Gravity vs. Expansion
To determine if the universe will expand forever, coast to a stop or collapse on itself, astronomers must decide which of two opposing forces will win a cosmic tug-of-war. One of these forces is the bang part of the big bang -- the explosion that catapulted the universe outward in all directions. The other force is gravity, the pull one object exerts on another. If the gravity within the universe is strong enough, it could reign in the expansion and cause the universe to contract. If not, the universe will continue to expand forever.
Although astronomers know the universe is expanding, they can't precisely gauge the force responsible for the expansion. Instead, they try to measure the density of the universe. The higher the density, the greater the gravitational force. Applying this logic, there must be a density threshold -- a critical limit -- that will determine if the gravity within the universe is strong enough to halt the expansion and reel everything back in. If the density is greater than the critical limit, then the universe will stop expanding and start contracting. If it's less than the critical limit, then the universe will expand forever. Astronomers represent this mathematically with the following equation:
Ω = actual average density/critical density
If omega (Ω) is greater than 1, then the universe will be closed. If it's less than 1, the universe will be open. And if it's equal to 1, the universe will be flat. Based on the matter we can see, such as galaxies, stars and planets, the density of the universe seems to be below the critical value. This would suggest an open universe that will expand forever. But cosmologists think there is another type of matter that can't be seen. This dark matter may account for much more of the universe than ordinary, visible matter and may have enough gravity to stop, and then reverse, the expansion.
Recently, astronomers have made some observations that indicate there's another invisible material in the cosmos: dark energy. Could dark energy profoundly affect the universe's fate?