- The Big Bang Theory is that the observable universe began as an expanding point, approximately fifteen billion years ago. Since then, the universe has continued to expand, gradually increasing the distance between our Galaxy and external galaxies. We observe this effect. Will this expansion continue forever, causing the universe to become ever colder and darker? Or will the expansion eventually halt and, perhaps, reverse leading to a "Big Crunch"? What came before the initial expanding point?

- The gravitational pull of matter in the universe slows the expansion. If the matter is dense enough, the expansion will eventually reverse - gravity will win - and the universe will recollapse. If it is not, the expansion will continue forever. The density of matter in the universe will determine the ultimate fate of the universe. The minimum density required to halt the expansion is called the "critical density". What is the density of matter in the universe? We don't know the density of the universe yet, but we know what observations and what space missions we need to find out.

- The density of the universe also determines its shape. If the density of the universe exceeds the critical density, space is finite and curved like the surface of a sphere. This implies that light rays could traverse the circumference of the universe and return to their point of origin. If the density of the universe is less than the critical density, space is infinite and curved like the surface of a saddle. If the density of the universe exactly equals the critical density, the universe is infinite and flat like a sheet of paper.

- The simplest version of the "inflationary theory", an extension of the Big Bang theory that draws on developments in modern particle physics, predicts that there was a very brief period of extreme expansion shortly after the Big Bang. If true, this exceptional growth caused the universe to become so large that any original curvature would be effectively unobservable in the region of the universe we can see from Earth. (This is equivalent to the fact that our small local view of Earth appears flat to us even though the Earth is really round.) Thus inflation predicts that the shape of the "local" universe is flat, like a sheet of paper.

- New space missions can answer the fundamental questions about the shape, origin, and destiny of the universe. Following up on the spectacular results from the COBE mission, new windows on the cosmic microwave background (the afterglow radiation from the Big Bang) are needed.

**Missions:** COBRAS,
DIMES, MAP, SAMBA