There's overwhelming evidence that the universe is accelerating. This means that the future is going to be a relatively lonely place, with galaxies getting ever further from one another, and no hope (if there ever was any) of traveling between them. But could the accelerating universe be even more grim than that?
For this week's column, I'm going to dig deep into the Ask a Physicist mailbag and pull out a question that I've been meaning to get to for quite a while. The question comes from Jeremy Brown who asks:
I've learned about the Big Rip recently, and now it's turned into a low-level nagging paranoia of mine. Thus my question: if the Big Rip were to happen, would we have time to know that it's happening and freak out, or would we all just be going along with our daily lives until we just stop existing?
It's been quite a while since I've done a column on the end of days, and I have to say, it's good to have the paranoia back.
Before getting into the Big Rip itself, I need to say a little bit about what happens to the universe as it expands. Your gut might tell you that as the universe expands, all of the matter in it becomes more diffuse. In this case, your gut is correct – at least for ordinary matter.
It turns out that what happens to the stuff in the universe depends very greatly on the behavior of that stuff. The higher the pressure, the more quickly the material becomes diffuse. It's kind of like a piston engine. Material with high pressure pushes on the cosmic piston as it expands, does work in the process and loses extra energy.
Radiation, for instance, has a really high pressure (1/3 of the energy density), which means that at the universe doubles in scale, radiation density drops by a larger factor than does matter density. Run the process in reverse, and it begins to make sense that at early times, the universe was totally dominated by radiation.
Cosmologists refer to the ratio of pressure to energy density as w or "the equation of state," and while for radiation, the number is fairly straightforward to compute, it's not out of the question to have components of the universe with negative w – a tension. This is how we believe Dark Energy works.
As the universe expands, the expansion essentially pours energy into the Dark Energy, keeping the total density constant. You've heard of a "Cosmological Constant"? If w=-1, that's exactly what you get; a fixed density. And for the most part, measurements seem consistent with w=-1. But what if it's not?
What's the Big Rip?
The universe is accelerating, but the truth of the matter is that we don't know exactly what the rate of acceleration is. As time goes on, distant galaxies will recede from us (and us from them) at an exponentially expanding rate. A cosmological constant (w=-1) will chill the universe as a whole, but on the individual galaxy scale, nothing particularly terrible will happen for a very long time.
Of course, eventually, all of our stars will burn out, our black holes will evaporate, and every photon will be chilled toward absolute zero, but you have about 10^67 years before that happens. Really, it's not worth stressing about.
But something strange happens when w < -1. In 2003, Robert Caldwell, a physicist at Dartmouth and his collaborators decided to explore what the universe would look like in the limit of what they called "Phantom Energy."
Infographic via New Scientist
Contrary to the normal situation, when matter becomes ever more diffuse, or even with a Cosmological Constant, where the Dark Energy stays fixed over time, Phantom Energy becomes more and more dense as time goes on. The acceleration becomes a runaway process, and as time goes on, the cosmic horizon becomes smaller and smaller and smaller. Individual galaxies will be thrown apart from one another, and then subsequently ripped apart individually. Solar systems will be torn apart as well, and then individual planets, and ultimately even individual atoms.
This is the Big Rip.
The good news, if this can really be called good news, is that since the whole process is hierarchical, going from large scales down to small scales, we actually will have some warning. The solar system (or a solar system very much like ours since our sun would have long since burned out) will be ripped apart several months before the singularity at the end, and we likely would have noticed individual stars getting pulled out of sight long before that.
The bad news is that there is nothing we could do about it. Surprisingly, phantom energy would actually be a little less pronounced in high density parts of the universe (like where we're likely to be at the time), but unfortunately, that's not going to buy us very much time. If it's coming for you, there's no escape from the Big Rip.
Is it going to happen?
Let me say first that according to our best guess, w=-1, which means no Phantom Energy and no Big Rip, but we can't say for certain. But honestly, we don't know. I need to you take deep breath and not overreact to what I'm about to tell you, but according to the latest results from the Planck Satellite, the best estimate of w is -1.13+/-0.13. If you're a really big fan of apocalyptic scenarios (or wormholes), then you might immediately latch on to the fact that it's less than -1, and say that we're in for a big rip – and soon (at least by cosmological standards). Plug in the numbers, and the universe is set to rip apart in about 80 billion years.
But this is why you can't forget to look at your errorbars. Most cosmologists, including me, think that it's very likely that w is exactly -1, as there are several physical phenomena – vacuum energy, a true cosmological constant, and others – that can produce that sort of result. Given that the observed errorbars are consistent with -1, I'm willing to put my money on that.
But supposing I'm wrong, there still is an upside to all of this, and that's time travel.
That's right, folks, if there is stuff in the universe with w, then it is unlike anything we've seen in physics before. For one thing, to so-called "weak energy condition" would be violated. In essence, this means that by flying around at the appropriate speeds, we can observe phantom energy with a negative density. This is a very big deal because if we could harness it, it behaves very much like exotic matter. Exotic matter is a crucial ingredient in the creation of wormholes, and subsequently in time machines, and it sounds a lot like Phantom Energy in that it has a w=-1.
So here's your silver lining: on the off chance that we are in for a Big Rip, our cyborg descendents might be able to opt out by hopping into a time loop.
Dave Goldberg is a Physics Professor at Drexel University, and the author of "A User's Guide to the Universe," and the upcoming "Universe in the Rearview Mirror" (coming July 11!). You should send him your questions about the universe, or even better become a fan of his awesome new facebook page.