Hubble’s law states that recession speed, v_R, is equal to H₀D, where H₀ is Hubble’s constant, 2.37E-18 s^-1, and D is the distance to the inter-galactic photon source.  

When the relativistic Doppler equation is solved for (v_R /c), it is well-known that (v_R /c) → 1 as the received wavelength → ∞.  The standard conclusion is that distant galaxies approach light speed. This is a pre-Hubble conclusion. However, using Hubble’s law, one gets instead, (H₀D /c) → 1.  Therefore, in the limit, D = (c/H₀) ≡ R₀ = 13.4 billion light-years.  R₀ is the constant radial limit of our observable universe:  photons emitted from beyond this distance can never reach us.  The size of the universe is unknown:  an observer 10 billion light years from us would be bound by this same limit.

The energy loss of inter-galactic photons is due to the universal deceleration, - cH_0, not to space  expnasion.  It has a magnitude of 0.711nm/s² and is denoted D-Alpha.   It is related to the constant acceleration used in MOND, and is comparable to the deceleration of the Pioneer spacecraft, 0.874nm/s².  Direct detection of D-Alpha may be possible by a variation of the LIGO experiment. When D-Alpha is used in a Newtonian approach to orbital motion in the solar system, non-Keplerian orbits are the necessary result.