What is the light-horizon problem of the cosmic microwave background?

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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 light-horizon problem refers to the difficulty in explaining why the cosmic microwave background (CMB) radiation is so remarkably uniform across the entire sky.1

When observing the CMB from opposite directions, the source regions—hot plasma—are separated by distances so vast that, according to standard models, they could never have been in causal contact since the beginning of the universe. Because photons travel at the speed of light, these regions are beyond each other’s light horizons, meaning they could not have exchanged information or reached thermal equilibrium. Consequently, it is puzzling how these distant regions "know" to maintain the same temperature to a precision of one part in 100,000.1

The theory of inflation resolves this by proposing that the entire observable universe was in thermal contact before a period of rapid, exponential expansion pushed these regions outside their individual event horizons, preserving their uniform thermodynamic characteristics.1