Tired light models invoke a gradual energy loss by photons as they travel through the cosmos to produce the redshift-distance law. This has three main problems:

- There is no known interaction that can degrade a photon's energy without also changing its momentum, which leads to a blurring of distant objects which is not observed. The Compton shift in particular does not work.
- The tired light model does not predict the
observed time dilation of
high redshift supernova light curves.
- The tired light model can not produce a blackbody spectrum for the
Cosmic Microwave Background without some incredible coincidences.
The local Universe is transparent and has a wide range of temperatures,
so it does not produce a blackbody, which requires an isothermal
absorbing situation. So the CMB must have come from a far away part
of the Universe, and its photons will thus lose energy by the tired light
effect. The plot below shows what happens if the CMB comes from
z = 0.1.

Assume that the CMB starts out as a T = (1+z)*T_o = 2.998 K blackbody, which is the blue curve. Because the photons only lose energy but do not decrease their rate of arrival, the resulting red curve is not a blackbody at T_o = 2.725, but is instead 1.331 times a blackbody. The FIRAS data limit this prefactor to 1.00001+/-0.00005, which requires that the CMB come from redshifts less than 0.00005, or distance less than 0.25 Mpc. This is less than the distance to the Andromeda Galaxy M31, and we know the Universe is transparent well beyond this distance. Note that the CMB cannot be redshifted starlight.

Tutorial:
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Part 4

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© 1996-2000 Edward L. Wright. Last modified 31-Jan-2000