CMB: Understanding Uniformity & Isotropy

[vincentm]

Ok, I'm reading up on the Cosmic Microwave background radiation guys. And I'm at a part that confuses me (actually i kinda don't understand the whole thing, but I'm trying) regarding the uniformity or isotropy. If someone can put this in a simpler form so that i can understand it easier i'd highly appreciate it, thanks in advance for your help

The following simple argument shows. Any radiation produced near the sun, in our galaxy, or even in nearby galaxies would undoubtedly be unevenly distributed. Therefore, we assume that the sources of the radiation are evenly distributed throughout space. Suppose we divide the universe into a large number of concentric and equally spaced spherical shells, all centered on and enclosing the earth. In this case, the amount of radiation coming from sources within any pair of adjacent shells is the same, because the area of a sphere increases with distance in just the same way as the intensity of the radiation decreases. The number of emitting sources increases with the area, and the flux from each source decreases as the square of it's distance. A uniform background radiation must come from mostly distant parts of the universe, where the majority of the sources are found. Very little of the radiation could originate in our local region of space and any isotropic background radiation must be produced at cosmological distances.

 

[Chronos]

The intensity of radiation from objects within each concentric 'shell' decrease proportionate to the square of the distance [from earth], however, the average number of objects within each shell [assuming uniform density as indicated by observation] increases with respect to the cube of the distance [from earth]. Hence, the total background radiation [CMB] is dominated by contributions from distant sources. What they lack in intensity is more than made up by sheer numbers.