Starplex

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.

“And if the opposite is true—if the mass of the universe exceeds the critical density—then gravity will overcome the force of the big bang, slowing down and eventually reversing the universe’s expansion. Everything will fall in on itself, collapsing in a big crunch into a single block of matter. If conditions are right, that block might eventually expand again in another big bang, creating a new, and probably radically different, universe—but everything that had been part of this universe would be destroyed.”

“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.”