The Last Word On Nothing

What is the fate of the universe? Cosmologists are converging on an answer, and it ain’t pretty. Or so I gather from people who, hearing that the latest science favors a universe that goes on forever, growing colder and colder, lonelier and lonelier, ask me, “Don’t you find it depressing?”

The short answer is, No. My feeling is, if you need astronomy to tell you that the universe is indifferent to your existence, you haven’t been paying attention. But I have heard the question often enough now that I understand a lot of people would prefer the other possibility: a universe that eventually collapses back on itself in a reverse Big Bang.

The question of the fate of the universe has been around forever—or at least “forever” in the sense of “for as long as our species has understood the concept of not-forever.” The question didn’t begin to pass from metaphysics to physics, however, until Newton suggested that gravity applies not only on Earth but in the heavens.

This reconception of gravity as action at a distance did a splendid job of explaining the motions of celestial objects, but it also came with a complication. In 1692, five years after Newton published the first edition of his Philosophiæ Naturalis Principia Mathematica, the theologian Richard Bentley pointed out in an exchange of letters with Newton that a universe full of matter interacting gravitationally with all other matter would be unstable unless each piece of matter occupied a position of absolute equilibrium in relation to every other piece of matter. Newton conceded: Bentley had him there.

“For I reccon,” Newton wrote back, “this as hard as to make not one needle only but an infinite number of them (so many as there are particles in an infinite space) stand accurately poised upon their points.” Less than a month later Newton wrote to the Reverend again, offering a way out of the problem: “it infers a Deity.” He formalized the argument for God in a later edition of the Principia: “And so that the systems of the fixed stars will not fall upon one another as a result of their gravity, he has placed them at immense distances from one another.” In other words: inaction at a distance.

In 1916, Einstein offered the general theory of relativity as a gentle corrective to Newton. While his reconception of gravity as the interplay between space and matter did an even more splendid job of explaining the motions of celestial objects, it couldn’t resolve Bentley’s paradox. The following year, in his paper “Cosmological Considerations on the General Theory of Relativity,” Einstein found himself acknowledging that a universe full of matter interacting gravitationally with all other matter should be unstable.

But it isn’t. Just look at it! All you had to do was consider “the small velocities of the stars,” Einstein wrote, and you could see for yourself that the universe was indeed stable. So he did what any stubborn theorist would do when offering a powerful argument that seems too good not to be true: He fudged. He inserted an arbitrary variable—the Greek symbol lambda, “at present unknown”—into the math to represent whatever is keeping the universe from collapsing. Like Newton, he feigned no hypotheses as to what that something might be. It was just…lambda.

The flaw in Newton’s and Einstein’s reasoning began to emerge in 1925, when Edwin Hubble announced his discovery that other galaxies exist beyond our own “island universe” of stars. Whether you conceive of the stars as “fixed,” as Newton did, or operating at “small velocities,” as Einstein did, the assumption is the same: The stars are all there is. Now here was Hubble saying: Not even close.

Not that the existence of galaxies themselves resolves the paradox. It only extends, spatially, the needles-standing-on-their-points metaphor. But it does leave open the possibility that while the stars are relatively stable, the universe is not—if only you could consider it on a large enough scale.

Which is what Hubble—as well as the Belgian priest and astronomer Georges Lemaître—began to do over the next few years. By 1929, Hubble and Lemaître had noticed that other galaxies seem to be receding from our galaxy at rates proportional to their distances, implying that the universe is expanding. Newton hadn’t needed God, and Einstein hadn’t needed lambda: The universe is inherently unstable.

How unstable? The answer depends on the density of matter in the universe, which determines the cumulative drag of gravity. Is the density of matter so great that the cumulative gravitational drag will eventually cause the expansion to reverse itself? Or not enough, so that the expansion will continue forever, more and more slowly, until it slows to a virtual halt? Two teams of scientists spent much of the 1990s trying to answer those questions, and the result they got was an option they hadn’t considered: The expansion isn’t slowing down at all. It’s speeding up, under the influence of something that scientists have nicknamed, for want for a better term, “dark energy,” and that they represent in their equations as, yes, lambda.

You might assume that this discovery settles the question of the fate of the universe. An expansion that’s speeding up certainly seems to point to a universe that will go on and on and on. And that was indeed the initial assumption made by the two discovery teams. But theorists soon sent them a message: Not so fast (so to speak). Before you can draw any conclusions about dark energy’s influence on the fate of the universe, theorists said, you first have to figure out how it behaves.

Is dark energy constant over space and time? If so, then it’s a property of space itself. In which case, its unflagging influence will dominate the flagging influence of matter, whose cumulative gravitational drag will grow weaker and weaker as the needles move farther and farther apart. In which case, the expansion will continue forever and ever.

Or does dark energy vary over space and time? If so, then it’s probably something that is not a property of space itself but…something else. In which case, all bets are off, and until scientists figure out what that “something else” is, the fate of the universe will remain in limbo.

This scenario seems to be the one that people prefer. And not just regular folks—scientists, too. If the universe continues to expand, and our little corner of it grows lonelier and lonelier, then a hundred billion years from now, all that astronomers will be able to study are a few local galaxies. As one cosmologist told me, mournfully, his profession will be extinct.

Of course, a hundred billion years from now, our planet will also be extinct, having long, long, long ago been incinerated by the Sun’s evolution into a red giant. And it’s precisely the prospect of the opposite of extinction that seems to motivate scientists and non-scientists alike who prefer a reverse-Big Bang scenario. “Rebirth” is a word I often hear in this regard, because a universe that undergoes a reverse Big Bang might emerge anew, in another Big Bang. To my ear, the urgency I hear in this response seems visceral, even instinctive.

Also, alas, doomed. Studies of the evolution of the structure of the universe on the largest scales all seem to indicate that the influence of dark energy hasn’t varied. Meaning it’s a property of space. Meaning its dominance over gravity will only increase. Meaning the universe will not experience a rebirth. Meaning it will instead experience an eternal descent into vaster and vaster realms of darkness, deeper and deeper iterations of cold, remoter and remoter variations on the concept of isolation.

To me, that’s meaning enough for something that has none.

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Credits, top to bottom: joycestake.blogspot.com; Beyonddichotomy; Dimitri Torterat (Diti)