We now know much about scientific laws that govern matter in all normal situations. But we still do not know the laws that govern matter in very extreme conditions. Unless we know those laws we cannot understand how the universe began.
However, those laws have nothing to do with the future of the universe, until and unless the universe re-collapses to a high-density state. Even if we knew these laws, we could not use them to predict far into the future. Solutions to the equations of physics may exhibit chaos. A slight change to the way a system is by a small amount at one time will make the later behavior of the system completely different.
There is a time scale on which a small change in the beginning will later grow into twice as big. The time scale for the earth’s atmosphere is of the order of five days. We can make a reasonably accurate weather forecast for periods up to five days. For forecasting the weather more than five days ahead needs an accurate knowledge of the present atmosphere and an impossibly complicated calculation. We cannot predict the weather six months ahead.
We know the basic laws that govern chemistry and biology. With that, we think we ought to be able to determine how the brain works. But the equations that govern the brain are chaotic. A very small change in the initial state can lead to a very different outcome. This is why we cannot predict the human behaviour. Science cannot predict the future of the human society. Nor can it say if human society has any future. Our power to damage or destroy the environment and one another is increasing much more rapidly than our wisdom in using this power.
Whatever happens on the earth, the rest of the universe will carry on regardless. The motion of the planets around the sun is chaotic, though with a long time scale. The errors in any prediction grow bigger as time goes on. After a certain time, we cannot predict the mention in detail. The motion of the sun and other stars around the galaxy and of the galaxy in the cluster of galaxies is also chaotic.
We observe that other galaxies are moving away from us. The farther they are from us, the faster they are moving away. The universe is expanding in our neighborhood. We observe a background of microwave radiation. This gives evidence that the expansion of the universe is smooth and not chaotic.
The amount of radiation seems to be very nearly the same from every direction. Even if the temperatures are different in different direction the variations are very small, only one part in one lakh. The microwave background is almost the same in every direction. We must assume that the background is also the same in any direction in any other galaxy. This is possible only if the average density of the universe and the rate of expansion are the same everywhere. On a very large scale, the behaviour of the universe is simple. It can therefore predict far into the future.
One can describe the expansion of the universe in terms of the distance between two galaxies. This distance is increasing at the present time. The gravitational attraction between different galaxies would be slowing down the rate of expansion. If the density of the universe is greater than a certain critical value, gravitational attraction will ultimately stop the expansion.
The universe will start to contract again and collapse to a big crunch, a state of infinite density at which the laws of physics would break down. Even if there were events after the big crunch, what happened at them could not be predicted. If the average density of the universe is less than the critical value, it will go on expanding for ever. After a certain time the density will become to low for gravitational attraction to have any effect on the slowing down of the rate of expansion. The galaxies will continue to move apart at a constant speed. Even if the universe is going to re-collapses it will not stop expanding for at least ten billion years.
Our observations can help us estimate the average density of the universe. If we add up the masses of all the stars we can see, we get less than one per cent of the critical density. If the masses of the clouds of gas are added to it, the total comes to only one per cent of the critical density. The universe also contains dark matters which are huge collections of stars and gas. They are rotating about their centers. The rate of rotation is so high that they would fly apart if they contained only the stars and gas.
There must be some unseen form of matter whose gravitational attraction holds the galaxies together as they rotate. Clusters of galaxies are an evidence of dark matter as are spiral galaxies. We observe that galaxies are gathered together in clusters. The speeds at which these galaxies are moving in these clusters are too “high. If not held together by some strong gravitational attraction, they would fly apart. The mass required for such attraction is far greater than the masses of all the galaxies. It follows that there must be extra dark matter present in clusters of galaxies outside the galaxies that we see.
We can estimate the amount of dark matter in the galaxies for which we have definite evidence. This estimate is only one tenth of the critical density needed to cause the universe to collapse again. Thus one would predict that the universe would continue to expand for ever.
After about five billion years the sun would use up its nuclear fuel. It would grow into a red giant, swallow up the earth and other planet and would there settle down to be a white little star. That time is too far ahead. By that time man would have mastered the technique of interstellar travel a small white star. The end of the world is too far away from us. By the time the sun blows up, we should have mastered the art of interstellar travel.