Final answer:
The early radiation from the universe, now observed as the cosmic microwave background, has been redshifted into the microwave region due to the universe's expansion, with a characteristic temperature of 2.725 K.
Step-by-step explanation:
The radiation from the early history of the universe has been stretched by the expansion of the universe until today it is in the form of cosmic microwave background radiation (CMB). This is well illustrated by the evolution of the universe, where approximately 380,000 years after the Big Bang, the universe became transparent to electromagnetic radiation. The radiation emitted at that time has been stretched or 'redshifted' due to the expansion of the universe, and now falls within the microwave portion of the electromagnetic spectrum, which includes radio waves, microwaves, infrared waves, visible light, ultraviolet radiation, X-rays, and gamma rays. This CMB radiation has a characteristic temperature of 2.725 K and is observed in any direction in space not obscured by other sources.