Why Cosmology Isn’t Boring

As promised yesterday, here’s a copy of the slides I used for my talk to the ~150 participants of the collaboration meeting of the Dark Energy Survey that’s going on here this week at Sussex. The title is a reaction to a statement I heard that recent developments in cosmology, especially from Planck, have established that we live in a “Maximally Boring Universe”. I the talk I tried to explain why I don’t think the standard cosmology is at all boring. In fact, I think it’s only now that we can start to ask the really interesting questions.

At various points along the way I stopped to sample opinions…

IMG-20141022-00439

I did however notice that Josh Frieman (front left) seemed to vote in favour of all the possible options on all the questions.  I think that’s taking the multiverse idea a bit too far..

 

44 Responses to “Why Cosmology Isn’t Boring”

  1. Interesting that in your historical rogues’ gallery, only Rien got the parameters right. I don’t think of him as a cosmologist, but rather as a large-scale structure person (and as a gourmet).

    • I recognize and/or know personally most of them, but who are VI and PK?

      • telescoper Says:

        Vincent Icke and Peter Katgert

      • I recognize Vincent’s name but don’t think we’ve actually met. I admit that I’ve never heard of Peter Katgert.

      • telescoper Says:

        I believe he retired a few years ago…

      • Yes, of course, but that in itself is not a reason I shouldn’t have met him. I’ve met the late Allan Sandage, who was born in 1926. Last December, I had the pleasure of meeting for the first time personally one of my heroes, Wolfgang Rindler, who is now 90. I met Jack Steinberger at the last Moriond meeting; he is now 93.

    • “large-scale structure person”

      Although Rien is quite tall (as are many if not most Dutch), I don’t know much about his structure, so that should be “large-scale–structure person”. rather than “large-scale structure person”. 🙂 Yes, things like this do make a difference. Don’t forget the comma when you announce “Let’s eat, children!”

      The case of the high-energy physicist, however, really is ambiguous, although both meanings are quite distinct from the high energy physicist, though I suppose that one could belong to all three categories at once. On the other hand, the difference between a cold-fusion physicists and a cold fusion physicist should be clear to all.

  2. Anton Garrett Says:

    Slide 25 giving the various meanings of Inflation forgot the economic one.

  3. And don’t forget problems with the small-scale universe.

  4. Peter, in your slides I am surprised that you didn’t say much or deleve too much into dark matter or baryo-genesis or (origin of matter anti-matter symmetry). Are these questions resolved according to you?
    Thanks

    • telescoper Says:

      I did talk about both of these issues; they both appeared in the list of “questionable” things about the standard cosmological model.

  5. Thanks, Peter. Anyhow what I meant was let’s say there is no laboratory evidence for cold dark matter (or physics models which predict these theories) in the next 10 years, then will there be another paradigm shift in standard model of cosmology from Lambda CDM to LAMBDA + B-E condensate dark matter (or something like that).

    And on the astronomical side besides core cusp issue, due to the simple regularities in cosmic structure such as Tully-fisher relation, Renzo’s rule, dark matter dominated structures have the same asymptotic values for centripetal acceleretaions (basically something on which Stacy for eg has written a lot) bother you?

    • MOND, of course. However, I think even most of the MOND people now say that there is a need for dark matter as well. (On the other hand, even the direct detection of dark matter should not lead one to the conclusion that MOND is not needed.)

      Laboratory detection? At best, non-detections could rule out specific theories, but CDM is so generic that a non-detection won’t rule out the idea per se. Think of neutrinos, where there was a pretty definite prediction: it was decades between theory and detection.

      B-E condensate dark matter seems even less motivated.

      Using the term “paradigm shift” is dangerous, since many people will think of Thomas Kuhn, though his bad caricature of science is completely unrealistic. Yes, one can speak of paradigms in cosmology, but not in the sense of Kuhn.

  6. Philip, I agree with you about MOND. However the observations
    I pointed and which you know about must be explained within context of standard cosmological model (or someone should
    point out why these observations are wrong be ignored).

    I don’t see any discussion or mention of these points in most cosmology talks. The only attempt I have seen is a paper about 14 years ago Mike Turner (who is a Lcdm advocate) proposed that LCDM explains milgrom’s low http://arxiv.org/abs/astro-ph/0107284
    But most cosmologists (whom I have asked) are unaware of this paper and you can check from the citations of this that there is very little follow-up to this.

    I think any hypothesis on which we built an edifice of standard cosmology should be based on evidence in a laboratory based experiment. Otherwise there is no difference between positing dark matter and positing ether.

    Also bose-einstein condensate is as well or ill-motivated from astrophysical observations as WIMP or axion(two candidates
    which are favorites of particle physicists). At any rate there are a whole zoo papers in last one year on dark matter with de-broglie wave length of order galactic scales (partly because of failure of LHC to detect anything) and I want to ask if we again are at a point
    where the standard model of cosmology is changing course from
    LCDM to LWave-like DM.

    • I think one has to distinguish between, on the one hand, LCDM as a basic framework, where it works way to well to be completely wrong and has survived many predictions, and, on the other hand, models of galaxy formation etc which include much more “gastrophysics”. Using “LCDM” for both is confusing. Some people use “big bang” to mean essentially all of cosmology, including speculative ideas, which is certainly not completely correct (if only because it contains some contradictory ideas among all the hypotheses). Actually, it means that the universe has been expanding from a much denser and hotter state, for which there is an overwhelming amount of evidence.

      One shouldn’t throw the baby out with the bathwater. If there are problems with someone’s galaxy-formation simulations, then one should criticize them, and not the underlying cosmology. (Alternatively, it could be that some observations and/or their interpretation is wrong.)

    • “I think any hypothesis on which we built an edifice of standard cosmology should be based on evidence in a laboratory based experiment. Otherwise there is no difference between positing dark matter and positing ether.”

      There is a difference. We can’t (yet) detect the expansion of the universe in the laboratory, but that is not a good reason to doubt it. As always, a theory is good if it makes testable predictions.

      • That’s because expansion of universe can never be verified in a laboratory based experiment and its very hard to do Sandage
        type tests to find if redshift has changed as a function of time.
        Same is true with dark energy. However, dark matter by definition is something by definition is based on fundamental physics and is a non-baryonic
        has a lifetime longer than age of universe, zero pressure .
        if you assume its a thermal relic then it has electroweak scale cross-sections. only problem is that there is so far there doesn’t seem to exist such a candidate in nature.

      • telescoper Says:

        The neutrino satisfies all the requirements you state…

      • There is currently an ESO key programme to do precisely the Sandage test you mention.

      • “The neutrino satisfies all the requirements you state…”

        But the mass is too small to be CDM.

      • Peter, an ordinary massive neutrino is only a hot dark matter candidate and not a CDM.
        Else of course the problem would have been solved long time back and no would invest so much in underground DM experiments of look for indirect DM annihilation signals in gamma ray and positron telescopes.

      • telescoper Says:

        But it is one particle that satisfies the list of conditions you specified. Why should there not be others?

      • Peter, my point is we should have seen “some” laboratory evidence for such a particle or the theories which predict such
        a particle. We have plenty of good data from accelerators.
        10 years ago most people would have told you that
        we would know what dark matter candidate is, by now
        (esp if it has weak scale cross-sections and is a thermal relic)

        and I think that is also one reason for the rise in no of papers on “quantum degenerate dark matter” (such as BE condensate of degenerate fermion matter)
        for example :
        http://arxiv.org/abs/1409.3167
        http://arxiv.org/abs/1409.7347
        http://arxiv.org/abs/1406.6586
        and probably many more

    • Also bose-einstein condensate is as well or ill-motivated from astrophysical observations as WIMP or axion(two candidates which are favorites of particle physicists).”

      A Bose-Einstein condensate, by definition, consists of bosons, so this theory has to explain not only what particles are involved and where they come from but also why it is in a Bose-Einstein condensate.

  7. “I don’t see any discussion or mention of these points in most cosmology talks. The only attempt I have seen is a paper about 14 years ago Mike Turner (who is a Lcdm advocate) proposed that LCDM explains milgrom’s low http://arxiv.org/abs/astro-ph/0107284
    But most cosmologists (whom I have asked) are unaware of this paper and you can check from the citations of this that there is very little follow-up to this.”

    I guess low—> law.

    The paper has 37 citations, which is not that bad. Of Turner’s approximately 700 papers, only 175 or so have more citations. 🙂

    • Should it be more highly cited? Who would cite it? Only someone wanting to demonstrate that Milgrom’s Law does not necessarily imply MOND. But, for various reasons, few people wish to demonstrate this (since, again for various reasons, they never believed it in the first place). From the paper:

      Separating the important clues from the misleading coincidences is at the heart of scientific
      creativity.
      Hoyle’s observation that the energy released in burning 25% of the Hydrogen
      to Helium is approximately equal to that of the CMB suggested a non big-bang origin for
      the CMB (see e.g., Burbidge & Hoyle, 1998). In the end, it turned out to be a misleading
      coincidence. Within the big-bang model, Holyle’s coincidence is explained by the near
      equality of the dimensionless amplitude of density perturbations ǫ and the product of the
      efficiency of nuclear burning times
      B: To make stars by the present epoch,
      γ must be ∼ ǫ,
      which coincidentally is equal to the energy that would be released in producing the observed
      Helium abundance (Martin Rees, private communication).

      CDM not only predicts Milgrom’s Law (at least over a order of magnitude range in luminosity
      from 0.1L⋆ to L⋆), but also accounts for a wealth of other cosmological observations.
      This suggests to us that Milgrom’s Law is a misleading coincidence rather than evidence for
      a modification of Newtonian dynamics.

      I have marked the key statement in bold. In an RAS talk, Turner mentioned the coincidence between the Hubble time and the age of the universe, a coincidence which holds only now during the history of the universe. Donald Lynden-Bell then asked whether he was worried about the fact that the angular sizes of the Sun and the Moon are the same, to which Turner replied that sometimes coincidences tell you something and sometimes they don’t.🙂

  8. Philip, I am not sure whether you will find any other cosmologist agreeing to this statement from Turner’s paper which you have mentioned below

    “CDM not only predicts Milgrom’s Law (at least over a order of magnitude range in luminosity
    from 0.1L⋆ to L⋆), but also accounts for a wealth of other cosmological observations.”

    • I would say that most cosmologists agree with this statement.

      • To be clear, this was written when I was thinking about “a wealth of other cosmological observations”.

        The wealth of other observations is also important, of course. If someone provides an alternative explanation for one thing, or for a limited class of phenomena, as MOND does, one still has to explain all the other things which LCDM explains.

  9. Sorry I meant only this part of the sentence
    “CDM not only predicts Milgrom’s Law (at least over a order of magnitude range in luminosity”
    Peter: do you agree this assertion by Turner and is this seen
    in people who do LCDM simulations with baryons?
    If so, I stand corrected. But all the people who work on LCDM, whom I have asked are not aware of this paper.

    (In fact 2 years ago I asked Rocky in a seminar about the same questions and he was dismissive of the observational pieces of evidence. If you are right, then he would have said that LCDM
    naturally predicts Milgrom’s law as shown by his colleague in 2001)

    • Rocky can be dismissive at times. He’s a nice guy, though. 🙂 As late as 1998 or even 2000 (I’ll have to check), his answer to one question was “Omega matter is 1, next question”, and this not long after Ray Carlberg in a previous talk had made a good observational case for a low Omega. 😐 On the other hand, I wouldn’t expect even Rocky to have all of Turner’s 700 papers in his head (and Turner was not even the first author of this paper).

    • Milgrom himself replies to the Kaplinghat and paper here: http://adsabs.harvard.edu/abs/2002ApJ…571L..81M

      Indeed, the KT paper addresses only one aspect of MOND. Milgrom claims that a) it doesn’t explain everything in MOND and b) that simulations don’t indicate this, at least at the time that the paper was written. Keep in mind, however, that only recently have realistic simulations with baryons and CDM become available and it does appear, at least in some areas, that better simulations have fewer discrepancies with observations.

  10. Ok, but if LCDM naturally predicts milgrom’s law is an accepted fact among all cosmologists (irrespective of who wrote the first paper) then surely he (or anyone else who talks about LCDM) should mention it on the very first slide of their talks.

    • The paper is a theoretical prediction. Whether the effect is seen in simulations now, I don’t know. I wouldn’t expect people to mention it as a matter of course, though, since most speakers and most in the audience are actually not interested in MOND at all.

      Having worked a couple of years at the same institute as Bob Sanders, my impression is that many MOND people know more about observational astrophysics than many cosmologists. My impression is that most cosmologists have not investigated MOND in detail. That doesn’t mean that MOND is right, of course, but, echoing Fermi, if it is wrong, then it is wrong on a higher level.

      • Philip, I agree that most speakers and most in audience are not
        interested in MOND. However my question is are the empirical observations which led Milgrom to propose MOND correct
        and if so are they consequence of LCDM model or a complete fluke or are they pointing to some physics we have missed in dark sector?

      • I think it’s not completely clear which of the three possibilities you mention is correct. I don’t think anyone serious doubts the observations. As such, any theory must reproduce them. Galaxy formation is not easy, and perhaps it is too easy to say if there is a natural explanation without MOND. On the other hand, MOND has no framework for galaxy formation either, mainly because there is no relativistic version which is still compatible with observations.

  11. Readers here might be interested in this poll. Of course, being anonymous and on the internet, the statistical significance of the results are questionable at best. 😐

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