The anthropic principle states that intelligent life that asks the question “Why are we here?” can only exist in universes compatible with complex life forms. Universes with different rules of physics might exist, but no life can arise within them, and they therefore can’t be observed. The strong anthropic principle declares that the only universes possible are those that can sustain life; the weak anthropic principle suggests that, of many possible universes, only those conducive to life are where human-like minds can exist. Hawking had to consider the anthropic principles when he worked on theories about the origin of the universe. His ideas needed to generate livable, observable universes, or they’d become moot. The Anthropic Principle also serves as an example of how science approaches philosophical or existential questions.

The Big Bang theory states that the universe started as an infinitely dense point, or singularity, which contained all the matter and energy of our universe and then rapidly expanded outward. The Big Bang expansion went through several stages, as matter and energy ballooned outward; these stages are better understood because of Hawking’s contributions.

The term “big bang” came from British astronomer Fred Hoyle, a believer in the Steady State theory of the universe who disagreed with the singularity theory: In a BBC interview in 1949, he dismissively referred to the proposed singularity expansion as a “big bang,” and the term stuck.