Final answer:
The incorrect statement is that the cosmic background radiation is a mixture of radiation from many sources; it actually is the afterglow of the big bang, discovered by Penzias and Wilson, and has a temperature of about 2.725 K and is isotropic.
Step-by-step explanation:
The statement about the cosmic background radiation (CMB) that is not true is the first one: it is the result of a mixture of radiation from many independent sources, such as stars and galaxies that formed within the first billion years of the big bang. The CMB is actually the afterglow of the hot, early universe and not a mixture of radiation from independent sources. It was emitted at the time of photon decoupling, a few hundred thousand years after the Big Bang, when the universe had cooled sufficiently for electrons and protons to combine into hydrogen atoms, allowing radiation to travel freely.
The CMB indeed had a much higher temperature in the past and currently has a temperature of approximately 2.725 K, slightly above absolute zero, which confirms the hypothesis of the expanding and cooling universe. Additionally, the CMB is remarkably isotropic, meaning it is nearly the same in all directions. This isotropy, combined with tiny fluctuations, provides insights into the early universe's structure and supports the Big Bang model. The CMB's blackbody spectrum was discovered by Arno Penzias and Robert Wilson in the mid-1960s, a finding that would lead to a Nobel Prize in Physics for their work.