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Susskind's story doesn't match Krauss'?

  1. Jan 23, 2010 #1
    I've watched some 2009 lectures by both Susskind and Krauss, and it sounds like they're telling different stories. Krauss seems to be saying that the origin of the universe is a plain-jane quantum fluctuation, but Susskind seems to be saying that at one point all matter in the universe was on top of everything else (meaning a singularity, right?). Also, Krauss says that using Type I supernovae as standard candles we now know the age of the universe to four places: 13.72B years. But then Susskind waffles and says that it's somewhere between 13B-14B years! Why do they seem to be telling different stories?
     
    Last edited: Jan 23, 2010
  2. jcsd
  3. Jan 23, 2010 #2
    As far as the origins of the universe, it's likely that they are telling different stories, since no one really knows what is going on, and people are just making guesses which might be different from people to people.

    As far as the age of the universe, it's not totally guesswork. The numbers are consistent so it may be that what happened is that Krauss remembered the exact number while Susskind didn't. Also you can get slightly different numbers based on whether you think that the acceleration is due to a constant cosmological constant where you can fix the age to four significant figures or if you think that the acceleration is due to something else at which point you have wiggle room.
     
  4. Jan 23, 2010 #3
    No, actually, Krauss was very clear in his lecture that everything he said is based on observation and experiment, not guesswork of any kind. Susskind didn't make such overt claims, but the video shows him teaching a class, so I have to assume that he's as informed as Krauss. Similarly, Krauss explicitly said that we know the age to four places, and Susskind really does waffle between 13B and 14B, which isn't wiggling--it's contradicting. I'm really confused about why these guys would be flatly contradicting each other.
     
  5. Jan 23, 2010 #4

    sylas

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    Without seeing the lectures myself, I can't say: or even judge whether there really is a true contradiction here. We certainly don't know the age of the universe to 4 decimal places. However, if you take a couple of parameters as given without worrying about the measurement uncertainties, you can calculate the age. I'd really need to see the lectures myself to know what Krauss means by knowing age to four places. As it stands, that simply isn't true, and my first guess is that isn't exactly what he said. But I'm guessing without seeing the lecture for myself.

    Cheers -- sylas
     
  6. Jan 24, 2010 #5
    Both lectures are on youtube: watch?v=7ImvlS8PLIo and watch?v=32wIKaLkvc4 . Krauss says, at 18:40, "We know the age of the universe to four decimal places: 13.72 billion years." Now I realize that he didn't really mean four *decimal* places. He just meant that we know four significant digits. Susskind is nowhere near this confident. Not only on the age, but on a lot of other things that Krauss is unambiguous about.
     
    Last edited: Jan 24, 2010
  7. Jan 24, 2010 #6

    sylas

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    I think Krauss is incorrect, then. I have not looked over the whole lecture, but he is almost certainly refering to

    This report quotes the age of the universe as 13.72 ± 0.12 Gyear. It may be that he has skipped the uncertainty bounds that were published. And even that is only at a one sigma confidence level. His claiming that we know the age to all those figures in 13.72 is a mistake.

    Furthermore, this report is not the whole story. It makes a few assumptions of its own, and so the error bounds should be seen as conditional on the "minimal 6-parameter LCDM". This is a very good bet, but work goes on, and the Hubble constant from that report was given as 70.5 ± 1.3. A more recent estimate has 74.2 ± 3.6.

    Cheers -- sylas
     
    Last edited: Jan 24, 2010
  8. Jan 24, 2010 #7
    Very cool, thanks. Any thoughts on this other big seeming contradiction? Krauss says that everything fits together into a perfect picture of the universe being created by a simple, everyday quantum fluctuation. But Susskind still talks about it as though the beginning was a singularity where all the mass occupied the same point in space. Is this just a semantic difference? The quantum fluctuation idea seems so much more elegant than the singularity idea; seems like we would have done away with it if Krauss is correct.
     
  9. Jan 24, 2010 #8

    sylas

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    I don't think that is really a contradiction.

    I've not seen Krauss' lecture, but given the venue (Atheist Alliance International) it is likely that he's emphasizing how terrific science is at answering questions. To say "everything fits" might be better understood as "everything is consistent with". That is, there's no indication of some great puzzle of the sort that a God can help to answer. The other side of the coin, of course, is that we don't actually know all the physics involved and so confidence that we actually understand how a fluctuation leads to a universe would be wrong. The point is, I think, that there's nothing in principle wrong with the idea and good old science will carrying on the hard work of trying to look at how things happen in the natural world.

    As for the singularity... the word "singularity" is actually a comment about mathematics, not a name for a certain kind of thing. A singularity is really a mathematical discontinuity of some kind. The thing about classical physics (relativity) is that it has one great big divide by zero when you try to apply it to the start of the universe. The proper implication of that is that existing physics breaks down under these conditions, and better physics is needed.

    We do know, with high confidence based on tested theory and repeated observations, that everything we see in the universe was once contained within a vanishingly small volume, with unimaginably large density and temperature and energy. Physics breaks down into a singularity as you push back even a fraction of second from those conditions. So what does go before is unclear. Quantum fluctuation of some kind is a pretty good guess, which fits with what we know about the physics that will need to apply in such conditions.

    But I don't think you can say a lot more than this, and there are some other interesting alternatives (colliding branes, for example).

    Cheers -- sylas
     
  10. Jan 24, 2010 #9
    Everything fits with the universe being created by mating sea turtles or a hyper-dimensional being with a long white beard. We have no real data for what when in the pre-inflationary era, and when you have no real data, then you can make anything fit.

    There are two big problems that I know of with the idea of universe as a quantum fluctuation:

    1) quantum fluctuations happen all of the time. So how come this one quantum fluctuation produced a universe?

    2) the Boltzmann brain problem. Suppose the universe was the result of a quantum fluctuation. Well then small fluctuations are more likely than large ones, and if you have a small fluctuation, you are much more likely to have an isolated self-aware brain pop out of the randomness than an entire universe. So chances are that if the universe was some quantum fluctuation, they odds are that you'd see an isolated brain that in a universe that otherwise made no sense at all.
     
  11. Jan 24, 2010 #10

    Chronos

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    Susskind is rambling, as he is wont to do. Sounds like he is talkiing about time now distance to the surface of last scattering - which obscurs the issue.
     
  12. Jan 24, 2010 #11
    Well, I'm trying to get some answers to specific questions, so I don't want to get sidetracked, but I have some thoughts on your problems with the fluctuation idea:

    1) Well, it's certainly an interesting question that we would like to ultimately answer. But I don't see how the question or the answer relates to the truth/falsehood of the quantum fluctuation idea.

    2) The fact that this problem has a name attached to it suggests that it is some well-known and unsolved problem. So I must be completely foolish in pointing out that there's a serious flaw in the argument: it assumes that the universe as it first appeared was as complex as it is now, or at least stupendously complex. But don't most agree that the universe, when it first appeared, was an entirely (or almost entirely) homogenous something occupied by a single force? Or maybe even just a single force and nothing else? Either one sounds way simpler to me than a brain. But I'm sure that Boltzmann is way smarter than I am.
     
    Last edited: Jan 24, 2010
  13. Jan 24, 2010 #12
    It actually does. We see quantum fluctuations all of the time, but we don't see universes popping into existence all of the time. So you have to explain why the quantum fluctuation that generated the big bang is "different" from the ones we see all the time.

    You run into the 2nd law of themodynamics. It's possible for complex structures to form out of nothing, but that involves an overall decrease in entropy.

    The problem is that in order for things to increase in complexity, the entropy has to decrease. If you have something that has high entropy, you aren't going to see complex structures appear. Boltzman pointed out that if you assume that the 2nd law of themodynamics holds true, you are more likely to see a blip with a small entropy difference than a high entropy one.

    I should point out that the fact that you can make arguments against the idea of the big bang being a quantum fluctuation is what makes it science. The other thing to point out is that there are several alternative ideas for what caused the big bang that are floating out there that have nothing to do with the universe being a quantum fluctuation, and there are ways to explain the fact that the universe seems to have net zero energy in ways that don't involve the universe being a quantum fluctuation.

    Personally my favorite is the "loop gravity cosmology" in which you have a preexisting universe that gets compressed to the point that gravity becomes repulsive.

    So this is hardly unknownable, but I do object strongly to presentations that suggest that we've figured it all out already, because we haven't.
     
  14. Jan 24, 2010 #13
    A lot depends on what you mean by the "quantum fluctuation" idea. If the idea is that the universe just appeared out of nothing, then I wouldn't say that this is a useful idea. If the idea is that the universe appears out of nothing in the way that virtual particles appear in quantum field theory, then this is a useful idea, but one that is *REALLY* hard to get to work, and I don't think that anyone has gotten close.
     
  15. Jan 24, 2010 #14

    marcus

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    A lot of people would agree that loop quantum cosmology (LQC) is one of the more interesting models to study and to try testing observationally.

    I'm pretty sure that in the past two years there have been more articles proposing or groping for ways to test LQC than about testing all other types of quantum cosmology combined. The issue of testing (being able to falsify if something's wrong) is at or near the top of the list for a growing number of people.

    My pick of the authors studing the LQC testing prospects are Aurelien Barrau and Julien Grain. Here are some sample papers of theirs.
    http://arxiv.org/abs/0911.3745
    http://arxiv.org/abs/0910.2892
    http://arxiv.org/abs/0902.3605
    http://arxiv.org/abs/0902.0145

    There are several other authors but these guys are beginning to stand out for me. Basically they are focusing on possibly observable quantum bounce effects that might be seen in the CMB map at higher resolution.

    Twofish, you seem likely to be well informed about this stuff, and you may have looked at some Barrau Grain papers! But I put up the links partly so that anyone else reading thread could get a taste of where QC phenomenology is at. It's rudimentary but beginning to emerge as a research topic.

    If anyone wants to take a look at the whole QC field (quantum cosmology including Loop plus all other approaches) here's a list of QC papers since 2005 ranked by cite-count.
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+cosmology+and+date%3E2005&FORMAT=WWW&SEQUENCE=citecount%28d%29 [Broken]
    Spires database comes up with 372 papers of which the top 50 (citation-wise) are mostly Loop. Probably the top 100 as well, I didn't look. Gives an idea of how the field is shaping up. Since it's a fairly new topic, only a small fraction of these papers would be concerned with observational testing, I expect.
     
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  16. Jan 24, 2010 #15
    LQC is an example how science is self-correcting, and what makes it different from most other fields of human inquiry. Basically, if LQC is the right approach, the string theory ended up being thirty years spent chasing a wild goose. But you don't know that you are chasing a wild goose until after you've chased it, and it's very possible that after looking at LQC for another 20-30 years, we'll come to the conclusion that it just doesn't work, and which point we'll just have to think of something, and if we keep eliminating ideas that don't work, we'll have something left.

    The thing about LQC is that with the LQC framework, the big bang was *not* a quantum fluctuation but something very different.

    One thing that scientists have to be careful about when communicating to laymen is what is speculation and what is accepted consensus. There is a lot of weird stuff where people are just guessing, but there is also a lot of weird stuff that you see in the lab every day. For example, having particles just pop in and out of nowhere is something that you pretty much see every day. Pair production is something that any decent atom smasher will demonstrate, and there are tons of experiments in which you can show that, yes "virtual particles do exist."

    In fact this is the problem that I have with the big bang as a quantum fluctuation. You see quantum fluctuations happen every day, and the math and physics that lets you calculate when and how particles appear out of nowhere is pretty well understood. I just don't see how you can get an entire universe to pop out of nowhere. One big problem is that when you do have quantum fluctuations, is that individual particles appear and disappear out of nowhere, but it's extremely unlikely that you have multiple pairs appear and disappear at the same time. You can argue that it's very unlikely, but you have an infinite universe in which something weird happened, at that point you end up with Boltzmann brain problems.

    Now it's possible that something weird happens with gravity that allows the entire universe to pop out as a quantum fluctuation. But unless you tell what that weird thing is, you are just guessing, and you aren't that much further than someone that just guesses that the universe pops out of nothing,
     
  17. Jan 25, 2010 #16
    Twofish-quant wrote

    I'm not sure I agree with your logic that small fluctuations are more likely than large ones. Yes that is certainly true now because small and large can be measured on some scale - electron-positron pairs yes, Ferrari-antiFerrari pair in my driveway no. However, if the Universe began as a quantum fluctuation there was no scale to measure this fluctuation against. Why would this initial fluctuation be "small"?
     
  18. Jan 25, 2010 #17
    It's not logic. It's observations on how thermodynamic and quantum fluctuations appear to work in the current universe. Now is it possible that near the big bang, thermodynamic and quantum fluctuations behave very, very differently, so that big fluctuations are more likely than small ones. Yes, it's possible, but at the point, you are pretty close to saying "the universe just happened".

    You can use the entropy of the fluctuation to measure the scale. Presumably the universe came from a bath of maximal entropy. If it didn't, then you have to explain where the low entropy region came from.

    Again, all of this assumes that the early universe had some rules that are similar to the ones that we can observe. This may not be true. In fact after playing this game for a few years, you may come up with the conclusion that for the BB to happen you *must* have a situation that large fluctuations happen more frequently than small ones, at which point you think about how that can happen.
     
  19. Jan 25, 2010 #18
    Also there are a lot of people thinking about these sorts of issues, and given a few more years, we'll make some progress on understanding the big bang. Even if it becomes totally obvious that all of the ideas we have on what caused the BB are wrong, that's progress.

    The idea of the BB as a quantum fluctuation is something that people are thinking about, but it's only one of several other ideas (ekpyrotic universe, quantum loop gravity, cosmological natural selection) and probably some more that I haven't heard of.
     
  20. Jan 25, 2010 #19
    Thank you for your answer.
    I get the impression that the idea that the Big Bang was a quantum fluctuation has little to recommend it. The fact that we are here to observe the CMB suggests that whatever happened was big enough for us to arrive on the scene. So we fall into a well of anthropic reasoning. Many other Small Bangs may have occurred which led to nothing. Big fluctuations wouldn't need to be more frequent. Just one really big one would do.
    I realise this sort of speculation is unlikely to bear scientific fruit.
     
  21. Jan 26, 2010 #20
    The good/bad thing about science is that you really don't know whether something works or not until you have lots working for about a decade working on it. One thing that you have to be careful about is how scientists communicate. When I say "this idea has a problem" it's not me saying "this is a totally stupid idea that you shouldn't work on", rather it's me saying "this idea has a problem, it would be wonderful if you go back to the drawing board to figure out ways around it."

    Actually, people are thinking along these lines. The problem with anthropic arguments is that you end up with the wrong answer. What you'd really like to see is that lots of small fluctuations end up nowhere, whereas only big fluctuations give rise to intelligent life. So if this happened, you could argue that there may be huge numbers of fluctuations, and we only see the big ones.

    The trouble is that this isn't the answer you get when you work through the numbers. You end up finding that if you use anthropic arguments, that you end up with an "disembodied brain" being far, far, far more likely than a rational universe. The good part of this is that you can then go through the argument and go point by point to figure out what assumption you are making is wrong.
     
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