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In cosmology, a Hubble volume or Hubble sphere, Hubble bubble, 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 10⁳ⁱ cubic light years. The proper radius of a Hubble sphere is c / H 0, where c is the speed of light and 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 3. However, the term is also frequently 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. The Hubble length c / H 0 is 14.4 billion light years in the standard cosmological model, equivalent to c times Hubble time. The Hubble time is the reciprocal of the Hubble constant, and is slightly larger than the age of the universe as it is the age the universe would have had if expansion was linear.