The universe began in the Big Bang 13.8 billion years ago. Since then, the universe has been expanding, yet what if that expansion were to stop? A theoretical model for how the universe will end assumes just that: eventually, the expansion of space will halt, and the universe will begin to contract. This model is known as the Big Crunch.

　　

Gravity And The Expansion Of Space

Most galaxies in the universe are moving away from us as the universe expands. Image credit: NASA/ESA How exactly does an entire universe just stop expanding? It has to do with the force of gravity, of course. The rate at which the universe expands depends heavily on the distribution of matter in the cosmos. All matter exerts a gravitational force, so the closer things are together, the stronger they pull on one another. Theoretically, the universe's gravity would eventually slow the expansion rate. The universe would come to a stop and start moving in reverse. The universe begins to contract, with everything in the universe moving closer and closer together. Over time this process would become exponential as objects with gravity move closer together. After many billions of years, the entire universe would collapse in on itself in a process that is precisely opposite to the Big Bang. The term Big Crunch comes from the fact that the whole universe is being crunched into a single point. 

　　An interesting result of the Big Crunch would be the existence of a cyclic universe. When the universe fully contracts into a single point, everything would begin again as another Big Bang. In this model, the universe itself never had a proper beginning, and there have simply been an infinite number of expansions and contractions. For all we know, our universe has existed an endless number of times, and time itself may be a circle rather than a straight line. 

　　

Will The Universe End In A Big Crunch?

For much of the 20th century, the Big Crunch was a reasonably popular model for how the universe will end. However, scientists were unaware at the time that the universe is composed mainly of dark energy. Dark energy is a mysterious form of energy that permeates all of space, and it is the force that causes the universe to expand. Before the late 1990s, astronomers assumed that the combined gravity of the dark matter and the visible matter would eventually slow the universe's expansion, eventually leading to a Big Crunch. However, astronomers discovered that, rather than slowing down, the universe is actually accelerating. The sheer amount of dark energy in the universe has overcome the force of gravity, and so space is expanding at an accelerated rate. This makes the Big Crunch model seem unlikely, yet it cannot be discarded just yet as the nature of dark energy remains one of the biggest mysteries in physics.