What Has Dark Matter Ever Done For Us ?

Well I suppose even Albert Einstein must have had an off-day, because if you ask me, a Universe full of giraffes and supernovae doesn't score highly on list of "things I definitely understand". In fact I wouldn't put it on the list at all.

I mean, Albert, what the hell were you thinking/drinking/smoking ? In what possible way is a Universe containing armadillos and exploding stars and porridge and black holes the slightest bit comprehensible ? Even very senior astronomers still insist on the old cliché that a frog is far more complicated than, say, a star or a galaxy. Which is a little bit weird considering just how little we understand about galaxies, and makes me wonder if biologists are all in the wrong profession.

ALL GLORY TO THE HYPNO-TOAD... but is he really more complicated than a galaxy ? I think not.

Nothing in astronomy seems to anger People On The Internet quite so much as the apparently simple notion of dark matter. Popular articles are universally greeted with a chorus of "scientists don't understand everything !" and "they're only looking for dark matter to keep themselves employed !" The sociology of this anti-science movement, even though dark matter is about as detached from any political or ethical concerns as it's possible to be, is almost as interesting as the physics itself. But today, let's stick to the science as much as possible.

(To digress from that just for a moment, I get kind of cheesed off when I read things like, "physicists who don't believe in dark matter are wrong." Such a level of certainty is not warranted. I do believe dark matter is the most likely explanation, but to claim that all other explanations have been falsified seems a bit strange.)


I've written about dark matter many times before, but to sum up : galaxies appear to be spinning too fast. Without some extra mass to hold them together, they should just fly apart.

Explanatory blog post.

My third-year undergraduate project has earned me a small degree of internet fame in some circles, while my fourth year project has never, until now, got a look in*. Today, in an effort to demonstrate how science is really done (and hopefully convince you that dark matter isn't such a crazy notion after all), I shall try and redress the balance.

* Unfortunately at the time I had yet to learn Python, which was a shame because it would naturally have lent itself to some pretty animations.

This was a project to try and simulate the formation of a disc galaxy without dark matter. Back then I was far less convinced about dark matter : probably, I would say, only at the 50:50 level. These days I'm more at the 75:25 level in favour - good enough to work with, but not nearly enough to bet my life on.

In my report, I described several arguments against the existence of dark matter. There are plenty more, of course, but these will do for now :


1) Halo conspiracy
Why are rotation curves* of galaxies flat as opposed to some other shape ? These days I'm a lot less convinced that this is really a problem - curves are seldom truly flat. They vary considerably, sometimes always rising, sometimes weakly declining (though not as much as if there was no dark matter). The only sense in that there's a "conspiracy" is that the curves don't agree with predictions from Newtonian dynamics. That's a lot less interesting than if they really did all have very similar shapes. Actually the details of this turn out to be very interesting, but I'll save that for a future post.

* A rotation curve just measures the speed at which a galaxy  is rotating at different distances from its center (usually by measuring its gas). That the speed doesn't drop with distance was the key piece of evidence in the discovery of dark matter.

If you're wondering about the name, dark matter clouds are usually known as halos, hence the "halo conspiracy" term - the idea being that the halo is somehow conspiring with the normal matter to always produce rotation curves of roughly the same shape.

Source - rotation curves of a sample of spiral galaxies. Irregular galaxies tend to show curves which never flatten off but just keep rising.

2) Surface brightness conspiracy
Surface brightness just means how bright galaxies are per unit area  - or to put it another way, some galaxies of a given size have more stars than others. Strangely, this doesn't seem to make much difference - it's only the total brightness that determines how fast the galaxy is rotating. But it's quite easy to show that surface brightness should make a difference. It's as though the dark matter and luminous matter are connected, and it isn't at all obvious why that should be.

The low surface brightness VCC 975 (left) compared with the normal VCC 1555 (right).

Similarly, dark matter "halos" (i.e. clouds) always seem to contain the same fraction of luminous matter, no matter their size. Galaxy formation theory says that ordinary matter falls into dark matter halos, so there's no obvious reason why the fraction of ordinary matter should be constant.


3) Dark matter inside galaxies is inferred from a single measurement - rotation curves.
If there were several observations that dark matter existed this would be a lot more convincing, but as far as we can tell dark matter does one thing, and one thing only : it keeps galaxies rotating more quickly. Even on scales just a bit smaller than the galaxy it doesn't appear to have much of an effect. And it seems a bit strange that 90% of the galaxy should do nothing very much except keep it spinning round a bit faster than it otherwise would.

Of course there are other observations in other situations where dark matter is also very important, like galaxy clusters and the large-scale structure of the Universe. But on scales smaller than galaxies, this is apparently not the case. Yes, even though galaxies come in many different sizes - dark matter only appears to be important over the whole galaxy. You may have heard recent reports to the contrary, but in my opinion that press release was actually pretty awful.

Shamelessly plugging my own work again.

It was that third point that motivated my undergraduate project - what would happen if we tried to simulate the formation of a spiral galaxy but without the dark matter ? Does the dark matter really do nothing at all except change the rotation curve ? If so, that would be a bit suspicious. You'd think that 90% of the mass of a galaxy ought to play some role beyond gathering the gas together and making it spin faster.

Yes, I know there are a whole bunch of other reasons to believe dark matter exists. But trying to test the entire dark matter paradigm at once is a monumental undertaking - so daunting a prospect that there's a risk no-one* will check it at all, thus establishing a false consensus. A much better approach is to break it down into smaller, manageable chunks. If we can say, "dark matter doesn't do anything expect make things spin faster", then we might want to start questioning other aspects of the paradigm. If, on the other hand, we can show that dark matter has some other essential role, then that strengthens the case for its existence.

* Especially a fourth-year undergraduate.

Neither scenario makes for a revolutionary breakthrough, but as I've tried to stress again and again and again : that's not how science usually works. What we'll get from the investigation is only one piece of the puzzle - nothing more, nothing less. Take enough baby steps and eventually you'll get to Outer Mongolia, write a novel, learn to play the piano, bring dinosaurs back to life, or whatever the heck else it is you want out of life.

Of course, some of us have more cerebral goals than others.

With that in mind, the simulations we ran were anyway somewhat unorthodox. Mainstream cosmology says that galaxies form by the merging of smaller galaxies. The model we used was much simpler : the collapse of a big cloud of gas straight into a rotating disc. It's called a monolithic collapse, though actually the process is a lot more complicated than a big hunk of gas suddenly turning into a galaxy.

More here. Dark matter is purple, gas yellow, and stars are red. The particles are initially in a 3D grid (filling a spherical volume), with small random motions. That's why at the start it looks like there are lots of discrete purple and yellow clouds - actually they are long columns.

I'm not sure why this approach doesn't get more attention. While dark matter is usually taken for granted, the idea of hierarchical merging is very much more openly controversial. In fact, pretty much every conference I've ever been to has left me with the distinct impression that it's only really a few prestigious diehards who actually believe it.

To me, the collapse of a single giant cloud into a galaxy - which quantitatively resembles actual spiral galaxies - seems like a much simpler idea than trying to build up a disc by lots of small galaxies that all collide in just the right way as to form a neat disc. It also has the advantage of not producing a "missing satellite" problem - unlike most other simulations, this one produces a spiral galaxy with about the same number of orbiting satellite dwarf galaxies as we observe in reality*. Unfortunately their orbits are all wrong, but meh. You can't have everything.

* That is no small achievement, but for some reason - God knows why - the paper has only 15 citations in 14 years.

The satellite galaxies of the Milky Way orbit at right-angles to
the disc, whereas the simulation predicts they should orbit in
the same plane. Ah, well, whatever, never mind.

Anyway, what you can't see too well in the gif is that on the large scale, the dark matter tends to fragment before it completely collapses. Any part of the halo which happens to become slightly denser than the rest (just through the small random motions of the particles) will have more gravity and tend to collapse faster. Remember that the gif is showing several billion years of simulation time - there's plenty of time for parts of the halo to fragment before the whole thing collapses.

Fragmentation of the collapsing dark matter (left) and gas (right). Both images are about 85 kpc across (280,000 light years).

Since the dark matter is collisionless - it doesn't interact with itself - these condensations tend to be pretty diffuse, but they're still quite massive. This means the dark matter fragments draw in some of the surrounding gas, which makes the whole collapse process very messy. Instead of a uniform sphere that just shrinks, you get a load of different clouds all heading only roughly toward the center of the cloud.

In the gif you can see a giant purple shockwave - after the dark matter collapses, most of it re-expands. That means the gravity near the center is reduced, and what you get is a bunch of gas fragments all sort of milling about, which eventually turn into a nice happy galaxy.

But... take the dark matter away and that early fragmentation doesn't happen. Instead, the gas just collapses uniformly until it reaches a very small size indeed, at which point it does, eventually, fragment*. But the final result looks nothing like a galaxy - what we found was a ring about 10% the diameter of the galaxy that formed in the case of dark matter, with the gas density being very much higher.

* The gas is much hotter than the dark matter. Temperature is basically velocity dispersion, i.e. random motions. For fragmentation to occur, gravity has to be strong enough to overcome these random motions. For the hotter gas, this means it only happens when the density is very high.

With dark matter, a galaxy around 50 kpc (163,000 light years) across is formed. Without it, what we got was this itty-bitty ring about 3 kpc (9,700 light years) across.

This lack of pre-collapse fragmentation was certainly important, but there was also another problem. The simulation lost angular momentum (which breaks physics - simulations do that all the time but usually just by a negligible amount), so everything was spinning more slowly than it should. The resulting ring was therefore smaller than it should have been. However, fourth-year undergraduate me neglected to write down exactly how much angular momentum was lost, so I can't tell you how much bigger the ring may have been if the code wasn't bugged. Well, we live and learn.

(all I did write was that while normally it's conserved to within 1% or less, in this run it was losing 8% per timestep - which is nothing short of catastrophic)

Of course we didn't just do one simulation - more like a couple of dozen, each taking a few days to run. We varied a whole bunch of parameters, which I won't go into here. Nothing worked. The one time we managed to create something that did resemble a galaxy, it was spinning much too fast - more than twice as fast as anything ever observed, and for once that's well above any measurement errors.


So, another triumph for the dark matter model then ? Heck no ! That's how the popular press would spin it... but I'm trying really hard to convince you that just because a result doesn't have Earth-shattering implications doesn't mean it's not interesting. The obsession in the media for decisive results is pretty damaging for science communication, but that's another story...

What we showed was that in this unconventional model, dark matter was necessary to forming the galaxy - it's not a case of it doing nothing much except making the gas spin faster. It doesn't merely help galaxies form more rapidly - it's essential for them to form at all. I don't know of anyone who's tried to do a more conventional galaxy formation simulation but with the dark matter simply turned off, but it would make for a fascinating comparison.

So we found one small piece of evidence in favour of dark matter. It's through a series of these small, incremental findings that I've become convinced that dark matter is the most likely explanation for what we observe. But that in no way means that there aren't still other problems, or other ways to form a galaxy without dark matter. I'll take a look at why I'm still not a True Believer in dark matter next time.

Berlin

Berlin ! The city of.... uhh, the Brandenburg Gate ! The Berlin Wall ! And probably some other stuff, too, I shouldn't wonder... Naturally, when invited to visit a friend on an internship in Berlin, I jumped at the chance. How could I refuse the chance to visit somewhere with both famous landmarks ?

So I found an inexpensive hotel - Berlin, somewhat surprisingly, is full of them - booked a bus ticket, and filled my suitcase with 10 litres (17.5 pints) of beer. Why on Earth would I take beer to Germany, I hear you ask. Isn't that like taking coals to Newcastle ? Not if your friend is Czech, it isn't. To suggest to a Czech that any other nation's beer is superior is like calling a Glaswegian English or telling an American that guns are really actually quite dangerous things and they should put them away. It's not something you want to do if you value your continued existence.

Anyway, said friend has been teaching me how to drink beer, a statement which is likely to have given several long-term readers heart palpitations. In fairness, there are - it seems - many beers which do not have the disgusting aftertaste that makes me think beer drinkers are otherwise completely mad.

Hauling near enough twenty beers up several flights of stairs was somewhat gruelling work. I was rewarded with a rather fine sunset at the train station in Berlin shortly after my arrival.

Unfortunately the pleasant sunset was in no way compensation for the massive inefficiencies of the Berlin public transport network. Instead of taking 30 minutes to get to my hotel it took an hour. Much worse than this was that instead of simply delaying the train from reaching the station, the train simply stops at the station. Everyone gets on.

Then the train doesn't go anywhere.

The train continues not going anywhere for about 15 minutes.

Yep, that's right, it just sits there.

It's 37 degrees Celsius and at this point it was still full daylight. It was not fun.

I also soon noticed other flaws in the public transport system. The front of the train labels the destination, but of course you only see that for a moment as it rushes past. Inside the train (or bus) the electronic screen doesn't say anything until the train leaves the station, so for a confused tourist it's of little help. When it does move, it only says the next station, not the final destination. And often it just doesn't work at all, and in one case it got stuck after about three stations.

The good news is that to get through this muddlesome mess, God created the German people. Anyone even looking like they might possible have any doubt is sure to rapidly encounter an extremely helpful German person who will make sense of it all, or at least try to. You don't even have to ask for help, just look slightly confused and someone will soon get your sorted out*. Perhaps the Berlin transport network was designed knowing that it wasn't necessary to make it simple to use because the Germans would happily explain it to everyone who didn't get it.

* I've begun to wonder if this would work in other circumstances. Like, if I could contrive an expression that very clearly says, "I'm really not sure if my smooth particle hydrodynamic code is giving me numerical artifacts or interesting results", maybe a German person would just magically turn up and explain it.

Eventually I reached my hotel and the situation rapidly improved. I was given a free upgrade to a larger room, for no reason, and though the air conditioning was underpowered the room was at least 15 C cooler than outside so my brain was able to re-congeal into something basically functional once again. Really, the last time I was this sweaty and disgusting I was in the frickin' jungle.

No, I'm not wet because I went swimming. Yes, I know that's disgusting. Topical paradise, my foot.

Shortly afterwards I began another full hour of travelling to reach the visitor's rooms near the DLR, where I was treated to a delicious Czech meal of potato pancakes followed by beer and more beer. I made it back to the hotel around about 2ish, where after a full eight hours of travelling (and precious little sleep the night before for some reason) I swiftly collapsed.

I suppose just for the sake of comparison I should also mention that the brief riverisde walk outside the hotel the next morning was a lot nicer than the harrowing death march to reach the Tanama river in Arecibo. It wasn't anything special, but at least when I took this photo I didn't feel like I was starting to liquefy. Unfortunately, I've lost most of my tropical climate adaptations, which means I'm back to complaining bitterly whenever it gets over 30 C.

I met up again with my Czech friend and housemate-whose-name-neither-of-us-can-pronounce for lunch (bratwurst) by the Brandenburg Gate. Gosh, that sounds classy, doesn't it ? It should. It's a classy place.

This being my first full day in Berlin, wandering around the city center was essentially obligatory. And a very nice place it is too. I mean, it's not Cardiff, or even Prague, but it is well worth seeing. Just around the corner from the Gate is the Holocaust Memorial. It is, of course, right and proper that a huge piece of real estate, in the center of what is essentially the capital of Europe, is given over to recording one of the worst atrocities in human history.

... but on the other hand the monument itself perhaps does not command the air of solemnity its designers probably had in mind. In the above picture I've recoloured it to give it some sense of atmosphere, but on a bright sunny day it isn't like that at all. Signs say "no jumping from one stele to the next", which is a rule continuously flouted because most people are, at heart, about five years old. And that's a good thing. Maybe it's even the reason we haven't gone extinct yet.

A little further on from this we came to Potsdamer Platz in search of the Cafe Gelato. Which we found. We also found part of the Berlin Wall and a small exhibition about it inside the shopping center.

Leaving the British sector ? Hah ! I AM the British sector !

Yeah ? Well, screw you, sign ! You're not the boss of me !

As shopping centers go, it's tough to beat having part of the Berlin Wall just outside and an exhibition about it inside. It really is astonishing to consider the scale and speed of the changes that have happened so recently. Less profoundly, it also had really good ice cream - although you weren't allowed to sit at a table to eat it. Because walking around with melting ice cream is obviously a much better idea.

From Potsdamer Platz we headed back to reach the Berlin Victory Column. Along the way we encountered the Soviet War Memorial.

The memorial is flanked by two tanks and small artillery pieces. Presumably the Soviet artists, like their later German counterparts, saw no reason a monument to some utterly horrific events couldn't also be a) intimidating and b) fun for tourists decades later. Because who's going to resist the chance to pose with a tank, right ?

Glamorous blonde scientists don't often pose with tanks for your amusement.

Moving swiftly on, the Victory Column. It's a dramatic sight - 67m tall, built to celebrate three 19th century German (Prussian) victories against the French, the Danes and the Austrians.... none of which I know the slightest thing about. I doubt anyone else does either. That would be like visiting Big Ben because you're fascinated by clockwork - entirely legitimate, sure...but, well... weird.

After the Victory Column, the TV tower. We didn't go up - it's expensive - but it is impressive to see, and less ugly than Prague's giant space rocket covered in deformed babies with giant heads (that is its official name). What I would have liked to have seen is the dome of the Reichstag, but unfortunately it was already fully booked for the weekend.

The church is one of the oldest in Berlin.

We ended the day near what I'm calling the half-church... because it's half a church. And whadayaknow ? Typing, "Berlin half church" into Google finds the correct result. It's great looking building, but it's almost a shame it's in Berlin. It would have made for a perfect movie location if it was out in the countryside somewhere.

However, as far as awesome movie locations go, the next day won hands down. Unlike the above, you won't find Teuflesberg listening station on any "must see in Berlin" lists. And I have absolutely no idea why, because it's an extraordinary place. It was just about visible from the Victory Column so we decided to try and get there.

I reserved a place online... but here's a word to the wise - I really don't think it's necessary. Their website may look flashy on a tablet but it's very amateurish on a regular PC, and I seriously doubt the place even has internet access. They probably just check their email on their phones. Also, though their site says the tours stop at 4, this didn't seem to be the case either. If you can't get there by 4pm, I'd email or call them to check. Possibly you do need a reservation for the 2.5 hour guided tour (weekdays only, once per day), though.

A bright white geometrical dome poking through the forest... quite a familiar sight, in some ways.

Teuflesberg ("Devil's Mountain") is an artificial hill 80m high built from the rubble of Berlin after the war. It's named after the nearby Devil's Lake, although it even has a rather fiendish connection of its own, despite being only a few decades old. I didn't have time to read up on it beforehand, but the wikipedia article is quite fascinating :

Its origin does not in itself make Teufelsberg unique, as there are many similar man-made rubble mounds in Germany (see Schuttberg) and other war-torn cities of Europe. The curiousness begins with what is buried underneath the hill: the never completed Nazi military-technical college (Wehrtechnische Fakultät) designed by Albert Speer. The Allies tried using explosives to demolish the school, but it was so sturdy that covering it with debris turned out to be easier. 

OK, so it's an artificial hill that covers the ruins of a Nazi military college. Appropriate name after all, then. Apparently the forest is also full of wild boar.

From the approach we took, the listening station "Field Station Berlin" was rarely visible through the forest. A very nice forest it is too, though methinks some of the holiday homes take their security a tad... seriously ?

Maybe those wild boar can jump.

Even when you reach the hill - after a brisk 45 min walk or so - you can't see much from the entrance. The site is now a sort of reservation for graffiti artists, presumably to keep them safe and protected from the real world. Which is exactly the same function universities serve for scientists.

The angry dog will guide you on your mystical forest journey.

OK, the scrawls at the front aren't up to much, though I did appreciate the ironic angry dog. But pretty soon you're rewarded with real artistry. It varies in quality, of course (not all of it was to my taste), and it certainly isn't family friendly. Much of it isn't friendly to anyone. In fact I would even say it's overtly hostile, but almost all of it is interesting.

Graffiti ? No, it's the original colour, honest.

Generally good advice.

Some of the doodles are little better than what you get scrawled on the side of a subway (though at this point I have to point out that Cardiff's graffiti can be amazing, seriously click that link). And some of it is genuinely good art.

Immortan Joe's forest-dwelling cousin ?

I was almost half-expecting just to be able to walk around the site, because the buildings are not in a good condition. But, nope, you get everything. Right to the top of the tallest tower*. You don't get much in the way of hearing about the history or function of the building, unfortunately, but what you do get is a graffiti art gallery built in the ruins of an NSA listening post built on the ruins of Berlin built on a Nazi military college. And come on, that's tough to beat for 7 Euros.

* Is it safe ? Well the guide described it as being "not idiot proof" which I take to mean "hell no".

The title of this painting is definitely, "Giant naked sea goddess attacks defenceless island using giant swans". Definitely.

"Fuck condoms" ought to win an award for irony.

At the top, the geodesic domes. The outer fabric is half-ripped away and billows in the wind, making the whole place sound like you're inside a giant tent with a view of the whole of Berlin. Actually what I was most impressed with was not Berlin itself (which is fine enough) but the fact that in some directions the forest stretches right to the horizon.

A.k.a the forest moon of Endor.

OK, it was an NSA listening station, so perhaps the anti-America stuff is to be expected. Still, I don't really get it... or the "freedom is beautiful like a brick in the face of a cop" slogan*, or the various anti-Obama messages. I mean, come on people : you've been given this huge structure overlooking Berlin as a toy ! And you've been allowed to do this by the system you're against ! It just strikes me as incredibly self-righteous and suffering from a massive dose of the Nirvana fallacy. If you've been given somewhere like this to do what you love, and all you can express is anger at the system, well... aaargh. Maybe I'm just not taking it as tongue-in-cheek enough.

* Thus spake the angry dog. I preferred his other slogans, "The man who does not read has no advantage over the man who cannot read" and especially, "Who left the bag of idiots open ?".

That's enough ranting. The finale at Teuflesberg is the very top of the tallest tower, where the dome is completely intact. It's lit only by a single window and you're greeted on entry with open arms :

But the weirdest thing by far is the sound. The acoustics in the dome are... well, awesome. Wherever you stand, any noise you make is reflected back to you - loudly. It's like listening to your own voice through a microphone and speakers, without the microphone and speakers. When we entered the dome it was filled with a cacophony of  whoops, whistles, laughing, animal noises... everything, really. I only wish I'd thought to record it sooner, but it was a such an arresting experience that it seemed more important to just listen.

The center chair is even more remarkable. Despite the din, any sound you make in that spot becomes audible to you but not necessarily anyone else. I looked on in bemusement while my friend just sat there, tapping her foot. Foolishly I yelled at her to say something. She kept tapping her foot, grinning like a crazy person*. Only when I tried it for myself did I finally understand - this extremely quiet sound becomes massively amplified to anyone sitting on the chair. It can't be recorded, you have to experience it. Lord knows what music would sound like at that spot.

* Oh thank Gods for anonymity.

Teuflesberg, then, ought to be very high on anyone's must-see in Berlin list. The only thing you don't get on the one hour guided walk is much in the way of any historical information about the place. I can't vouch for the 2.5 hour tour but if I ever go back there for more than a weekend (why this option is weekdays only I have no idea) then this would be top of my list.

This extremely pleasant, more sedate day was followed by more delicious Czech food* and quite a lot more beer. I reached my hotel room around 3:30am, learning the hard way that the S-Bahn doesn't run on Sunday evenings ! Although really it was Monday morning... grrr.

* Fried cheese. The Czechs have great sympathy for vegetarians, and they're determined that they should be able to enjoy incredibly unhealthy fattening food as much as the rest of us.

Anyway, though its public transport service doesn't hold a candle to Prague - I suspect nowhere does - Berlin is a fine city, completely different to Prague or Vienna. While I wish Teufelsberg had provided a bit more information on the place, being kept as a reservation for graffiti artists gives it really unique character. Maybe that angry dog should occasionally quote snippets of interesting information about the place. He could even keep his trademark anti-capitalist, anti-American ranting, I wouldn't mind it so much if I got a bit of an education as well.

Tracing the outline of the angry dog took a surprisingly long time for a cheap joke, but meh.