What is the thermal radiation assumed to be left over from the "Big Bang"?

The thermal radiation left over from the "Big Bang" is known as cosmic microwave background radiation. This radiation filled the universe after the initial expansion and cooling following the Big Bang approximately 13.8 billion years ago. As the universe expanded, the intense heat and light from that early time were stretched to longer wavelengths, resulting in what we now observe as microwave radiation. This cosmic microwave background radiation is extremely uniform across the sky and serves as a critical piece of evidence supporting the Big Bang theory.

The other types of radiation mentioned, such as gamma rays, X-rays, and ultraviolet radiation, originate from different astrophysical processes and do not represent the leftover thermal radiation from the Big Bang. Gamma radiation typically comes from radioactive decay or high-energy cosmic events, while X-rays can be produced by hot gases in space or around black holes. Ultraviolet radiation is primarily generated by stars and celestial objects that emit high-energy light but is not associated with the primordial conditions of the early universe. Thus, cosmic microwave background radiation is the appropriate choice in this context.