Big Bang

The Big Bang is the name scientists give to the event that started our universe. It happened about 13.8 billion years ago. At that moment, everything in the universe was packed into a tiny, hot, dense point. Then it began to expand, and it has been expanding ever since.

The name is a little misleading. The Big Bang was not an explosion in empty space. It was the sudden stretching of space itself. Before the Big Bang, there was no "outside." There was no space to explode into. Scientists are careful about this. They will tell you the Big Bang is a story about how the universe grew, not about how it was lit on fire.

The first moments happened unbelievably fast. In less than a second, the universe grew from smaller than an atom to bigger than a galaxy. It was also incredibly hot, trillions of degrees. No atoms could hold together in that heat. There were only tiny particles zipping around.

As the universe spread out, it cooled down. After about 380,000 years, it was cool enough for the first atoms to form. Mostly hydrogen and helium. The universe became clear instead of glowing. Much later, gravity pulled clouds of gas together into the first stars and galaxies. Our own Sun and Earth formed much later still, about 4.6 billion years ago.

How do we know any of this happened? Two main clues. First, almost every galaxy in the sky is moving away from us. The farther away a galaxy is, the faster it is moving. That means the universe is getting bigger, so it used to be smaller. Second, there is a faint glow of warm radiation coming from every direction in space. It is the leftover heat from when the universe was young. Scientists call it the cosmic microwave background. It matches exactly what the Big Bang theory predicted.

Still, many big questions remain open. What caused the Big Bang? What was there before it, if "before" even makes sense? Why did the universe expand so fast in its first second? Scientists have ideas, but no clear answers. Some physicists wonder if our universe is one of many. Others think our rules of time and space just stop working at the very beginning.

The Big Bang is how everything you can see got started. Every star, every planet, every atom in your body began as part of that first hot, tiny universe.