How will the universe end? One possibility is that it will eventually suffer an implosion, as time, light and space collapse. In this theory, the expansion of the universe that started with the Big Bang will run out of momentum and the universe will start to contract back into itself.
This process has been called the Big Crunch, and this may in turn lead to another Big Bang, perhaps only the latest in an unimaginably long cycle. Alternatively, if there is not enough matter in the universe for gravity to bring about a Big Crunch, it has been conjectured that entropy might lead to the ‘heat death of the universe’, with all energy dissipated and the cosmos left for ever cold and lifeless.
More recently, in response to a range of observations of gravitational effects, cosmologists have proposed that over five-sixths of the mass of the universe may consist of something they call ‘dark matter’ – matter that has mass, but which cannot be observed using current technology. This would make the mass of the universe far greater than hitherto thought. It would also make the Big Crunch the more likely scenario.
Against this, observations of distant supernovae indicate not only that the furthest parts of the universe are moving away from us (as the Big Bang theory would expect), but that the rate of acceleration is increasing. Previously, physicists would have expected the momentum of the original expansion to slow down. Nothing has explained this rising acceleration, and scientists have posited a mysterious ‘dark energy’, acting counter to gravity, which might lead to the universe expanding indefinitely. This in turn has led to the suggestion of another possible way the universe might end – in a ‘Big Rip’, in which all objects in the universe no matter how large or small eventually disintegrate into elementary particles and radiation.
There are some obvious philosophical difficulties in contemplating the end of the universe. Firstly, none of the present scenarios – no matter how careful the mathematical modelling – are provable. They depend on current observations, which have been revised before and will be revised again. Secondly, the very act of thinking about the universe is a difficult one for the human mind. The universe is ‘everything there is’, and it is all but impossible for us to truly imagine it not existing. It is conceivable that the whole search for a start and an end to the universe is a misleading metaphor based on our own experience of all creatures being born, living for a lifetime and then dying. It is even possible that there are many parallel universes, of which our own is just one. But it would be impossible for a conscious being in this universe to have any direct knowledge of any of these other alternative worlds.
One of the advantages of studying big history, as opposed to human history, is that it gives us a humbling insight into how contingent and fleeting our existence is. Each one of us exists for a tiny fraction of the time that humans have lived on this planet. And the human species has existed for only a tiny fraction of the life of our solar system. Our solar system has in turn existed for only a fraction of the life of the universe and only came about because of a particular set of forces and matter combining in a certain way in a chaotic earlier period in our galaxy.
It is something of an achievement that we can know as much as we do about the universe and the history of our planet, but there comes a point where we have no choice but to accept that we will never know the full story of the universe in which we live.