Cosmology Conundrum

March 26, 2013

Some colleagues have suggested that my posting on Planck Day was overly frivolous, given the huge importance of what Planck has achieved. (Nicely written up by Andrew Jaffe.) Other colleagues have suggested that Planck Day was bad press, as it was such a huge public non-event, with a big fuss about mild parameter adjustment. I find both these things true, leaving a weird sense of tension and excitement. I’d better explain myself.

First, lets be clear about the technical achievement. Planck is an absolute triumph of technology, engineering, management, and organisation. An amazing machine that has worked beautifully. It also represents a stunning scientific achievement. The lamda-CDM model, and its beautifully articulated engine of prediction, is a conceptual and analytic triumph. Of course this triumph belongs not just to the Planck team, but represents the accumulated achievement of many scientists all over the world over a number of years.

Lets just look at that power spectrum fit. It is not just a question of the theoretical curve going vaguely up and down, in more or less the same way as the data. The detailed agreement is gob-smacking – multiple peaks, their positions, their sizes, their widths, their second and third order curvature. Any scientist will look at this and think “no way is this a fluke”. Don’t let doubters trot out that coffee-time stuff about being able to fit anything with enough parameters. Firstly, that old chestnut is largely nonsense, and secondly, the fit quality is way beyond that.

Parfait. Everybody love parfait

And yet – outside the world of the CMB, the CDM paradigm has problems, as we were reminded here at ROE the day after Planck Day, in a nice wee coffee talk by Jorge Penarrubia.The best known problems are that CDM predicts far too many dwarf galaxies, and galaxy profiles that are much cuspier than observed,  but there are other claims, such as the existence of a very unlikely polar structure of dwarfs surrounding the Milky Way,  and of suspicious uniformities in galaxy rotation curves. All these problems may still get patched up by astrophysical fixes to do with feedback, IMF games, etc… but we don’t know yet.

Its common to hear people say that Particle Physics and Cosmology are in a similar situation – a model rather than a theory – a perfect fit but with no explanation. Why do all those parameters have those particular values? It seems so arbitrary. And what are dark matter and dark energy anyway? In this view, even the boringly successful fit is tantalising because it tells us there must be a deeper theory in waiting that will explain the perfect fit.

That may describe particle physics, but it doesn’t describe cosmology. Rather, what we have is perfect but fishy. How can the model be so perfect in some places and so poor in others? Have we missed something simple? It may well be that the astrophysical fixes do the trick, and then cosmology does look indeed like particle physics – successful but unsatisfying. Or it could be that some conceptual change is needed, and a revolution is waiting.

I’d give the revolution 2:1 against. But thats good enough odds that your eyes are glued to the table… Mesdames et Messieurs, faites vos jeux.


I am that young man

November 17, 2010

Allen Sandage just died. An article in the New York Times is here. Not someone I knew or indeed ever met, but he has always hovered in my psyche, like he has for most astronomers my age. In 1961 he founded modern cosmology by writing  “The ability of the 200inch telescope to discriminate between selected world models” and in 1962, with Eggen and Lynden-Bell, he kicked off the subject of galaxy formation by writing “Evidence from the motions of old stars that the Galaxy collapsed“.  Anyone who can write two such epochal papers in successive years ought to have a warm glow for the rest of their lives. Maybe he did, but of course he didn’t sit back; he pursued his program relentlessly for decades.

In some ways I miss the old days when the warring H0=52 and H0=85 camps would have bust ups at every conference, led by Sandage and his arch-rival Gerard De Vaucouleurs. A well known astronomer, who may or may not be Richard Ellis, once told me that sometime in the 1990s, by which time Sandage was 70ish and De Vaucouleurs was 80ish,  he went to yet another conference where each camp was convinced of its rectitude. Richard claims he stood up and said “I am getting fed up with these debates. This subject needs some young man to come in and sort things out”. (Pardon the sexism.) Apparently De Vaucouleurs swelled his chest and said “Indeed. I am zat young man.”

(Richard can feel free to unclaim the story.)

So I just checked out those two classic papers. What really hit me was that the classic 1961 paper has 375 citations. Well that’s pretty good of course, but plenty of people these days clock up papers with more citations than that. Certainly wouldn’t guarantee you a Faculty position at a good university these days…


Everything you know is wrong – or is it ?

December 1, 2008

There is a paradox at the heart of science. The root of science is scepticism; doubt authority and do the experiment. However the result of all that scepticism and experiment over four hundred years is the most secure body of knowledge ever produced. If you go around doubting Newton’s Laws every day you’re a nutter.

Around the edges of the clearing, where we still face the darkness of the forest, its a different story. We could be hacking uselessly into dense growth, when the path to the meadow is hidden just a few yards away to the left. Its fashionable now to be sceptical about String Theory  – see Lee Smolin’s book The Trouble With Physics – but inflationary cosmology worries others, and some folks are still nervous about the logical basis of quantum mechanics. Three years ago in Warsaw I saw David Gross and and Roger Penrose  give alternating public talks. This was very entertaining. Penrose’s title was “Faith, Fashion, and Fantasy in Modern Physics” – thats Quantum Mechanics, Inflation, and String Theory. You could almost hear David Gross’ teeth grinding. More recently, Robert Simpson (the Orbiting Frog) wrote a lovely post called Five Scientific Ideas that could just be Bullsh*t . Do give it a read.

Of course some people are just career long rebels. Recently on astro-ph Geoff Burbidge set out his dish of sour grapes yet again. Mostly this was quietly ignored, but over at Cosmic Variance Sean Carroll picked it apart. But its not just Burbidge you know. Accomplished, distinguished and terribly serious young cosmologist Douglas Scott, along with his possibly even more distinguished colleague Dr Frolop, has written a series of three papers questioning many of the key assumptions of modern cosmology : see here, here, and here.

So here’s some fun. Below I paraphrase a few statements from Burbidge and Scott, not saying which is which. A big no-prize for who-ever can give the best one-sentence rebuttal or confirmation of each statement. John Peacock has to wait until everybody else has finished.

(1) Dark Energy is just Hoyle and Narlikar’s C-field, so the Steady State Universe can come back.

(2) H_0 * t_0 is consistent with 1.0 to within 3%;  there is no reason this should be the case, so it tells us we are missing something obvious.

(3) We believe in a hot Big Bang because there isn’t time to make the Helium in stars; but if the Universe is cyclic, with a bounce before the Big Crunch, some stars could be much much older, and the Helium can be easily made.

(4) We believe that black holes powering radio galaxies are 10% efficient; however, man made particle accelerators like SLAC are a thousand times less efficient than this, so we are almost certainly kidding ourselves.

(5) The biggest galaxies have the oldest stars; younger galaxies are smaller; therefore galaxies are coming apart.

(6) If likewise clusters of galaxies are breaking up, the virial theorem doesn’t apply, and there is no need for dark matter.

(7) The energy density in the CMB is almost exactly the energy density associated with converting Hydrogen to the observed density of Helium; therefore the latter is the cause of the former.


Girlfriend still alive but only just

November 18, 2008

Crikey. Just got home after hard day cutting through the undergrowth at the frontiers of knowledge, only to find girlfriend has been in undergrowth in Foothils Park, where she encountered a mountain lion. Five minutes of staring back and trying to look big, and eventually it drifted off .. Jeez. Difficult seminars suddenly seem tame…