In his book On Time: The Physics That Makes the Universe Tick, physicist Jim Al-Khalili explores the idea that time might not have had a beginning with the Big Bang. One of the theories he discusses is eternal inflation, which challenges the conventional view that the Big Bang marked the start of time. According to this theory, our universe could be just one of many "bubbles" in an infinitely expanding space that has been inflating forever. In this scenario, the Big Bang is not the start of time, but rather an event that occurred at a particular moment in an already existing infinite space, where inflation suddenly stopped and expansion began more slowly. Eternal inflation suggests that our universe is part of a larger multiverse, where other universes may have their own Big Bangs and are separated from each other by the rapidly expanding space between them. However, some scientists argue that this and similar theories are not yet testable, placing them outside the realm of traditional science. Other alternatives to the Big Bang as the beginning of time include Roger Penrose's conformal cyclic cosmology, which proposes that our universe goes through an endless cycle of expansions and contractions, and the ekpyrotic universe model, inspired by ancient Greek philosophy and modern string theory. The ekpyrotic model suggests that our universe is a three-dimensional "brane" floating in a higher-dimensional space. A collision with another brane would create the Big Bang, leading to the expansion of our universe. This model also implies that time may not have had a beginning, as the universe could be part of an ongoing cycle of collisions. Another idea, the "mirror universe" theory proposed by Neil Turok, suggests that a universe exists on the other side of the Big Bang, with time flowing in the opposite direction. This could explain the imbalance between matter and antimatter in our universe, as the mirror universe would have the reverse. These theories are fascinating, but without observational evidence—such as a signature of events before the Big Bang in the cosmic microwave background—they remain speculative. Scientists continue to search for ways to test these ideas, but for now, they remain in the realm of theoretical physics.