Regarding the topic of my OP: Why Planck's formula for black body radiation is used to measure the CBR, the answer lies in another thread that nobody took care about:
What Do You Think About That A 1879 Stefan's Law Be Used To Calculate Star Radius Or Temperature? (Apr 30 2019 01:11 AM)
Both threads are related because 1879 Stefan's Law can be derived by integrating 1901 Planck's Law over the entire frequency spectrum and over the entire space (by using Planck's formula as expressing the spectral flux density at the aperture in a black body cavity, and extending the integration over the entire spherical space of 4Pi radians around the aperture).
Along with my rejection to the use of Planck's spectrum, I also wanted to make evident that, in spite of all the knowledge that modern physics has developed, science still relies on XIX century thermodynamics for derivations of values in astronomy. Quantum physics and relativity, the foundations of the deranged XX and XXI century physics, have proved that nothing of value can be provided by them, in the grand scheme of things (stars, galaxies, universe).
So, we should celebrate the insight and wisdom of the XIX century scientists, in particular: Fourier, Joule, Faraday, von Helmholtz, Classius, Kelvin (Thomsom), Kirchoff, Maxwell, Mach, Weber, Hertz, Heaviside, Wien, Stefan, Lummer, Rayleigh (Strutt) and dozen of others (well known or not), which allowed mankind to reach this point of development.
Electrical energy, nuclear physics and radioactivity aside (Tesla, Fermi, Curie and the like), I'm still waiting for practical applications of relativity and quantum physics.
Please, don't bother to mention GPS, laser, maser or microelectronics, which are developments originated by applied engineering and classic physics and chemistry, starting at the end of the XIX and early XX centuries (diodes, triodes, semiconductors, etc.).
In conclusion: Planck's formula was used because astronomers had nothing else to use from "modern science". The aftermath of this is a "petitio principii" fallacy. As Planck theory was used at the beggining, after decades of following this particular use of the theory, it HAD TO be verified. The conception of the big bang theory NEEDS with dispair the verification that the CBR is planckian one. Everything else is built around this.
If Planck's spectrum fails to be verified in the CBR, the big bang theory collapses.
Easy, isn't it?
P.S.: By the way, the CBR has a value which is so low that results can easily be falsified, as its power is BELOW the NOISE of the most advanced amplifiers and antennae. Both the CBR and equipment noise are gaussian random, and no correlation exists between them, so they CAN'T be separated.
For instance: A 30°K state of the art receiving system can not detect a 3°K CBR noise. The noise is expressed in terms of its temperature, as is commonly used in radio-astronomy or sensitive microwave applications. Multiplying the noise temperature by the Boltzmann's constant and an arbitrary constant gives the energy level per unit frequency at the entrance of this particular radio-astronomy receiver. If you multiply this value by the bandwidth of the receiver gives the total power at the input of the radio system. As it applies for both noise sources, It will be easy to understand that, in this case, the SNR=-10dB (the power of the state of the art equipment noise is at least 10 times higher that the power of the CBR noise, which makes it UNDETECTABLE due to uncorrelated stochastic behavior of both noise sources).
Whoever claiming that can detect CBR with a comercial device or just by arbitrarily pointing an antenna to the sky doesn't get this.
For those interested, there is a classic paper about noise in microwave systems (R. Dicke, 1946):
"The measurement of thermal radiation at microwave frequencies"
Edited by rhertz, 03 June 2019 - 07:06 PM.