The Zel’dovich Universe – Day 1 Summary

I’m up possibly bright but definitely early to get ready for day two of IAU Symposium No. 308 The Zel’dovich Universe. The weather was a bit iffy yesterday, with showers throughout the day, but that didn’t matter much in practice as I was indoors most of the day attending the talks. I have to deliver the conference summary on Saturday afternoon so I feel I should make an effort to attend as much as I can in order to help me pretend that I didn’t write my concluding talk in advance of the conference.

Day One began with some reflections on the work and personality of the great Zel’dovich by two of his former students, Sergei Shandarin and Varun Sahni, both of whom I’ve worked with in the past.
zeldovichZel’dovich (left) was born on March 8th 1914. To us cosmologists Zel’dovich is best known for his work on the large-scale structure of the Universe, but he only started to work on that subject relatively late in his career during the 1960s. He in fact began his life in research as a physical chemist and arguably his greatest contribution to science was that he developed the first completely physically based theory of flame propagation (together with Frank-Kamenetskii). No doubt he also used insights gained from this work, together with his studies of detonation and shock waves, in the Soviet nuclear bomb programme in which he was a central figure, and which no doubt led to the chestful of medals he’s wearing in the photograph. In fact he was awarded the title of  Hero of Socialist Labour no less than three times.

My own connection with Zel’dovich is primarily through his scientific descendants, principally his former student Sergei Shandarin, who has a faculty position at the University of Kansas, but his work has had a very strong influence on my scientific career. For example, I visited Kansas back in 1992 and worked on a project with Sergei and Adrian Melott which led to a paper published in 1993, the abstract of which makes it clear the debt it owed to the work of Ze’dovich.

The accuracy of various analytic approximations for following the evolution of cosmological density fluctuations into the nonlinear regime is investigated. The Zel’dovich approximation is found to be consistently the best approximation scheme. It is extremely accurate for power spectra characterized by n = -1 or less; when the approximation is ‘enhanced’ by truncating highly nonlinear Fourier modes the approximation is excellent even for n = +1. The performance of linear theory is less spectrum-dependent, but this approximation is less accurate than the Zel’dovich one for all cases because of the failure to treat dynamics. The lognormal approximation generally provides a very poor fit to the spatial pattern.

The Zel’dovich Approximation referred to in this abstract is based on an extremely simple idea but which, as we showed in the above paper, turns out to be extremely accurate at reproducing the morphology of the “cosmic web” of large-scale structure.

Zel’dovich passed away in 1987. I was a graduate student at that time and had never had the opportunity to meet him. If I had done so I’m sure I would have found him fascinating and intimidating in equal measure, as I admired his work enormously as did everyone I knew in the field of cosmology. Anyway, a couple of years after his death a review paper written by himself and Sergei Shandarin was published, along with the note:

The Russian version of this review was finished in the summer of 1987. By the tragic death of Ya. B.Zeldovich on December 2, 1987, about four-fifths of the paper had been translated into English. Professor Zeldovich would have been 75 years old on March 8, 1989 and was vivid and creative until his last day. The theory of the structure of the universe was one of his favorite subjects, to which he made many note-worthy contributions over the last 20 years.

As one does if one is vain I looked down the reference list to see if any of my papers were cited. I’d only published one paper before Zel’dovich died so my hopes weren’t high. As it happens, though, my very first paper (Coles 1986) was there in the list. That’s still the proudest moment of my life!


We then went into a Dick Bond Special, with a talk entitled: From Superweb Simplicity to Complex Intermittency in the Cosmic Web. The following pic will give you a flavour:


It’s all very straightforward, really. Um…

The rest of the day consisted of a number of talks about the Cosmic Web of large-scale structure using techniques inspired by the work of Zel’dovich, particularly the Zel’dovich approximation which I’ve mentioned already. There were many fascinating talks but I had to single out Johan Hidding of Groningen for the best use of graphics. Here’s a video of his from Youtube as an example:

Well, I must get going for the start of Day Two. The first session starts at 9am (7am UK time) and the day ends at 19.30. Conferences like this are hard work!

PS. If anyone reading this either at the conference or elsewhere has any questions or issues they would like me to raise during the summary talk on Saturday please don’t hesitate to leave a comment below or via Twitter using the hashtag #IAU308.


19 Responses to “The Zel’dovich Universe – Day 1 Summary”

  1. George Jones Says:

    Astrophysicist Kip Thorne, who spoke Russian, was a good friend of Zel’dovich. An interesting sub-theme of Thorne’s book “Black Holes and Time Warps” is physics in the Soviet Union. Pages 241-243 detail an absolutely fascinating story about Thorne’s efforts to have, on separate occasions, Zel’dovich and American expert Stirling Colgate explain flame propagation.

    Thorne wanted to know for astrophysical applications, but both Zel’dovich (uncharacteristically) and Colgate clammed up when pressed for details.

    A quote about Zel’dovich from page 270 of Thorne’s book follows. Considering the physicists that Thorne has met and worked with, the last sentence is quite something.

    “Impatient with perfectionists like me, who insist on getting all the details of a calculation right, Zel’dovich cared only about the main concepts. Like Oppenheimer, he could scatter irrelevant details to the winds and zero in, almost unerringly, on the central issues. A few arrows and curves on the blackboard, an equation not longer than half a line, a few sentences of vivid prose, with these he would bring his team to the heart of a research problem.

    He was quick to judge an idea or a physicist’s worth, and slow to change his judgments. He could retain faith in a wrong snap judgment for years, thereby blinding himself to an important truth, as when he rejected the idea that tiny black holes can evaporate. But when (as was usually the case) his snap judgments were right, they enabled him to move forward across the frontiers of knowledge at a tremendous pace, faster than anyone I have ever met.”

  2. “It’s all very straightforward, really. Um…”

    I’m surprised that no-one has sued Dick Bond for discriminating against colour-blind people. 🙂

  3. “I have to deliver the conference summary on Saturday afternoon so I feel I should make an effort to attend as much as I can in order to help me pretend that I didn’t write my concluding talk in advance of the conference.”

    When delivering the conference summary, it should be obvious that you wrote it the night before (or at least wrote about the day before the night before and wrote none of it before you had arrived). With a regular talk, it’s probably not a good idea that it is obvious you wrote it the night before. (Actually, the best talk I ever gave I wrote the night before, but I don’t think this was obvious.) I was once at a conference where a talk had to be cancelled because the speaker had broken his arm. 🙂 (Bonus points to anyone who can guess the speaker, especially if you weren’t at said conference.)

  4. “If anyone reading this either at the conference or elsewhere has any questions or issues they would like me to raise during the summary talk on Saturday please don’t hesitate to leave a comment below”

    These days, probably almost all talks are in some electronic form (though I do recall Penrose with a table of transparencies at the last Texas Symposium). One thing I have seen some of the Great Summarizers do (I assume you aspire to join their noble ranks) is to get one slide from each talk to show at the summary (and, of course, say something about it).

    Even if there are proceedings (which, being an IAU Symposium, there probably will be), it would be nice to put the actual talks on the web, or at least the slides. Many conferences now do this.

  5. Do tell us about “Conference Dinner House of the Blackheads, Pikk street 26”.

    • telescoper Says:

      I thought you might spot that.

      • I just now appreciated the pun.

        Interestingly, in German the term for “blackhead” in the sense of an oxidized pimple is “Mitesser”, literally someone who eats with someone else. 🙂

        The etymology (which is the same in Latin) comes from the belief that they were caused by worms eating into the skin. Certainly some worms do eat into the skin, and since blackheads look like worms when squeezed out, it was believed that they really were worms eating into the flesh.

        I’m sure you’ll remember this during the conference dinner. 🙂

      • telescoper Says:

        I just got back from Lunch. Thanks for that.

  6. Shantanu Says:

    I hope you or someone else brings up the issue of whether one should still believe in CDM if LHC and direct dark matter detectors continue to see nothing for the next 10 years.
    Stacy often points that he has been hearing the statement “dark matter will be seen in the next 5 years for the past 25 years”.
    In fact the recent DM conference at CFA, many people are pushing for self-interacting dark matter

    • Would you have rejected belief in the neutrino before it was detected?

      • A more perceptive person with adequate scientific training would know that one does not “believe” in hypothetical entities before there is observational evidence for them. The scientist would consider the existence of the hypothetical entities a possibility. If observations appeared to detect the entities, then and only then would the scientist begin to believe in their likely existence.


  7. Shantanu Says:

    Every idea has some “expiry” date.
    Neutrinos were detected about 20 years after they were proposed
    (which is not too much) and certainly we had a good idea of what mass range and cross-section a neutrino is expected to be found.
    However at some point if neutrinos failed to be detected people would have rejected belief in neutrinos.

    To give another particle physics example, at one point people
    were confident of detecting magnetic monopoles. but failure
    to detect them in the expected mass range has rejected belief
    in their existence and I doubt any particle physicist believes
    they will ever be found.

    unfortunately with Cold dark matter its a case of a paradigm
    where we are completely blind. The CDM mass and interaction cross-section ranges over 30 orders of magnitude.
    however a some point you need invoke occam’s razor and ask
    what is the most plausible CDM candidate and physics principle
    which would give rise to it. and the answer to that is WIMP miracle
    and that idea is already in trouble given the LHC results.

    Anyhow at the texas conference (which you were also present)
    Rocky Kolb mentioned that WIMPs would be seen this decade(
    but then added that he had said the same thing last decade also).
    Astrophysicists really need to introspect whether they would
    still believe in CDM if nothing is found after 2020.

    But given the large no of talks on SIDM at the CFA dark matter conference, maybe a paradigm shift is already happening
    from Lambda CDM to Lamda Self-interacting dark matter.

    • “To give another particle physics example, at one point people were confident of detecting magnetic monopoles. but failure to detect them in the expected mass range has rejected belief in their existence and I doubt any particle physicist believes they will ever be found.”

      I don’t know enough about particle physics to say whether we should expect to see magnetic monopoles or not. Certainly the GUTs of the 1970s which predicted them have been ruled out due to lack of observed proton decay. On the other hand, Dirac’s argument is quite elegant.

      However, since the lack of observable monopoles is often cited as something which inflation explains, probably some people still believe in them (and probably some have not updated their arguments since the 1970s).

  8. Shantanu Says:

    Philip, today no one mentions the lack of magnetic monopoles
    as motivation for inflation.

    • Some people do, even some high-profile physicists who work on inflation.. OK, you could say they are misguided, but that’s a different claim. I’ll see if I can find a true Scotsman who still mentions them.

  9. Shantanu Says:

    Philip can you point me to a recent talk or paper which still mentions the absence of magnetic monopoles as an argument
    for inflation? Anyhow there maybe a few isolated cases, but largely
    no one mentions the issue of magnetic monopoles. also GUT
    theories are also not that fashionable anymore.

    Anyhow in 2007 there was a 25 years after inflation meeting in Cambridge
    and in Mike turner’s summary talk he quoted Demos Kazanas
    comment in the meeting
    “whatever happened to the magnetic monopole problem”
    which best summarizes the point.

    Peter: maybe you could summarize some issues discussed
    in the concluding panel debate.

    • I’m not a particle physicist, so I don’t know how common it is to think of the monopole problem as a problem. However, I recently read Sean Carroll’s From Eternity to Here, which is a semi-popular–science book, and he definitely mentions the monopole problem in connection with inflation.

      As for Mike Turner, he’s a funny speaker, but I wouldn’t interpret an off-the-cuff quote as evidence that no-one thinks it is a problem anymore.

      You might be right, of course, and, as I said, “probably some have not updated their arguments since the 1970s”.

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