The "scientific" Big Bang (~13.8 billion years ago) marked the birth of space, time, energy, and matter. In the earliest moments (Planck time, 10⁻⁴³ seconds), all four fundamental forces (gravity, electromagnetism, strong & weak nuclear forces) were unified.
Other theories, such as multiverse theories, assume potentially countless Big Bangs – each the beginning of its own universe.
Models such as the ekpyrotic universe or the Big Bounce theory posit that our universe expands and contracts periodically. Then each Big Bang would be the beginning of a new cycle – that is, many Big Bangs occurring sequentially in time, not simultaneously in space.
