The End of Everything (Astrophysically Speaking)

by Katie Mack

Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. Robert Frost, 1920

Perhaps the promise of a final judgment serves to somehow make up for the unfortunate fact that our imperfect, unfair, arbitrary physical world cannot be relied upon to make existence good and worthwhile for those who live right. In the same way a novel can be redeemed or retroactively ruined by its concluding chapter, many religious philosophies seem to need the world to end, and to end “justly,” for it to have had meaning in the first place.

Do we still have to take the trash out next Tuesday if the universe is going to die someday?

We are a species poised between an awareness of our ultimate insignificance and an ability to reach far beyond our mundane lives, into the void, to solve the most fundamental mysteries of the cosmos.

Acknowledging an ultimate end gives us context, meaning, even hope, and allows us, paradoxically, to step back from our petty day-to-day concerns and simultaneously live more fully in the moment. Maybe this can be the meaning we seek.

Everything you see is in the past, as far as you’re concerned. If you look up at the Moon, you’re seeing a little over a second ago. The Sun is more than eight minutes in the past. And the stars you see in the night sky are deep in the past, from just a few years to millennia.

Only, a singularity doesn’t have to be a point—it could just be an infinitely dense state of an infinitely large universe.

To sum up where we’ve gotten so far: there may have been a singularity. If there was, it was immediately followed by an era we can’t really say much about called the Planck Time.

In about four billion years, Andromeda and our own Milky Way galaxy will collide, creating a brilliant light show.

But if you were to throw the ball incredibly fast—specifically, 11.2 km/s, the escape velocity of the Earth—you could in principle give the ball so much of a push that it leaves the Earth entirely, slowing down slightly all the while, and only comes to rest infinitely far in the future (or, I suppose, when it hits something else). If you throw it even faster, it’ll be completely unbound from the Earth and just coast away forever.

Because we don’t know whether it’s really a cosmological constant or not, we generally call any hypothesized phenomenon that could make the universe accelerate in its expansion dark energy. To throw some more terminology into the mix, an evolving (i.e., nonconstant) dark energy is often called quintessence, after the “fifth element,” a mysterious something-or-other that was popular to philosophize about in the Middle Ages and is not really much more precisely specified now. A nice thing about the quintessence hypothesis is that it could lead us to a theory with some parallels to the cosmic inflation at the beginning of time. We know that whatever it was that caused cosmic inflation eventually turned off, so perhaps a similar accelerated-expansion-causing field could have turned on since then, causing the acceleration we see today.

At early times, when the cosmos was more compact, there just wasn’t enough space for the cosmological constant (which is a property of empty space) to do very much, so the expansion at that time was slowing down, just as we would have expected. But about five billion years ago, matter got so diffuse due to ordinary cosmic expansion that the inherent cosmological-constant-induced stretchiness of space started to really become noticeable.

While nothing can travel faster than light through space, there’s no rule that limits how quickly things can happen to find themselves farther apart because they are sitting still in a space that’s getting bigger between them.

The laws of physics generally have no regard for the direction of time; in most situations, reversing the equations in time makes no difference to the physics. The only part of physics that seems to care at all about which direction time is going is entropy. In fact, it’s possible that the only reason we can remember the past and not the future is that “things can only get worse” is a truth so universal that it shapes reality as we know it.

The Boltzmann Brain problem is the assertion that this unfortunate brain, doomed to quantum-fluctuate back into the vacuum almost instantaneously after its creation, is so vastly more likely to occur than a whole universe that, if we want to use random fluctuations to build our universe, we have to accept that we’re much more likely to be just imagining the whole thing.

The core then gets hot enough to start fusing helium into oxygen and carbon and turns the Sun into a huge bloated red giant star.

None of the things one frets about ever happen. Something one’s never thought of does. Connie Willis, Doomsday Book

Over and over again, scientists have developed new theories of nature by recognizing patterns in their observations and then looking for a hidden property that could give them insight into what was really going on.

HAMLET: O God, I could be bounded in a nutshell, and count myself a king of infinite space, were it not that I have bad dreams. William Shakespeare, Hamlet

Wherever we end up looking for data, in the quest for a theory of everything, we have two complementary approaches. One is to examine the phenomena we already see in nature that don’t fit into established physical theories, so we can build new and better theories to explain them. The other is to just try to break the theories we have—write down hypothetical extreme cases that might not have been tested yet, and see if we can find a new way to look at the data that will show us if the theory still works there. A combination of these two approaches is pretty much always how we move forward in physics. It’s how we went from Newtonian gravity, which works extremely well in everyday situations, to Einstein’s general relativity.

If we want to learn anything about the far future of the cosmos, we’d better address the giant invisible ever-expanding killer elephant in the room: dark energy. When the accelerated expansion of the universe was discovered in 1998, the new paradigm placed us squarely in the path of a dark-energy-dominated future: one in which the cosmos gets progressively emptier, colder, and darker until all structure decays and we reach the ultimate Heat Death. But this is just an extrapolation, one that’s predicated on dark energy being an unchanging cosmological constant. As we’ve seen, if whatever is responsible for cosmic acceleration falls into the category of phantom dark energy, or if it somehow changes over time, the implications for the cosmos are drastically different.

One of the things I love about cosmology is how much it requires thinking creatively, trying to approach the physics of the universe from a totally new direction. This doesn’t mean fully unconstrained flights of fancy. You can’t just randomly make stuff up. But what you can (and must) do is constantly find new ways to look at problems to wring a little more insight out of whatever data the universe has to offer. This kind of creative thinking becomes especially important when we’re faced with a conundrum like “How do we improve on Concordance Cosmology or the Standard Model?” Everything we’ve tried so far has been frustratingly consistent with predictions; where are we supposed to find clues leading us to new models if we can’t get something in the current model to break?

Researchers I spoke to at CERN were adamant that increasing energy is essential to move us forward, even if only to better understand the Standard Model. Which does, after all, present us with the specter of vacuum decay. If that Sword of Damocles is going to be hanging over our heads, it would be nice to know exactly what it’s doing up there.

Some theorists have tried to reassure me. Sean Carroll, a cosmologist at Caltech who’s interested these days in the underpinnings of quantum mechanics, thinks we are all being a little rash to dismiss spacetime as not strictly real. “It’s real but not fundamental,” he tells me. “Just like this table is real but not fundamental. It’s a higher level of emergent description. That doesn’t mean it’s not real.” Basically we shouldn’t get too hung up on this because it’s not like spacetime isn’t there, it’s just that if we really understood what it was made of, it would look, at a deeper level, like something else entirely. This does not, in fact, reassure me.VII As a physicist I always try to maintain some level of dispassionate reserve when it comes to my subject, but the notion that spacetime is only real in the sense that it is something we can talk about and sit on but not in the sense of being what the universe is actually made of still makes me feel like it might collapse underneath me at any moment.

There’s a famous equation in astrobiology called the Drake Equation. In theory, it’s a way to calculate the number of civilizations in our galaxy with whom we might be able to communicate. All you have to do is input the number of stars, the fraction of those with planets, the fraction of those with life, the fraction with intelligent life, and so on, and in the end you get the number of messages you should expect on your interstellar voicemail. Of course, many of these input numbers are, at least with current data, completely impossible to determine, which means that the final answer isn’t meaningful. The thing that’s useful about the Drake Equation is that it makes us think about our assumptions about extraterrestrial life, and to figure out what we do and don’t know about this whole question.

“But if nothing we do here has any guarantee of lasting, if even the best gestures have only a slim chance of outliving us, is there any reason not to just give up?” “Every reason in the world,” Rudd said. “We’re here, and we’re alive. It’s a beautiful evening, on the last perfect day of summer.” Alastair Reynolds, Pushing Ice

Jonathan Pritchard, a cosmologist at Imperial College London whose work has run the gamut from cosmic inflation to the evolution of galaxies, finds hope in the idea that in some other distant, unconnected region, something might exist long after we’re nothing but waste heat. “Somewhere out there, there is a multiverse where stuff is always going on,” he says. “Emotionally, I like the idea of that.” But we still die, I say. He’s unfazed. “It’s not all about us, you know.”

Neil Turok points out that the prospect of an end of time in the future, combined with the existence of our cosmic horizon, places hard boundaries on the universe, and thus helpful limits to the problem of understanding it all. A light wave traveling across a limited, expanding, accelerating universe can undergo only so many oscillations, even into the infinite future. “We live, effectively, in a box, okay? Which is finite. And if that’s true, I think it’s to be welcomed because we could understand it. The problem of understanding the universe just got a whole lot easier because it’s finite,” he says. “Finite to the past, finite in space because of the horizon, finite to the future because everything will only oscillate a finite number of times. Wow! I mean, that’s understandable. I’m an optimist by nature, but I think the world is our oyster.”

Pedro Ferreira is way ahead of me on that one. “I think it’s great,” he says. “It’s so simple and so clean. “I’ve never understood why people get so depressed about the end, the death of the Sun and all,” he continues. “I just like the serenity of it.” “So it doesn’t bother you that we ultimately have no legacy in the universe?” I ask him. “No, not at all,” he says. “I very much like our blip-ness… It’s always appealed to me,” he continues. “It’s the transience of these things. It’s the doing. It’s the process. It’s the journey. Who cares where you get to, right?” I admit it, I still care. I’m trying not to get hung up on it, on the ending, the last page, the end of this great experiment of existence. It’s the journey, I repeat to myself. It’s the journey. There is perhaps some solace in the fact that whatever happens, it’s not our fault.