Cosmic Microwave Background
Cosmic Microwave Background
One-line summary: The afterglow of the Big Bang — light from when the whole universe was as bright as the Sun's surface, now stretched to microwaves, and the same temperature in every direction.
The insight
Looking far away means looking back in time. ~400,000 years after the Big Bang the entire universe was so hot and dense it glowed and was opaque to light; that "surface" is the farthest we can see with light. The light from it has been stretched by cosmic expansion into a faint microwave signal arriving from every direction — the cosmic microwave background (CMB). Its near-perfect uniformity (identical to fractions of a degree everywhere) is a profound puzzle, because those regions should never have been in causal contact — a key motivation for inflation and for the idea that everything was once in a tiny volume. See expanding-universe.
Evidence
- michelle-thaller in 2026-05-28-youtube-powerfuljre-joe-rogan-experience-2506-michelle-thaller: "What they had discovered was the afterglow of the Big Bang... it is exactly the same, down to fractions of a degree in every direction on the sky."
- Discovery (1960s–70s): Penzias and Wilson at Bell Labs found an unremovable background "noise" in every direction; they suspected pigeon droppings in the antenna, cleaned it out, and the signal remained — it was the CMB.
- NASA's COBE then WMAP (Wilkinson Microwave Anisotropy Probe) measured it to hundred-thousandths of a degree; the tiny temperature variations correspond to sound waves propagating across the early universe.
Contradictions / tensions
The "horizon problem" — uniformity across regions that couldn't have exchanged heat — is unresolved by the basic Big Bang picture and motivates inflation (see expanding-universe). Thaller treats sub-second early-universe timelines (Planck epoch) with "a big chunk of salt."
Open questions
- Why is the universe so uniform? Does inflation fully explain it?
- Can gravitational waves (via ligo-type detectors) let us see past the CMB, beyond light's opacity limit?