Chapter 18, Problem 1QFR

Why do astronomers think that the Universe is expanding?

To determine

The reason for astronomers thinking the Universe is expanding.

Answer to Problem 1QFR

The reason for astronomers thinking the Universe is expanding is because the light from the remote galaxies are redshifted.

Explanation of Solution

Astronomers observed that the light from the remote galaxies are redshifted, that is light from the stars observed at time $t$ is not the same but shift to a longer wavelength when it is observed at time $t+x$.

This is later interpreted using Hubble’s law that all the galaxies are moving away from each other. And wherever the observers in the Universe, all will observe that everything is moving away from then in the same away.

This proves that the Universe is expanding. Also the point of the center of the Universe cannot be identified and for every observer in the various points on the Universe feels that they are at the center of the Universe.

Conclusion:

The reason for astronomers thinking the Universe is expanding is because the light from the remote galaxies are redshifted.