“The Sun and you and me (and all the stars that we can see) are moving at ten million miles a day, in an outer spiral arm at 400,000 miles an hour, in a galaxy we call the Milky Way.” Astronomers prefer to work with speeds in kilometres per second, and Monty Python’s Galaxy Song was a factor of ten off, but it’s true, we are passengers on a very high speed merry-go-round. We don’t notice this motion, of course, just like we don’t notice our speed going round the Sun: the accelerations are tiny.
The really surprising thing is that there are stars in the Milky Way that are ten times further out than we are, but that are still orbiting the galactic centre at about the same speed as us. If you think 200 kilometres a second sounds a bit quick, you’re right: there aren’t enough stars in the Milky Way to prevent, by gravity, these distant outliers from flying off into intergalactic space. Something else is holding our galaxy together, and for want of a better name this mysterious, heavy, invisible, space-filling stuff is called “dark matter”.
Dark matter is both a triumph of science and an embarrassment at the same time. On the one hand, we know it must be there, thanks to a lot of painstaking stellar speed measurements and some careful model-building. On the other hand, it seems that there is about 6 times more matter in the Universe than we can see, and we have little idea what the surplus is.
Astrophysicists are on a quest: to figure out what this new form of matter is. Having ruled out the possibility of dark matter sitting around in star-sized clumps, they began to suppose that dark matter might exist in the form of a gas of ‘weakly-interacting massive particles’, or WIMPs. If you look up in the night sky towards the constellation of Cygnus (“The Swan”), you’ll be looking straight ahead as we fly through the non-rotating dark matter that pervades our Galaxy. Just as you feel the wind in your face as you cycle to work, we ought to feel a dark matter wind in our faces as we look towards Cygnus – except that since dark matter seems not to interact with anything, we won’t actually feel it on our skin.
Dark matter streams right through us (with no ill effects), but we might be able to detect WIMPs if they were to occasionally crash into the atoms of specially-designed particle detectors. A number of teams have built experiments like this, in deep underground mines, to try and detect the dark matter wind. The mines’ thick rock ceilings (and floors) shelter these sensitive sensors, making it easier for the scientists to pick out signals that might be the reverberations of a dark matter particle impact.
Three teams of physicists have now seen hints of what might be dark matter: most recently the CoGeNT experiment detected a seasonally-varying number of particle impacts, similar to what you might expect to see as the Earth orbits the Sun while the Sun circles the Galaxy. When the Earth is moving in the same direction as the Sun, the dark matter wind should be a bit stronger; six months later, when the Earth is moving in the opposite direction, the breeze should be a little weaker. A periodic change like this is what CoGeNT sees – like the DAMA/LIBRA experiment did before it. The trouble is that other teams, using different detector technology, don’t see the effect.
It’ll take a while for the picture to become clearer; right now we don’t know for sure what’s causing the DAMA and CoGeNT seasonal variation. But it might be the dark matter wind.
Picture: The constellation of Cygnus, marking the direction in which our Solar System is travelling around the Milky Way galaxy. You can explore the night sky with this labelled, zoomable all-sky photograph by Nick Risinger. I made the above picture from a piece of a different, reorientable all-sky photo mosaic, put together by Axel Mellinger.