Subject: will the universe eventually cease to be?

According to the conservation principle, matter/energy can be neither created nor destroyed, but can and do, however, change form. I can see how something cannot be created from nothing, but do not understand why it would be impossible for something to cease to exist. The conservation principle evokes an analogy of a circle representing existence, having no definitive beginning or ending point. However, this does not seem to preclude a breaking of the circle. A break in the circle, of course, would destroy it. Can the circle be broken? Can an independent existence, e.g., matter or energy, cease to be?

Ah, astronomy and philosophy. The ending point of one is the starting point of the other...

The Universe will never cease to be. It may have had a starting point and time (the Big Bang), but like an avalanche, once set in motion it's pretty hard to stop. The Universe is popularly defined as Everything There Is, but a more accurate description might be Everything We Can See. What we see may be part of a much larger universe we cannot detect; as the Universe expands, the parts most distant expand away from us so quickly that they actually move faster than light, and as such we can never see them (this motion is not forbidden by relativity since it is the fabric of space-time itself that is expanding, and not the objects embedded in it that are actually moving). This is a gross oversimplification, by the way, but for this argument it'll do.

Still with me? The point is, the Universe will probably expand forever. Recent findings suggest that the expansion is even accelerating, though that conclusion is far from definite. In the very distant future, trillions of years from now, the Universe will have expanded so much that it may be hundreds or thousands of light years on average between atoms in it, but those atoms will still exist. So it will never cease to exist, it will sort of always fade away, never quite disappearing.

For more detail about the Universe, its expansion and other fun topics, try taking a look at Ned Wright's Cosmology Tutorial. It's a great place to start. You can also try the USENET newsgroup sci.astro FAQ, which has answers to lots of astronomy related questions.