<human>: What is the most famous physics question?
<bot>: That would be “How do you know that the universe is expanding?” This is a question about the Big Bang theory, which is a scientific theory that explains how the universe began. The Big Bang theory states that the universe began as a hot, dense, and rapidly expanding state. This theory is supported by many observations of the universe, such as the cosmic microwave background radiation and the Hubble expansion.