What is the cosmic microwave background?
The cosmic microwave background is ancient light from the hot early universe, now observed as microwaves across the sky.
How it works and what the evidence shows
Core idea In the early universe, light repeatedly scattered off free electrons. When cooling allowed atoms to form, the universe became transparent about 380,000 years after its hot beginning. Expansion stretched the released light to microwave wavelengths. Evidence and research Space instruments map the radiation and its tiny temperature differences. These variations preserve traces of early density differences and constrain models of cosmic structure.
Uses and limitations
Why it matters The background is major evidence for the hot Big Bang model and helps establish the starting conditions for later galaxy formation. It is one line of evidence among several. Common misunderstandings It is not a photograph of the exact beginning: it records the later transparent era. Its mapped temperature variations are not visible colors in the sky.