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The most widely accepted theory for the origin of the Universe is called the Big Bang Theory. This argues that the Universe began as almost a single point with a temperature and density of near infinity. The Universe has been expanding ever since. We know almost nothing about the first fraction of a second of the Universe because the theories of physics do not apply in such extreme conditions.
Very early on, the Universe probably went through a short period of very rapid expansion called inflation. When this inflation ended a fraction of a second later, the size of the Universe had been increased by about 1,000,000,000,000,000,000,000,000,000 times. At the end of inflation the normal expansion of the Universe took over, and light and particles were created. As the expansion continued, the light and material cooled, forming protons, neutrons and electrons together with three sorts of another particle called a “neutrino”.
The Universe continued to cool and the protons and neutrons came together to form nuclei. At this time, only hydrogen and helium were formed. These simple nuclei are the building blocks from which all other elements are created in stars.
At 300,000 years after the Big Bang, when the Universe was at a temperature of 3000C, the last of the protons, neutrons and electrons finally came to form neutral atoms. A snapshot of this time can be seen in the Cosmic Microwave Background (see Structure of the Universe). At this time some areas had slightly higher densities than others, and these eventually collapsed to form galaxies. The earliest of these can be seen as faint blue dots in the Hubble Deep Field. This image covers a tiny patch of sky - only 1/30 the size of the full moon - which seemed to be completely empty before the Hubble Space Telescope took this picture.
This is an alternative model for the Universe that used to compete with the Big Bang theory. It states that the Universe is expanding, but that material is continuously being created so the Universe always has the same density. It is no longer a serious rival to the Big Bang theory because it cannot explain the Cosmic Microwave Background and the formation of galaxies.
The true size of the Universe isn’t really known because it is only possible to see to the edge of the visible Universe. Galaxies can only be seen because of the light coming from them. Although light moves very quickly, it still takes a long time to reach us from distant galaxies. The farthest that can be seen is the distance light has travelled in the age of the Universe, i.e. 15 billion years. Light from more distant objects hasn’t had time to reach us yet, so we are unable to see them.
Studying the light from distant galaxies is like looking back in time to the early Universe. The starlight that is seen in the night sky left the stars millions of years ago, so each star does not appear as it is now, but as it was when the light set out from it. It will be another few million years before the stars can be seen as they look today.
It is not yet known what is going to happen to the Universe. Some cosmologists think that it will stop expanding and collapse again in a “Big Crunch”. Others believe that it will carry on expanding forever. Recent measurements suggest that the expansion is speeding up, so the Universe is more likely to expand forever.
Click on the links below to find out more about the Universe.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001