The grim endgame: the heat death of the universe.
That…that can't be it, right? That's the fate of the universe as predicted by modern physics? That's all we get, a slow winding down of energy differences and the dismemberment of structures? We've worked so hard over the past few centuries to plumb the deepest mysteries of the cosmos, and this is all we can show for it?
Sadly, as depressing as this scenario is, it's a simple extrapolation of the physics of the universe as we know it. If you know where your car is and how fast you're driving, you've played the same game to figure out when you'll get to that party. Except there's no party here, just a miserable end to an enfeebled cosmos.
It's easy to get caught up in the melodrama of the long-term fate of our universe because the outcome is just so dang morose. I'm guilty of it too, though I don't know what else you were expecting—the title of this chapter is “The Long Winter,” after all. Still, we shouldn't get too comfortable with this telling. There are a lot of assumptions and conditionals baked into our current forecasts, and it makes it all seem so safe and predictable and boring and simple.
And if there's one thing the universe has taught us these past few centuries, it's that complexity has a way of taking its revenge.
For one thing, dark energy. The accelerated expansion depends on dark energy behaving like a cosmological constant, applying its accelerating pressure with dumb eternal insistence. But we're not sure if it really is constant—at best we hope to measure it to within a few percent accuracy in the coming decades. Even if we were to apply all our methodological might and constrain the properties of dark energy to one part in—let's go crazy here—a thousand, that still won't be enough. Given the protracted timescales involved in discussing the ultimate fate of the universe, tiny variations can add up.
Indeed, over the past few years some small tensions have arisen between measurements of Hubble's constant when using early-universe probes (like the relic cosmic microwave background) and contemporary-universe tools (like supernova). That's quite possibly explained by operator error on the part of astronomers, but it could be a sign that something fishy is going on in the world of dark energy.4
If dark energy is constant, it's not necessarily a good thing, so don't get too excited. For example, if dark energy is actually increasing with time—a scenario called phantom dark energy because that sounds totally awesome—then the expansion of the universe will become overwhelmingly coercive, ripping apart clusters, galaxies, and even solar systems. In short order, the small patches of vacuum inside atoms tear them apart, dissolving structures in a cascade of doom. If that's the case, then we only have about another ten billion years or so before our entire universe rips itself apart at the seams.
While violent, at least it's quick.
This scenario puts a big red line under the phrase “we don't know what dark energy will do in the future.” As time goes on and accelerated expansion continues apace, the universe will look more and more like the inflationary epoch of old. And we think that rapid expansion transformed the cosmos and flooded it with all the cool subatomic toys and gadgets that we love today. Perhaps an encore performance is in store for the far-future universe, refreshing and reigniting the feeble flickering candle of our fate?
Dunno.
What about dark matter? You know, the stuff that makes up most of the stuff? If it's anything like we suspect it to be, then it does occasionally interact with regular matter and even itself. Over time, this drains energy from the dark matter particles, allowing them to settle into any nearby gravitational wells, like a brown or white dwarf. Continued interactions can keep them warm (for very minimalistic definitions of the word “warm”) through the twilight of the degenerate era. It's not much to go on, but when faced with the ultimate heat death of the universe, we've got to count our blessings.
Or the universe could just up and change in a flash. Seriously, poof, it's gone and replaced with something else. Don't put the book down, I'm not kidding around. Here's why it's a very real possibility: it's already happened! It would just be another phase transition, like the one that sent the strong nuclear force (and before that, gravity) splitting off from the remaining forces. During that energetic, exotic process, the universe transformed from one state with a certain population of particles and fields to a completely different one.
If you lived through that transition, you wouldn't: the forces and interactions that you depended on would be up and gone, replaced with strange and unfamiliar new species.
Here's the kicker: what if the phase transition of the universe isn't done? What if the current universe, with its four fundamental forces and array of leptons and hadrons, isn't the true ground state of the governing equations? What if the universe got “stuck”?
Imagine skiing downhill, racing to the bottom of the mountain—the ground state—and watch out for that rock! and taking a tumble, getting jammed on a slight rise. You can see the rest of the mountain below you, but you're not moving. You're stable, but only in a meta sense—a swift enough kick (avalanche, abominable snowman, I'm not really familiar enough with winter sports for more examples) would send you tumbling down to the true base of the slope. But if that swift kick doesn't come, you can just chill out and relax; you're not going anywhere.
So maybe—emphasizing that word as hard as I can—the universe is in just such a state. Metastable, it can maintain its current arrangement of forces and physical constants for a very long time. Indeed, it's already managed to do so for more than thirteen billion years. So everything looks nice and comfy…for now. All it would take to trigger a new vacuum decay is a random blip or jiggle in the wrong place at the wrong time. Good thing there's nothing in the vacuum of space-time providing a minimum energy level capable of destabilizing the local patch of reality.
Oh, right. Vacuum energy. Microscopic quantum fluctuations. If you the skier started shaking uncontrollably, a violent jerk might send you continuing on your downward way.
Maybe the universe has already done it. In such a nucleation event, just as in any other phase transition, the cosmos reconfigures itself from a single point in an outwardly expanding Sphere of Doom. Traveling at the speed of light, there's literally no way to see it coming. By the time it overwhelms you, it's already replaced all your electrons, photons, and anything else-ons with…with whatever comes next, I guess.
Calculations are rough here, since they depend on physics beyond the standard model. If we stick to what we know so far (it's worth a shot, I guess), the stability of the universe depends on the nature of the Higgs field, since that field was involved the last time the cosmos underwent a phase transition, giving us the clean separation between the weak nuclear and electromagnetic forces. And you thought we were done with exotic subatomic physics.
Now that the Higgs boson has been confirmed to exist, thanks to the tremendous rock-smashing powers of our particle colliders, looking at how the Higgs particle behaves gives us insights into its future. Is it done, stable for all eternity in its ground state? Or does it have more room to fall? Current measurements of the Higgs put us right on the line of metastability, which is of little comfort.5
Hey, at least the universe isn't unstable (but we knew that already).
Maybe the end isn't an end at all. Quantum mechanics teaches us that reality is ruled by random chance. An electron can just so happen to be on the opposite side of a wall the next time you look at it. Two protons can just so happen to cohabitate the same volume, and voilà, you have a fusion reaction. But the larger and less quantumish an object or system, the less you expect it to behave weirdly. I can lean against a wall all day long without expecting to pass through it spontaneously. I can sit on my couch all day long and not occupy its same volume (hopefully).
Even in the not-quantum world, gamblers still rule the day. For example, there's nothing in the laws of known physics to prevent all the air molecules in the room you're sitting in to spontaneously end up crammed into a tiny corner, leaving you
to asphyxiate helplessly in the vacuum. The only reason thoughts like this don't keep physicists up at night is that these conditions are exceedingly, exceedingly, exceedingly rare. There are so many more ways for the air molecules to be jumbled around the room compared to the number of ways they can be crammed in the corner that the random jostling and jiggling at the molecular level almost always leads to an air-filled room.
That was the briefest summary of the concept of entropy that I could concoct, so consider yourself spared a more long-winded metaphor. Entropy itself is a way to count the number of ways a bunch of particles can rearrange themselves, and the second law of thermodynamics—that entropy always goes up in closed systems—comes from the fact that there are way more disorderly states (like air spread evenly throughout a room) than orderly ones (crammed into a corner).
When you throw out a possibility like bodies spontaneously jumping through walls or air molecules conspiring against you, a proper physicist would immediately scoff and say, “Pshaw, yes, it's technically possible, but not likely in a bajillion years.”
Well, now we're dealing with a bajillion years. The absurd and unlikely are bound to happen. Given an infinity of time, anything that could happen must happen. What does this mean for the long-term fate of the universe? It's hard to say because we're operating far outside the normal bounds of known and generally accepted physics. It could mean that a new universe—big bang and all—simply pops into existence through a new inflationary event triggered by a random fluctuation. That universe would be effectively cut off from its parent, with its citizens blissfully ignorant of what came before their own bang.
This new universe—which might or might not have its own set of physical laws—would eventually lead to the formation of new cosmo-babies, on and on and on. That would imply that our big bang was neither the first nor the last of those dramatic events, but simply one bead along an infinite glittering strand of…beads, I guess.
It could mean that a random patch of the universe might spontaneously decrease in entropy, so much so that a complex structure—say, for example, something like a brain capable of something like conscious thought—would get to contemplate its lonely existence before subliming back into the mean. Preposterous? Yes, but in ten to the hundred to the hundred years, the preposterous becomes plausible.
If the universe is infinite in size, or at least capable of infinitely generating new inflation events, and if matter can only arrange itself in a finite number of ways (which just might be true due to the quantum limits on measurements), then that means that all possible combinations and permutations of arranging matter in the universe have been realized. Infinity is a tough concept to deal with, and scenarios like this certainly aren't helping. In this picture, not only is every possible organization of galaxies, stars, planets, rocks, and molecules brought to fruition somewhere (or somewhen), each possibility is realized an infinite number of times.
That means there's a literal copy of this exact situation, either of me sitting in my pajamas typing this sentence, or you wearing who-knows-what reading it. If the universe is infinite in size, the nearest copy is somewhere out there, well beyond our observable horizon (thankfully). If the universe is infinite in time, then this scenario has already occurred and is fated to happen again. An infinite number of times. Cripes, this is getting embarrassing.6
I'll be the first to admit that this picture is a bit hard to swallow, but we should remember that, well, the universe doesn't care what we think about the issue, and it's the (extreme) logical conclusion if we're to take our most modern theories at face value.
Or maybe we're just wrong about all of it. It's not like it hasn't happened before.
We don't know if inflation is correct. We don't know how the rules of quantum mechanics can be extended to incomprehensible timescales. We don't know if the technology of entropy can be applied to the whole entire universe, let alone over the course of an exceedingly exponential number of years.
And don't even get me started on braneworld cosmologies or string theories or whatever the kids are calling it these days. The more hypothetical the physics, the more room for creative explorations of the end state (states?) of the universe.
Our knowledge of the universe at 10100 years isn't much different from our knowledge at 10−100 seconds: woefully incomplete. In both cases it's the energies involved. In the young cosmos, the temperatures are so high and pressures so extreme that the physics of the familiar are melded together into some strange chimera that eludes understanding. In the remote future, temperatures are so low and processes so agonizingly slow that the statistical rules that govern our daily lives lose their identity. In both cases the universe is extreme, exotic, and potentially unknowable. At its core, after centuries of searching, we don't know how the universe began or how it will end—or if those are even reasonable scientific questions to ponder.
But at least there is symmetry.
So here's the score. Our entire lives, our entire existence save a few all-too-brief excursions, are confined to a thin, fragile shell on the surface of the Earth. Space, and all the threatening emptiness and vaguely malevolent vastness that goes with it, is a mere sixty miles away. That's right: sixty miles. One hundred kilometers. By International Agreement of People Who Know These Things,1 space is just a leisurely hour's drive away, if your car could drive straight up.
The most generous definition of the entire biosphere—the oceans and land, the otters and terns, the people and bacteria, the dung beetles and Douglas firs, the lot of it—puts our livable home at around 1 percent of the radius of the Earth. That's roughly the thinness of the shell of the egg you cracked open for your omelet this morning.
Our home planet is but one of eight (or eight thousand, depending on your definition) planets, the largest of the inner rocky worlds but dwarfed by the outer gas giants. The sun, that great luminous ball of fusing hydrogen, is but one of hundreds of billions swimming through the Milky Way galaxy, of roughly middle size and middle age—nothing remarkable there. It sits near the edge of what's called the Local Bubble, the blown-out cavity of a supernova that detonated long ago. Lying about halfway out from the dense galactic core at a radius of twenty-five thousand light-years, the sun is perched on a small spur splintering off the much larger, but comparatively minor, Orion-Cygnus spiral arm.
The Milky Way too is just one among a vast number of galaxies in the observable universe, numbering between five hundred billion and two trillion, subject to how quality you think the estimates for counting dim galaxies are. It's one sparkling but relatively small jewel embroidering the great cosmic web. A member of the Local Group, a faction with the Virgo Supercluster, which itself is nested within the hierarchy of our universe, just a branch of the grander Laniakea Supercluster.
Our place in the universe. ’Nuff said. (This and the next seven images courtesy of Wikimedia Creative Commons; author: Andrew Z. Colvin; licensed under CC BY-SA 3.0.)
The observable universe itself is roughly ninety billion light-years across, with the cosmic web stretching across is breadth and depth. The Milky Way is twenty-five thousand times wider than the distance from the sun to Proxima Centauri; our patch of the visible universe is a million times wider than that. Of course, the actual universe is far larger. Perhaps infinitely so, but at the very least…well, numbers are already meaningless here, so let's just go with significantly so.
And here we are. After 13.8 billion years of (known) cosmic evolution, from the nuclear maelstrom that birthed the fundamental elements of our existence, the deliberate growth of the galaxy, past generation after generation of stellar births and deaths, comes one particular little star with a family of planets. One of those planets, a blue-colored gem against a backdrop of night, is home to something quite unique and even more surprising in the universe: life.
From the perspective of physical cosmology (which, if you haven't noticed, is the subject of this book), there was no plan, no grand design. The heavens did not single this planet o
ut among all the others. The stars did not whisper to themselves over the eons to conspire and arrange this lucky chance. By all accounts, we're just here, and the universe had better get used to it, whether it cares about us or not.
But then, we must be a little bit special, because where is everybody else? We're still in the early days of needing more than our fingers and toes to count all the planets outside the solar system, but rough estimates land within the ballpark of one trillion for total planets in the Milky Way.2 That's more than one, on average, per star. Of course most of those obviously aren't good candidates for life (and henceforth I'll use the word “life” to mean “life as we know it,” you know, based on carbon and liquid water and all that. Otherwise we have basically no clue what to look for, so we would have no confident idea of whether we would actually see it even if we had our telescopes pointed right up their…never mind, this parenthetical is getting way too long).
Anyway, most planets aren't homes for life. A good number, perhaps most, of those trillion or so planets are unbounded, homeless rogues, not attached to any parent star. Orphaned by ejection events in the chaotic early days of a system's formation, they're doomed to wander aimlessly through the long night. Of the planets lucky enough to call a star home, many are too big, or too small, or too hot, or too cold. The chances of life appearing in any one place are exceedingly, frighteningly slim.
How do we know? Because if life were easy, we would have noticed.
Space is big; space is empty. We've already covered that. But it's also lonely. Tens of thousands of detected planets. Probes and rovers and scanners sent to every planet and moon we can reach. Relentless searches for a twin of our Earth circling a distant sun. Countless sleepless nights, monitoring the heavens for the faintest whisper of an alien radio signal.
Your Place in the Universe Page 26