The discussion revolves around deriving the Cosmic Microwave Background (CMB) flux density in the context of present cosmology, with a focus on calculations assuming a constant temperature for the CMB.
Participants express differing views on the application of Planck's law to the CMB and the appropriate considerations for calculating flux density, indicating that multiple competing views remain without consensus.
There are unresolved aspects regarding the assumptions made in applying Planck's law to the CMB and the implications of isotropy on the calculations. The discussion also highlights the dependence on measurement methods for determining the area involved in calculations.
Well, the CMB is almost a perfect black body as T=2.725K. So you can compute it directly from the black body spectrum (Planck's Law):Nabeshin said:Does anyone know where I can find numbers (or how to derive) the CMB flux density (W/m^2)? I'm only really interested in our present cosmological time, so a solution may assume the CMB to be at a constant temperature.
Also the energy absorbed per unit surface area. This works for the CMB because it's isotropic (as opposed to the light from a star which only comes from a small area of the sky). So you don't multiply that value by any area to get the flux density of the CMB.Nabeshin said:First, using Planck's law would give the energy radiated per unit surface area at that temperature,
Yup. Just bear in mind that "A" there would be dependent upon how you are doing the measurement, and would typically be the area of the beam of the detector on a telescope.Nabeshin said:So just [tex]A \sigma T^4[/tex] should work for total power absorbed, then? Interesting that it should turn out to be so simple!