There is far more to the universe than meets the eye. The matter and energy we observe constitutes no more than 5 per cent of that known to be present. The other, invisible 95 per cent is called "dark matter" and "dark energy", and science does not yet know its composition. The dark energy that suffuses the universe appears to drive galaxies apart. Michael Turner, a cosmologist at the University of Chicago, brought us up to date on this fascinating topic in a recent edition of The Sciences.
Our earth, living organisms and the stars and planets are made of ordinary matter, which is to say atoms, each with a tiny nucleus composed of protons and neutrons and a surrounding cloud of electrons. There are 92 natural elements, ranging from hydrogen, the lightest, to uranium, the heaviest. Amazingly, this ordinary matter makes up only the visible 5 per cent of the universe.
The invisible dark matter provides the large-scale infrastructure of the universe. The dark energy is associated with the vacuum, or emptiness, that exists outside the matter components of the universe and is powerful enough to determine the shape and fate of the universe.
Whereas dark matter and dark energy have little effect on ordinary matter over familiar distances, they are detectable because of their huge gravitational effects over immense distances, such as when huge assemblages of matter are studied over spans of millions of light years.
In the 1930s Fritz Zwicky, the late Swiss astronomer, inferred the presence of massive quantities of dark matter in the universe. The great clusters of galaxies, and each individual galaxy, are held together by the gravitational attraction of some unseen mass. Our own Milky Way, for example, would fly apart were its stars solely responsible for the gravitational forces gluing it together.
Early suggestions were that dark matter could be accounted for by extremely dim stars, vast clouds of rarefied gas and planets that are invisible because they do not emit light. Such stars, clouds and planets exist, but not nearly in masses great enough to account for the gravitational forces that hold galaxies together.
The most promising candidates for dark matter come from particle physics, and for a considerable time the neutrino was a favourite. These little fellows were generated in enormous numbers in the big-bang origin of the universe, and about 10 trillion of them rush through every square centimetre of the universe every second, travelling close to the speed of light and almost never interacting with ordinary matter.
If they have any significant mass at all, it could add up to account for the gravitational effect of dark matter. The mass associated with the neutrino, however, which was eventually measured in 1998, turned out to be too small to account for more than a fraction of the dark matter.
There are currently two candidates that could have been born in the big bang in numbers sufficient to account for dark matter: the neutralino and the axion. Neither particle has yet been experimentally detected. The neutralino has about 100 times the mass of a proton. Axions have a very tiny mass - about one million million millionth of the mass of the neutralino - but are so copious in number (if they exist) as to be capable of accounting for the dark matter. If neutralinos exist, millions should be passing every second through every teacup-sized volume in the universe. Ten trillion axions should whizz each second through every cubic centimetre of the universe.
By definition, however, dark matter emits neither light nor other radiation and scarcely interacts with ordinary matter, apart from exerting a gravitational pull, so it is extremely difficult to track down neutralinos and axions. Sophisticated experiments are in progress in an effort to detect them.
The universe has been expanding ever since the big bang. The rate of expansion is determined by the amount of matter in the universe, because the mutual gravitational attraction will counteract the momentum of expansion and slow it down. Too much mass would cause a great deceleration, eventually reversing the expansion and causing the universe to collapse, in a big crunch. Too little mass would cause only a slight slowdown, and the universe would expand forever.
The best measurements so far of the rate of expansion of the universe were published in 1998. Surprisingly, they showed that the expansion is not slowing down; rather, it is accelerating. This seems to be at odds with what we understand about gravity. It is explained by the presence of an antigravitational drive that suffuses the universe, insignificant on a local scale but important on a cosmic scale, which Turner calls dark energy.
The energy may originate in the vacuum of space. According to quantum mechanics, the vacuum is not empty but filled with particles living on borrowed time and energy. Particles and antiparticles pop into existence for an instant, then disappear again, reverting to pure energy. Quantum vacuum energy has been known about for some time, but physicists do not know how large it is. It would have to be of a significant size to account for the acceleration of cosmic expansion.
This is not the easiest stuff to grasp, but it is of great importance. If you have been struggling with my clumsy attempts at explanation, perhaps you will be comforted by the words of St Thomas Aquinas: "The slightest comprehension of the highest things is worth more than the surest grasp of lesser things."
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC