I’m on record as predicting that we’ll understand what happened at the Big Bang within fifty years. Not just the “Big Bang model” — the paradigm of a nearly-homogeneous universe expanding from an early hot, dense, state, which has been established beyond reasonable doubt — but the Bang itself, that moment at the very beginning. So now is as good a time as any to contemplate what we already think we do and do not understand. (Also, I’ll be talking about it Saturday night on Coast to Coast AM, so it’s good practice.)
There is something of a paradox in the way that cosmologists traditionally talk about the Big Bang. They will go to great effort to explain how the Bang was the beginning of space and time, that there is no “before” or “outside,” and that the universe was (conceivably) infinitely big the very moment it came into existence, so that the pasts of distant points in our current universe are strictly non-overlapping. All of which, of course, is pure moonshine. When they choose to be more careful, these cosmologists might say “Of course we don’t know for sure, but…” Which is true, but it’s stronger than that: the truth is, we have no good reasons to believe that those statements are actually true, and some pretty good reasons to doubt them.
I’m not saying anything avant-garde here. Just pointing out that all of these traditional statements about the Big Bang are made within the framework of classical general relativity, and we know that this framework isn’t right. Classical GR convincingly predicts the existence of singularities, and our universe seems to satisfy the appropriate conditions to imply that there is a singularity in our past. But singularities are just signs that the theory is breaking down, and has to be replaced by something better. The obvious choice for “something better” is a sensible theory of quantum gravity; but even if novel classical effects kick in to get rid of the purported singularity, we know that something must be going on other than the straightforward GR story.
There are two tacks you can take here. You can be specific, by offering a particular model of what might replace the purported singularity. Or you can be general, trying to reason via broad principles to argue about what kinds of scenarios might ultimately make sense.
Many scenarios have been put forward among the “specific” category. We have of course the “quantum cosmology” program, that tries to write down a wavefunction of the universe; the classic example is the paper by Hartle and Hawking. There have been many others, including recent investigations within loop quantum gravity. Although this program has led to some intriguing results, the silent majority or physicists seems to believe that there are too many unanswered questions about quantum gravity to take seriously any sort of head-on assault on this problem. There are conceptual puzzles: at what point does spacetime make the transition from quantum to classical? And there are technical issues: do we really think we can accurately model the universe with only a handful of degrees of freedom, crossing our fingers and hoping that unknown ultraviolet effects don’t completely change the picture? It’s certainly worth pursuing, but very few people (who are not zero-gravity tourists) think that we already understand the basic features of the wavefunction of the universe.
At a slightly less ambitious level (although still pretty darn ambitious, as things go), we have attempts to “smooth out” the singularity in some semi-classical way. Aguirre and Gratton have presented a proof by construction that such a universe is conceivable; essentially, they demonstrate how to take an inflating spacetime, cut it near the beginning, and glue it to an identical spacetime that is expanding the opposite direction of time. This can either be thought of as a universe in which the arrow of time reverses at some special midpoint, or (by identifying events on opposite sides of the cut) as a one-way spacetime with no beginning boundary. In a similar spirit, Gott and Li suggest that the universe could “create itself,” springing to life out of an endless loop of closed timelike curves. More colorfully, “an inflationary universe gives rise to baby universes, one of which turns out to be itself.”
And of course, you know that there are going to be ideas based on string theory. For a long time Veneziano and collaborators have been studying what they dub the pre-Big-Bang scenario. This takes advantage of the scale-factor duality of the stringy cosmological field equations: for every cosmological solution with a certain scale factor, there is another one with the inverse scale factor, where certain fields are evolving in the opposite direction. Taken literally, this means that very early times, when the scale factor is nominally small, are equivalent to very late times, when the scale factor is large! I’m skeptical that this duality survives to low-energy physics, but the early universe is at high energy, so maybe that’s irrelevant. A related set of ideas have been advanced by Steinhardt, Turok, and collaborators, first as the ekpyrotic scenario and later as the cyclic universe scenario. Both take advantage of branes and extra dimensions to try to follow cosmological evolution right through the purported Big Bang singularity; in the ekpyrotic case, there is a unique turnaround point, whereas in the cyclic case there are an infinite number of bounces stretching endlessly into the past and the future.
Personally, I think that the looming flaw in all of these ideas is that they take the homogeneity and isotropy of our universe too seriously. Our observable patch of space is pretty uniform on large scales, it’s true. But to simply extrapolate that smoothness infinitely far beyond what we can observe is completely unwarranted by the data. It might be true, but it might equally well be hopelessly parochial. We should certainly entertain the possibility that our observable patch is dramatically unrepresentative of the entire universe, and see where that leads us.
Inflation makes it plausible that our local conditions don’t stretch across the entire universe. In Alan Guth’s original scenario, inflation represented a temporary period in which the early universe was dominated by false-vacuum energy, which then went through a phase transition to convert to ordinary matter and radiation. But it was eventually realized that inflation could be eternal — unavoidable quantum fluctuations could keep inflation going in some places, even if it turns off elsewhere. In fact, even if it turns off “almost everywhere,” the tiny patches that continue to inflate will grow exponentially in volume. So the number of actual cubic centimeters in the inflating phase will grow without bound, leading to eternal inflation. Andrei Linde refers to such a picture as self-reproducing.
If inflation is eternal into the future, maybe you don’t need a Big Bang? In other words, maybe it’s eternal into the past, as well, and inflation has simply always been going on? Borde, Guth and Vilenkin proved a series of theorems purporting to argue against that possibility. More specifically, they show that a universe that has always been inflating (in the same direction) must have a singularity in the past.
But that’s okay. Most of us suffer under the vague impression — with our intuitions trained by classical general relativity and the innocent-sounding assumption that our local uniformity can be straightforwardly extrapolated across infinity — that the Big Bang singularity is a past boundary to the entire universe, one that must somehow be smoothed out to make sense of the pre-Bang universe. But the Bang isn’t all that different from future singularities, of the type we’re familiar with from black holes. We don’t really know what’s going on at black-hole singularities, either, but that doesn’t stop us from making sense of what happens from the outside. A black hole forms, settles down, Hawking-radiates, and eventually disappears entirely. Something quasi-singular goes on inside, but it’s just a passing phase, with the outside world going on its merry way.
The Big Bang could have very well been like that, but backwards in time. In other words, our observable patch of expanding universe could be some local region that has a singularity (or whatever quantum effects may resolve it) in the past, but is part of a larger space in which many past-going paths don’t hit that singularity.
The simplest way to make this work is if we are a baby universe. Like real-life babies, giving birth to universes is a painful and mysterious process. There was some early work on the idea by Farhi, Guth and Guven, as well as Fischler, Morgan and Polchinski, which has been followed up more recently by Aguirre and Johnson. The basic idea is that you have a background spacetime with small (or zero) vacuum energy, and a little sphere of high-density false vacuum. (The sphere could be constructed in your secret basement laboratory, or may just arise as a thermal fluctuation.) Now, if you’re not careful, the walls of the sphere will simply implode, leaving you with some harmless radiation. To prevent that from happening, you have two choices. One is that the size of the sphere is greater than the Hubble radius of your universe — in our case, more than ten billion light years across, so that’s not very realistic. The other is that your sphere is not simply embedded in the background, it’s connected to the rest of space by a “wormhole” geometry. Again, you could imagine making it that way through your wizardry in gravitational engineering, or you could wait for a quantum fluctuation. Truth is, we’re not very clear on how feasible such quantum fluctuations are, so there are no guarantees.
But if all those miracles occur, you’re all set. Your false-vacuum bubble can expand from a really tiny sphere to a huge inflating universe, eventually reheating and leading to something very much like the local universe we see around us today. From the outside, the walls of the bubble appear to collapse, leaving behind a black hole that will eventually evaporate away. So the baby universe, like so many callous children, is completely cut off from communication with its parent. (Perhaps “teenage universe” would be a more apt description.)
Everyone knows that I have a hidden agenda here, namely the arrow of time. The thing we are trying to explain is not “why was the early universe like that?”, but rather “why was the history of universe from one end of time to the other like that?” I would argue that any scenario that purports to explain the origin of the universe by simply invoking some special magic at early times, without explaining why they are so very different from late times, is completely sidestepping the real question. For example, while the cyclic-universe model is clever and interesting, it is about as hopeless as it is possible to be from the point of view of the arrow of time. In that model, if we knew the state of the universe to infinite precision and evolved it backwards in time using the laws of physics, we would discover that the current state (and the state at every other moment of time) is infinitely finely-tuned, to guarantee that the entropy will decrease monotonically forever into the past. That’s just asserting something, not explaining anything.
The baby-universe idea at least has the chance to give rise to a spontaneous violation of time-reversal symmetry and explain the arrow of time. If we start with empty space an evolve it forward, baby universes can (hypothetically) be born; but the same is true if we run it backwards. The increase of entropy doesn’t arise from a fine-tuning at one end of the universe’s history, it’s a natural consequence of the ability of the universe to always increase its entropy. We’re a long way from completely understanding such a picture; ultimately we’ll have to be talking about a Hilbert space of wavefunctions that involve an infinite number of disconnected components of spacetime, which has always been a tricky problem. But the increase of entropy is a fact of life, right here in front of our noses, that is telling us something deep about the universe on the very largest scales.
Update: On the same day I wrote this post, the cover story at New Scientist by David Shiga covers similar ground. Sadly, subscription-only, which is no way to run a magazine. The article also highlights the Banks-Fischler holographic cosmology proposal.
Sean, what would it take for the physics world to achieve concensus that any particular “pre-Big Bang” model is correct? I mean, a direct experimental test seems out of the question. Are you proposing that our knowledge of quantum gravity will progress to the point that only one possible Big Bang scenario will be compatible with it? Or that one model will arise that is so much “simpler” or “more beautiful” than all others that its correctness is self-evident? If not, then what are you saying?
TimG — we probably won’t really know until we know….
There was a time in the early 20th century when the notion of making observations that could probe back to an early Universe that was opaque, and thus from which no light has escaped, would have seemed absurd. And, yet, we’ve done just that. The light element rations in relatively pristine gas clouds have fingerprints left over from the era of nucleosynthesis a mere 10-20 minutes after the Big Bang, long before the Universe became transparent at 400,000 years (or whatever it was).
What I hope, myself, is that any serious “pre-big-bang” theory that gets developed will end up being something that will have left similar fingerprints in our observable Universe, allowing it to really be tested.
We’ll see when we see if we see….
-Rob
Neil B, Time is a measure of movement
If Earth, the planets and the Sun, were not moving, and strings were not vibrating, and particles were ‘static’ and not moving – how would you measure Time
The singularity would have existed in Time
The singularity would have existed in SPACE
from the big bang emerged the observable universe
You could simply divide Time into
pre big bang epoch or era, and post big-bang
Mind you I’m still looking for that SPACE where there is NO Time, ergo no decay, no ageing, no death, and no phase transitions …
or at least where the lifespan is longer (almost eternal)
but don’t hold your breath
Sean said “NCndL (27)— I don’t think I’m saying anything is not possible, as much is up in the air in this game. Ordinarily, “Coleman-deLucia bubble) refers to tunneling from a large vacuum energy to a small one, while I’m suggesting that the inverse process is ultimately going to be more important.”
OK, so the answer to my question [“am I misunderstanding you?] is “yes” 🙂
Right, I get it now. Next question: *why* do you think that “going uphill” is ultimately going to be more important? Doesn’t the idea of dS bubbles nucleating inside higher-energy dS bubbles, working your way downwards, seem more natural? Is it because that process apparently cannot explain the arrow of time?
Sean (39):
Thanks!
Origin of Big Bang is really an interesting question.
Hyperbolic systems tend to give expanding solutions (where some `arrow of time’ can be drawn).
What about the following 5D picture ?
A model of repeatable Big Bangs:
large-size non-linear `etwas’ (the largest-size topological soliton), which is living and rotating in `the very center’ of 5D world, generates from time to time an expanding single big-wave, which becomes more and more O_4-symmetrical during its moving from that center; small-size quasi-solitons (and other stuff)
settle this big-wave.
The expansion (with velocity near the speed of light) will never stop, and sooner or later any such big-wave will dissolve, but subsequent big waves will be also good (and hopefully appropriate for some blogs).
So why is the universe speeding up? What causes dark energy? Nothing? You have to explain what a large % of the universe is doing.
Nigel,
I appreciate your enthusiam for thinking about these problems. However, it seems clear that you haven’t had any formal education on the subjects. The bare mass and charges of the quarks and leptons are actually indeterminate at the level of quantum field theory. When they are calculated, you get an infinities. What is done in renormalization is to simply replace the bare mass and charges with the finite measured values. When this is done, one get sensible answers. What all this means is that quantum field theory cannot explain the values of the masses and charges. There must be a deeper theory which does, and this is where string theory (or other candidate theory of everything) enters the picture.
Thanks for this! I’m still having trouble with the your “arrow of time” discussion. In your arrow of time linked article (Oct 2004, Preposterous universe), you say:
This is the bit just above the diagram.
I don’t follow this. Can you really compare the directions of the arrow in time in different “Big Bangs”?
By the way; there have been a couple of interesting science fiction stories on this kind of idea.
Isaac Asimov wrote one about how all through time “Multivac” and his successors were being asked if it was possible to reverse entropy. In the end, when all life had gone and entropy had run its course and the universe was featureless; only Univac was left in some kind of higher dimensional exitence, reflecting on this last question. And eventually, having sorted all the data, Univac found the answer. But there was no-one left to be told. So the best Univac could do was demonstrate the answer. And Univac said: “Let there be light”.
Another one (I can’t recall the author!) concerned beings of some sort who were building universes for a teacher. Some of them got nice cyclic universes going, but this one student built one that was too finely balanced to collapse. He started with the usual featureless point that explodes into a universe, but then it just continues on and on, until after unimaginable time even all the protons had decayed, and nothing was left but a featureless uniformity. And then, at last, it exploded… If anyone recognizes the story, please speak up!
Quasar9: As I said, I can’t really hash out those time-twisting quantum creation scenarios. However, just looking at the concept of a static world (no processes etc.) which is wrongly imagined to “then” form something or change: you seem to have fallen into the mistake of considering a static state of affairs, which would really have to stay that way since there is no actual time for any act or chance to play out, and imagined time to still be acting while it waits…because you really do have time going on in our universe while you look at a diagram or think about the situation. Then you can imagine “something happening” to that static state, because anything really sitting arond here could be acted on by something else. But really, there wouldn’t be that extra time of the rest of the universe acting on a true static condition, and it would just have to sit there.
Neil B
I wasn’t looking at a diagram, but at the Universe.
If you freeze frame the universe – like in a photo
The moment is frozen in Time. No movement no time.
Time is a measure of movement –
Are you assuming there was no movement in the singularity prior to the big bang?
Are you assuming there was no movement before the big bang which gave rise to the Quark Gluon Plasma, which then gave rise to light and clarity in the observable universe.
With no movement – we would not have reached where we are today … or now … or here … a measure of time.
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I’ve deleted a bunch of off-topic comments.
NCndL (54)– Basically it is because of the arrow of time, yes. A high-energy false vacuum is a much lower-entropy state than a low-energy vacuum is; the entropy of a de Sitter patch is given by its horizon area in Planck units. It’s much more natural to start in a high-entropy state than in a low-entropy one.
Chris (59)– You might be able to compare the arrow of time in different baby universes, if there was a sensible way to continue the spacetime metric from one region to another. The causal structure of a Lorentzian spacetime defines a “timelike dimension” throughout spacetime, while the gradient of the entropy defines a directionality along that dimension; there’s no reason why that gradient needs to be the same in very far-apart regions, a priori.
Sean, I notice you deleted a comment explaining where V’s claims in comment 58 are not even wrong. If anyone wants to see it, there’s a copy of it on my blog.
“It’s much more natural to start in a high-entropy state than in a low-entropy one.”
Can you elaborate on that? I can see how it would make sense if you were starting from, say, an ensemble of universes in some sort of equilibrium, but otherwise it doesn’t seem obvious. If there turns out to be a good physical reason for initial conditions to prefer a low-entropy state, it wouldn’t be unnatural, surely?
Well, there are good physical reasons for the final conditions to prefer a high-entropy state, and as a matter of empirical fact that’s the direction in which we seem to be evolving. And the laws of physics seem to be time-reversal invariant (or at least CPT invariant, which is good enough). I therefore conclude that high-entropy conditions are natural for the initial state, as well. Anything else is secretly sneaking in time-asymmetry by hand, without any dynamical justification.
anon, this may be of interest:
http://realityconditions.blogspot.com/2006/08/on-price-on-arrow-of-time.html
and this link:
http://aeolist.wordpress.com/2007/03/17/dynamical-explanations-of-entropy-increase/
I believe Sean is stating some aspects of Huw Price’s thoughts with regard to initial conditions?
There are interesting cosequences that have not yet been discussed here.
I don’t see why this entropy issue is such a big deal. The entropy of some system can be defined as the number of (extra) bytes you need to use to specify the exact physical state of that system given the macroscopic state (e.g. it’s volume, pressure etc.) it is in.
Now suppose you start with a universe specified by some wave function that completely specifies it. Then everything there is to know about the universe is specified by that wave function. The number of bytes you need to specify the universe at any time is constant, as it can be specified by the initial condition.
For observers living in this universe the situation is different. They don’t have access to all the information about their universe. They could calculate the entropy of their universe by just ading up the contributions from the background radiation, baryons etc. etc. But what does that mean? It means that there are Exp[S/k] possible physical states their Hubble volume could be in given their observations.
But why should model builders care about how much information “internal observers” who evolve inside the model universe won’t have access to?
Of course, it may help to determine the arrow of time the observers experience etc. but I don’t see how the entropy (as defined by the internal observes) is of such fundamental importance. Surely the “entropy” of the model itself (the complexity of the initial state) is much more a quantity of of interest.
Sean,
Perhaps all of this talk about eternal inflation and what the Universe looks like beyond the Hubble volume is moot. WMAP observations clearly show a lack of structure on very large scales. The simplest and most logical interpretation of this observation is that the Universe does not contain long wavelength fluctuations because the Universe is not larger than a certain size. What other explanation is more likely for these missing fluctuations than the purely geometric one briefly outlined above?
An interesting study which (to my knowledge) has not been published by cosmologists, and would crucial to understanding what the Universe looks like on very large scales, would be to determine what is the smallest and largest universe compatible with the long wavelength cut-off in the CMB angular power spectrum.
Sean,
I was under the impression that CPT invariance governed microscopic particle interactions as opposed to the statistical properties of many body systems. How is it related to entropy then?
Josh– Stat mech is supposed to derive laws of macroscopic behavior from microscopic laws; so why are the microscopic laws reversible, and not the macroscopic ones? Boltzmann thought he had shown how it could happen with his H-theorem, but it was a cheat. The real answer is that boundary conditions are responsible — the observable universe’s initial configuration is very low-entropy, for reasons that remain mysterious.
How do you get processes when the bulk is, as far as I can tell from my limited reading on and understanding of the subject, not the time dimension? Folks talk about things colliding, tunneling, expanding, inflating…they use some verb which seems to imply a change occurs, but how do you get change when there is no time? How do pre-big bang cosmologies organize different states of this “larger” structure, whatever it is? How do we say we’re in the baby, and not the parent universe? How do you even establish a relationship of that sort when “before” and “after” are not meaningful concepts? Or maybe that’s a complete misinterpretation of the nature of “the bulk” or whatever structure the entire “multiverse” is supposed to have?
Sean,
Just today a new paper was submitted to the preprint archive by Luminet et al which strongly suggests that the Universe is indeed a gigantic expanding dodecahedron (http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.0217v1.pdf). Interestingly, one of the authors of this paper, Jeff Weeks, came out with a paper last fall which concluded that the dodecahedral space “completely fails to explain” the low-l anomalies in the WMAP3 data. The resolution of this paradox comes from, according to Weeks et al, the fact this very latest analysis is state-of-the-art. This is just my opinion, but it appears that Weeks et al are suffering from a classic case of what is known as confirmation bias–that is, they tended to ignore evidence against their view whilst stubbornly clingling to it. Again it all comes down to a simple and important question in modern cosmology: What other explanation is more likely for these missing CMB fluctuations than the purely geometric one briefly outlined above?
Spaceman, I saw that paper too. What you say about the authors could be said about any speculative idea. While I don’t think that this idea is likely to be right, I wish the authors well: their idea is a truly beautiful one. We should not always assume that the most boring possibilities are always the right ones. I’m amused by the plethora of papers declaring solemnly that some data set is “compatible” with the notion that the cosmic acceleration is due to a plain vanilla CC. What they neglect to mention is the fact that their data are *also* “compatible” with many other suggestions. What they are really doing is striving to establish their bona-fides as respectable members of the community who [apparently by definition] always prefer boring explanations to interesting ones….by the way, what has this to do with the topic of the thread? I suppose you could say that if they are right, then inflation is probably wrong, and that would certainly change our views as to how the universe began…..