Why is the space expanding

How fast is the universe expanding? [Marla, 13 years old]

100 years ago Albert Einstein found out how quickly the universe is expanding with his general theory of relativity. According to Einstein's theory, the universe expands in such a way that it is mainly the large empty spaces between collections of galaxies that enlarge. However, within a galaxy there is so much matter that space does not expand. The distance to distant galaxies is therefore constantly increasing, and the faster it grows the further away they are. Because far away there are more such large empty spaces between us and these other galaxies than in our neighborhood. Currently the universe is expanding at about 20 kilometers per second per million light years away. That's 72,000 kilometers per hour - about three times as fast as a rocket flies. This means that galaxies that are a million light years away are moving away from us at an average of 20 kilometers per second. A light year is the distance that light travels in one year, that's about 10 trillion kilometers. And the galaxies that are 2 million light years away are moving away from us at 40 kilometers per second, and so on ...

Dr. Jean-Luc Lehners heads the “Theoretical Cosmology” working group at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam. He researches the early universe and the big bang.


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