“That hardly sounds much better?” said Rissa.
“True?” said Jag. “But if—if!—the universe has precisely the critical density of matter then, and only then, can our universe go on in a viable state forever. The expansion caused by the big bang will be slowed to a virtual halt by gravitation—the expansion will asymptotically approach a zero rate. The universe will not die cold, empty death, and it will not collapse back in on itself. Instead, it will exist in a stable configuration for trillions upon trillions upon trillions of years. For all practical purposes, this universe will be immortal.
“And which is it?” asked Rissa. “Is the universe above or below the critical density?”
“Our best current estimates are that the mass in the entire universe of all we can see, plus the mass of all that we cannot, including all dark matter, alls five percent short of the critical density.”
“Meaning the universe will expand forever, right?” said Lianne.
“Exactly. Everything will continue to fling away from everything else. The cosmos will die with all of creation ending up the merest fraction of a degree above absolute zero.”
Rissa shook her head.
“But it doesn’t have to happen,” said Jag. “Not if they can pull it off.”
“Not if who can pull it off?” asked Keith.
“The beings in the future—the descendants of the Commonwealth races. You said it yourself, Lansing, you are going to become vastly old, live for billions of years. In other words, immortal. Well, truly immortal beings would eventually have to deal with the death of the universe; it’s the one thing that could indeed end their lives.”
“But what about entropy?” asked Lianne.
“Well, yes, the second law of thermodynamics does predict an eventual heat-death for any closed system. But the universe may not be entirely closed; there are, after all, good theoretical reasons to believe our universe is only one of an infinite number. It may be possible to pull in energy from another continuum, or to simply conserve energy here, producing minimal entropy, so that this continuum will be viable virtually forever. In any event, they would have untold trillions of years before that issue would have to be faced—trillions of years to come up with an answer.”
“But—but—it’s and inconceivable project,“ said Keith. “I mean, if we’re currently five percent below the critical density, how many stars would have to be pumped back? Even one from every shortcut wouldn’t be enough, would it?”
“No,” said Jag. “Our best estimate is that there are four billion shortcuts in our galaxy. Let’s assume that that’s typical—that they’ve built one shortcut for every hundred stars not just in the Milky Way, but in every galaxy in the universe. Stars account for roughly ten percent of the mass of the universe; the other ninety percent is dark matter. So, if you pumped one average star through each shortcut, you’d increase the mass of the universe by one one-thousandth of its current total. To increase the mass by one twentieth—which is five percent—you’d need to pump fifty stars through each shortcut.”
“But—but surely if you have time travel, you don’t need to save the universe,” said Keith. “You could live for ten billion years, then time travel back to the beginning, live another ten billion, travel back again and so on, forever.”
“Oh, indeed—and who knows how many cycles might go through before they work up the nerve and technology to undertake the project? The endless time-jumping method gives a pseudo-immortality—it’s clearly inferior to actually making the universe that last forever. Not only does it mean no building or other structure can have a lifespan longer than ten billion years, but it limits immortality to those beings who actually have time travel.”
“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.
Thorald 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.”