Q. How old is the universe, and will it continue to expand or eventually contract back in, like a bungee cord?

A. Give or take a few minutes, the universe is about 10 billion years old, or 14 billion, or 18 billion, depending on who you talk to. Harvard astrophysicist Robert Kirshner says we can calculate its age by looking at bright objects in space, including galaxies and supernovae.

If we can figure out how far away they are, and how fast they're moving, we can figure out how long they took to get to where they are, when they started their journey, and how long things have been travelling since the Big Bang.

But we can't tell precisely how far away things are, because we're not sure just how bright they're supposed to be. Kenneth Brecher, professor of physics and astronomy at Boston University, explains: ``If we know, for example, that a 100-watt light bulb gives off a precise amount of light right at the bulb, we can tell how far away from it we are by measuring how much light has been lost by the time it hits us. But we don't know precisely how bright galaxies and supernovae are supposed to be.'' As to whether everything will keep spreading apart, many scientists think it will. But some disagree. ``The data for the last 100 years suggest a [a universe] that will keep expanding,'' Brecher says, but some think it will ultimately contract.''

Kirshner, to illustrate how the space between objects in space is growing, uses the analogy of a playground jungle gym:

``It's not as though everything is moving out from a single spot in the center. Imagine a big jungle gym made of living, growing bamboo. The space between every crossbar is getting bigger. Each one is moving away from all the rest. If there were kids on it, every kid would be getting farther away from all their neighbors. That's what's happening to the galaxies.''