Well, it’s Sunday morning and it’s raining in Tallinn. I’ve got a few hours to kill but fortunately don’t have to check out of the hotel until noon so I thought I’d briefly summarize Days Five and Six of IAU Symposium No. 308, The Zel’dovich Universe just to complete the story.
Day Five (Friday) began with a talk by Jaan Einasto, recent winner of the Gruber Prize for Cosmology. As you can see from this picture I took before his talk commenced, the topic was Yakov Zel’dovich and the Comic Sans Cosmic Web Paradigm:
The following talk was by the ebullient Rashid Sunyaev, whose name is associated with Zel’dovich in so many contexts, including the Sunyaev-Zeld’ovich effect. Sunyaev is such a big personality that he is unconstrained by the banal notions of time, and his talk set the schedule back for the rest of the morning. Among the things I remember from his contribution was a discussion of the Berkeley-Nagoya distortion. This was a hot topic during the time I was a graduate student, as it was a measurement that suggested the spectrum of the cosmic microwave background departed significantly from a black-body (Planck) curve in the Wien part of the spectrum; this is now usually known as a y-distortion. Anyway, lots of theorists wrote papers explaining the measured excess in terms of this that and the other and then it was shown to be an error; the excess emission came not from the Big Bang but from the exhaust of the rocket carrying the measurement. The thing I remember most strongly about this was that as soon as the error was identified it ceased to be the Berkeley-Nagoya distortion and became instead the Nagoya-Berkeley distortion…
Rashid Sunyaev was himself a winner of the Gruber prize some years ago, as indeed were Dick Bond and Brent Tully who spoke erlier in the conference, so the organizers decided to form a Gruber-panel to discuss various topics suggested by the audience. Here is Sunyaev, hogging the microphone:
Carlos Frenk is also a Gruber prize winner, but he only arrived after lunch so wasn’t part of this discussion. The afternoon was all about cosmological simulations of various aspects of the Cosmic Web. This gives me an opportunity to repeat how the Oxford English Dictionary defines “simulation”:
1. a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.
b. Tendency to assume a form resembling that of something else; unconscious imitation.
In the World Cup players can even get sent off for simulation, although regrettably they seldom are.
Anyway, Friday evening found us at the famous House of Blackheads (aptly on Pikk Street) for an evening of very long speeches punctuated by small amounts of food and wine (and of course some very lovely music as I described yesterday). When the party was over a group of us adjourned to a local bar, from which I returned to my hotel at about 2am.
Day Six was a half-day, with some very interesting talks about gravitational len-sing in the first session and “superstructures” in the cosmic web. Then we were into the final furlong as it were. Nick Kaiser was put in Session (No. 21) all of his own. As usual, given how annoyingly brilliant he is, Nick gave fabulously interesting talk full of insights and ideas. The organizers had definitely saved the best for second-to-last.
Then, after five-and-a-half days and almost 100 talks, it was down to me to give the conference summary. Obviously I couldn’t really summarize all that such I just picked up a few things that occurred to me during the course of the conference (some of which I’ve written about over the last week or so on this blog) and made a few jokes, primarily at the expense of Carlos Frenk. I was interested to see that signs like this had been put up around Tallinn advertising my talk:
The OMG and WOW are self-explanatory, but I was a bit confused about the SAH so I googled it and found that it means the Society of Architectural Historians. I’ve never heard it put quite like that before, but I guess that’s what we cosmologists are: trying to understanding the origins and time evolution of the architecture of the Universe.
A number of speakers at this conference referred to a conference in Hungary in 1987 at which they had met Zel’dovich (who died later that year). I was a graduate student (at Sussex) at that time and owing the shortage of travel funds I wasn’t able to go; I went to a meeting in Cambridge called The Post-Recombination Universe instead. If memory serves that’s when I gave my first conference talk. Anyway, Carlos Frenk gave a talk at that meeting in Hungary which he decribed in his talk at this conference on Friday afternoon. Somebody back in 1987 had written a series of limericks to describe that meeting, so I was challenged to come up with one to conclude this one. Here’s my effort, which is admittedly pretty feeble, but at least the sentiments behind it are genuine..
In Tallinn (IAU 308)
The sessions invariably ran late
But despite being tired
We still much inspired
By Yakov Zel’dovich (the Great).
. Their (Eulerian) positions at some later time 
are taken to be![\vec{r}(\vec(q),t) = a(t) \vec{x}(\vec{q},t) = a(t) \left[ \vec{q} + b(t) \vec{s}(\vec{q},t) \right].](https://s0.wp.com/latex.php?latex=%5Cvec%7Br%7D%28%5Cvec%28q%29%2Ct%29+%3D+a%28t%29+%5Cvec%7Bx%7D%28%5Cvec%7Bq%7D%2Ct%29+%3D+a%28t%29+%5Cleft%5B+%5Cvec%7Bq%7D+%2B+b%28t%29+%5Cvec%7Bs%7D%28%5Cvec%7Bq%7D%2Ct%29+%5Cright%5D.&bg=000000&fg=B0B0B0&s=0&c=20201002)
coordinates are comoving, i.e. scaled with the expansion of the Universe using the scale factor 
. The displacement 
between initial and final positions in comoving coordinates is taken to have the form
is a kind of velocity potential (which is also in linear Newtonian theory proportional to the gravitational potential).If we’ve got the theory right then the gravitational potential field defined over the initial positions is a Gaussian random field. The function 
is the growing mode of density perturbations in the linear theory of gravitational instability.
In the Dark 