In the beginning of Woody Allen's Annie Hall, an anxious young Alvey Singer visits a psychologist's office. When asked what's wrong, he says, "the universe is expanding." His mother replies, angrily: "What is that your business?"

Well, it is MIT professor Alan Guth's business.

Speaking that the 2009 AAAS Annual Meeting in Chicago, Guth, a theroretical physicist and cosmologist, said that the Big Bang Theory-- that the Universe began as a hot, dense mass and rapidly expanded following an explosion some 13-15 billion years ago-- has reached near scientific consensus.  

Now, physicists have begun exploring the next big question: What caused the Big Bang?

Guth said that the Big Bang Theory is a successful model for the Universe's origins because it explains what happened during the expansion, including how the Universe cooled, formed the light chemical elements, and produced stars and planets.

But it does nothing to explain what caused the explosion. In his quest to find the "Big Bang's prequel," Guth has proposed that gravity could have caused the massive blast that formed just about everything.

But this isn't the kind of gravity that made a Red Delicious hit a sleeping Isaac Newton. Termed repulsive gravity, Guth said that this force could have existed in the early universe; it would have driven the Big Bang by causing all matter to separate from itself.

"Physicists aren't supposed to speak of miracles," said Guth,"but this is a very miraculous force."

This force is still working, said Guth, leading scientists to believe that the Universe is still expanding; it's a process also known as cosmic inflation. But it's getting tired. Guth said that because repulsive gravity is unstable--similar to decay from a radioactive element--its expansion could slow.

In a talk entitled "Our Miserable Future," Lawrence Krauss, professor of physics and director of the Origins Initiative at Arizona State University, said that the universe may eventually expand so fast that objects are moving faster than the speed of light.

Krauss said that when this happens, observers on Earth would lose sight of the pillars of evidence used to support the Big Bang Theory--the closest galaxies used to trace expansion; the cosmic microwave background will disappear; and the light atomic elements, such as hydrogen and helium, will become undefinable.

Krauss clarified a common misconception about an expanding universe: While the universe as a whole is getting larger, matter inside a galaxy is not. So, while our home galaxy--the Milky Way--is getting further from her neighbors, distances between objects within the galaxy are staying more or less the same.

"At some point in the next millions of years, observers on Earth will think that we are alone in this universe," said Krauss. "Everything we used to let us know there was something else out there will be gone."

But there's hope--scientists are constantly able to see things once thought impossible. "Sometimes," Krauss said, "the observers surprise us with what they come up with."