Cosmological Redshift Photons propagating through the expanding space are stretched between their point of emission emission (em) and their point of observation (obs), creating an increase in wavelength: the cosmological redshift. This effect is prescribed by the current cosmological model as an observable manifestation of the time-dependent cosmic scale factor a(t) in the following way: (1+z) is the factor by which the universe has expanded while the photon was travelling towards the observer. If tobs is the present time, then a(tobs)=1, and the cosmic scale factor at the time of emission is e. g.: for a cosmological redshift of z=3 the cosmic scale factor at the time of emission was a=1/4. The discovery of the linear relationship between redshift, interpreted as recessional velocity, and distance yields a straightforward mathematical expression for Hubble's Law as follows: v = recession velocity, in km/sec H0 = Hubble Parameter (today), H0 = (70 +/- 7) km/sec/Mpc D = (comoving) distance from the galaxy to the observer, in Mpc H0 may be interpreted as the rate of expansion of the Universe, and the "Hubble time" 1/H0 as the current age of the universe.

Web Links

Hubble's law (Wikipedia) Redshift (Wikipedia) Rotverschiebung (Wikipedia) The Expanding Universe (Ohio State University)