In 1998, astronomers studying the brightness of light emitted by dying stars called supernovae discovered that the stars had moved significantly and were farther away from our galaxy than had ever been observed before. This led scientists to conclude that the universe's expansion is accelerating at a faster rate than they had previously thought [source: Kirshner].
While most astronomers and scientists agree that the universe is definitely expanding, no one is quite sure exactly what will happen in the future. However, they do believe that there are only three possibilities, which are a closed universe, a flat universe or an open universe. But what exactly is an open universe?
Some think of an open universe as saddle-shaped, curving up on one side and down on the other, never closing in on itself [source: Wong]. As the universe expands, the matter within it will begin to thin out, and galaxies will continue to separate from each other over time. There could come a time when it reaches the state known as an open universe, which would keep expanding forever [source: Sloan Digital Sky Survey].
Of course, if the universe keeps expanding over an infinite period of time, there will not be sufficient materials to create new stars, and the existing stars will be extinguished. In fact, when the universe expands far enough, everything inside it will be cold and lifeless. Don't worry, though -- when it comes to studying cosmology, no one can be certain of what will happen billions of years from now.
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