Physically, the Hubble 'horizon' is mostly meaningless. What you have in mind is the cosmological event horizon. Both can be defined in terms of certain veloci

Physically, the Hubble 'horizon' is mostly meaningless. What you have in mind is the cosmological event horizon. Both can be defined in terms of certain velocities reaching c: The hubble sphere is located where the change in proper distance at constant cosmological time reaches the speed of light (aka recession velocities), the cosmological event horizon is located where relative velocities as evaluated by parallel transport along the light path does so.