We are used to the idea of the universe being around forever, but forever is a long time. The important thing about infinity is that it is not just a very big number. Infinity is qualitatively different from something that is merely stupendously, unimaginably huge.Suppose the universe were to have no end. For it to endure for all eternity means that it would have an infinite lifetime. If this were the case, any physical process, however slow or improbable, would have to happen sometime.

Given enough time, processes that are negligible on a human time-scale, but are nevertheless persistent, may eventually come to predominate and thus serve to determine the ultimate fate of physical systems.Let us imagine the state of the universe a very, very long time in the future - say, in a trillion, trillion years The stars have long since burned out; the universe is dark But it is not empty. Amid the black vastness of space lurk spinning black holes, stray neutron stars, and black dwarfs - even a few planetary bodies. At this epoch, the density of such objects is exceedingly low: the universe has expanded to 10,000 trillion times its present size.Gravity would play out a strange battle. The expanding universe attempts to pull every object farther apart from its neighbours, but the mutual gravitational attractions oppose this and try to bring bodies together. As a result, certain collections of bodies - for example, clusters of galaxies, or what pass for galaxies after eons of structural degeneration - remain gravitationally bound, but these collections drift ever farther from neighbouring collections.

The ultimate outcome of this tug-of-war depends on how fast the rate of expansion decelerates. The lower the density of matter in the universe, the more 'encouragement' these collections of bodies get to disengage from their neighbours and move apart independently.Not everything in the universe continues to move apart, however. What is left of the galaxies and other objects in 'gravitationally bound' systems are still subject to the slow but inexorable processes of gravity. Gravitational radiation, feeble though it is at any one moment, insidiously saps the system's energy, causing a slow spiral of death. Ever so gradually, dead stars creep closer to other dead stars or black holes, and coalesce in an orgy of cannibalism.Astronomers have good evidence that there already exist monster black holes at the centres of some galaxies, greedily gobbling up swirling gases and releasing huge amounts of energy as a result. Such a feeding frenzy will await most galaxies in time, and will continue until the material surrounding the black hole has been either sucked up or ejected, perhaps to fall back again eventually or to join the dwindling intergalactic gases. The bloated black hole will then remain quiescent, with only the occasional rogue neutron star or small black hole plunging in.Eventually all black holes - even the supermassive ones - will probably disappear too, their death pangs creating momentary flashes of light in the inky blackness of eternal cosmic night, a fleeting epitaph to the erstwhile existence of a billion blazing suns.IF STARS and galaxies end in this way, what will be left? What will happen to matter itself? Certainly, not all of it falls into black holes.

We need to think about the neutron stars and black dwarfs and planets that wander off alone into the vast intergalactic spaces, not to mention the rarefied gas and dust that never got itself together into stars, and the asteroids, comets, meteoroids, and odd chunks of rock that clutter star systems. Do these things survive forever? Ultimately we need to know whether ordinary matter - the stuff of you and me and the Earth - is absolutely stable.This is where quantum mechanics - the branch of physics that decribes the microworld - comes into play. Although quantum processes are normally associated with atomic and subatomic systems, the laws of quantum physics should apply to everything, including planets. This means that, given enough time, even these exceptionally small effects, when they occur in large objects, will bring about major changes in the universe.The hallmarks of quantum physics are uncertainty and probability.