Loading…
Loading contentLoading…
Loading contentThe boundary beyond which light has not had time to reach us — the edge of the observable Universe.
The particle horizon marks the farthest distance from which signals could have reached us. Because the expansion is accelerating, there is also an event horizon: galaxies beyond a certain distance are receding so fast that light they emit now will never reach us.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.