Question

how is the \big bang\ described?
a contraction
an expansion
an explosion

Explanation:

The Big Bang theory describes the origin and evolution of the universe, stating that the universe has been expanding from an extremely hot and dense initial state. "Contraction" is the opposite of what the theory proposes. "Explosion" implies a sudden, violent release in a limited space, while the Big Bang is a continuous expansion of space itself. So the correct description is "an expansion".

Answer:

an expansion