Archive for the ‘Science’ Category

I realise I’ve been neglecting the blog a bit the last few months, with only a few posts that link to other things, but that’s because I’ve been solving a nasty problem for my research and I think it’s now time to update what’s going on with a ‘real’ post.

For the past year or so, we’ve been working on modelling a galaxy using a set of models developed by my supervisor, Larry Widrow, and his collaborator, John Dubinski, who is at Toronto. The models assume a standard exponential disc for spiral galaxies, but the curious thing is that, because discs appear to contain a luminous bulge at the centre (which is generally considered to be a dynamically distinct entity) there is degeneracy in how surface brightness profiles can be broken down: typically, we break them down into a part due to the bulge (which is often assumed to be a de Vaucouleurs-type r^1/4 profile) and a part due to the disc, which is assumed to be exponential. In the centre, the bulge and the disc contribute to the total light seen in a galaxy. There is some theory behind assuming that discs form exponential density distribution, but in principle other functional forms are allowed for disks. One of these is called a Kormendy disc (named after John Kormendy who proposed this in 1977).

A Kormendy disc is basically an exponential disc with a hole in the centre. So the centre of the disc emits no light, because there are no stars in the centre. This sounds weird, but there is nothing preventing us from attributing all the central light to the bulge, and letting the disc take over as the bulge falls off – we can find fits using the Kormendy functional form as well as the standard exponential. In fact, a siginificant minority of galaxies appear to be better fit by a bulge + Kormendy disc than by a bulge + exponential disc.

The presence of a hole is highly counterintuitive, and you may wonder if ‘seeing’ a hole is merely a measurement artifact or a real measurement due to some other effect. Neither of these appears to be the case; measurement artifacts have been ruled out, and effects such as central dust (which could obscure disc light) are unlikely to be the cause. But, there are good reasons to think it reflects a real physical effect; many of the galaxies that display holes are known to be barred (i.e., they have a central bar of stars out of which typically emanate spiral arms), and bars could be responsible for the effect, but even some that don’t have bars are well fit by holes.

So what has this got to do with my research? Well, the galaxy we’re looking at might in fact be fit better by a central hole than by the exponential disc in our models. Originally we accounted for this by pretending that the real mass distribution was exponential while the light distribution had a Kormendy hole. This is reasonable, because the galaxy appears to have some dust at the centre, although, as I mentioned above, holes are not likely due to dust. So we decided to fundamentally change the structure of the models to generate a Kormendy disc instead, and that’s what I’ve been doing for the past two months… and it’s a pain, because the code is written in Fortran (the F-word again) 77, and Fortran 77 is just plain annoying.

Fortunately, the code is structured in such a way that the radial profile can be changed separately without altering anything else, so it was in principle just a matter of changing the density wherever it was called in the code. Unfortunately, the code itself has been changed several times over the past 15 years and the syntax and variable names used across different subroutines are inconsistent. Moreover, just changing the density profile at first did not work, partly because there is a mathematical (though not a physical) singularity at the centre; to rectify this, I just redefined the central density (and the first two derivatives) to be zero. Debugging was annoying because outputting important quantities in subroutines ran into fortran’s absurd I/O recursion error, which really seems to be an annoying bug that has just never been fixed (as evidenced by the fact that it would pop up sometimes and not at other times when I had made no changes to the code). Finally, the code has a bothersome habit of giving different results for the same input when compiled on different machines… I’ve quietly decided to ignore that since it seems to work on my department machine (but on my home desktop or my laptop). Regardless, after two months of fighting through oceans of NANs and INFs, outputting dozens of different quantities at all points of the program, and banging my head against the wall (not literally, but it wasn’t going to stay not literal for long), I tried an arbitrary but well-behaved radial profile and found that it worked perfectly, which indicated that the main problem was likely a coding bug or something having to do with the centre of the Kormendy disc. I finally traced the problem to the calculation of certain spline quantities at the centre… where the density was zero. Instead of fighting my way through tedious, possibly buggy numerical subroutines, I changed the density (and the first two derivatives) to a very small nonzero quantity… and presto! We have self-consistent Kormendy discs.

So now we have real Kormendy discs, and we can model galaxies with either a Kormendy profile or an exponential profile, and the sun is shining, and life is grand (well, sort of, but that’s another story :P).


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Sean Carroll over at Cosmic Variance has succintly explained why it is reasonable to believe in dark matter, in spite of the spate of recent modified gravity-inspired publications that purport to explain the Bullet Cluster result and the emergence of a relativisic verison of modified gravity over the past few years. Quoth the article:

The dark matter hypothesis provides a simple and elegant fit to the Bullet Cluster, and for that matter fits a huge variety of other data. That doesn’t mean that it’s been proven within metaphysical certainty; but it does mean that there is a tremendous presumption that it is on the right track. The Bullet Cluster (and for that matter the microwave background) behave just as they should if there is dark matter, and not at all as you would expect if gravity were modified. Any theory of modified gravity must have the feature that essentially all of its predictions are exactly what dark matter would predict. So if you want to convince anyone to read your long and complicated paper arguing in favor of modified gravity, you have a barrier to overcome. These folks aren’t crackpots, but they still face the challenge laid out in the alternative science respectability checklist: “Understand, and make a good-faith effort to confront, the fundamental objections to your claims within established science.” Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source, and why your theory miraculously reproduces all of the predictions of the dark matter idea (which is, at heart, extraordinarily simple: there is some collisionless non-relativistic particle with a certain density).

And people just don’t do that. They want to believe in modified gravity, and are willing to jump through all sorts of hoops and bend into uncomfortable contortions to make it work. You might say that more mainstream people want to believe in dark matter, and are therefore just as prejudiced. But you’d be laboring under the handicap of being incorrect. Any of us would love to discover a modification of Einstein’s equations, and we talk about it all the time. As a personal preference, I think it would be immeasurably more interesting if cosmological dynamics could be explained by modifying gravity rather than inventing some dumb old particle.

But the data say otherwise….

The basic probelm is that, while modified gravity proponents tend to argue that dark matter is a kludge designed to fit the data even though there is no indication of what it might actually be composed of (and until recently hadn’t even been directly detected), modified gravity itself proposes an ad hoc modification to the law of gravity without any theoretical motivation. Why should modified gravity then be considered more realistic hypothesis than dark matter?

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“The mysterious dark matter that fills the universe could be made of the same particles that put the ‘big’ in the big bang, explaining both inflation and dark matter in a single stroke.”

read more | digg story

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Here’s an interesting read I found last night.

‘Forgive me if it takes me a little time to get up to speed here, but it’s not everyday I get to question a deity’

The Deity’ he interrupted.

ooh. Touchy!’ I thought.

Not really – just correcting the image

Now That takes some getting used to!

I tried to get a grip on my thoughts, with an internal command – ‘Discipline Harry. You’ve always wanted to be in a situation like this, now you’re actually in it, you mustn’t go to pieces and waste the opportunity of a lifetime

You won’t’ he said.

Tell you! That’s the bit that made it feel unreal more than anything else – this guy sitting across the table and very obviously accurately reading my every thought. It’s like finding someone else’s hand inside your trouser pocket!

Nevertheless, something made me inclined to accept the invasion, I had obviously begun to have some confidence in his perception or abilities, so I distinctly remember the effect of his words was that I suddenly felt deeply reassured and completely relaxed. As he had no doubt intended. Man must have an amazing seduction technique!

So then we got down to business…

Maybe we can get to level two…

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Update to my previous post about the declining bee population: it appears that the real culprit is a parasitic fungus, and not cell phones:

A fungus that caused widespread loss of bee colonies in Europe and Asia may be playing a crucial role in the mysterious phenomenon known as Colony Collapse Disorder that is wiping out bees across the United States, UC San Francisco researchers said Wednesday.

Since the quality of science reporting in the Independent seems to be something of a fiasco, I promise not to rely it anymore to relay interesting stories.

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Interesting question, and the answer may be cell phones.

[Scientists] are putting forward the theory that radiation given off by mobile phones and other hi-tech gadgets is a possible answer to one of the more bizarre mysteries ever to happen in the natural world – the abrupt disappearance of the bees that pollinate crops. Late last week, some bee-keepers claimed that the phenomenon – which started in the US, then spread to continental Europe – was beginning to hit Britain as well.

The theory is that radiation from mobile phones interferes with bees’ navigation systems, preventing the famously homeloving species from finding their way back to their hives. Improbable as it may seem, there is now evidence to back this up.

Interesting answer even though the article is half informative and half alarmist claptrap.

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Bits and pieces:

  • Paul Ohm on Eugene Volokh has a couple of nice posts discussing the price of digital music and the analog hole (the analog hole costs exactly 23.9828 cents) here and here.
  • MiniBooNE saves the standard model of particle physics: it finds no neutrino oscillations where LSND did, which means there is no reason to believe any new physics (such as sterile neutrinos) is required by LSND.
  • Quote of the day: ‘Emo – punk music on estrogen.’ From Urban Dictionary.

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