The cosmic microwave background is radiation observed from across the sky, left from the early universe when it became transparent. Expansion stretched its wavelengths, and it is now observed mainly in the microwave region. Its near-uniform background with small variations is consistent with an early hot universe and later growth of structure. It is not human-transmitted radio.
The observed broad abundance of light elements, especially hydrogen and helium, also agrees with calculations for a hot early phase. Together with galaxy redshift patterns, these observations support the Big Bang model. A scientific theory is a well-supported explanatory framework, not a casual guess. Models still have limits and open questions, such as the detailed nature of dark matter; an unanswered question does not erase the observations they already explain.
Example 1
Which Big Bang prediction matches radiation arriving from all sky directions with a cooled thermal spectrum?
The cosmic microwave background is the relic of an early hot phase, stretched and cooled as the universe expanded.
Example 2
Name three independent evidence categories supporting the model.
Large-scale galaxy redshift–distance patterns, the cosmic microwave background, and the abundance of early light elements. They test different consequences of the model.
Watch out: In science, a theory is an explanatory framework supported and repeatedly tested by evidence.