When we observe the cosmic background, commonly known as the cosmic microwave background (CMB), we do not see a black sky. In fact, the CMB appears as a uniform glow of microwave radiation coming from all directions in the universe. This radiation is a remnant of the early stages of the universe, specifically from a time about 380,000 years after the Big Bang.
The reason we perceive the CMB as a uniform glow rather than a black sky is due to the nature of the radiation and the expansion of the universe. The CMB radiation is isotropic, meaning it comes from all directions equally. As the universe has been expanding since the Big Bang, the wavelengths of this radiation have stretched over time, shifting towards longer wavelengths, specifically into the microwave region of the electromagnetic spectrum. This stretching phenomenon is known as cosmological redshift.
The redshifted CMB radiation fills the entire sky, and its intensity is nearly the same in all directions. The uniformity of the CMB is one of the key pieces of evidence supporting the Big Bang theory. If the CMB were not isotropic or uniform, it would indicate significant deviations in the early universe's density distribution, which would contradict our current understanding of cosmology.
So, when we observe the CMB, we see a uniform glow rather than a black sky because the radiation is isotropic and fills the entire observable universe.