Professor of Astrophysics, Princeton University
What is the ultimate fate of the universewill it expand forever or will it eventually collapse in a Big Crunch? The answer to this fundamental question depends on two critical observations: How much matter or gravity exists in the universe? And, does the universe contain other forms of energy that affect its expansion? Answers to these questions will reveal not only the fate of our universe but will also shed light on the evolution of the universe since the Big-Bang. Combining recent observations of different sources we find evidence that we live in a surprisingly lightweight universe, with mass-density well below the critical density needed to halt the universal expansion. Furthermore, the observations suggest that the expansion rate of the universe is not slowing down as expected if only matter exists in the universe; rather, the expansion appears to be speeding up! This suggests the existence of additional energy that opposes the attractive pull of gravity. The combined observations suggest a universe that will expand forever, growing ever larger, colder, and darker.