Pyrotex said:
Dear Kmarinas,
I checked several websites on Olber's Paradox, and I am afraid your initial equation and assumptions above, do not work. Sorry.
You say "there are x/2 photons from r=[1,2]", but that only from a point. Photons come not from points in a flat sky, but from spherical SHELLS that surround us. If we assume the shells are rather thin, then photons come from the AREA of the shell with radius r.
Actually, as the radius r increases, the area of each shell goes UP as the square of r. On the other hand, the inverse law says that the amount of radiation we recieve from an object goes DOWN as the inverse square of the distance.
These two principals exactly ballance each other. The areas of the shells go up as r^2 and the radiation from any given point on a shell goes DOWN as 1/r^2. A little simple calculus will combine these two principals to show that the
total radiation from each shell should be the same.
In other words, this is how the math really works:
If there are x photons from shell with r=[0,1], then
there are x photons from shell with r=[1,2]
there are x photons from shell with r=[2,3]
there are x photons from shell with r=[3,4]
Therefore, if the Universe were infinite, the sky would blaze infinitely bright in all directions. Of course, this is NOT what we see.
Your statement that Olber's Paradox only works with uniform star density is correct, but it doesn't make the Paradox go away. We only have to calculate the AVERAGE star density for the visible Universe and use that for the radiance of each shell. We get the same result.
Take a look
here.
The cosmological principle is not assumed in the opening post (OP). Rather, a decreasing density is assumed as distance from greater clusters increases.
If we throw the cosmological principle into the trash bin, it is possible to get an infinite universe that is not infinitely bright.
If you take the integral of 1/r from r=1 to r=infinity, you'll get infinity.
But if you take the integral of 1/r^2 from r=1 to r=infinity, you'll get 1.
In general, integrating 1/r^2 from r=a to r=b gives us 1/b - 1/a.
If the density drops with distance at a sufficient rate, then you won't have infinite brightness. In this case, the brightness provided by each radii must be 1/r^a ... where a is number greater than 1. In this way, what we end up with is a finite brightness.
When integrated from r=1 to r=infinity, the integral of 1/r^p is finite as long as p>1.
If p=2, starting from the sun (its distance from earth), the brightness by other stars would that of 1 sun.
If p=1.5, starting from the sun (its distance from earth), the brightness by other stars would that of 2 suns.
If p=1.25, starting from the sun (its distance from earth), the brightness by other stars would that of 4 suns.
If p=1.2, starting from the sun (its distance from earth), the brightness by other stars would that of 5 suns.
If p=1.1, starting from the sun (its distance from earth), the brightness by other stars would that of 10 suns.
If p=1.01, starting from the sun (its distance from earth), the brightness by other stars would that of 100 suns.
etc.
For brightness to drop with the square of the distance, overall density at a particular radii must drop with the square of the distance. The mass at each radii in this case would be a "constant" as the derivative of volume increases with the square of the radius - i.e. M is proportional to R so that additional brightness added is inversely proportional to the square of the distance. This is the case with galaxies with flat rotation curves, as a result of having M proportional to R for significant ranges of R. If a galaxy were infinite in size but having the same 1/r^2 density gradient, some parts of the sky would be dark, depending which way we looked. And for very small r, we have black holes, which are not necessarily bright as long they are not taking in large amounts of matter.
It is by theories such as the big bang which make it possible to see immediate regions of the universe of having a density

times greater than more far off regions of the universe.
Olbers' paradox - Wikipedia, the free encyclopedia
Wikipedia said:
A different resolution, which does not rely on the Big Bang theory, was offered by Benoît Mandelbrot. He postulated that if the stars in the universe were fractally distributed (e.g. like a Cantor dust), it would not be necessary to rely on the Big Bang theory to explain Olbers' Paradox. This model would not rule out a Big Bang, but would allow for a dark sky even if the Big Bang had not occurred. This is merely a demonstration of the consequences of fractal theory, rather than a serious resolution of this paradox. Astronomical observers have found no evidence to support a fractal distribution of the stars.
The idea of a hierarchical cosmology - what would now be called a fractal cosmology - had been proposed in 1908 by Carl Charlier.