The cosmic microwave background (CMB) radiation is indeed a remnant of the early universe and is commonly regarded as strong evidence for the Big Bang theory. However, it is important to clarify a few points regarding its temperature and its implications for the cooling of the universe.
The CMB radiation has a temperature of approximately 2.7 Kelvin, or -455 degrees Fahrenheit. This temperature corresponds to the average energy of the photons in the microwave background. While this temperature is low, it does not mean that the universe cannot get colder.
The CMB radiation represents the thermal equilibrium state of the universe at the time when photons decoupled from matter, about 380,000 years after the Big Bang. Since then, the universe has continued to expand and cool. In fact, the expansion of the universe causes the wavelengths of the CMB photons to stretch, leading to a redshift. This redshift is one of the key pieces of evidence supporting the Big Bang theory.
The cooling of the universe is an ongoing process due to the expansion, and it has not yet reached its lowest possible temperature. As the universe expands, it continues to cool, and if the current understanding of the expansion remains valid, the universe will become colder over time. However, it's worth noting that the universe's cooling process is intricately linked to other factors, such as dark energy, which plays a significant role in the expansion dynamics.
So, while the CMB radiation provides evidence for the Big Bang and the early thermal state of the universe, it does not signify that the universe has reached its coldest possible state. The cooling process is ongoing, and the future evolution of the universe's temperature depends on various cosmological factors.