A most common question is how big is the universe? But we cannot say exactly how big the universe is, only how far we can see out into space. But the observable universe is immensely vast-about 200,000 million million million km across, in fact.
How big is the universe
The farthest objects that we can detect from the earth are 8,000 million light-years distant. They are so far away that, when the light waves or radio waves we detect them by left these bodies 8,000 million years ago, the earth and sun were not even formed. The waves have traveled for this great length of time at speed of 300,000 km every second before reaching us- a practically impossible concept to grasp!
The diagram showing the scale of the universe (right) may give some idea of the immense distances involved. Each division on the scale is 10,000 times bigger than the division above it. From this, we can work out that the universe is 10 million million million million times bigger than a man. And a man is 10,000 million times bigger than each atom of which he and everything in the universe is made.
How did the universe form?
Scientists disagree on the origin of the universe. They observe that all the galaxies are moving away from each other, rather like spots painted on a balloon do when it is blown up. The universe is therefore expanding.
The ‘steady state’ theory suggests that the universe is infinitely large and that it has always existed. As it expands, new galaxies are formed in the spaces left as the existing galaxies move apart. If this theory is correct, the universe would always have looked to us as it does now.
The big bang theory disagrees. It suggests that the universe formed about 20,000 million years ago with a gigantic explosion. The galaxies produced started to move out into space in all directions.
Towards the edge of the observable universe, there seem to be more galaxies moving away than astronomers would expect. This evidence suggests that the ‘big bang’ theory may be correct. But another theory holds that the universe alternately increases and decreases in size.
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