Some theorists suggest that the Big Bang was not so much a birth as a transition, a ''quantum leap'' from some formless era of imaginary time, or from nothing at all. Still others are exploring models in which cosmic history begins with a collision with a universe from another dimension.

All this theorizing has received a further boost of sorts from recent reports of ripples in a diffuse radio glow in the sky, thought to be the remains of the Big Bang fireball itself. These ripples are consistent with a popular theory, known as inflation, that the universe briefly speeded its expansion under the influence of a violent antigravitational force, when it was only a fraction of a fraction of a nanosecond old. Those ripples thus provide a useful check on theorists' imaginations. Any theory of cosmic origins that does not explain this phenomenon, cosmologists agree, stands little chance of being right.

Fortunately or unfortunately, that still leaves room for a lot of possibilities.

''If inflation is the dynamite behind the Big Bang, we're still looking for the match,'' said Dr. Michael Turner, a cosmologist at the University of Chicago. The only thing that all the experts agree on is that no idea works -- yet. Dr. Turner likened cosmologists to jazz musicians collecting themes that sound good for a work in progress: ''You hear something and you say, oh yeah, we want that in the final piece.''

One answer to the question of what happened before the Big Bang is that it does not matter because it does not affect the state of our universe today. According to a theory known as eternal inflation, put forward by Dr. Linde in 1986, what we know as the Big Bang was only one out of many in a chain reaction of big bangs by which the universe endlessly reproduces and reinvents itself. ''Any particular part of the universe may die, and probably will die,'' Dr. Linde said, ''but the universe as a whole is immortal.''

Dr. Linde's theory is a modification of the inflation theory that was proposed in 1980 by Dr. Alan Guth, a physicist. He considered what would happen if, as the universe was cooling during its first violently hot moments, an energy field known as the Higgs field, which interacts with particles to give them their masses, was somehow, briefly, unable to release its energy.