Which theory posits that the universe does not change and has no beginning?

The steady-state theory is a cosmological model that asserts the universe is eternal and unchanging on a large scale. According to this theory, rather than originating from a singular event or having a beginning, the universe has always existed in a constant state. The steady-state theory suggests that as the universe expands, new matter is continuously created to fill the gaps left by the expanding universe, resulting in a constant density over time.

This idea contrasts sharply with models like the Big Bang theory, which posits a distinct beginning of the universe approximately 13.8 billion years ago, alongside the notion of an evolving cosmos. Additionally, the recurrent universe theory supports the idea of cyclic models where the universe goes through an infinite series of expansions and contractions, indicating change and beginnings for each cycle.

In summary, the steady-state theory is unique in its assertion that the universe remains static over time, with no defined beginning or end, making it the correct answer to the question.