Archive for Marc Davis

The Zel’dovich Universe – Day 4 Summary

Posted in History, The Universe and Stuff with tags , , , , , , , on June 27, 2014 by telescoper

And on the fourth day of this meeting about “The Zel’dovich Universe”  we were back to a full schedule (9am until 7.30pm) concentrating on further studies of the Cosmic Web. We started off with a discussion of the properties of large-scale structure at high redshift. As someone who’s old enough to remember the days when “high redshift” meant about z~0.1 the idea that we can now map the galaxy distribution at redshifts z~2. There are other measures of structure on these huge scales, such as the Lyman alpha forest, and we heard a bit about some of them too.

The second session was about “reconstructing” the Cosmic Web, although a more correct word have been “deconstructing”. The point about this session is that cosmology is basically a backwards subject. In other branches of experimental science we set the initial conditions for a system and then examine how it evolves. In cosmology we have to infer the initial conditions of the Universe from what we observe around us now. In other words, cosmology is an inverse problem on a grand scale.  In the context of the cosmic web, we want to infer the pattern of initial density and velocity fluctuations that gave rise to the present network of clusters, filaments and voids. Several talks about this emphasized how proper Bayesian methods have led to enormous progress in this field over the last few years.

All this progress has been accompanied by huge improvements in graphical visualisation techniques. Thirty years ago the state of the art in this field was represented by simple contour plots, such as this (usually called the Cosmic Chicken):

chicken

You can see how crude this representation is by comparing it with a similar plot from the modern era of precision cosmology:

chicken

Even better examples are provided by the following snapshot:

IMG-20140626-00352

It’s nice to see a better, though still imperfect,  version of the chicken at the top right, though I found the graphic at the bottom right rather implausible; it must be difficult to skate at all with those things around your legs.

Here’s another picture I liked, despite the lack of chickens:

IMG-20140626-00353

Incidentally, it’s the back of Alar Toomre‘s head you can see on the far right in this picture.

The afternoon was largely devoted to discussions of how the properties of individual galaxies are influenced by their local environment within the Cosmic Web. I usually think of galaxies as test particles (i.e. point masses) but they are interesting in their own right (to some people anyway). However, the World Cup intervened during the evening session and I skipped a couple of talks to watch Germany beat the USA in their final group match.

That’s all for now. Tonight we’ll have the conference dinner, which is apparently being held in the “House of Blackheads” on “Pikk Street”. Sounds like an interesting spot!

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The Zel’dovich Universe – Day 3 Summary

Posted in History, The Universe and Stuff with tags , , , , , , on June 26, 2014 by telescoper

Day Three of this meeting about “The Zel’dovich Universe” was slightly shorter than the previous two, in that it finished just after 17.00 rather than the usual 19.00 or later. That meant that we got out in time to settle down for a beer in time the World Cup football. I watched an excellent game between Nigeria and Argentina, which ended 3-2 to Argentina but could have been 7-7. I’ll use that as an excuse for writing a slightly shorter summary.

Anyway we began with a session on the Primordial Universe and Primordial Signatures led off by Alexei Starobinsky (although there is some controversy whether his name should end -y or -i). Starobinsky outlined the theory of cosmological perturbations from inflation with an emphasis on how it relates to some of Zel’dovich’s ideas on the subject. There was then a talk from Bruce Partridge about some of the results from Planck. I’ve mentioned already that this isn’t a typical cosmology conference, and this talk provided another unusual aspect in that there’s hardly been any discussion of the BICEP2 results here. When asked about at the end of his talk, Bruce replied (very sensibly) that we should all just be patient.

Next session after coffee was about cosmic voids, kicked off by Rien van de Weygaert with a talk entitled “Much Ado About Nothing”, which reminded me of the following quote from the play of the same name:

“He hath indeed better bettered expectation than you must expect of me to tell you how”

The existence of voids in the galaxy distribution is not unexpected given the presence of clusters and superclusters, but they are interesting in their own right as they display particular dynamical evolution and have important consequences on observations. In 1984, Vincent Icke proved the so-called “Bubble Theorem” which showed that an isolated underdensity tends to evolve to a spherical shape.Most cosmologists, including myself, therefore expected big voids to be round, which turns out to be wrong; the interaction of the perimeter of the void with its surroundings always plays an important role in determining the geometry. Another thing that sprang into my mind was a classic paper by Simon White (1979) with the abstract:

We derive and display relations which can be used to express many quantitative measures of clustering in terms of the hierarchy of correlation functions. The convergence rate and asymptotic behaviour of the integral series which usually result is explored as far as possible using the observed low-order galaxy correlation functions. On scales less than the expected nearest neighbour distance most clustering measures are influenced only by the lowest order correlation functions. On all larger scales their behaviour, in general, depends significantly on correlations of high order and cannot be approximated using the low-order functions. Bhavsar’s observed relation between density enhancement and the fraction of galaxies included in clusters is modelled and is shown to be only weakly dependent on high-order correlations over most of its range. The probability that a randomly placed region of given volume be empty is discussed as a particularly simple and appealing example of a statistic which is strongly influenced by correlations of all orders, and it is shown that this probability may obey a scaling law which will allow a test of the small-scale form of high-order correlations.

The emphasis is mine. It’s fascinating and somewhat paradoxical that we can learn a lot about the statistics of where the galaxies are fom the regions where galaxies are not.

Another thing worth mentioning was Paul Sutter’s discussion of a project on cosmic voids which is a fine example of open science. Check out the CosmicVoids website where you will find void catalogues, identification algorithms and a host of other stuff all freely available to anyone who wants to use them. This is the way forward.

After lunch we had a session on Cosmic Flows, with a variety of talks about using galaxy peculiar velocities to understand the dynamics of large-scale structure. This field was booming about twenty years ago but which has been to some extent been overtaken by other cosmological probes that offer greater precision; the biggest difficulty has been getting a sufficient number of sufficiently accurate direct (redshift-independent) distance measurements to do good statistics. It remains a difficult but important field, because it’s important to test our models with as many independent methods as possible.

I’ll end with a word about the first speaker of this session, the Gruber prize winner Marc Davis. He suffered a stroke a few years ago which has left him partly paralysed (down his right side). He has battled back from this with great courage, and even turned it to his advantage during his talk when he complained about how faint the laser pointer was and used his walking stick instead.

IMG-20140625-00351

D+E+F+W=$500000

Posted in The Universe and Stuff with tags , , , , , , , on June 2, 2011 by telescoper

Just a quickie this fine summer morning to pass on the news – for those of you who haven’t heard yet – that this year’s Gruber Prize for Cosmology has been awarded to Marc Davis (Berkeley, USA), George Efstathiou (Cambridge, UK), Carlos Frenk (Durham, UK) and Simon White (Garching, Germany). This prestigious award is given for their pioneering work on the Cold Dark Matter model of structure formation, which included some of the first large-scale N-body computer simulations. The “Gang of Four” produced a number of papers during the 1980s that established the idea that galaxies form by hierarchical clustering from small initial fluctuations in a matter distribution dominated by massive collisionless non-baryonic particles, the most famous of their papers being pretty universally referred to as DEFW.

In fact, if you’ll forgive me going on a trip down memory lane, that paper, published in 1985, was one of the first papers I read when I started my research degree the same year at Sussex. It was back in the days when everyone seemed to use a VAX for big computing jobs and the simulations presented in that paper involved a mere 323 = 32768 particles. You could probably run that kind of simulation on a mobile phone these days!

This early work on Cold Dark Matter wasn’t the final word, of course. Subsequent observational evidence for an accelerating Universe resulting in our standard cosmological model being modifiel to include an additional (large) component of dark energy in addition to dark matter. Nevertheless, the core ideas presented by DEFW established the basic foundations of structure formation upon which the current standard model is built.

Incidentally, you can read an interesting account of the discovery of the accelerating universe here; a cosmologist by the name of “George F. Stathew” plays a prominent role in that piece and it’s curious I’ve never heard of him before now.

Each of the four winners gets a share of the $500000 Gruber Prize, i.e. in “normalized” terms, they get $125000 each. Why is it so controversial to suggest dividing citation counts the same way? The DEFW paper has about 1500 citations according to ADS, so I think it’s quite reasonable to award the authors 370-odd each towards their respective h-indices. That’s still a pretty good result by any bibliometric standard!

The four also get a Gold Medal each to wear at parties, although by my previous logic they should have to share one between them. Perhaps George might consider donating his to Arsenal Football Club, as their trophy cabinet is looking rather empty these days?

None of the winners are Australian undergraduates, so this award probably won’t be considered newsworthy by the mass media. Believe it or not, however, the Gruber Prize is held in even higher regard by cosmologists than the Templeton Prize, so I’d like to take this opportunity to congratulate them myself for their thoroughly well-deserved honour!