The Formation of the Universe
How The Universe May Have Started
Fifteen thousand million years ago the Universe suddenly came into existence in a tremendous explosion which is called the Big Bang. The gases rushing outwards from this explosion eventually turned into galaxies, stars and planets, including the Milky Way, Sun and the planet Earth.
In the 1920s, the American astronomer Edwin Hubble discovered that galaxies are rushing away from each other and that the more distant galaxies are moving away faster than the near ones, as if they were the shrapnel from a huge cosmic explosion. By dividing the galaxies' distances by their speeds, Hubble could calculate when this explosion took place. The answer is 15,000 million years ago which he calculated using the very modern and accurate observations.
To check on this result, astronomers have measured the ages of clusters of stars. Stars change in color and size as they get older, turning into red giants and then into white dwarfs. Stars of different masses or weights change at different rates, so even though all the stars in a particular cluster were born at the same time, some stars have 'aged' faster than others.
Within each cluster, astronomers look for the most massive ordinary stars that are about to turn into red giants. They know, in theory, how long a star lives before becoming a red giant. For example: the Sun will live for 10,000 million years, whereas a star 20 times heavier will turn into a red giant after only 20 million years. From the mass of stars about to become red giants, astronomers can calculate the age of the whole cluster. Some star clusters are quite young on the cosmic scale which is around 70 million years old and the Sun is around 5000 million years old. Many are very much older, in particular the globular clusters. This type of cluster is a giant swarm of about a million stars. Astronomers have found that the stars in these clusters are very old - around 12,000 million to 14,000 million years old. They were probably the first stars to form the gases of a Big Bang, and suggest that it occurred about 15,000 million years ago.
There is a final, and extremely persuasive, piece of evidence that there was a Big Bang, even though it does not reveal when it occurred. Radio telescopes can pick up a faint background noise that permeates the entire Universe. The most likely explanation is that this is caused by electromagnetic waves which emanated from the hot gases of the Big Bang and are still pulsating through the Universe. Although astronomers agree on the birth of the Universe, they do not yet agree on the way in which it will end. There are two possibilities. The momentum from the Big Bang may be sufficient to keep the galaxies moving apart for ever. The stars in each galaxy will reach the end of their lives, as black holes or dark solid objects called black dwarfs or neutron stars. Eventually, about a million million years in the future, the Universe will just die.
On the other hand, the pull of gravity between the galaxies may slow down the impetus of the Big Bang and start to draw the galaxies together again. The Universe will then begin to contract. This theory is less popular at the moment, but some astronomers have calculated the consequences if it is true. In around a hundred thousand million years' time, the galaxies will rush together to a single point, in the Big Crunch. What happens after that is even less certain. The concentration of matter may explode again, as another Big Bang, giving birth to another Universe.
The Edge of The Universe
Astronomers and scientists are considering building a telescope ten times more powerful than any that exists. With it, they hope to look farther into the Universe than ever before. Even so, no one really expects to see to the edge of the Universe.
Modern studies of the Universe are based on Einstein's General Theory of Relativity. This theory states that matter has a gravitational field which distorts space and time so that space becomes curved and time runs fast or slow. The gravity of matter also bends light.
In testing the effects of the General Theory, scientists have found that it accounts for the motion of planets circling the Sun and stars orbiting other stars.
Accepting that the theory can be applied to the Universe as a whole, cosmologists also accept one final prediction of Einstein's theory - that the universe has no edge. The theory in fact says that there are two possibilities for the Universe. One is that it curves round on itself, like the surface of the planet. Although it has no edge, it is finite. A space traveler setting off in one direction and never changing course would eventually arrive at his starting point. This is a 'closed' Universe.
The other possibility is that the Universe is infinite, that space goes on for ever in all directions. In this 'open' Universe, however far you traveled you would always come across new regions of space.
Whichever possibility is correct depends on the amount of matter in the Universe. If there is enough matter, its gravity will bend space around so that the Universe is closed. In this case, the gravity is strong enough eventually to halt the expansion of the Universe, and draw galaxies together into a Big Crunch.
The most recent estimates of the amount of matter indicate that there is not enough matter to 'close' the Universe. The Universe is thus likely to be infinite in size, with no end. This also means that the Universe will keep on expanding for ever.
