The universe we live in is expanding. We know this because we see galaxies and groups of galaxies steadily moving apart in the universe. This expansion has been occuring since the universe was formed 15 billion years ago in a very hot, dense event known as the Big Bang.
Here are six Frequently Asked Questions about our expanding universe:
There no centre of the universe because there is no edge of the universe. In a finite universe, space is curved so that if you could travel billions of light years in a straight line you would eventually finish back where you started. It is also possible that our universe is infinite. In both examples, groups of galaxies completely fill the universe and are moving apart at all points making the universe expand (see question 2).
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| An example of a very small universe containing only 48 stars. A spaceship flying among these stars cannot find the edge of this universe. If it exits on one side of the universe it reemerges on the other side. The people in the spaceship see an infinite number of stars all around them. This universe has no boundary and no centre. |
There is a common assumption that the Big Bang was an explosion that occured in empty space and that the explosion expanded into the empty space. This is wrong.
| Space and time were created in the Big Bang. At the beginning of the universe the space was completely filled with matter. The matter was originally very hot and very dense and then expanded and cooled to eventually produce the stars and galaxies we see in the universe today. |
Although space may have been concentrated into a single point at the Big Bang, it is equally possible that space was infinite at the Big Bang. In both scenarios the space was completely filled with matter which began to expand.
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There is no centre of the expansion, the universe is simply expanding at all points. Observers in any galaxy see most of the other galaxies in the universe moving away from them.
The only answer to the question "Where did the Big Bang happen?" is that it occured everywhere in the Universe.
The Earth is not expanding and neither is the solar system, nor the Milky Way galaxy. These objects formed under the influence of gravity and stopped moving apart. Gravity also holds galaxies together into groups and clusters. It is mainly the groups and clusters of galaxies that are moving apart in the universe.
Space was created in the Big Bang. Our universe has no edge or boundary - there is no 'outside' of our universe (see question 1). It is possible that our universe is part of an infinity of universes (see question 5), but these universes do not necessarily need a 'space' to exist in.
Time was created in the Big Bang - we do not know if it existed before the Big Bang. This question is therefore hard to answer. Some theories suggest that our universe is part of an infinity of universes (called a 'multiverse') which are being continuously created. This is possible but very hard to prove.
It is probable that our universe is infinite and has been filled with matter everywhere since the Big Bang (see question 2). There is also good evidence that in the early universe that the universe may have expanded much faster than the speed of light. It is possible to inflate space so that although particles are not travelling fast, the space between particles increases enormously.
We can imagine galaxies are like balls sitting on a rubber sheet which represents space. If we stretch the sheet, the balls move apart. Balls which are close together will only move apart slowly. Balls which are widely separated will seem to move apart very quickly. There is no limit on how fast space can expand.
Space is the geometry of the universe. Changes in the size or shape of space occur because of the movement of matter and energy in the universe or because of changes to the matter and energy content of the universe.
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