Big Bang (noun, “Big Bang”)
This theory explains how our universe began. About 14 billion years ago, all the matter that makes up the universe was squashed into an incredibly small space. Because the matter was so condensed, it wasn’t in any form we would recognize today. No atoms, or even particles. Suddenly, though, that matter went through a rapid inflation — an explosion, in a way. That’s the Big Bang. The result was a super-hot mass of matter, including light and charged particles such as protons and electrons. The matter cooled slowly over billions of years. As it cooled, it formed elements such as hydrogen. The matter also began to clump together into stars and planets. At the same time, the universe kept on cooling and expanding. In fact, the universe is still cooling and expanding today.
No one, of course, was around at the time of the Big Bang. And so scientists are always looking for more evidence that the Big Bang really occurred. For example, the Big Bang’s huge explosion should have sent ripples through space and time — gravitational waves. Using laser beams traveling down extremely long tubes, scientists have previously detected gravitational waves from two black holes colliding. In 2014, scientists thought they had detected gravitational waves from the Big Bang itself. Unfortunately, they were wrong. But don’t worry, they’re still looking.
In a sentence
The best evidence for the Big Bang is the remnants of the light — called cosmic background radiation — released from the original explosion.