Physics – 6.2.3 The Universe | e-Consult

The CMBR was produced during a period in the early Universe called recombination, which occurred approximately 380,000 years after the Big Bang. Initially, the Universe was a very hot, dense plasma of protons, neutrons, and electrons. Photons were constantly interacting with these charged particles, preventing them from travelling freely.

As the Universe expanded and cooled, the temperature eventually dropped to a point where electrons and protons could combine to form neutral hydrogen atoms. This process is recombination. With fewer free electrons to scatter them, photons were able to travel freely through space. This "first light" released during recombination is what we observe today as the CMBR.

The expansion of the Universe has significantly affected the wavelength and energy of the CMBR. As the Universe expanded, the wavelengths of the photons have been stretched. This phenomenon is known as cosmological redshift. The greater the expansion, the greater the redshift.

This stretching of the wavelength corresponds to a decrease in energy. The photons that were initially very high-energy have been cooled to the microwave region of the electromagnetic spectrum. The current temperature of the CMBR is approximately 2.7 Kelvin, which is a direct result of this expansion and redshift. The observed spectrum of the CMBR is a nearly perfect blackbody spectrum, consistent with the prediction of a uniformly expanding Universe.