Black Beauty

I found out something beautiful a month or so ago. Something so brilliant, so satisfying, so dazzlingly perfect that I gasped, stared at the book, re-read the passage again a few times and then held it away from me - and then texted a few people to tell them how happy I was.

It was about dark matter.

It's also a book that I'm reviewing for Astronomy Now, so I won't be tell you too much about it or reproduce what's in my review. It's called Horizons of Cosmology and it's by Joseph Silk, once a colleague of ZookeeperKevin. It's about galaxies, star life, dark matter, dark energy, the acceleration of the expansion of the Universe - and the study of all this and the way humans look at astronomy. I think the most interesting human point Silk makes is that discoveries are generally made by doing something new with an existing set of data, rather than thinking up a theory and then trying to derive it.

That's actually very significant for science in the public too. Alternative medicines such as homeopathy and chiropractic were born of rather random ideas which then try to get data to fit them. And although I guess much astrology has a basis in "when the Pleiades are so high in the sky, then it's the best time to plant your grain", statements such as "when Jupiter comes near the Moon from the Earth's perspective it isn't the best time to annoy your lover" are certainly based on ideas rather than concrete evidence.

Yes, of course ideas are a good thing. But they're not the only thing - not when it comes to science, and probably not when it comes to most other things.

The trouble with science in the public is that only the results (and usually only a small part of the results) are seen - not the method, and not the data. The Daily Mail, for instance, has just claimed that going to the toilet in the middle of the night will give you cancer. They certainly don't explain what the hell has led anyone to this idea. Sure, they might give you some guff about cell division and throw in a scary word or two, but what was going on? What experiments have been done? How have they organised the data and is it absolutely unbiased? And is it something for which it is seriously, statistically significantly worth making a change? When I was at school we had to write a hypothesis before our experiments, and then our results were supposed to match it. That's not science. Today the emphasis appears to be the ability to give reasons why there are "two sides" to issues such as vaccination and global warming. That's not science either.

Back to the dark beauty.

I've heard plenty of people voice the opinion that dark matter is a myth. They might have heard of MOND (modified Newtonian dynamics); they might just have a gut feeling that it sounds too unlikely or that some clever dick is over-complicating things; or they might be a bit too used to scientists sounding sure of themselves, so hearing "we don't know what Dark Matter is" comes as a bit of a cop-out (though it shouldn't - the most exciting science is the unknown). So I'll quickly summarise the evidence for dark matter's existence - although I should add we still can't be entirely and absolutely sure it even does!

The Solar System works perfectly for Newtonian dynamics. Planets nearest the Sun go round fastest. (Check out this gorgeous musical representation of how it works.) It's the same with a whirlpool or a tornado. So it should be the same with a galaxy. This one, for instance, NGC 4535:


You'd expect those stars on the very edge, the blue ones, to be going round far slower than the ones nearer the middle. Surprisingly, that isn't the case. Although of course they do take longer than the centre ones to complete the circuit, they still ought to fly straight out of the galaxy. They don't, because there's something else holding them there. It's the same with galaxy clusters. Zwicky examined the Coma Cluster to find the same effect. More evidence for there being more matter than we can see includes gravitational lensing and the rate of the Universe's expansion. To the best of my knowledge we haven't confirmed we've trapped any dark matter particles yet, but there are definitely experiments on the way and I believe one just might have succeeded.

Dark matter behaves differently from baryonic matter, the matter we can see and touch. It doesn't interact, except gravitationally - otherwise we'd be able to feel it. It also doesn't interact with light, or we'd see it. (It might just as well be called invisible matter, rather than dark.) And yet we're able to decipher dark matter haloes around galaxies, and map it out around galaxy clusters:

(You can read more about this here.)

What I found most mysterious was: if a spiral galaxy is flat, why on earth is the dark matter halo much more spherical? And in any case, as a few people have sensibly asked me, why should dark matter be so spread out that it only becomes apparent at galaxy-or-bigger-sized scales? For instance, why can't we have dark matter stars or planets or black holes?

Horizons of Cosmology answered me.

It's so simple. I actually told you earlier, and I could have told myself a lot earlier, but I had never picked it up. It's because it does not interact with the electromagnetic spectrum. Therefore, it cannot throw off photons, and therefore, it cannot lose heat.

Stars can only form when their parent dust clouds lose heat. When a dust cloud is hot, its atoms are not going to clump together. In fact about half the baryonic matter in a galaxy cluster is hot gas that cannot condense; the heat from stars alone stops it condensing. It is ironic that the hottest objects we know have to be born from something so cold. But there you have it.

All matter has been at about a billion degrees, in the moments and days and millenia after the Big Bang. Because baryonic matter has been interacting with light - first from all the photons from the Big Bang and from annihilation between matter and antimatter and then from stars and so on so forth, and from shelter amongst dust clouds and even, in minuscule quantities, in planets, the baryonic matter has been changing temperature, so we wouldn't know what temperature it would "naturally" be on its own. And you can't really stick a thermometer into dark matter, either. But because it can't condense, as can the atoms we know, it's like a ghostly fluid, floating around the more solid objects.

As far as I know, we don't know what would happen if we could cool it enough so that it can: for instance, we don't know if it's atoms like baryonic ones, which can interact with each other chemically, forming bonds and different types of atom and so forth. It may be much simpler. Or could it have its own, different complexities? I have no idea.

What happens in the formation of a galaxy, I also learnt from Horizons of Cosmology, is much more complex - but much more satisfying and beautiful - than I thought. I won't go into most of it, only a few basics. Remember the Cosmic Microwave Background had very, very tiny unevenness. I don't know if this was simply due to inevitable random movements of particles or something else. But in any case the slightly denser areas had a slightly stronger gravity, so they were able to attract more material, so the gravity got stronger . . . and so on. (A popular joke about this is the rich getting richer and the poor getting poorer.) This was actually mostly dark matter. It can't clump together, but it's still affected by gravity. So it pooled into huge, more or less spherical haloes. And because baryonic matter can clump, it trickled right into the middle of those haloes.

And after that I picture the spiral galaxies flattening and spinning in their centres just like the foam on my coffee.

The simplest thing imaginable, the difference between hot and cold, lit up dark matter in my mind. It's the purest, most dazzling beauty. I feel so lucky to be here to appreciate it all.