Hubble radius, Hubble sphere, Hubble volume, or Hubble horizon is a conceptual horizon defining the boundary between particles that are moving slower and faster than the speed of light relative to an observer at one given time. Note that this does not mean the particle is unobservable; the light from the past is reaching and will continue to reach the observer for a while. Also, more importantly, in the current expansion models, light emitted from the Hubble radius will reach us in a finite amount of time. It is a common misconception that light from the Hubble radius can never reach us. In models assuming decreasing H with time (some cases of Friedmann universe), while particles on the Hubble radius recede from us with the speed of light, the Hubble radius gets larger over time, so light emitted towards us from a particle on the Hubble radius will be inside the Hubble radius some time later. In such models, only light emitted from the cosmic event horizon or further will never reach us in a finite amount of time.