The cosmic microwave background (CMB) radiation has not dissipated or run out of steam because it is the residual radiation from the early universe, specifically from a time known as the "recombination era." The CMB is the afterglow of the Big Bang, which occurred approximately 13.8 billion years ago. It is the oldest light we can observe in the universe.
When the universe was about 380,000 years old, it had expanded and cooled enough for neutral atoms to form. Before this time, the universe was filled with a hot, dense plasma of charged particles that strongly scattered and absorbed light. Photons, the particles of light, could not travel freely through this plasma. However, during the recombination era, the universe had cooled sufficiently for electrons and protons to combine and form neutral hydrogen atoms. This process allowed photons to decouple from matter and travel freely through space, creating the CMB.
The CMB radiation permeates the entire universe and is observed in all directions. It is incredibly uniform, with slight temperature fluctuations that provide valuable information about the early universe's conditions. The CMB has redshifted over time due to the expansion of the universe. As the universe expands, the wavelength of the CMB photons also stretches, causing them to shift towards longer, lower-energy wavelengths. This is known as cosmological redshift.
The redshift of the CMB does not mean that the radiation has dissipated or lost energy. Rather, it is a consequence of the expanding universe and the stretching of the photons' wavelengths as space itself expands. The CMB is still present today, and it is detected as a faint microwave radiation with a nearly uniform temperature of about 2.7 Kelvin (-270.45 degrees Celsius or -454.81 degrees Fahrenheit). The redshifted CMB provides valuable evidence for the Big Bang theory and has been studied extensively to understand the universe's early evolution and structure formation.
In summary, the CMB has not dissipated or run out of steam because it represents the radiation from the early universe that has traveled across vast distances to reach us. Its redshift is a consequence of the expansion of the universe, and it continues to exist as a remnant of the Big Bang.