In the field of cosmology the Hubble volume, or 'Hubble sphere', is the region surrounding an observer beyond which objects recede from the observer faster than the speed of light, making them unobservable. The speed of recession is due to the expansion of the Universe, and so different models give different values for our Hubble volume, and whether it will increase or decrease over time. However, the Hubble volume is approximately 1031 cubic light years.