Final answer:
The CMBR is thermal radiation that dates back to the Big Bang when the universe was hot and bright; as it expanded, radiation cooled and shifted into microwaves, observable as the cool 2.725 K blackbody radiation from any direction in space.
Step-by-step explanation:
The Cosmic Microwave Background Radiation (CMBR) is a thermal radiation that was left behind after the Big Bang and its intensity follows a blackbody radiation curve. The best description of the CMBR is that after the Big Bang, the universe was incredibly hot and bright, but as it expanded, the light stretched out and cooled, shifting into microwave wavelengths. Thus, option A) is the correct choice as it states: 'After the big bang, the universe was bright, but as the universe expands, the light wave stretches out into a microwave.'
Initially, these photons had enough energy to produce particles upon colliding with each other. As the universe expanded and cooled, protons and neutrons formed first, followed by other atomic particles. Eventually, the universe cooled to the point where electrons could combine with protons to form neutral hydrogen atoms, at which time it became transparent to radiation, releasing the CMBR.
The CMBR can be observed from any direction in space and is considered as the most perfect blackbody spectrum ever detected. Scientists have made detailed measurements of the CMBR, which supports that the universe is 13.8 billion years old, with 31% of its critical density composed of matter, including dark matter, and 69% consisting of dark energy.