One of the fun things I get to do as part of my job is to be involved with the annual British Science Festival, getting involved in organising the physics and astronomy events. This year the festival is in Bradford, where I am now, and I sit outside composing this at a rather attractive new campus, sitting outside in the mid-September sun.
The first event I went to, on Tuesday evening, was about something quite different from the Sun. The Sun, of course, is the brightest thing we see, and is largely responsible for illuminating everything else we see. Surprisingly enough, it turns out, or at least so we think, that the stuff we can see only accounts for about 4% of all “stuff” in the Universe, and by “stuff” I mean things that affects the gravity of the Universe.
This is quite a new result in physics and astronomy, and is quite a revelation. To think that we only understand 4% of stuff in the whole Universe is quite a blow, in some ways, but quite exciting in other ways. It certainly gives us a lot to think about.
Apart from the 4%, the rest is around 22% of so-called dark matter and 74% so-called dark energy. One of the talks I heard, by Carlos Frenk of Durham University, summed up the entire world knowledge of what Dark Energy is rather succinctly – because we don’t know much: It’s not “stuff” or particles and it’s an inherent property of the vacuum of space. It acts as a kind of anti-gravity… but more than that, we don’t really know much. We infer it exists because of the observation of expansion of the Universe as evinced by galactic redshift. More than that, we have some ideas, but not much more.
So what of dark matter? Well, it’s “matter” – stuff – particles – things you could touch and make things out of. Except that it is dark in the sense that they don’t interact with light, which is quite unlike normal matter made up of atoms (we can see them -they absorb and reflect light). Quite what it is is unsure, although there are various theories from particle physics. What is sure, though, is that galaxies don’t behave anything like they should unless there is dark matter in them. According to the laws of gravity, the planets furthest away in the solar system take a long time to orbit the sun. Not only are they travelling in longer orbits, they also travel more slowly. That’s what Newton’s (and Einstein’s) theories of gravity say should happen. If you look at galaxies, though, and measure the velocities of stars far away from the centre, they seem to be orbiting more quickly than they should. There are a few ways in which you might try to explain this, but the existence of lots of extra mass in galaxies – mass that we can’t directly see – is the front-runner.
Perhaps the most exciting thing to come from this session of talks (with Dr Hendry from Glasgow and Spooner from Sheffield as well as Frenk) was the admission by Frenk, that perhaps this idea of Dark Matter might not be right after all. There is some evidence from direct observation of the motion of stars in nearby “dwarf galaxies” – small galaxies orbiting ours, that the standard dark matter picture is not right after all. This is no great disaster – it’s just how science works: we have hypothesis, we test them and see whether they work. If not, we think about why not and come up with new hypotheses. But still, it is exciting to know that there’s lots of work to do on the 96% of the Universe that we don’t understand very well.