The future of the universe

This month’s provocative results on the acceleration of the universe raise an interesting issue: what can we say about our universe’s ultimate fate? In the old days (like, when I was in grad school) we were told a story that was simple, compelling, and wrong. It went like this: matter acts to slow down the expansion of the universe, and also to give it spatial curvature. If there is enough matter, space will be positively curved (like a sphere) and will eventually collapse into a Big Crunch. If there is little matter, space will be negatively curved (like a saddle) and expand forever. And if the matter content is just right, space will be flat and will just barely expand forever, slowing down all the while.

Fate of the universe This story is wrong in a couple of important ways. First and foremost, the assumption that the only important component of the universe is “matter” (or radiation, for that matter) is unduly restrictive. Now that we think that there is dark energy, the simple relation between spatial curvature and the ultimate fate of the universe is completely out the window. We can have positively curved universes that expand forever, negatively curved ones that recollapse, or what have you. (See my little article on the cosmological constant.) To determine the ultimate fate of the universe, you need to know both how much dark energy there is, and how it changes with time. (Mark has also written about this with Dragan Huterer and Glenn Starkman.)

If we take current observations at face value, and make the economical assumption that the dark energy is strictly constant in density, all indications are that the universe is going to expand forever, never to recollapse. If any of your friends go on a trip that extends beyond the Hubble radius (about ten billion light-years), kiss them goodbye, because they won’t ever be able to return — the space in between you and them will expand so quickly that they couldn’t get back to you, even if they were moving at the speed of light. Meanwhile, stars will die out and eventually collapse to black holes. The black holes will ultimately evaporate, leaving nothing in the universe but an increasingly dilute and cold gas of particles. A desolate, quiet, and lonely universe.

However, if the dark energy density actually increases with time, as it does with phantom energy, a completely new possibility presents itself: not a Big Crunch, but a Big Rip. Explored by McInnes and by Robert Caldwell, Marc Kamionkowski, and Nevin Weinberg, the Big Rip happens when the universe isn’t just accelerating, but super-accelerating — i.e., the rate of acceleration is perpetually increasing. If that happens, all hell breaks loose. The super-accelerated expansion of spacetime exerts a stretching force on all the galaxies, stars, and atoms in the universe. As it increases in strength, every bound structure in the universe is ultimately ripped apart. Eventually we hit a singularity, but a very different one than in the Big Crunch picture: rather than being squashed together, matter is torn to bits and scattered to infinity in a finite amount of time. Some observations, including the new gamma-ray-burst results, show a tiny preference for an increasing dark energy density; but given the implications of such a result, they are far from meeting the standard for convincing anyone that we’ve confidently measured any evolution of the dark energy at all.

So, it sounds like we’d like to know whether this Big Rip thing is going to happen, right? Yes, but there’s bad news: we don’t know if we’re headed for a Big Rip, and no set of cosmological observations will ever tell us. The point is, observations of the past and present are never by themselves sufficient to predict the future. That can only be done within the framework of a theory in which we have confidence. We can say that the universe will hit a Big Rip in so-and-so many years if the dark energy is increasing in density at a certain rate and we are sure that it will continue to increase at that rate. But how can we ever be sure of what the dark energy will do twenty trillion years from now? Only by actually understanding the nature of the dark energy can we extrapolate from present behavior to the distant future. In fact, it’s perfectly straightforward (and arguably more natural) for a phase of super-accelerated expansion to last for a while, before settling down to a more gently accerated phase, avoiding the Big Rip entirely. Truth is, we just don’t know. This is one of those problems that ineluctably depends on progress in both observation and theory.

Of course, it’s thoroughly remarkable that we can even think about these possibilities in a scientific way, even if we haven’t yet isolated the ultimate answer. Before the dynamical spacetime of Einstein’s general relativity replaced Sir Isaac Newton’s view of absolute space and time, we couldn’t have been having this conversation. Newtonian spacetime is there once and for all, fixed and unchanging in its structure, only permitting the matter within it to evolve. It’s really quite difficult to come up with any sensible long-term history of the universe within Newtonian cosmology; in any finite region of space, the matter will ultimately settle down to some equilibrium — so why is evolving now, if spacetime is infinitely old? In Einstein’s universe, spacetime can evolve and change, and we have grounds for speculating about where it came from and where it’s going. In the last ten years we’ve learned an amazing amount about the universe that bears directly on this question.

But the old picture of collapse vs. perpetual expansion is wrong in another way, too: it takes a feature of our observable universe, namely that it is homogeneous and isotropic, and extrapolates it to the entire universe, even the unobservable bits. This is a reasonable first guess on grounds of simplicity, at least if you didn’t have any reason to suspect that the ultra-large-scale structure of the universe were wildly different from place to place. But these days we do have such a reason: eternal inflation. The idea of cosmic inflation invokes a period of rapidly accelerated expansion in the very early universe, that takes a tiny patch of space and blows it up to fantastic size. Eternal inflation is simply the realization that this phase doesn’t end everywhere at once: inflation typically stops in some part of the universe, but continues on somewhere else — forever! In other words, somewhere far away, inflation is still going on. And this idea isn’t some baroque kind of model that requires a handful of miracles to set in motion; to the best of our current understanding, it’s very easy for inflation to be eternal once it begins at all. That means that the ultimate expansion or contraction of our meager patch of universe is just a tiny part of the big picture — so we shouldn’t take the fate of our little neighborhood too seriously.

Meanwhile, of course, there’s the issue of the distant past of our universe, as well as the distant future. It’s becoming more and more popular to contemplate the idea that the Big Bang wasn’t the start of it all, but simply a dramatic moment in a much larger picture. For a long time now, Gabrielle Veneziano and others have been investigating the idea of a pre-Big-Bang phase in the context of string theory. Steinhardt and Turok have suggested that the universe is cyclic, repeating an infinite pattern of expansions and collapses. And of course Jennie Chen and I have been arguing (following the exhortations of Huw Price about the arrow of time) that the far past should look like the far future, only backwards.

Thinking about what happened before the Big Bang is precisely as respectable as, although admittedly more difficult than, thinking about what the ultimate future of our universe will be. In each case we have to extrapolate into unknown territory, relying on a combination of observational clues and theoretical predictions. And in each case the current state of the art isn’t nearly good enough for us to make any definitive statements. But no need to invoke the God of the Gaps just yet! The amazing thing is not that these questions are hard, but that they are legitimate scientific issues, and that we are increasingly able to address them in the context of established (or at least plausible) physical theories. Stick around, we’ll figure it out.

38 Comments

38 thoughts on “The future of the universe”

  1. ….or look at current experimental processes…..how you got there.:)

    For such wide speculation on the stringevangislistic views, it is not done without recognizing experimental basis, like a everyone saids, “does not exist.”

    They never considered how they might handle it in extra dimensions, and such. It is just called a fantasy for some so a “real debate” is needed by the experts to help us lay people contend with letting our minds run amuck.:).

    Maybe that’s an Edge question that John Brockman could handle?

  2. Sorry again to change the subject., talking about provocative papers (since Jim
    mentioned astro-ph/0601581), Sean (and others) any comments on gr-qc/0511160?
    (however it hasn’t yet been published).

  3. What I want to know is how a paper got on to arXiv.org with ether in the title? They deleted my paper in 2002 and I had to change the title to get it on CERN doc server.

  4. Sent: 02/01/03 17:47
    Subject: Your_manuscript LZ8276 Cook
    {MECHANISM OF GRAVITY}
    Physical Review Letters does not, in general, publish papers on alternatives to currently accepted theories…. Yours sincerely, Stanley G. Brown, Editor, Physical Review Letters

    ‘… the innovator has for enemies all those who have done well under the old conditions, and lukewarm defenders in those who may do well under the new. This coolness arises partly from fear of the opponents, who have the laws on their side, and partly from the incredulity of men, who do not readily believe in new things until they have had a long experience of them. Thus it happens that whenever those who are hostile have the opportunity to attack they do it like partisans, whilst the others defend lukewarmly…’ – http://www.constitution.org/mac/prince06.htm

    ‘(1). The idea is nonsense. (2). Somebody thought of it before you did. (3). We believed it all the time.’ – Professor R.A. Lyttleton’s summary of inexcusable censorship (quoted by Sir Fred Hoyle in ‘Home is Where the Wind Blows’ Oxford University Press, 1997, p154).

  5. 1. Feynman diagrams, the physics behind the maths of quantum field theory, show that forces arise from the exchange of gauge bosons (coming from distances at light speed, hence coming from times in the past).

    2. The big bang mass has an increasing speed, in our observable spacetime, from 0 toward speed of light c with times past of 0 toward 15 billion years (or distances of 0 to 15 billion light-years), giving outward force by Newton’s 2nd empirically based law: F = ma = m.dv/dt = m(c – 0) / (age of universe) = mcH, where H is Hubble’s constant (based on v = HR, where R is distance).

    3. Newton’s 3rd empirically based law suggests equal inward implosion force, carried by gauge bosons, which shielded by mass, proves gravity and electromagnetism to within 1.65% (proof below). This mechanism also predicts particle masses and other observables, and eliminates most of the unobserved ‘dark matter’ speculation and the need for a cosmological constant / dark energy (the latest data suggest that the ‘cosmological constant’ and dark energy epicycle would need to vary with time!

    These are all existing accepted facts; the Feynman diagrams are widely accepted, as is the spacetime, the big bang, Newton’s laws of motion. The result, that apples fall at the measured acceleration, is apparently ‘only a personal pet theory that should be suppressed from arXiv.org and ignored’. Drs Lee Smolin and Peter Woit could sit under an apple tree to verify that existing ‘string theory’ gravity is ‘speculative gibberish’: it is an effort to destroy science using untestable hocus pocus ‘string theory’!

    Update: Lee Smolin has now kindly acknowledged the possibility of using this type of argument (that quantum field theory gauge boson exchange process predicts magnetic moments and Lamb shift, so an attempt to unify the spacetime fabric with Feynman path integrals is an empirically defendable physical reality, unlike ‘string theory’ speculation). This applies for some kind of spin foam vacuum in loop quantum gravity, as mentioned on Peter Woit’s blog. Smolin is committed to the very difficult mathematical approach, but was decent enough say:

    Nigel Says: January 14th, 2006 at 2:18 pm

    Some kind of loop quantum gravity is going to be the right theory, since it is a spin foam vacuum. People at present are obsessed with the particles that string theory deals with, to the exclusion of the force mediating vacuum. Once prejudices are overcome, proper funding of LQG should produce results.

    Lee Smolin Says: January 14th, 2006 at 4:41 pm

    … Thanks also to Nigel for those supporting comments. Of course more support will lead to more results, but I would stress that I don’t care nearly as much that LQG gets more support as that young people are rewarded for taking the risk to develop new ideas and proposals. To go from a situation where a young person’s career was tied to string theory to one in which it was tied to LQG would not be good enough. Instead, what is needed overall is that support for young scientists is not tied to their loyalty to particular research programs set out by we older people decades ago, but rather is on the basis only of the quality of their own ideas and work as well as their intellectual independence. If young people were in a situation where they knew they were to be supported based on their ability to invent and develop new ideas, and were discounted for working on older ideas, then they would themselves choose the most promising ideas and directions. I suspect that science has slowed down these last three decades partly as a result of a reduced level of intellectual and creative independence avaialble to young people.

    Thanks,
    Lee

  6. Sean said
    “First explored by Robert R. Caldwell, Marc Kamionkowski, and Nevin
    Weinberg”….
    No, it wasn’t first explored by these gentlemen. See for example

    http://arxiv.org/abs/hep-th/0112066

    Which, in fact, they cite. Note too [from this paper] that phantom
    cosmologies *do not* have to be singular.

  7. I was describing this post to my roommate, to relate to her my favorite quote in it:

    the far past should look like the far future, only backwards.

    Quoth my roommate: “And in heels.”

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