You may have heard the idea that the universe is expanding. How do we know this, and what does it mean?
The idea is explained in this video:
As the video explains, the expanding universe concept was first demonstrated by Edwin Hubble. He looked at a number of galaxies in our vicinity and noticed that most galaxies are moving away from us - their light is red-shifted. He also noticed that galaxies located farther away from earth are red-shifted more than closer galaxies. In other words, virtually everything in the universe is moving away from us. Thus the conclusion - the universe is expanding.
That part is relatively easy to understand, but the video also explains that space itself appears to be expanding. This video is helpful in understanding the expansion:
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