How Did the Universe Start?

By Sean Carroll | April 27, 2007 11:53 am

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.

Landscape

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.

CATEGORIZED UNDER: Science, Time
  • http://quasar9.blogspot.com/ Quasar9

    Hi Administrator, great post.

    I think we can discount the singularity at the Big Bang
    The big bang itself would have proceeded from the singularity. This would be a true cyclical universe, unlike Turok’s et al. However the Timescale in which these cycles could or would occur, would be even beyond anything conceived to date – effectively making the universe for all intents and purposes eternal or infinite, since its beginning has ‘gone’ and there is no need of a foreseeable end – other than as an attempt to justify or verify this or that theory.

    But that is different from the existence of singularities in This Universe.
    The fact that we have Earth Gravity,
    The fact that we have the Sun’s Gravity
    and the fact that there is a gravitational field in the Milky Way on which the Sun orbits are enough to satisfy matter can become so compressed that it will reach blachole singularity properties.

    More like the galaxies themselves are the product of unlimited numbers of mini-bigbangs, thus they are the bubble universes or pocket universes that Susskind et al theorise about – but in This Universe.

  • http://scipp.ucsc.edu/~aguirre Anthony A.

    Sean,

    A very nice post on a very interesting question. The part where you sort out “big-bang” (well-tested evolution of an expanding homogeneous hot patch) from “big-bang” (an initial singularity) is important, as the two are often conflated even by people who know better.

    A couple of brief comments, hopefully more later. First, Borde, Guth & Vilenkin did *not* prove that eternal inflation has singularities to the past. As you know, most singularity theorems prove geodesic incompleteness, and this is the case here. What all of their theorems do are (a) write out a set of conditions which they consider to correspond to eternal inflation, then (b) show that the region in which these conditions hold is geodesically incomplete. This would indeed be consistent with eternal inflation “emerging from a primordial singularity”, but it is also consistent with eternal inflation just being grafted onto some spacetime region that is not eternally inflating by their definition. This is exactly what Steven & I did in various ways in our paper; and in most cases we argued that the ‘extra’ region was indeed eternally inflating, just not in accord with their criteria for eternal inflation.

    Second, I agree that it may be too constrictive to think of highly homogenous boundary conditions for the universe (then again, one person’s “fine-tuned” is another’s “simple and symmetric”), but a lot of the thinking of that paper — and yours with Chen — is independent of that. In particular, the notion that there is a surface of ‘minimal entropy’ (however large one would like to make it) from which the arrow of time points away in two directions, was treated in our paper using a homogeneous slice, but is clearly much more general, as you showed in your paper with Chen. And while I think you will agree that it is sensible, it appears to be largely new. Steven and I when writing our paper were amazed to find that aside from a few obscure references from decades ago, people discussing “time symmetric universes” essentially always talk about low entropy at the ‘beginning’ and ‘end’ of the universe, rather than high entropy at the ‘beginning’ and ‘end’, (i.e. low entropy in the ‘middle’). (Please correct me if I am wrong and if you know of other references.)

  • spyder

    This thread post reminded me of that other singularity: the Vinge-Kurzweil nodal point for human transformation. Sean mentions, in his opening paragraph, his prediction for post-2050 theoretical realizations. If there is any reliability to V-K’s prescient visions, then those realizations will be made by, or at least through, some transhuman conscious interactions. The computing power of self-programming AI’s are predicted to exceed human capacity by 2029, and “supposedly” by 2050 the capability of these “intelligences” will exceed the mental capacity of all humans put together. Thus, we could expect a transhuman consciousness to program and utilize vast networks of circuits (biological and mineral–meat and chip) to determine much of the probable theoretical validities regarding the cosmology of our Universe. I wonder though, just what manner of universes such consciousnesses perceive????

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Anthony, thanks for the clarification about the BGV results. About high entropy in the past and future, as far as I know you’re exactly right. Previous attempts to take seriously a time-symmetric cosmology have simply taken for granted that the past boundary condition is low-entropy, so the future one must be also (the Gold universe). As far as I know, your paper with Steven was the first in recent years to explore the obvious alternative.

  • EDT

    I know this isn’t on-topic, but you mentioned it here so I’m pouncing. Am I the only one irked by the use of “weightless” and “zero-gravity” wrt the Hawking story? There’s a big difference between being weightless and these Vomit Comet rides.

    Neither Hawking’s mass nor the Earth’s gravity field ceases to exist at 32k feet.

    -EDT

  • Sam Gralla

    Hi Sean,

    I love the classical interpretation of the big bang, because it falsifies all major religions while affirming theism. Thus, it makes nobody happy (except me).

    Next time I see you, let’s agree to bet an encyclopedia of religion that we won’t understand t

  • Sam Gralla

    oh… my less-than sign is being interpreted as an html tag.

    well… I mean tto say..

    Next time I see you, let’s agree to bet an encyclopedia of religion that we won’t understand t is less than or order of 0 in the next fifty years.

    My witty piths ruined by technology!

    -Sam

  • Aaron Bergman

    There’s a big difference between being weightless and these Vomit Comet rides.

    Not really.

  • EDT

    Aaron,

    You still need to overcome gravitational forces to move the mass of your body. The weight of my left hand doesnt become zero simply because I’m embedded in a co-accelerating reference frame. In order to move my hand “up”, I still need to overcome its downward gravitational force. Furthermore, the gravitational effects on blood pressure and other physiological aspects don’t cease to exist. It may “look” like I’m weightless because of the reference frame, but it won’t “feel” like it.

    -EDT

  • Aaron Bergman

    So, tell me how this is different from being in orbit, then.

  • Pingback: jkwiens.com

  • http://mollishka.blogspot.com mollishka

    Heh. jkwiens thinks “religion and politics” aren’t discussed here. I’m amused.

  • EDT

    I see the problem here. It’s not different than being in orbit. The inverse square law of gravity allows the stable Earth orbit. If you compute GM/r^2 for the ISS (~350km above sea level) you get g=~8.8m/s^2.

    Since gravity is required for orbit, you cannot be weightless and you cannot be in “zero-g”.

  • http://www.sunclipse.org Blake Stacey

    Mollishka:

    Having spent far too many hours prowling through the Pharyngula comment threads, I’ve found that people who complain “why don’t the scientists talk about science, instead of religion or politics” are people offended that their own religious and/or political beliefs are being fisked. Of course, if you do write about some actual science, many of them will reject it from their brains post-haste because it conflicts with those self-same religious and political beliefs. Funny world, ain’t it?

    Sean:

    Now that the Horrendous Space Kablooie has come up, I wonder if you (or any of the other cosmologists in attendance) could take a look at the sidebar in Wikipedia’s article Bogdanov Affair. The text in the image caption (“Origin of the affair” section) appears to cover the same ground as this post, and AFAIK doesn’t have any egregious errors. It’d be nice to get a professional opinion.

  • Haludza
  • http://www.sunclipse.org Blake Stacey

    Edit: “are often people offended . . . .”

  • Pingback: Cosmic Variance: Speculating on the beginning of the universe « Identity Unknown

  • http://theeternaluniverse.blogspot.com Joseph Smidt

    Hands down this is one of the best posts I have ever read. Thanks a lot for posting it and for the links. I always like trying to find papers on this stuff.

    Sean, I know you like to post about more than science when you do you really hit the nail on the head. Thanks again.

  • http://tyrannogenius.blogspot.com Neil B.

    One of the questions associated with string theory/extra dimensions type approaches to origination is, why three large space dimensions, N =3? Normally, that is thought to derive from some process selection outcome, like Brandenberger and Vafa talking about number of dimensions constraining expansion of dimensions to larger size. Then there are Lisa Randall et al. with large hidden dimensions etc. Well, those approaches assume that spaces of other than three large dimensions are inherently self-consistent/non-contradictory (could exist without violating basic laws like conservation of energy, even if weird about signal transmission, atomic physics, bad for stable orbits and for organisms, etc.) There have been attempts in the past to find actual inconsistencies when N ≠ 3, which apparently aren’t convincing enough as actual prohibitions – however, it is now appreciated that 3-D space is indeed “special” in many ways.

    However, I have reviewed some apparently novel reasons for there being genuine self-contradiction where N ≠ 3 at Link, for anyone interested. If that pans out, then having N = 3 is not ultimately the result of a selection process that at least could have in principle turned out differently (and coincidentally, such processes could also still make N = 3 more likely, but they’d be “moot.”) It would instead be forced by a sort of logical constraint, if we think that the universe somehow tries to keep some basic principles going.

  • V.

    EDT,
    This point is actually the cornerstone of general relativity, aka the equivalence principle. The effects of being in a gravitational field are indistinguishable from being in an accelerating frame. In the free-falling frame, the gravitational field has been locally transformed away. If you’re in the vomit comet which is in free fall and you let a ball go, you do not see it fall.

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    Since gravity is required for orbit, you cannot be weightless and you cannot be in “zero-g”.

    …what V said.

    The reason Hawking was weightless was that he was falling at the same rate as the room around him.

    Another synonym for “zero-g” is “free fall,” and that term is perhaps a bit more accurate in its implications of what we really mean when we say weightless.

    You can always achieve weightlessness within a small “enough” region of spacetime. You don’t need to have “no gravitational fields” (or, perhaps to be more precise, the Riemann tensor all 0) in order to achieve the effects that we see as weightlessness and zero-g. You just have to be confined to a small enough region of spacetime that things freely falling don’t accelerate measurably relative to each other.

    -Rob

  • Josh

    I think it’s worth clarifying a little about “gravity vs. acceleration”. Uniform acceleration for an extended object is only equivalent to a uniform gravitational field. The gravitational field around the Earth is not uniform. Since the gravitational force is straight into the Earth, only the center of mass “feels” acceleration and gravity in an equivalent way. There are tidal forces on an object near a spherically symmetric object which will stretch it in the “vertical” direction and squish it in the “horizontal” directions. These are the forces that would mess you up if you were falling toward a dense object like a neutron star or black hole.

    Having just typed this, I see that Rob has made part of this point already.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Stephen Hawking’s weightlessness has ruined my thread.

  • Sourav

    Sean,

    Is there a compelling reason why the cosmological constant must be positive? Instead of a false vacuum, perhaps we’re in an de Sitter bubble in an AdS true vacuum. Wouldn’t this make the birthing process less torturous?

  • Ijon Tichy

    After reading a long and informative post that summarises our ignorance about the origin of the universe, I find it difficult to understand how any reasonable and rational human being can believe that…

    …we’ll understand what happened at the Big Bang within fifty years.

    But I find lots of things difficult to understand so I wouldn’t read anything into that.

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    Dear Sean,
    What will figuring out the big bang tell us about what is currently (within the constraints of our light cone, etc) happening in our universe?

    Would it have any predictive value for currently operating physical processes?
    cheers,
    LL

  • Neither Coleman nor deLuccia

    Hi, I’m a bit confused. In most of the literature, for example in Freivogel et al
    http://arxiv.org/abs/hep-th/0505232

    it is assumed that the universe began by means of a Coleman-DeLuccia bubble in a dS background. As far as I know, Susskind and co continue to believe that this is the most likely way for the universe to begin. But you seem to be saying that this is not possible. Am I misinterpreting you in some way?

  • agm

    “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”

    I’m confused. Exactly how is one supposed to do that in a quantum mechanical (i.e., statistical) paradigm, as opposed to a classical dynamics one?

  • http://quantumfieldtheory.org nigel

    Sean, you getting into pseudoscience in even asking questions like that, because whatever theory people may find for the initial conditions, it won’t be possible to test it. The best you will be able to do is to say you have a consistent ad hoc theory for how the universe began which includes qantum gravity effects.

    It won’t be a potentially falsifiable theory, so it isn’t doing science, unless somehow the theory is based entirely on solid facts as input.

    It reminds me of the time you claimed that the universal gravitational constant G had a value within 10% of the present day value a minute after the big bang. It turned out that claim was based on the assumption that the electromagnetic force (which resists fusion, due to Coulomb repulsion) remained static while G varied. If there is a unification of long range forces like electromagnetism and gravity, however, they’d both be likely to vary. If gravity and electromagnetism vary the same way, fusion rates won’t vary because an increased gravitational compression helping fusion will be offset by an increased Coulomb repulsion between approaching charged nuclei.

    So it’s very wishy-washy to be investigating this stuff, especially when there are loads of implicit but unstated assumptions involved. Basically, it amounts to assuming something then claiming to have evidence from a calculation based on those assumed assumptions. This is what things like religion and string theorists do. It’s not very interesting because it’s really just orthodoxy.

    Take Hawking’s radiation as another specific example. He implicitly assumes that there is pair production occurring in spacetime everywhere. In quantum field theory, spontaneous pair production in a steady field say of a black hole needs an electric field strength exceeding Schwinger’s limit of 1.3*10^18 v/m (equation 359 in http://arxiv.org/abs/quant-ph/0608140 or equation 8.20 in http://arxiv.org/abs/hep-th/0510040 ).

    So for Hawking radiation to be possible due to one fermion in pair production near the event horizon falling into the hole while one escapes, you need the black hole to have a minimum electric charge of Q = 16*Pi*(m^4)(G^2)*(Permittivity of free space)/(c*e*h har).

    In general, massive black holes will swallow up as much positive as negative charge, so they’re be neutral, there won’t be any pair production near the event horizon, and they can’t radiate. The situation where Hawking radiation can occur which is most interesting is where you treat fundamental partucles like electrons (which have the maximum charge to mass ratio of all matter) as radiating black holes. Then you have something to act as a source of the exchange radiation.

    So it’s by examining assumptions and rejecting false assumptions that progress is made, not by theorizing willy-nilly with foundations consisting of quicksand (a host of unchecked speculation).

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    Stephen Hawking’s weightlessness has ruined my thread.

    Sorry, man :/

    Here’s a question for you:

    Max Tegmark convinced me once that in the eternal inflation picture, you still could have infinite post-inflation-ending Universes embedded within the inflating bulk. The argument goes something like this:

    The people inside the bubble Universe are going to interpret a line of simultaneity– the “t” parameter in their FRW metric– as not what might be the most natural “t” parameter in the bulk, but as the end of inflation. If inflation stops in a bubble, it’s likely to stop at one point, stop a little later at another point, etc. etc. etc., decaying smoothly away from that central point. (Where, implicitly here, I’ve chosen some sort of coordinate system to work in. I just hope I’m being clear enough.)

    As long as the decay is smooth, farther out from the center of the bubble Universe there will be points where the end of inflation is delayed arbitrarily far into the future. However, because the people in the bubble Universe are defining “t=0″ as the end of inflation, they will call those things arbitrarily far away in space.

    In other words, given that space and time are all relative, it becomes possible to embed an infinite spatial volume within a finite spatial volume as long as eternal inflation really is eternal.

    This is in the bubble universe picture, no the baby universe picture, and I haven’t even tried to think about entropy here, which of course is your main point.

    The question at the end of all of this is: isn’t it possible, then, that isotropy and homogeneity can be strictly true within our Universe and still have this picture of the inflating bulk?

    Of course, I don’t think I really believe that– I’m fully with you on your point that we only know that homogeneity and isotropy are good approximations to a spatial volume that must be a lot larger than our own Hubble Volume. Larger than that we haven’t really measured.

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    nigel–

    I disagree with that.

    This is not willy-nilly speculation. This is extrapolation. Granted, extrapolation is also scary, but it’s very different from willy-nilly speculation.

    Suppose you have a theory that makes predictions (a) through (e). You’re able to test (a) through (d) but completely unable to test (e). If predictions (a) through (d) are all borne out, you now must take prediction (e) seriously. You don’t consider it proven, but you take it seriously because you take the theory seriously. The “default assumption” becomes that (e) is right, until we have a better theory that predicts something different.

    Much (if not all) of what Sean is talking about is grounded in something that is connected to reality.

  • http://quantumfieldtheory.org nigel

    Rob, thanks for defending him.

    “If predictions (a) through (d) are all borne out, you now must take prediction (e) seriously. You don’t consider it proven, but you take it seriously because you take the theory seriously. The “default assumption” becomes that (e) is right, until we have a better theory that predicts something different.”

    Yes, that’s perfectly sensible, at least the way you put it. Problem is, there can be more than one theory which agrees with the facts, as in the case of Ptolemy’s earth centred universe with epicycles (150 AD), versus Aristarchus’ s solar system (250 BC).

    Ptolemy’s theory was wrong, but it was made to fit observations better because more effort was put into it, and it was more popular because it seemed more sensible to people at the time. I’d no doubt that if you went up to one of Ptolemy’s students and mentioned Aristarchus’ theory, you’d be laughed at, or treated as a crank. Ptolemy in The Almagest actually ridiculed the solar system in a patronising, saying the earth would have to rotate which would make people fly off an make the clouds circle the equator at 1000 miles per hour, etc. You can’t defend the facts against this sort of thing, because those mainstream people will either take offense or just ignore you. Their idea of talking about science is talking about mainstream science, and non-mainstream ideas are boring or stupid to them. So your recipe which concludes “until we have a better theory that predicts something different” needs to explain exactly how such a better theory can be developed. Otherwise, the first idea that fits your recipe will assume a dictatorial prestige and brainwash everyone for centuries, allowing little space for alternatives to be developed to the point of being a serious challenge.

  • http://quantumfieldtheory.org nigel

    (sorry for poor my typing and proof reading above)

  • http://evolutionarydesign.blogspot.com/ island

    Wow, Sean, we’re actually in agreement, “in principle”, for the same strict reasoning, and Lumo is calling you nuts, so you must be right… ;)

    I think that it is also a supportable claim that it is a mistake to assume that the observed accelerating expanding “flat” universe isn’t the eternal norm, as well. Eddington thought that the cosmological constant version of the general-relativistic field equation expressed the property that the universe was “self-gauging”, and this is also true of the self-regulating anthropic ecospheres/ecobalances that produce the goldilocks enigma across the universal board.

    If the anthropic constraint means that the universe is “Darwinian”, then there is an analogous mechanism that enables it to leap/bang to higher orders of entropic efficiency, (as evidenced by the fact that we did this when we lept from apes to harness fire, and beyond…), so the second law is never violated and the arrow of time is fixed.

    Of course, this advocates a NON-singular beginning to our universe, as traits or characteristics are *necessarily* “convolved” forward in a Darwinian universe.

    It seems to me very simple that an inherent asymmetry in the energy of the universe will necessitate such a configuration, but again… you’ve made an assumption about the infinite nature of the universe that is not a proven fact.

    So you forgot the most natural explanation, given only what we actually know.

  • Torbjörn Larsson

    This is in the bubble universe picture, no the baby universe picture, and I haven’t even tried to think about entropy here, which of course is your main point.

    The question at the end of all of this is: isn’t it possible, then, that isotropy and homogeneity can be strictly true within our Universe and still have this picture of the inflating bulk?

    Great post and great question.

    On that question, I have a possible follow up question or two which perhaps fits the subject of the thread:

    If I understand it correctly Linde presents another possible way around the Borde, Guth, Vilenkin results that could work for bubble universes as well.

    Roughly, it seems he proposes that one should always be able to find “past-going paths” of the inflating bulk that goes further back than the BGV bound for a specific set of paths (perhaps only describing the local end of inflation and forward). I.e. the global BGV bound can be pushed unboundedly back, as I understand it. I’m not sure if others accepts this possibility.

    So how does one think of entropy within “past-going paths” in the inflating bulk anyway? Specifically on the above, assuming it could be always expanding instead of having an initial condition within a bounded past, is it required and/or possible?

  • Torbjörn Larsson

    Hmm. Not a very clear formulation, I’m afraid.

    First, “a specific set of paths (perhaps only describing the local end”, should be “a specific set of paths (the later perhaps only describing the local end”.

    Second, “I’m not sure if others accepts this possibility.” was meant to be the first question: I’m not sure if others accepts this as a possibility?

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  • V.

    Nigel,
    The problem with your argument is that fitting the model to observation is not the only criteria modern scientists use to make judments. Practically any model can be made to fit observations. However, the subset of such models which are internally consistent with known laws of physics as well as mathematical consistency is very small. Take your example of the Ptolemaic model. Sure, it fits the observational data very well, but it violates known physical laws. Thus, modern scientists would know it is wrong on this basis. String theorists and cosmologists construct models to explain the universe, however they too are tightly constrained by physics and mathematics. One of the marvels of string theory is that satisfies so many constraints coming from mathematics, quantum mechanics etc. that many people cannot believe that it doesn’t have relevance to our universe, despite the fact that string theorists cannot as of yet make firm predictions.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Blake (14)– As far as I can tell, the Bogdanovs are just charlatans who strung some cool-sounding words together. But I haven’t looked at their papers closely, so I could be wrong. Their ideas certainly haven’t affected the mainstream conversation in any substantial way. The Wikipedia sidebar is perfectly sensible, however.

    Sourav (24)– All options are open, but I’m not sure why that would make the birthing any less tortuous.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    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.

    agm (28)– If you know the complete state, you just use Schrodinger’s equation to evolve it in time. I’m assuming that wavefunction collapse is a perception of local observers rather than a fundamental law of physics, but that’s another debate entirely.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Rob (30)– Yes, formally speaking, you can embed an infinite open universe within a single bubble inside de Sitter. Whether that actually happens in practice is a different matter; for example, Aguirre, Johnson, and Shomer just wrote a paper about observational effects from bubbles colliding into each other. (Yet another reminder that it’s premature to indulge in self-righteous babble about what is and is not observable before we fully understand the theories we are talking about.)

  • http://quantumfieldtheory.org nigel

    “The problem with your argument is that fitting the model to observation is not the only criteria modern scientists use to make judments.” – V

    I’m in favour of building theories on the basis of facts and making predictions, checking the predictions, etc. That’s science.

    “Practically any model can be made to fit observations. However, the subset of such models which are internally consistent with known laws of physics as well as mathematical consistency is very small.” – V

    No, the new theory has to disagree with the known laws of physics in order to get anywhere. For example, the known force laws in the standard model predict that forces don’t unify at 10^16 GeV.

    If a new theory must be consistent with the old theory, the new theory is just a carbon copy and – unless it is covering an area of physics which is devoid of any existing laws (there aren’t any such empty areas of physics) – it will come into conflict with existing laws.

    For example, supersymmetry predicts – contrary to the existing laws of the standard model – that electromagnetic, weak and strong force strengths unify at 10^16 GeV. That blows your argument sky high, if you think string theory is science and is consistent with existing laws.

    “… Take your example of the Ptolemaic model. Sure, it fits the observational data very well, but it violates known physical laws. Thus, modern scientists would know it is wrong on this basis. …” – V

    Evolving dark energy would violate conservation of energy, so you’d dismiss it out of hand for being inconsistent with known laws? Basically your argument would also ban progress in quantum gravity, since any final theory would need to be inconsistent with known physical laws in either general relativity or quantum gravity. (To start with, a modification of general relativity is needed to allow for quantum effects on the gravity constant Glike redshift over massive cosmological distances of force-mediating radiation being exchanged between distant gravitational charges, i.e. receding masses.)

  • http://quasar9.blogspot.com/ Quasar9

    If we presume that the whole universe could be compressed into a singularity
    and we presume the original singularity dispersed matter, mass and gravity – after the big bang …

    Then the gravity is out there (and here)
    The gravity from singularities in blackholes
    The gravity in galaxies (including the Milky Way)
    The gravity in the Sun, Earth & Moon, …

    And possibly Sean some portion is in your favourite place

    However the observable universe is not the limit of the Universe, the periphery or cosmic event horizon, is the perimeter of an ‘event’ NOT OF SPACE or the Universe.

  • V.

    Nigel,
    Your argument that SUSY would violate the know laws of physics because it predicts that the gauge couplings unify whic is in conflict with the non-SUSY standard model doesn’t make any sense to me. First, I would hardly charactize the failure of the gauge couplings to unify in nonSUSY SM as a law of physics, in particular since we do at present have the ability to check the values of the gauge couplings at 10^{16} GeV. The standard model is an effective theory, which may be extended by SUSY.

    Regarding general relativity, it is also an effective field theory and does not hold ‘all the way’. However, I would expect any quantum theory of gravity to reproduce general relativity in the regime for which it is valid. I would point out that most physicists believe that quantum mechanics is something which should hold all the way and we should expect any theory to be consistent with it.

  • http://quantumfieldtheory.org nigel

    V, Supersymmetry is a good example. It modifies the existing extrapolations of three experimentally based force laws at unobservably high energy, without good reason (unless you think that unification is beautiful and a good reason), it introduces unobserved superpartners, and 6 extra dimensions. Dr Woit explains on page 177 of Not Even Wrong that using the measured weak SU(2) and electromagnetic U(1) forces, supersymmetry predicts the SU(3) force incorrectly high by 10-15%, when the experimental data is accurate to a standard deviation of about 3%.

    All these things can be viewed as incompatibilities with existing knowledge. But they are excused because each is just a little difficulty as seen in isolation (just like the debtor with a million little debts of $1 each, who refused to see the big picture – and his big problem).

    A question that should be answered is what happens to the electromagnetic charge energy which is “shielded” by dielectric of the Dirac sea, composed of radially polarized pairs of fermions in loops between the IR and UV cutoffs? The electric charge energy of the bare core of an electron is considerably higher than the observed (screened) value. Does the attenuated electric charge energy power short ranged forces? I.e., do the loops of W weak gauge bosons result from energy being screened by the polarized vacuum? If the attenuation of the electric charge at small distances from an electron causes the weak force, then (by analogy) you’d expect the strong force between hadrons to be caused by energy absorbed from the vacuum by radially polarized virtual electric charges at small distances. If the weak and strong forces are indeed being powered by the attenuation of the electromagnetic charge by polarized vacuum dielectric, then as you get to energies exposing the unshielded bare core charge of the electron like 10^16 GeV, the weak and strong forces should drop to zero because there’s no shielded energy to power them.

  • Paul Valletta

    “How Did the Universe Start”?

    There are many intepretations, one might just as well ask:DId the Universe start from nothing?..if so where did this nothing come from?

    The nearest answer is another question:How much of the previous Universe is present in this/our Universe?

    I thing the Universe came from a previous Universe, and as Einsteins GR states, you cannot create something from nothing, so there must have been a previous “something”, a little(remnant or background), of the previous Universe left_over to bounce from.

    I think also that there is evidence that the Universe evolves Dimensionally, more Dimensions(late_times) evolve from less dimensions (early_times). As our Universe is still evolving, I believe that the “next” Universe will, at some point in the far of higher dimensional future, tunnel out, or phase change into the next Universe.

    Seen from the next future universal generation of inhabitants, it would appear that their Universe was created from almost nothing, and it would also appear that it tunneled/emerged out of another higher dimension?

    A protective process that guarantee’s evolving continuation, from low dimensional process’s,1-D,2-D,3-D, to high “extra” dimensional process can allow diemnsional tunneling to occur.

    A late time higher dimensional tunnel function in this Universe, becomes a early time lower dimensional emergence in the next Universe?

  • V.

    Nigel,
    The main motivation for introducing SUSY is that it provides a natural resolution to the gauge hierarchy problem. As an added bonus, one gets gauge coupling unification and has a natural dark matter candidate. Plus, if you make SUSY local you get supergravity. These are all very good reasons why we expect SUSY to be a feature of nature, besides mathematical beauty.

    Regarding your questions about vacuum polarization, this is in fact what causes the gauge couplings to run with energy. Contrary to your idea, the electroweak interactions are a result of local gauge invariance under SU(2)_L x U(1)_Y, while the strong interactions are a result of an SU(3) local gauge invariance. The specific way in which the SU(3) gauge coupling runs with energy results in ansymptotic freedom, so that quarks are bound inside hadrons. The observation that the gauge couplings unify (if one includes SUSY) within experimental errors is support in favor of grand unification, although it does not prove it. Hopefully, a lot of our questions will be answered by LHC.

  • Thomas Larsson

    The main motivation for introducing SUSY is that it provides a natural resolution to the gauge hierarchy problem.

    This is no longer strictly true. Already a couple of years ago, it was often stated that SUSY requires fine-tuning on the percent level; this was, I believe, the main motivation for split SUSY. Further null results from Tevatron, SuperKamiokande and PEDM experiments have hardly decreased the amount of fine-tuning necessary.

    Anyway, we will soon know. Even in the focus point scenario, several sparticles must be available to the LHC.

  • http://eskesthai.blogspot.com/2007/03/i-should-also-emphasize-that-while-book.html Plato
  • http://tyrannogenius.blogspot.com Neil B.

    A not very original reminder about the very idea of “universe formation”, in which for example “our universe” (implying perhaps the possibility of there being others….) somehow erupts from some sort of superspace or background: I won’t pass judgment on weird orobouros style self-looping schemes, which look fishy to me. However, if you imagine that in order for an “event” to happen, even one like the formation of a/the “universe,” then here has to be a probability of occurance within some range, even if you don’t want to call it just like time. (Otherwise, the pre-condition would just have to stay that way, wouldn’t it, unless something like “time” could operate to allow events….) Unless that background/mother reality etc. itself is confined or only existed for awhile, in which case it was created at some time-like moment, then any probability of any universe has to be played out over and over again – there can’t be just one. That doesn’t explain the principles behind the background, or prove that some universes would have different laws (i.e., ways for things to act) than others etc., or answer the modal realists’ attacks on the very idea of what “existing” means in distinguishing some possible realities from others, or the problems therof, etc. Just some background framing.

  • TimG

    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?

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    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

  • http://quasar9.blogspot.com/ Quasar9

    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

  • http://www.susskind.org Neither Coleman nor de Luccia

    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?

  • http://www.sunclipse.org Blake Stacey

    Sean (39):

    Thanks!

  • http://zhogin.narod.ru Ivan

    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).

  • http://eskesthai.blogspot.com/2007/03/i-should-also-emphasize-that-while-book.html Plato

    So why is the universe speeding up? What causes dark energy? Nothing? You have to explain what a large % of the universe is doing.

  • V.

    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.

  • http://duoquartuncia.blogspot.com/ Chris Ho-Stuart

    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:

    So in the super-far past of our universe, before our “Big Bang” (which is nothing special in this picture), we will find other Big Bangs for which the arrow of time is running in the opposite direction. On the very largest scales, the entire universe is symmetric with respect to time.

    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!

  • http://tyrannogenius.blogspot.com Neil B.

    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.

  • http://quasar9.blogspot.com/ Quasar9

    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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  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    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.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    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.

  • http://quantumfieldtheory.org nigel

    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.

  • anon.

    “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?

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    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.

  • Paul Valletta

    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.

  • http://countiblis.blogspot.com Count Iblis

    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.

  • spaceman

    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.

  • Josh

    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?

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    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.

  • Dumb Biologist

    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?

  • spaceman

    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?

  • Neither Coleman nor deLuccia

    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…..

  • Neither Coleman nor deLuccia

    By the way, I would not have it thought that I believe that there is something boring about vanilla. I am amazed and appalled, as they say in the letters sections, that Max Tegmark goes about talking about “plain vanilla” this and “plain vanilla” that. Au contraire, vanilla, the real thing I mean, is one of the most subtle of all flavors. If Max and the others who affect this terminology are reading: please in future refer to “plain caramel” or [ugh] “plain butterscotch” cosmological scenari. Thanks.

  • spaceman

    The issue brought up in the Luminet et al paper is very relevant to the topic of this thread. If the Universe is small and spherical, then this would put constraints on how it began. First and foremost, the vast majority of inflationary theories predict a flat infinite universe. Right away these theories would have to be abandoned. We would be forced to ask the question: What quantum gravitational process could have brought about a dodecahedral universe?

    Actually, Luminet et al have shown more of a confirmation bias than most authors I have known to put forth speculative ideas. They refuse to accept evidence which would make one seriously doubt the validity of their model– even some of their own evidence. Rather than search for matched circles in the CMB they keep relying on indirect evidence to buttress their view even though they put forth this model four years ago.

  • Paul Valletta

    This may go quite a way in answer to Sean’s post # 63
    :
    http://arxiv.org/abs/0705.1178

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  • CAPT

    Do you think that god created all

  • CAPT

    Because the thoery if the big bang and creationism both have a chance to prove each other wrong. I am with Evolution but if there was a big bang then how did the life START

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  • 4491

    I don’t believe the so called “theories” on how the universe first started. I think that we shouldn’t start accepting these theories as facts. I also hate when scientist try to descredit critics that dissect their theories instead of trying to descredit the flaws critics point out.

    Were still finding more information out all of the time.

    http://www.sciencedaily.com/releases/2006/09/060905104549.htm
    http://www.apologeticspress.org/articles/30

    I came to realization that all of these theories on how the universe started have no reasonable explanation of how nothing caused the occurance from happening. So many of the how the universe started theories don’t try to explain what happened before the event or try to say that an infinite amount of events happened before the occurance that started the universe.

    What theory can explain a finite amount of energy appearing out from nothing without cause? Why that certian amount? If an event caused the event that created the universe then what caused the event that caused the event that created the universe? and so on.

    Why do so many people believe in the existance of so called dark matter / dark energy when they is very very little if any evidence of its existance? Pathetic! Dark matter now suddenly exists to try and give more evidence to big bang! So what caused the first event from nothing?

    We simply don’t have enough evidence to start making conclusions on how the universe began. We don’t even know exacly what energy, space/time is even though its right infront of us just like we can’t imagine what another spacial dimension or color (not in the colorwheel) would look like first hand. The fact that something from nothing happened isn’t being stressed enough.

    Big bang doesn’t explain the specific amount of energy we observe. Why would a singularity pop out a specific amount of energy? Why do we have these specific parameters?

  • Bernard McCarthy

    Hi guys, Hope I can join this debate? I don’t have a degree in any scientific discipline but I have studied well and I am always willing to learn!. What I would like to postulate is this,.. Since we have no awareness of why our brains really function and therefore have no real scientific proof of the beginning of intelligence which has taken our race so far? ..Then how come, from a simple exchange of chromosones and a passing on of DNA, we have reached a level of understanding that belies our simple chemistry? I submit, That the universe is and always shall be a playground in which any intelligent lifeform may take form.

    However, I will say this! I could not have been able to think this far because some guys gave up their sons on the beaches of Normandy! You Americans are why I still live and why I am still here and I will for one ..Will ..Never forget that your sons gave me the chance to live .God bless you America..And all the brothers who died on that Day. Now that dissolves any thought about what I have been told to do by my father who held an American in his hands and watched him die! Let now be shown that the boy died for us and told my father through dreams that a man called by the name Allah gore must not be made pure! And not to be made a prophet! Another number! Be that he is able to change things Before us,and he will always be encouraged to do so?..And must be impelled to do so! .I applaud all scientists who reach out and search for the clues to the belonging and well being of the sphere but He must not be allowed to take the higher ground! I implore you to take a stand! That boy of yours who died on the fields of Europe did not die for nothing! He was An American and he believes in truth. If Gore gets in then the end days are coming! God bless America and put Hilary in the whitehouse because then Mary will be happy.

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  • collin237

    Infinite since the beginning???

    I thought the universe began with a point. Isn’t that what “past singularity” means?

  • Fred Gangstad

    check it out- you tube or google ‘cosmic bubbles/mother rock’ and see the start of the only true GUT. Fictional story set in real universe at
    http://www.angelfire.com/planet/cosmicbubbles Fred Gangstad

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  • http://itsnotauniverseanymore.blogspot.com/ Darrell

    It’s Not A Universe Anymore

    A Whole Bunch of Guesses At:

    The Formation of (Many) Irregular Galaxies
    The Function of Black Holes
    The Explanation of a Multi-Singularity Universe

    “Irregular galaxies feature neither spiral nor elliptical morphology. They are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by gravitational action. Irregular galaxies also contain abundant amounts of gas and dust.”

    For over ten years I have been puzzled by the explanation of our universe. We have this “Big Bang Theory” which states it all started with the ole singularity. I was only smart enough to come up with two questions regarding this:

    1) Isn’t a Black Hole also a singularity?
    2) Where did all that energy and matter come from?

    If a Black Hole is a singularity then wouldn’t it blow up in our face into another universe? So I asked my scientific type friends and they had no answers. I sometimes read that it’s “a different type of singularity”. Huh? Doesn’t that destroy the definition of singularity itself?

    Then the other day I watched Nova. I went into a dreamy state like when I was in fifth grade and the hot teacher read to us from a good book. A feeling I have not felt in a long time. The Nova episode was about Fractal Geometry.

    It would seem that fractals explain a great deal about the structure of nature. Life and non-life. Nature is the universe. The universe is nature. Why then would it not explain the structure of the universe? What if one were to apply fractals to our universe? Then one would probably have to come up with at least one “point of self similarity”. The reproduction, the branching, the duplication. If one would apply fractals to the structure of the entire universe it would have to be a point of self similarity on a grand scale. But what if we went to the biggest production we know, the Big Bang?

    And that’s where it seems to all come together for me. What if, just what if, singularities are the points of self similarity in our MULTIVERSE? What if the “trunk singularity” is what our conceptualized universe comes from? The “branch singularities” are the ones we see in back holes now. The black hole eventually creates a new universe and sort of “disappears” in the process. Seem impossible? Allow me to quote again, “Most irregular galaxies were once spiral or elliptical galaxies but were deformed by gravitational action…..with neither a nuclear bulge nor any trace of spiral arm structure”. A spiral galaxy is at the end of the guessed “galactic evolution”. What happened to its nucleus? What happened to that black hole? Is there any evidence that a black hole has never “gone away”? What is the evidence of a “disappearing” black hole?

    If it’s “Exact Self-Similarity” these would be smaller replica universes or “mini universes” (I type as I point my pinky to the corner of my mouth). If it’s “Quasi-Self-Similarity” it would be smaller copies in distorted and degenerate forms. Or it could be a combination of both.

    So if we were made from a big ole black hole that’s where all the wonderful energy and matter came from. Maybe the singularities are very much alike and perhaps only differ in “scale”. Like branches on a tree. The answer to “Why don’t the singularities we see around us blow up into new universes themselves” perhaps can be answered by the simplest yet hardest to accept answer of, “well, they do”. Perhaps we don’t see that “explosion” (or “implosion”) from our position. Maybe we just see a little part of it and then the black hole fades away. The “explosion (implosion)” forms into “another branch universe” that we just can’t see yet (or maybe ever). Let us remember that when one looks at a fractal like: http://en.wikipedia.org/wiki/Image:Mandel_zoom_00_mandelbrot_set.jpg
    One sees that in this model the reproduction could sort of happen “outside of us”. Singularities would represent sort of the “points connecting” the replications. Like the points connecting the smaller black circles to the bigger black circles seen in the drawing.

    Perhaps this can help with the concept of parallel universes. Can it help us understand the dark stuff?

    If I were to tell you that a universe was created from a singularity then pointed to another singularity and asked you, “What do you think that singularity is going to do?”, what is your natural response? What is your inclination? How about the only thing we know for sure…that there would seem to be the distinct possibility that it creates another universe! If I told you an x created a y, and there’s another x, what do you think is going to happen?

    Universe makes Stars
    Stars make Galaxy
    Galaxy makes Black Hole
    Black Hole makes Universe
    (REPEAT)
    Universe makes Stars
    Stars make Galaxy
    Galaxy makes Black Hole
    Black Hole makes Universe
    (REPEAT)
    Universe makes Stars
    Stars make Galaxy
    Galaxy makes Black Hole
    Black Hole makes Universe
    (REPEAT)

    The thing that scares me about this argument is it is so incredibly simple but tackles a great deal. Things like:

    Why is our universe a multi-singularity universe? (Instead of just a one singularity universe – the singularity our universe itself was made from).

    What is the function of black holes? WHY do they exist?

    Why do black holes “eat”? Could it be they eat for the same reason everything else in the universe eats? To grow and reproduce?

    Why do we see galaxies that have appeared to have lost their nucleus? (Is there something else going on other than galaxies smashing into each other).

    Is everything cyclical? People make people. Branches make branches. Clouds make clouds. Stars make stars. Universes make universes. No matter where you are everthing goes to the virtually infinitely small and the infinitely large.

    What we must remember is the proof of the multiverse being a fractal (or fractals) is that the proof does not appear a million light years away inside a black hole but instead all around us. The reflections we see in all things make it extremely hard to toss this theory out the window. These reflections are the agents of self similarity. The way a galaxy looks like a hurricane that looks like sudsy water circling a drain. The way blood vessels look like the outline of a tree which looks like the outline of a mouth of a river. The way the corona of a star looks like iris of your eye. All the circular shapes that dominate our existence.

    Scientists will gnash their teeth when they find out the stuff that makes particles is made from other stuff that is made from other stuff that is made from other stuff and so on and worse yet going both ways. What’s that, the atom isn’t the smallest? Now you’re talking about strings and other stuff? LMAO! Aint it great?

    Any thoughts?

  • William

    Wow! I am a student studying physics and I have a large presentation to write about the universe. However, it is not about how it started, but rather if the universe were to restart, would it be the same? What is your oppinion on this?

  • William

    (the Wow! was because i was so impressed with the subject and posts. I dont understand a few terms, but i get the main idea of it)

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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