The cosmic egg - that tiny, dense mass from which the universe exploded into existence - is an uncrackable puzzle. It is a riddle which now looks definitely like it has no answers, and it is driving astronomers politely bananas.

In the past few decades this puzzle of the origin of the cosmic egg has bumped astronomers unexpectedly, and a little irritably, straight into the problem of God.

So, it was no small coincidence yesterday that Robert Jastrow, physicist, astronomer and author of "Until the Sun Dies" (a survey of cosmic evolution), spoke on "God and the Astronomers" at the convention of the American Association for the Advancement of Science.

Just before Jastrow came down to the Sheraton Park Hotel to give the talk, he mentioned the topic of the lecture to a friend in New York. His friend quickly reminded him that "it is well known among scientists that when a physicist or astronomer begins to write about God, he's over the hill or is going bonkers."

Jastrow came anyway. "I would like to say at the start that I am a complete agnostic on religious matters," he told the crowd of 800, as he stood before them in a black suit, with dark tie, short-cropped hair and a facial expression which suggested fervent detachment. The lean, bouncy man in his 50s looked the part of a bright young cleric.

The problem, as Jastrow sees it, is the strong evidence that the universe is "open." That means it started at one point, at one time, and that all the pieces from the original bang will continue to race apart from one another forever.

The universe will eventually dissipate all its energy and die a cold, lonely death, in fragments.

Scientists had expected, and hoped, that the universe would stop expanding. They hoped it would finally be shown to have settled into a "steady state," or to be permanently oscillating between exploding and collapsing back agin.

"The galaxies were all in one place about 20 billion years ago," Jastrow said. "At that moment, the universe was packed into an infinitely dense and hot state. The universe exploded outward - and formed stars, planets, and then life."

All the recent evidence points to this scenario, but scientists are unhappy with it. It presents them with the problem of having to account for The Creation: Where did the dense egg of mass come from? And what was it doing while it was waiting to be scrambled?

If there was really a beginning, a first event without a cause, then suddenly scientists are forced to take up just the positions the theologians have held for centuries - that there was a First Cause, and It or He made the universe from nothing.

The reaction of scientific minds to this situation, Jastrow said, has been strange. "It turns out that the man of science reacts as the rest of us do when our beliefs conflict with the evidence. We become irritated. We pretend the conflict does not exist. We paper it over with meaningless phrases."

Take Einstein, for example, Einstein first made some errors in his algebra in his General Theory of Relativity. His error eliminated the chance of a Creation. When he was caught, he first got angry, denied the error, and then finally admitted it.

Physicist Arthur Eddington wrote in a beautiful self-contradiction, "I have no philosophical axe to grind, but the notion to a Begining is repugnant to me. The expanding universe is preposterous, incredible. It leaves me cold."

Allen Sandage, whose work has made the Big Bang theory more firm, has said that the universe with one beginning "is such a strange conclusion. It cannot really be true."

Jastrow: "Something was bothering these people . . . there is a ring of feeling and emotion in these reactions. They come from the heart, whereas, you would expect judgments to come from the brain."

Jastrow himself is not bothered by the paradox. His faith in science is on another plane entirely.

After an early career in a concentrated area of study in physics, he left it at age 30 for the space program. He was enthralled with the idea of space travel, and even more, of an escape from his narrow discipline.

He has kept a giddy child-like sense of enthusiasm ever since.

If his faith in science is as fervent as a religious faith, then his version of prayer is probably mathematics. "I have loved doing mathematics since I was a student. It is an extraordinary pleasure. It is as strong a pleasure as the physical things I do, like handball or like gardening at my New Hampshire farm."

There is perhaps something alike in the arc of a well-placed handball shot and the spare, functional lines of a neat equations.

Jastrow feels more warmly toward religion than many of his colleagues, more warmly than the classic scientific view of Einstein, that God is nothing more than the grand machinery of the universe taken together.

"That's a little too cold for me," Jastrow said. "My feeling lies somewhere between that and faith. He finds himself siding up to religion, and does not reject the idea that there are forces in the cosmos outside the reach of science.

'I keep coming close to the edge of faith, but I never quite make it over."

He said that physicists and astronomers are reacting today just as the great theologian Saint Augustine did when he was lecturing on cosmology and was embarrassed by an unanswereable question. Someone asked him what the Lord was doing before he created the heaven and the earth. Augustine replied, "He was making hell for people who ask questions like that."

It is as if scientists, after scaling Himalayan masses of ignorance, tangled information and plain hard work, are about to conquer the final peak - a complete and clear picture of the cosmos, from Creation, to life, to the ultimate end.

But as scientists pull themselves over the final rock, they are greeted by a band of grinning theologians who have been sitting there for centuries, legs crossed, talking to themselves about The First Cause.

The scientists' evidence now supports just what the theologians have been saying all along - there was a First Cause, a force from which everything came.

Now, Jastrow said, the astronomers are halted. They cannot find out if their was a First Cause, because they would have to trace a cause-and - effect chain to the cosmic egg and before. But, of course, all the evidence was destroyed in the Big Bang.

"Science cannot bear the thought," Jastrow said, "that there is an important natural phenomenon which it cannot hope to explain even with unlimited time and money. Scientists hold that every effect has its cause.

"But every item of evidence that might have given a clue to the cause of the great explosion was destroyed in the holocaust. This is a very strange development."

Jastrow added that the barrier to further progress on the cosmic egg "seems insurmountable. It is not a matter of another year, another decade of work, another measurement, another theory. It seems that science will never be able to raise the curtain . . ."

This state of affairs violates the sense of order which is at the bottom of scientists' faith. "The sense of cause is violated by a Beginning . . . known laws of physics cannot be valid, under circumstances which we cannot now discover . . ."

"When something like that happens, the scientist has really lost control," Jastrow said.

After his talk, Jastrow rushed from the ballroom to catch a taxi to the airport. By the time he opened the taxi door, he had 17 minutes until his flight. He dove in, head first. Then with a jerk, he pulled is his umbrella and briefcase.

During the ride, Jastrow speculated. He rode with his head cocked back a little, with a smile appearing and disappearing as he looked into the distance ahead.

Science's reaction to the cosmic egg is important, he said. "It suggests we have peeled back all the layers to a certain point. Now we come to another layer. That means a revolution of thought . . ."

The resolution of the problem of the universe, he said may not come from scientists on earth. "I really believe, even though this sounds like a gag, that the answer will come from outside the earth.

"You know, for about the last 13 years, we have been broadcasting out into space at the million-watt level. "I Love Lucy,' things like that. We are the hottest broadcasting source in our in our end of the cosmos, and our messages have gone put past 50 other suns.

"I think." Jastrow smiled, "we can expect to get an answer back in about 14 years."