Hubble volume

The volume of a sphere whose radius is the Hubble radius, the distance light travels in one Hubble time. It is a useful cosmological scale, not necessarily the observable universe’s full volume.

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Hubble constant: The present-day rate at which the universe’s expansion speed increases with distance. Its value sets the Hubble time and Hubble radius used to define this volume.
Observable universe: The region of the universe from which signals have had time to reach an observer since the early universe. Its present-day radius is larger than the Hubble radius, so its volume exceeds the Hubble volume.
Hubble sphere: The sphere centered on an observer with radius equal to the Hubble radius. Its enclosed spatial region is the Hubble volume.
Superluminal recession: The increase of proper distance between sufficiently remote galaxies faster than the speed of light due to cosmic expansion. Objects beyond a Hubble radius can recede faster than light without locally violating relativity.
Hubble time: The inverse of the Hubble parameter, giving a characteristic timescale for cosmic expansion. Light travels one Hubble radius in this characteristic time.
Particle horizon: The greatest distance from which light could have reached an observer since the universe began. It marks a causal boundary, unlike the Hubble radius’s expansion-defined scale.
Cosmic microwave background: The relic radiation released when the early universe became transparent, now observed at microwave wavelengths. Its anisotropies are analyzed across angular scales related to the Hubble scale at last scattering.
Cosmic inflation: A proposed early epoch in which space expanded exponentially or nearly exponentially. It drove the Hubble radius to shrink in comoving coordinates, changing which regions could communicate.
Hubble radius: The distance light travels in one Hubble time, approximately the speed of light divided by the Hubble parameter. This is the sphere’s defining radius.
Cosmological event horizon: The greatest distance from which signals emitted now can ever reach an observer in the future. It is a future causal boundary, not the radius used for a Hubble volume.