Answer:
Both the Big Bang and Steady State theories accepted that the universe is expanding because there was observational evidence supporting this idea. In the 1920s, the American astronomer Edwin Hubble observed that distant galaxies were moving away from us, and the farther away they were, the faster they appeared to be moving. This observation suggested that the universe was not static but was expanding.
Supporters of the Big Bang theory interpreted this expansion as evidence that the universe had a beginning and that it started as a small, hot, dense state that has been expanding and cooling ever since. Supporters of the Steady State theory, on the other hand, proposed that new matter was continuously being created in the expanding universe so that the overall density of the universe remained constant over time. However, they still accepted that the universe was expanding based on the observational evidence available at the time.
Therefore, despite their different interpretations of the implications of the expansion, both theories accepted the idea that the universe is indeed expanding based on the available observational evidence.