Final answer:
The cosmic microwave background provides evidence for the Big Bang theory and helps astronomers understand the universe's expansion history. It is the thermal radiation left over from the time the universe became transparent to electromagnetic radiation, approximately 380,000 years after the initial expansion.
Step-by-step explanation:
The topic pertains to the cosmic microwave background (CMB), a crucial piece of evidence in the study of the universe expansion history. The Big Bang theory describes the origin of the universe as an incredibly energetic explosion that occurred approximately 10 to 20 billion years ago. After the initial expansion and cooling, matter began to form, including protons, deuterium, helium, and lithium.
Around 380,000 years after the Big Bang, the universe became transparent to electromagnetic radiation, which is now observed as the CMB. Instruments like COBE, WMAP, and Planck have measured the CMB, providing a blackbody spectrum that strongly supports the Big Bang model and the continued expansion of the universe as indicated by the galactic red shifts.
The existence and characteristics of the CMB not only confirm that the universe was initially very hot but also offer insights into its age, the rate of expansion, and the composition of the early universe. The understanding of this cosmic phenomenon has been enhanced by observations from both space and ground-based telescopes, providing a comprehensive view of the universe's evolution over the last 13.8 billion years.