In this era galaxies collided more often, massive black holes burned as quasars, heavy-element production intensified, and the foundations of modern galactic morphology were laid down. The disks of spiral galaxies matured then; a substantial fraction of the Milky Way's stars were born then.

We see cosmic noon through deep-field imagery — the Hubble Ultra-Deep Field and the James Webb deep fields. These images expose a small patch of sky for many hours to gather light from 10–13 billion years ago; each speck is a galaxy of that era. The present-day sky, by contrast, is in a much quieter, preserving phase: the star formation rate has fallen, and existing stars are simply living out their lifetimes.

Why a peak followed by decline? Two reasons dominate. First, gas: stars need cold gas to form, and during this era the inflow of cosmic gas into galaxies was at its richest. Afterward most of the gas had either turned into stars or been blown out of galaxies by stellar radiation and supernovae. Second, dark energy: the accelerating expansion that switched on around 5 Gya weakened the inflows that fed galaxies. Cosmic noon is the fullest leaf on the universe's tree, set in the balance between fertility and expansion.