Dark Matter Decrypted

October 30, 2007

I am starting to believe in dark matter. I don’t mean “believe” as in “rationally favour” – thats been true for a long while. I mean it feels right. Genuinely convincing evidence is starting to pile up. But what IS dark matter ?

Last week we had a workshop at the ROE whose title was Decrypting the Universe. The poster seemed to be a pun on “decrypting”. Matrix-like strings of numbers are descending from the top, but it looks like green slime dripping into a dark background, giving the whole thing a sort of Monster Mash-ish flavour. The workshop outing was a trip to the spooky Mary Kings Close – the street that Edinburgh buried during the plague, so I guess this was a deliberate theme. (Peder, are you a Hammer Film fan ??). Anyhoo, the workshop itself was really good. As usual these days, I was nipping in and out of very boring University meetings and so only got to half the talks, but I caught some excellent stuff.

Cosmology consistency checks

David Spergel of WMAP fame summarised the overall state of cosmology. As he emphasised, if the concordance cosmology came only from fitting the CMB, you’d be worried we were all having a communal fantasy. However the constraints from several independent techniques – CMB spectrum, large scale galaxy clustering, supernova redshifts, cluster counts – intersect at the same spot – ordinary matter 4%, dark matter 21%, dark energy 75%.

Mapping the dark matter

Nowadays we can map out the invisible dark matter by tracing the distortions in background galaxies caused the gravitational lensing effect of the foreground matter. A few months back there was a lot of publicity about the COSMOS maps and how they showed the galaxies and dark matter tracing the same patterns. But this wouldn’t convince a sceptic – two things that look the sameish derived from the same dataset … hmmm. Enter the Bullet Cluster, stage left, and a lovely paper by Clowe et al. Here they have both X-ray and optical data on two clusters merging together. The galaxies and the dark matter go together, but the X-rays, which trace the hot gas representing most of the baryonic matter, looks different. This makes sense as the gas can kinda slop about, but the starlight is essentially a tracer for the dark matter halos, where the galaxies form. More importantly, modified gravity theories like MOND, which try to dispense with dark matter, can’t explain all this.

Hot or cold ? Galactic Archaeology

Next up, what type of dark matter ? Many moons back, the rivals were light fast moving particles like neutrinos (hot dark matter, HDM) or weakly interacting massive particles – WIMPS, otherwise known as cold dark matter, CDM. From the 1980s on, galaxy clustering studies have argued that it can’t be HDM, as this would make too much large scale structure and not enough small scale structure. But this is a rather indirect and statistical argument.

A consequence of CDM is that galaxies build themselves bottom up, by the gradual merging of smaller things. Clearly, galaxy mergers happen occasionally today, but what about lots of merging way back in the past ? This history of ancient merging should be encoded in the structure of the Galaxy. Looking for this is known as Galactic Archaeology. At the workshop, Amina Helmi reviewed the evidence and showed stunning pictures from 2MASS and SDSS, where very old, metal poor stars have been picked out, to look at the structure of the halo, the oldest part of the Galaxy. The halo of the Galaxy is not smooth – it has lumps and streams and ripples. The fossil history of the Galaxy just stares out at you.

There was a problem – when Majewski et al 2003 studied the Sagittarius stream with 2MASS, they found it to lie on a great circle, suggesting the gravitational potential of the Galaxy is pretty much spherical, whereas CDM predicts distinctly non-spherical halos. However looking at a picture projected in 2D can hide a lot of stuff… Helmi showed new data from RAVE, which collects velocities of those halo stars; this showed very clear departure from the spherical potential prediction. The best fit model has a dark matter halo thats a prolate ellipsoid with axial ratio 5/3. Just about on the money for CDM.

Dark matter on Planet Earth

So CDM is looking pretty solid. But the snag is nobody has ever actually seen one of these particles. The theoretical suggestions for what could fit the bill all come from unproven speculations in particle physics, like supersymmetry. But this is just what excites folk of course – its not just astronomy, there’s PHYSICS in them thar hills. If dark matter is universal, it should be streaming past the earth, and we should be able to detect the efffect of these particles colliding with regular earthly particles.

At the workshop, Alex Murphy reviewed the attempts to do this, which normally involve putting detectors way underground, to avoid confusion with plain ole cosmic rays, and re-deploying nuclear physicists who would otherwise be doing all that dull stuff about exotic nuclei and hadron structure. Oops, sorry Alex. Anyway. The British Effort is in Yorkshire, at the bottom of the Boulby mine. Despite a false alarm from DAMA, so far nowt has been seen. But our intrepid scientists are building bigger detectors with better discrimination. Alex reckons we will be in detection land in 5-6 years. That would be fantastic. Watch – this – space.

Err.. what about dark energy then ?

I’m tired now. Perhaps another day.

More than coincidence ?

October 23, 2007

This flaming pope thing has been all over the internut, including at Ye Bad Astronomer, but I suddenly twigged what it was reminding me of…

Exhibit A : Pope John Paul II seen at bonfire :

flaming pope

Exhibit B : One of my favourite Kliban cartoons :

angled saxophone - kliban

This one is from “Never eat anything bigger than your head”. Kliban died some while back, but his ghost inhabits Larson and Baxter. In memory of Kliban, go buy the book, second hand, or new. Here is a fan-site with more pictures..

My thumb is not a watch

October 17, 2007

I have arrived at a theological insight through a combination of academic duties, astronomical management responsibility, and clumsy cookery. This comes after spending last week watching and fostering the growth of astronomical e-science (VOTECH DSRP meeting), while having to drop out of the meeting every couple of hours to deal with my duties as Head of School, dragging our sorry academic carcasses towards the Research Assessment Exercise.

The amazing thumb

The RAE panic is getting more intense daily. The thumbscrews are being applied by the University, I found myself thinking, and actually stared at my thumb while I was supposed to be listening to Francois Bonnarel . I was very pleased with my thumb. A week before that I had cut it while trying to chop peppers. This was slightly more than the usual nick; I sliced off a contact-lens sized piece, which was left dangling. Rather than merging back together, the contact-lens gradually shrivelled into a thin hard cap, while a new piece of thumb grew up from underneath. Eventually the hard cap fell off, and voila ! A new thumb. This is amazing of course. But as I stared at my thumb, I realised that the process was even more astonishing than I had realised.

My thumb print had come back. How does it do that ?

Faced with something so gobsmacking, the cliched phrase “Miracle of Nature” is on the tip of your tongue. Indeed some people would brandish such a “miracle” as an example of Why God Exists. Actually, my initial thought was that God really wants us to live in a police state. Otherwise, why would our bodies be so insistent on an unchangeable identifying feature … ? Hem. My second thought was more Dawkins-ish. God is irrelevant.

My thumb is not a watch.

God the Designer ?

Paley argued that if you come across a watch on the ground, you would assume that someone created it; something like that just couldn’t happen by chance. It’s too intricate and perfect. But suppose you came across a flower ? Or, as you dropped out of hyperspace during your journey across the cosmos, you suddenly saw the spiral galaxy M101 in the distance ? The complexity in these things is of a different order and of a different kind. You don’t build something like that; you grow it. Its complexity arises from accretion and meandering over vast aeons. If you started from scratch with a purpose in mind, thought it through, and carefully put every piece in place, even if that were possible, you wouldn’t get something that looked like a galaxy; you would get something that looked like a watch.

My thumb and its print are like this. They were not built by a conscious being. Evolution is the only way.

OK. So maybe God doesn’t build things, he designs them. But what does this mean ? I found myself puzzling over three types of creation I have been involved in recently and how they differ. Warning : we will be doing metaphor here, not mathematical proof…

First, writing a blog post.

Well, I sit down and create. Its all me. And yes, believe it or not, it is designed. Some of my posts are diary-like and spontaneous, but others, like this, are written top down. Once the design is there, the words are placed carefully; and I go back, delete sections, and start again. I design, build, and polish. The result is a watch, not a flower. (Maybe a Timex rather a Rolex …)

Second, writing the RA5A document.

This is an 11,000 word document that goes with our RAE submission. It has been led by me, but it is a group effort. I drew a blueprint, wrote a few bits, then commissioned colleagues to write the other sections. The stitched together result got circulated round, people suggested changes, sections got re-commissioned for editing, and so on in repeating cycles. I have tried to control the document as a whole, but its hard work keeping track of who is working on which bit, let alone what changes have been made. It really is my design, but I can’t say I built it. The whole thing is just on the edge of what you can achieve by command and control.

Third, creating the Virtual Observatory infrastructure.

I am project leader for both the UK AstroGrid project, and the Europe-wide VOTECH project. So do I direct every detail from on high ? You must be joking. Do I design an architecture, and let the engineers fill in the detail ? Nope, and if I did, the result would be awful. It is so complex it is obvious you can’t even try to micro-manage. What I am is the gardener. I provide the soil (money, meetings) ; buy some good plants from the Nursery; and maybe sprinkle some fertiliser (visionary words etc). Then I stand back. Its magic. Ideas and software grow before my eyes. Somehow the trick is to achieve as much as possible by doing as little as possible. (Wu Wei).

God the Gardener ?

This doesn’t make much sense either. I am proud to be the gardener, but there is nothing special about this. You need a gardener, you need plants, you need the guy who works at the Nursery, you need soil, you need water. You need all of it. Its an undivided whole. Nobody is in charge. We just have different jobs.

So at the end of this musing, I can just about see enough room left over for a sort of vague Einsteinian pantheism, or for some sort of mystical “God” which is the same thing as the whole Universe. But an entity separate from, and controlling, the Universe ? Doesn’t make sense.

So ?

As Physicists, in this situation, we typically echo Pauli and say “Its not even wrong”. But as the Eastern Wuss that I am, instead I recommend unasking the question.

Dunkin’ Dylan Shocker

October 14, 2007

Google “Newport 1965” and you know what you will get. The infamous, the mythic, the revolutionary moment in time when Dylan went electric. I did this very thing, just after watching the Murray Lerner movie on TV and coming over all nostalgic. This is odd, as not only was I not there, I was only eleven at the time. Still, four years later, listening to the Incredible String Band and buying my first Pink Floyd album, people were still talking about it. (Try this and this)
Next, instead of searching for “Newport 1965” I just looked for “Newport Festival” and woohooo its still going… but there was a bit of a shock. Now, as the story goes, the folkies were distressed because their politically perfect festival had been sullied by commercialism… So how fitting that, as you will see at this web page, it is now the Dunkin’ Donuts Newport Folk Festival.

Feeds, Readers, and Feedburner

October 13, 2007

I finally got round to figuring out how Feedburner works, and have added a subscription button (“chicklet”) to my sidebar. Now you can read me three ways. (1) Just drop in every so often; (2) Read the content through your favourite feedreader; (3) Get an email when I post something new. Some of you are already subscribed to me through some feedreader or other. I can’t easily get the stats on these; but if you unsubscribe and then re-subscribe (using your same favourite feedreader) then you will get the same stuff routed through the feedburner server, and I will know you are there ! (For bloggers who run the WP software themselves, feeds can be automatically re-directed, but for blogs hosted at wordpress.com you can’t do this – see below).

I have cleared my head by explaining below what I now understand… hope it helps somebody else !

What is a feed ?

I know that I have geeky readers who will say “you are explaining what a feed is ? why ?“, and other readers who will say “eh wot pardon ?” – so this is for them.

Its like Reuters for the Web. Modern web pages can change quite often. Its annoying to have to keep checking whats new. This is true of blogs a fortiori of course. So the technique is to collect new material and make it available (syndication) through a standardised document (feed) ; an application (feedreader) can then subscribe to the feed and display for you. Your reader can collect lots of different feeds in one place (aggregation) and so you can check all your favourites for updates in one visit rather than forty.

The syndication stage is done automatically for me by the WordPress software. (Many other types of software, like Wikis for example, also automatically construct a feed.) The feed itself is an XML document that is basically a set of links to material, plus typically some extracts from the material. There are (sigh) at least two standards for the format – RSS and Atom – but luckily the feedreaders usually figure this out. You can learn more about all that at this wikipedia article, and at this O’Reilly article. (There is nice general article by mezzoblue here.) For my blog, at https://andyxl.wordpress.com, the standard feed document provided by WP is at


and my new Feedburner feed document is at


You can click on these to take a look, and do “view source” to see how the XML works.

So how do you set about subscribing and reading ?

First, you can just read feeds in a standard web-browser like Firefox and IE – they understand the XML. “Subscribing” is just a matter of collecting the bookmarks as usual. Often (but not always), FF or IE knows when a website contains a feed and adds a wee symbol in the address bar. If you click on this, you can see the feed and bookmark it; but you may also be given a list of web services through which you can subscribe to the feed. (Depends how your browser is set up)

Second, you can register with a web service, like Bloglines or Newsgator or Google Reader. You subscribe to each feed you want, their server trawls the feed docs, and you read them at their web site. I use Bloglines myself.

Third, you can install a client application on your own PC, like NewzCrawler or FeedDemon . These work like Outlook Express – they suck down the new stuff from your subscribed feeds, and save it on your hard disk for reading at your leisure.

There is friendly explanation of this stuff in a BBC news site article.

Why this change to feedburner now ?

Until some months back, WordPress used to provide stats on how often your blog was read through a feedreader, rather than by viewing the actual page – but they withdrew this, for reasons I never quite understood. As a result its quite hard to know what one’s actual readership is. From the old stats, it looked like I was getting roughly equal numbers of feed reads and page reads. Of course some of those page views were people clicking through to the real thing if something looked interesting (I do this all the time), and the feed-reads were just the services collecting updates, which doesn’t prove anybody read them (likewise, guilty). But even this dodgy info disappeared when WP withdrew the FeedStats feature.

From the point of view of the reader, Feed Burner is just the same as using the original feed. (If you compare the feed docs linked above you can see they are the same; its just that one is sitting on the WP server and one on the FB server.) But the publisher gets the stats; and because it is all routed through the FB server regardless of which reading method your readers are using, you don’t miss anything.

Sounds fun, how do I do this too ?

I won’t wade through this in detail, but rather point you at some other web pages I found handy.

If you want to just put an icon on your blog linking to your regular WP feed, this is explained in the WP FAQ on this page.

I found a general article article about setting up FeedBurner by Marshall Kirkpatrick, and another specifically about Feedburner and WP blogs by HarisTV; however FeedBurner itself has a very clear Word Press Quick Start page.

Key issue that I gradually realised : there are two types of WordPress blog. You can install the WP software on your own server (self-hosted) or you can set up and run your blog using the WP server and its interface (WP-hosted). Some things can only be done for self-hosted blogs. Example one is improved auto-discovery – at the moment Firefox seems to know my feed exists, but IE doesn’t. You can fix this by putting some extra lines of code in your blog template, which FB give you. But if you are WP-hosted, you can’t edit your blog template. Example two is automatic re-direction. As I explained earlier, new subscribers will be routed through Feedburner whichever reader they use, but Feedburner won’t know about people who already subscribed to my existing WP feed. However, somebody terribly clever called Steve Smith wrote a plug-in which would automatically re-direct readers. This has now been officially absorbed by Feedburner and called FeedSmith. However, once again this works by plonking the right bit of code in your WP installation – so can’t be done by WP-hosted bloggers.

Phew. Think I am done.

And in closing…

As John Ebdon used to say, if you have been, thanks for listening.

The pale cast of thought

October 7, 2007

Discovered a great new blog last week – the AstroStat Slog. It reminded me how Astronomy has some things in common with social science. Pause for spluttering by hard nosed physical scientists. What I have in mind is that its hard to do controlled experiments. You have to let Nature do the experiments for you, and then work out whats going on. This causes all sorts of problems, but in particular you often need to be good at statistics, to stop yourself being a sucker for all those fascinating flukes.

Even worse, often you don’t even know the underlying probability distributions of your variables. Assuming everything is a Gaussian is a dangerous mistake. Cue non-parametric statistics. Years back in the 80s chum Martin turned me on to the Mann-Whitney U-test, the Spearman rank correlation co-efficient, and other delights. I was hooked. Martin’s secret weapon was a book called “Non parametric statistics for behavioural scientists” by Sidney Siegel. It was full of arcane secrets but still written in plain English. Impressive. An updated version with John Castellan is available.

Why is doing statistical analysis so addictively satisfying ? I see it as a kind of merger of thought and action.

As an Experimental Scientist, you need to think hard and slow; to be absurdly rigorous and avoid the traps set by sloppy thought; to root out the assumptions you didn’t realise you were making; to experiment and test in a carefully controlled manner, and conclude only when you are sure. Don’t jump in. Stop, Think, Test.

The Busy Executive (or the Neolithic Hunter..) laughs. Pause and you are dead. Sicklied o’er wi’ the pale cast of thought you do not want to be. Sure, you don’t want to be dumb. But over the years, you build up your experience, knowledge, and instinct. It waits inside you, a coiled spring, ready for release when you need a Decision. Even if you have plenty of time, you never have all the information you need. Assemble what you have, run it through your judgement mill, and go for the best bet.

The Astronomer’s instinct is to be just like the experimental scientist but its impossible. You just can’t do the experiments you want. You have to make do with the ragbag of facts Nature has provided. Like the executive you need to take a decision with incomplete information, but you want it to be somehow impartial and rigorous. Well tough. You can’t absolutely and safely decide. You just can’t. But given any possible decision, you CAN say how likely it is you are being fooled. I can’t tell you which horse will win, but I can tell you whether I will accept that wager.

I’d better stop now or I am in danger now of drifting into the sticky morass that is the Frequentist-Bayesian debate.

Once I almost read a paper called “Are probabilities propensities ?” but it was lunchtime so I decided not to.