Penzias and Wilson’s 1965 result identified pervasive microwave radiation, interpreted as a relic of the early hot universe. Later missions measured its spectrum and tiny temperature variations. We place it third because this evidence links cosmological models to an observable early epoch. The background comes from when the universe became transparent, not from a photograph of the initial instant. The WMAP map uses colours to exaggerate very small temperature differences.
Source: The cosmic microwave background reveals a hot past.
Image credit: NASA / WMAP Science Team · Public domain. Resized and JPEG compressed.
