Quantum mechanics demonstrates unevenness that would have taken place at the instant following t=0. The unevenness was excessive for what we observe, so along came Inflation theory that smoothed things out, pun intended.

IIRC, we had to get better than 1 part in 100,000 in temperature to notice the unevenness (anisotropy). It first came from a U2 flight, I think, followed by a balloon study (BOMERANG?), then finally COBE, then WMAP.