A distance defined as the ratio of the velocity of light, c, to the value of the Hubble constant, H₀. This gives the distance from the observer at which the recession velocity of a galaxy would equal the speed of light. Roughly speaking, the Hubble radius is the radius of the observable Universe. Current observations place the Hubble constant at around 70 km/s/Mpc, which gives a Hubble radius of 14 billion l.y.