Originally posted by Clavicula
The critisisms are not completely detrimental to the idea.
In fact, they are. There is no physically possible version of the "tired light" hypothesis. Here's the most conclusive reason why:
Consider the Cosmic Microwave Background (CMB). According to the current model (and current observations), the CMB is blackbody radiation emitted
~13.7 billion years ago. To emit blackbody radiation, a region must be absorb light evenly throughout (which, in astrophysics, is called "opaque").
In the current theory, this condition was met by the early universe, so, at that time, it was emitting blackbody radiation. When the universe then
became transparent, as it expanded and cooled, the last of the blackbody radiation was allowed to travel freely, and this is what we are seeing now in
the CMB.
However, let's assume a static universe (where there is no cosmological redshift - so, the overwhelming majority of redshifting is attributed to
"tired light"). This requires that the blackbody radiation of the CMB be coming from some region of the universe as we see it today. The problem is,
our local area of the universe is transparent, so it cannot emit blacbody radiation. However, some will say that it's a far-away region of the
universe that the blackbody radiation is coming from. Well, let's find out how far away it would have to be...
The temperature of the CMB, which we can measure directly, is 2.725 K. In a static universe, space is not expanding, so the density of the blackbody
radiation is constant, and, therefore, its temperature decreases only as a result of the redshifting.
Now, the spectrum represented by any blackbody radiation must show a very specific distribution (it peaks at a certain wavelength and falls off on
either side). Given this, we can find at what redshift/distance an object must be to produce the observed spectrum (and how much it deviates from a
pure blackbody spectrum). Sparing you the math, to obtain a temperature that deviates from the blackbody spectrum even at the limit of our
observational error (which shows the CMB is a pure blackbody spectrum within a prefactor of 1.00001 +/- 0.00005), the maximum distance the source of
the CMB must be at is 815,000 light-years. This 1/3 the distance to the Andromeda Galaxy. Obviously, the universe is transparent well beyond this
point.
The source of the CMB cannot be an opaque region of our contemporary universe. This eliminates the tired light theory, which requires a contemporary
source for the CMB.