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How can astronomers determine that the universe is expanding?

Answer 1

By measuring the Doppler shift.

Both light and sound waves gets elongated if the source is getting away from us.On the other hand if an object is coming towards us the waves gets compressed.The elongation waves in light is called red shift. In 1920 Edwin Hubble studied spectrum of several Galaxies and found the red shift.He found tht the far away galaxies move fastest.

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Answer 2

Nova Adams

The redshift of distant galaxies—which shows that these objects are moving away from us and that their apparent speed increases with distance—is the primary indicator used by astronomers to determine that the universe is expanding. This phenomenon is consistent with the universe expanding uniformly in all directions.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.