The universe is expanding, which means it had a beginning

Hawking builds his cosmology on Edwin Hubble's 1920s observation that distant galaxies are all moving away from us, and the farther a galaxy is, the faster it recedes — a pattern consistent everywhere we look, implying the universe itself is expanding uniformly rather than us sitting at some special center.

Run that expansion backward in time, and everything converges toward a single point of infinite density roughly 13.8 billion years ago: the Big Bang. Before Hubble's discovery, most scientists, including Einstein himself, assumed the universe was static and eternal, and Einstein even added a fudge factor (the cosmological constant) to his equations specifically to cancel out the expansion his own theory predicted.

Hawking treats this as a case study in letting observation correct even the most brilliant theorist's assumptions — Einstein later called the constant his "biggest blunder," though Hawking notes modern cosmology has revived a version of it. Takeaway: a theory's elegance doesn't excuse ignoring what the actual data shows.