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What is the surface of last scattering in cosmology?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The surface of last scattering refers to the period in the early universe, often called the "decoupling time," when the cosmos cooled sufficiently to allow the formation of neutral atoms.1

Prior to this event, the universe was filled with a hot, dense plasma where matter was completely ionized, rendering the cosmos opaque to electromagnetic radiation. As the universe expanded and cooled, electrons and nuclei combined to form neutral atoms, which allowed radiation to travel freely through space for the first time.1

The "surface" is not a physical boundary but rather a snapshot of the universe at that moment of transition. The radiation released at this time—originally of very high energy and short wavelengths—has been redshifted by the ongoing expansion of the universe into the cosmic microwave background (CMB) observed today. This radiation provides a vital record of the conditions of the early universe, including the small temperature fluctuations that eventually led to the formation of galaxies and large-scale structures.21