Horizon problem

The puzzle that widely separated regions of the cosmic microwave background have nearly identical temperatures despite lacking time to exchange signals in standard Big Bang cosmology.

Cosmic microwave background: Electromagnetic radiation released when the early universe became transparent, now observed as a nearly uniform microwave glow. Its nearly uniform temperature across distant regions is the puzzle’s central observation.
Cosmological redshift: The increase in light’s wavelength caused by the expansion of the universe. It connects the microwave background’s present wavelength to conditions in the early universe.
Hot Big Bang: The model in which the universe expanded from a hot, dense early state and cooled over time. Its standard non-inflationary version contains the causal-contact puzzle.
Flatness problem: The puzzle of why the universe’s spatial curvature is so close to zero today. Inflation addresses this fine-tuning puzzle alongside the horizon problem.
Particle horizon: The maximum distance from which signals could have reached an observer since the beginning of cosmic expansion. It defines which early-universe regions could have exchanged signals.
Scale factor: A measure of how the relative distances between comoving points in the universe change over time. Its evolution determines how the observable horizon grew relative to cosmic distances.
Standard Big Bang cosmology: Cosmology describing expansion from a hot, dense state without requiring an early inflationary phase. Without inflation, distant microwave-background regions were not in causal contact before emission.
Cosmic inflation predictions: Observable consequences of inflation, including primordial fluctuations and possible gravitational waves. A proposed solution gains testability through its predicted signatures.
Cosmic inflation: A hypothesized period of extremely rapid expansion in the early universe. Inflation can stretch a once-connected region across the observed cosmic microwave background.
Conformal time: A time coordinate that expresses the distance light can travel in an expanding universe. It makes the limited causal reach of early regions explicit.

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