"My offer still stands," said Keith.

Jag bowed his head. "I — we — shall consider it." And then Jag did something Keith had never seen any Waldahud do before. He added the words, "Thank you."

It was evening; the corridor lighting was dimmed, As he always did just prior to dinner, Keith dropped by the bridge, and had a word with the gamma-shift director, a Waldahud named Stelt. Everything was running smoothly, Stelt said.

Not a surprise; Keith would have been called at once had something been amiss. Keith wished everyone a pleasant night and left the bridge, heading toward the central shaft.

Lianne Karendaughter was there, sitting on a bench in the widened part of the corridor just before the elevators. She looked lithe and sexy in a skintight black jumpsuit.

Surely a coincidence, thought Keith. Surely she didn't know his murine — know that he passed this way every evening at this time. She must be waifng for somebody else. Lianne had her hair down; Keith had never realized that it went halfway down her back. "Hello, Keith," she said, smiling warmly.

"Hello, Lianne. Did — did you have a good day?"

"Oh, yes. I mean, you saw alpha shift today — a breeze.

And I got to do some swimming and fencing during beta shift. How about you?"

"Fine. Just fine."

"That's good," said Lianne. She paused for a moment, and looked down at the rubberized flooring. When she lifted her head again, she didn't quite meet Keith's eyes. "I, ah, understand Rissa is away today."

"That's right. She's taken a pod back to Grand Central. I think she's trying to find a way not to have to accept a medal, or have a parade in her honor."

Lianne nodded. "So I was thinking," she said, after a moment, "that perhaps you'd be all alone for dinner."

Keith felt his pulse quickening. "I — I suppose I am," he said.

Lianne smiled at him. She had perfect white teeth, perfect alabaster skin, and the most beautiful dark, haunting, almond-shaped eyes. "I wondered if you'd like to join me. I've got a wok in my apartment; I could make that stir-fry I promised you."

Keith looked at… at the girl, he thought. Twenty-seven.

Two decades younger than himself. He felt a slight shifting in his shorts. It was probably just an innocent invitation. She felt sorry for the old guy, or maybe was trying to ingratiate herself with the boss. Just some stir-fry, mayhe some wine, maybe…

"You know, Lianne," said Keith, "you are a very beautiful woman. He held up a hand. "I know, I'm not supposed to say things like that, but we're both off duty. You're a very beautiful woman." She lowered her eyes: He paused and chewed on his bottom lip. And a thought welled up in his brain.

Don't hurt Rissa.

You'll only hurt yourself.

"But," he said at last, "I think it's better if I just admire you from afar."

She met his eyes for a moment, then dropped hers again.

"Rissa is a very lucky woman," Lianne said.

"No," said Keith, "I'm a very lucky man. See you tomorrow, Lianne."

She nodded. "Good night, Keith."

He went home, made himself a sandwich, read a few chapters in an old Robertson Davies novel, then went to bed early.

And slept like a log, absolutely at peace with himself.

Alpha shift the next day started uneventfully. Rhombus had arrived precisely on time, of course; Thor came in, put his feet up on the helm console, and started dictating instructions into the navigational computer; Lianne was hard at work briefing little holographic heads of her engineers on the day's proposed work. In the back row, Keith was talking quietly to Rissa, who had just returned from Grand Central.

But then the starscape split, and Jag came in, moving with more of a run than a waddle.

"I've got it!" he said — although from the excited waving of his fur, perhaps "Eureka!" would have been a more appropriate translation.

Keith and Rissa turned to look at Jag. He didn't go to his workstation; instead, he moved to the front of the room, standing about two meters ahead of Thor's console.

"What have you got?" asked Keith, resisting the potential straight line.

"The answer!" barked Jag excitedly. "The answer!" He caught his breath. "Bear with me for a moment; this will take some explaining.

But I'll tell you one thing up front — we do matter! We do make a difference. Gods of the mountains, rivers, valleys, and plains — we make all the difference!" His eyes diverged, one falling on Lianne, a second on Rhombus, a third on Rissa, and the fourth on Thor and Keith, who were lined up one behind the other from Jag's point of view.

"We know now that time travel from the future into the past is possible," he said. "We've seen it happen with the fourth-generation stars, and with the time capsule Hek and Azmi built. But consider the implications of that. Suppose that at noon tomorrow, I used a time machine to send myself back in time to today. What would we have then?"

Keith said, "Well, there'd be two of you, right? The Jag from today, and the Jag from tomorrow."

"That's right. Now think about that: if you have two of me, you've doubled the mass. I mass one hundred and twenty-three kilograms, but if there were two of me here, then there'd be two hundred and forty-six kilos of ]ag-mass aboard this ship."

"But I thought that was impossible," said Rissa, "because of the law of conservation of mass and energy. Where did the extra hundred and twenty-three kilos come from?"

Jag looked triumphant. "From the future! Don't you see? Time travel is the only conceivable way to overcome that law. It's the only way to increase the total mass in the system." His fur continued to dance.

"And what about the stars from the future? As each arrives, the mass of the present-day universe is increased. After all, even fourth-generation stars are made up of preexisting recycled subatomic particles. Pushing them back in time means that those particles have essentially been duplicated, doubling their total mass." "An interesting side effect no doubt," said Rhombus. "But it still doesn't explain why the stars are being sent back."

"Oh, yes it does. The doubling of mass is not just a side effect — not at all! Rather, it's the whole point of. the operation."

"Operation?" said Keith.

"Yes! The operation to save the universe! These stars are being pushed back in time to increase the mass of the entire universe."

Keith felt his jaw dropping. "Good God."

All four of the Waldahud's eyes converged on Keith.

"Exactly!" barked Jag. "We've known for over a century that the visible matter in the universe accounts for less than ten percent of the total that must be present. The rest is neutrinos and dark matter, like our giant friends outside the ship. We now know what all the matter in the universe is, but we don't know how much there is in total. And the fate of the universe depends on how much mass it has, on whether the total is above, below, or precisely at the so-called critical density."

"Critical density?" asked Rissa.

"That's right. The universe is expanding — and has been ever since the big bang. But will that expansion go on forever?

That depends on gravity. And how much gravity there is, of course, depends on how much mass there is. If there isn't enough — if the mass of the universe is less than the critical density — gravity will never overpower the original explosion, and the universe will continue to expand forever, all the matter in it spreading out farther and farther.

Everything will grow cold and empty, with light-years separating individual atoms."

Rissa shuddered.

"I suppose," said Keith. "But what a project!"

"Indeed," said Jag. "And it might be even greater in scope than it first seems. Tell me: How old is this universe right now?"

"Fifteen billion years," Keith said. "Earth years, that is."