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A clown walks on stage. Maybe he pulls a cowboy pistol from a holster. Maybe he pops a balloon. Either way, the punch line is unavoidable: His pants fall down. Children roar with laughter, but this is far more than a mere vaudevillian sight gag. This is one of the four fundamental forces of nature in action.
Yes, it's gravity -- a force so constant and ubiquitous that we rarely notice it. Yet without gravity, the universe as we know it could not exist. As such, gravity plays a starring role in the theory of the big bang, the immense expansion event from which the universe's billions of galaxies herald.
According to Sir Isaac Newton's law of universal gravitation, gravity is an attractive force that acts on every particle of matter in the universe. The strength of the attraction depends on distance and mass however. If they're close enough, two particles of cosmic dust will gravitate toward each other. Meanwhile, the gravitational force of a planet will pull on objects much farther away.
In the early 20th century, physicist Albert Einstein built on Newton's findings with his general theory of relativity, which, among other things, explained gravity not as a force but as a distortion in the shape of space-time. A particularly massive object like a star warps both the time and space around it. Time itself passes measurably slower in close proximity to such an object and curves the otherwise straight path of speeding light waves. Gravity dictates the structure of the universe, from the way cosmic bodies form to the way they orbit more massive planets or stars.
Einstein also proposed that the universe began as a singularity, a point with zero volume and infinite density containing all the matter of the universe. Then the big bang occurred, rapidly expanding all that matter with enough ferocity to overpower the inward pull of gravity. Einstein also predicted that we'd be able to tell gravity was present during those early moments, thanks to gravitational waves (or changes in a gravitational field). All the resulting gas and dust eventually formed into the universe we know today due to gravity as well.
Gravity is one of the four forces of nature, along with electromagnetism, strong force and weak force. All of these forces are tied up in the big bang theory. Furthermore, Einstein's groundbreaking theories about the nature of gravity were central to the understanding of the universe he presented with general relativity.
So remember: Gravity isn't just the force that makes a clown's pants fall down. It's a key aspect of the universe, all the way back to the big bang.