Margot Brouwer is looking for nothing
The hunt for dark matter
Door Christien Boomsma
By Christien Boomsma
What is dark matter?
Approximately 95 percent of all energy in the universe is ‘missing’. Astronomers calculated that this energy must exist. However, no one knows where it is.
Galaxies spin at a certain speed. Their speed is determined by gravity, which makes sure the galaxy mass holds together. Then there are the centrifugal forces, which push the mass apart. However, galaxies spin way too fast for the amount of gravity exerted by the matter visible. Astronomers think that the mysterious dark matter is responsible for this extra gravity. At least 20 percent of all energy in the universe consists of dark matter; only 5 percent is normal matter.
Then there is such a thing as dark energy. Astronomers came up with this to explain that the galaxy is expanding at an increasing rate. According to the current laws of physics, this is only possible if there’s another unknown source of energy that is causing this rapid expansion. This dark energy makes up 75 percent of the mass in the universe.
There are astronomers who spend their research down in an abandoned mineshaft, where they set up large containers filled with the noble gas xenon, as well as a bunch of detectors. Far away from anything that can distort their readings, they hope to detect particles of the mysterious dark matter that explains why faraway galaxies spin so fast.
Other astronomers make use of the kilometre-long particle accelerator at the Swiss research organisation CERN. They collide high-energy particles, hoping the collisions will create dark matter.
And some scour the universe for unexplainable peaks in photons, or light particles. ‘That’s an indirect method. We assume that normal particles are created when two dark matter particles collide. These normal particles might be photons. If that’s true and someone detects an unexplainable peak at a certain wavelength, that could be an indication of dark matter’, says RUG astronomer Margot Brouwer.
High risk, high gain
She scoured the skies for those peaks when she was writing her master’s thesis at the University of Amsterdam. She quickly realised she didn’t have the patience to do the job. Just like her fellow astronomers, she never found anything. ‘This kind of research is high risk, high gain’, she explains. ‘If you do find something, blam, you’re a shoo-in for the Nobel Prize. At the same time, it’s like you’re looking everywhere for your keys and of course you find them in the last place you’re looking.’
Not that the search is useless, Brouwer says. Finding your keys doesn’t mean looking in all those other places was useless. And yet. ‘When I still hadn’t found anything after a year, I figured I would never get that Nobel Prize.’
She settled for a method that was used a lot in Leiden: gravitational lensing. The idea is not to look for single particles, but to figure out where in the universe the dark matter is.
Curved space-time
‘Einstein said that gravity is caused by curved space-time’, Brouwer explains. Light that travels long distances through the universe doesn’t move in a straight line, because gravity causes it to bend. ‘The more gravity there is, the more distortion’, says Brouwer.
I figured I would never get that Nobel Prize
Aided by pictures taken by the VLT telescope in the Chilean Atacama Desert, which captured no fewer than 1,500 square degrees of the sky, Brouwer calculated the distortion of faraway galaxies. ‘That allows me to inventory the dark matter’, she says. It didn’t lead to the discovery of any actual particles, but it might just be a step in the right direction.
Then, something happened in the world of theoretical physics. Amsterdam physicist Erik Verlinde had come up with a radical new theory, which posed that there was in fact no such thing as dark matter.
Hypothetical particles
‘There are good reasons to assume that dark matter does exist’, Brouwer emphasises. ‘Einstein’s model of gravity explains almost all phenomena on earth and in our solar system. It’s accurate to ten decimals.’
Nevertheless, there are reasons to doubt the existence of dark matter. We’re still ‘missing’ gravity to explain the behaviour of certain galaxies. We’ve substituted hypothetical particles, of which we only know what they don’t do: they’re not visible, they don’t interact with other matter, they don’t transmit any visible light or radiation, and they don’t reflect anything.
Moreover, gravity doesn’t work on a scale of elementary particles. For that, we invented quantum mechanics, but so far no one’s succeeded in connecting the two theories.
I didn’t understand much of what Verlinde said
Let’s not forget the fact that the universe is rapidly expanding. That, too, is in fact impossible. Astronomers solved that particular riddle by assuming the existence of the similarly mysterious dark energy.
Sea of information
Verlinde proposed a different solution. ‘He says that gravity isn’t a natural force, but rather a result of other forces in physics. Instead, he calls it “emergent gravity”.’ You can look at it like hot tea’, says Brouwer. ‘The heat you feel doesn’t actually exist; it’s caused by the molecules’ behaviour. But the single molecules themselves aren’t warm.’
According to Verlinde, the universe isn’t a sea of particles, but a sea of information, of fundamental units that make up everything else. After some calculations, it turns out that these information packages behave in accordance with laws that resemble those of thermodynamics.
Brouwer was present at the lecture where Verlinde presented his theory. ‘The room was packed’, she says, ‘I was squished against the back wall. To be honest, I didn’t understand much of what he said. I’m not a theoretical physicist.’
Verlinde’s formula
At the end of the lecture, Verlinde wrote a formula on the blackboard. The formula predicted how much dark matter could be found surrounding normal matter. ‘That got me excited’, says Brouwer. ‘That was something I could test. After all, I had measurements of the gravity surrounding faraway galaxies.’
I was really fucking excited!
It only took her a few days to apply Verlinde’s formula to her data. ‘It all made a lot of sense!’ she says. Not that the ‘old’ gravity theory didn’t work, but you’d need to add the hypothetical dark matter. ‘Verlinde’s theory didn’t need any free parameters’, says Brouwer. ‘So I was really fucking excited!’
Since then, Brouwer has been working on figuring out which theory best works with her data. Verlinde’s theory is a contender, but so is MOND – Modified Newtonian Dynamics. Supporters of that particular theory also don’t believe in dark matter. Their proposed solution is that gravity is stronger at lower accelerations, which means those galaxies spin faster.
Right now, Brouwer is finishing up a paper in which she tests several theories through her data on faraway galaxies. Verlinde’s theory comes out on top, although MOND is also a contender. Nevertheless, the ‘classic’ dark matter theory can also still be applied.
Good results
What’s clear is that the farther removed you are from the centre of a galaxy, the more dark matter you’ll find. The most important finding is the relationship between dark and normal matter. ‘These are really good results’, Brouwer says happily. ‘I’m really curious to know what the community has to say about it.’
So is there a clear winner in her paper? There is not. That’s a bit of a shame, because she secretly hopes that Verlinde is right. ‘It’s a whole new idea that revolutionises the theory of space-time and it’s really interesting’, she says. She chuckles. ‘It would certainly solve a lot of problems.’