How do we know the universe is constantly expanding?
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Technically what is expanding is the space itself, picture it as a balloon being inflated and we are just a dot in the surface of the balloon. The larger the balloon, the more separation between us the dot, and another dot or star.
We know the space is expanding because when the stars move further apart from each other, like the dots in the balloon, they change their light to become more red. If a star is blue, then the light can turn yellow, then orange then red, this is called the red shift.
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