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 Jag-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.'
Jag moved his lower shoulders. 'Actually, although that is the most commonly cited figure, no astrophysicist believes it. Fifteen is a compromise, halfway between the ages of the universe suggested by two different lines of reasoning. The universe is either as young as ten billion years, or as old as twenty. Since the mid-1990s, the accepted value of the Hubble constant — which measures the rate of expansion of the universe — has been about eighty-five kilometers per second per megaparsec. That means the universe is still flinging apart at a great rate from the original big bang — that gravity has done little to slow the expansion so far — and therefore it can't be much more than about ten billion years old. 'But spectral studies of extreme first-generation stars, especially those in globular clusters, suggest that such stars have been undergoing fusion for almost twice that length of time.
We've long assumed that one calculation or the other must be wrong.
But perhaps neither is. Perhaps what we're seeing now is merely the most recent phase of a multistage project.
Perhaps I was premature in rejecting Magnor's suggestion earlier about pushing globular clusters through shortcuts.
Perhaps such clusters, each containing tens of thousands of stars, have already been shoved back from the future. It's possible that originally this universe contained far, far less than ninety-five percent of the critical density of matter, and that the current phase of the project is just some fine-tuning.'
'But — but surely the mass doubling is only temporary,' said Lianne, 'To go back to your original example, if you traveled back from tomorrow to today, there'd be two of you today — but tomorrow, one of them would presumably disappear back into the past.'
'Perhaps so,' said Jag. 'But for the entire span between the departure point in the future and the arrival point in the present, you have doubled the mass. And if those two points were separated by ten billion years, then you've doubled the mass for a very long time indeed — long enough for its effects to put the brakes on the universe's expansion. If you calculate with great care, you don't need to permanently increase the mass of the universe. You only need to do it long enough for gravitational attraction to halt the rate of expansion of the original explosion. If you do it just right, even without a permanent increase in mass, you could end up with a universe in the far future that is indeed precisely balanced — a universe that will live forever.'
Jag paused for breath. 'It's the most massive engineering project ever undertaken,' he said. 'But it sure beats the alternative — which was to let the universe die.' He beamed at the members of the bridge staff.
'We did it. Regular-matter creatures — creatures with hands! In the end — correction, to prevent the end — the universe needed us!'
The ceremony, held in their favorite Waldahud restaurant, was short.
The audience was much bigger than their original family-only wedding in Madrid; any sort of celebration was welcomed aboard Starplex.
Thoraid Magnor had been promoted to acting director for the day so that he could perform the service. 'Do you, Gilbert Keith,' he said, 'again take Clarissa Maria, to love, honor, and cherish, in sickness and in health, for richer or poorer?'
Keith turned to face his wife. He remembered the day twenty years ago, the day they had first gone through this ritual, a wonderful, happy day.
It had been a good marriage — stimulating intellectually, emotionally, and physically. And she was, if anything, more beautiful, more challenging today than then. He looked into her large brown eyes, and said, 'I do.' Thor turned to face her, but before he could speak, Keith squeezed his wife's hand and added, loudly, for all to hear, 'For as long as we both shall live.'
Rissa smiled at him radiantly.
Hell, thought Keith, twenty years was just scratching the surface.
EPILOGUE
Keith Lansing had been sleeping well for weeks now. He lay in bed, next to his beautiful wife, drifting off to sleep. So what if he and Rissa and Jag and Longbottle and Rhombus and all the other billions of Commonwealth citizens didn't yet amount to a hill of beans in this crazy universe? So what if they were a cosmic afterthought, an unexpected by-product of dark-matter art? Someday they would make a difference — they would make all the difference…
Keith woke with a start. He pulled back the little card covering his clock face; it was 0143. He sat up in bed
