Imagine you are watching a movie of the universe's history. You see galaxies drifting apart, stars forming, and space stretching. Now, hit the rewind button. Everything starts rushing back together. The universe gets smaller, hotter, and more crowded. If you keep your finger on that rewind button long enough, you eventually hit a wall: Absolute t = 0. But what was actually that moment, or before that moment?
The traditional Big Bang theory says that if you go back far enough, the entire universe was squeezed into a point of infinite heat and density called a singularity. Physicists often say that asking "What happened before the Big Bang?" is like asking "What is north of the North Pole?" At the North Pole, every direction you look is South. Similarly, at t = 0, "before" might not even exist because time itself hadn't started yet.
Stephen Hawking had a different idea. He suggested that as you get closer to t = 0, time stops acting like a straight line and starts acting like space. Think of the universe like a bowl. If you walk toward the bottom of the bowl, you don't hit a sharp "point" or a "crack." It's just a smooth, rounded surface. In this theory, the universe is finite, but it doesn't have a jagged starting line. It just... is.
Some scientists don't like the idea of time starting from nothing. They suggest the Big Bounce. In this version, our Big Bang wasn't the start. Instead, a previous universe collapsed under its own gravity (a Big Crunch), reached a maximum density, and then "bounced" back out—like a compressed spring suddenly releasing. In this world, t = 0 was just a very narrow hallway between two giant rooms.
Another theory suggests we are living in a "Multiverse." Imagine a giant, ever-expanding foam of bubbles. Now and then, a new bubble forms. Our Big Bang was just the moment our specific bubble started growing. In this case, time has been "ticking" forever in the background; we just joined the party at t = 0.
We don't actually know. Every theory we have eventually hits a "math wall." Let's not go there. Let's just say that our theory of the "Big" (General Relativity) says the universe started as a point, and our theory of the "Small" (Quantum Mechanics) says nothing can be infinitely small.When we try to combine them at t = 0, the equations break. We are like explorers looking at a map that ends at the edge of the ocean. We can see the water, we can guess what’s across it, but until we build a better boat (a theory), Absolute t = 0 remains the universe's greatest mystery.