In cosmology, a Hubble volume (named for the astronomer Edwin Hubble) or Hubble sphere, subluminal sphere, causal sphere and sphere of causality is a spherical region of the observable universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the universe. The Hubble volume is approximately equal to 10³¹ cubic light years (or about 10⁷⁹ cubic meters).

The proper radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is ${\displaystyle c/H_{0}}$, where ${\displaystyle c}$ is the speed of light and ${\displaystyle H_{0}}$ is the Hubble constant. The surface of a Hubble sphere is called the microphysical horizon, the Hubble surface, or the Hubble limit.

More generally, the term Hubble volume can be applied to any region of space with a volume of order ${\displaystyle (c/H_{0})^{3}}$. However, the term is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.

The center of the Hubble volume and observable universe is arbitrary in relation to the overall universe; instead it is centered around its origin (impersonal or personal "observer").