Our universe is about 13 billion years old and, like an anorexic teenager, it looks severely underweight. In fact it looks much more underweight than the severest anorexic. According to Gerry Gilmore of the Institute of Astronomy in Cambridge, the weight of all the visible matter in the universe is only a few per cent of the weight astronomers would like the universe to be.
One possibility is that the universe really is underweight. Its ideal weight comes from a desire for a simple theory rather than from any health guidelines for universes. Theoreticians would like what they call a flat universe, one just heavy enough for the gravitational pull generated by its weight to stop its expansion given infinite time. But “there’s no evidence that it’s that simple” says Gilmore.
But it’s just possible that it could be. Astronomers now know that the universe is actually a lot heavier than it looks. When they measure the weight of galaxies directly, insead of adding up the weights of the stars they contain, they appear to be many times heavier. The extra weight is provided by something invisible and almost completely mysterious that the astronomers have christened “dark matter”.
Weighing planets and stars is straightforward according to Gilmore. Newton worked out the principles. Planets are held in their orbits by the gravitational pull generated by the weight of the sun. “The strength of the pull you need depends on the speed” says Gilmore “it’s like whirling a conker on a string”. So from the orbits of the planets, we can calculate the relative weights of the sun and all the planets.
Outside our solar system the same principle works, Gilmore says. Our sun orbits the centre of our local galaxy, the milky way, as does all the other visible matter in the galaxy. The only force to make averything stay in orbit, is the gravitational pull of the rest of the matter in the galaxy. However, when astronomers calcuate the forces required to keep everything in orbit at the right speed, there isn’t anything like enough visible matter in the galaxy to generate the gravitational pull.
The only possible answer is that there must something else – the dark matter – that boosts the weight of the galaxy so gravity can hold it together. “It’s the same wherever you look in the universe” says Gilmore. Whether you look at the motion of individual galaxies or at large clusters of galaxies the amount of visible matter isn’t nearly enough to generate the gravitational pull to hold everything in place.
The measurements of weight are very indirect, but two different ways of calculating how much matter was created when the universe was formed both confirm the shortage of visible matter. The calculations use well tested theories of nuclear physics to calculate the amount of matter in the universe from measurements of the background radiation and from measurements of the relative abundances of different elements in the universe. There’s no getting away from the fact that there isn’t anything like enough ordinary matter to hold the universe together. The question is not whether dark matter exists, but what form does it take.
To answer the question astronomers depend less on the precision of their telescopes than on the fertility of their imaginations. They are trying systematically to search for all the possible forms of matter that might bulk up the universe without having shown up on telescopes. The two leading contenders so far have the catchy acronyms wimps and machos.
Wimps, or weakly interacting massive particles to give them their full name, are elementary particles smaller than an atomic nucleus. The universe would have to be full of them – “about one in every patch the size of your thumb” says Gilmore – for them to make up the weight. Nuclear physicists have a list of several particles that could be the missing wimps. The favourite candidate is called the tau neutrino, but so nobody has been able to weigh one.
Machos, massive compact halo objects, are bigger than wimps – they would be as heavy as a small star or a large planet. It is unlikely that much of the dark matter comes in sizes in between the macho and the wimp because we be able to see it, or to bump into it. However larger objects would be able to make up the weight while remaining just rare enough not too have been seen.
For the last few years astronomers have been systematically scanning the skies looking for the brief disturbances that would be caused when a macho blocks of the view of something more distant. So far they have found less than ten, Gilmore says. It is beginning to look as if, when it comes to holding the universe together, just as in everything else, the wimps make a bigger contribution than the machos.