How do we know about the expansion of the universe?

Changing the spectrum of light towards red as the distance from an observer

There are astronomical observations showing that the rate of receding of galaxies from each other is approximately proportional to the distance between them. This law, named after astronomer Edwin Hubble who discovered it, indicates that distances between any two points in space are increasing. The rate of the receding of galaxies has been calculated on the basis of the spectral-line redshift of light from stars of the galaxies. In order to determine the distances, Hubble used the visible brightness of Cepheids — pulsing stars whose luminosity can be accurately determined as it is completely dependent on the pulsation period. In recent decades, Hubble’s law has been adjusted using the data of remote supernovae that can be seen at much greater distances, so that their light brings information about the deep past of the universe. Information on the expansion of the universe in even deeper past is given by observations of quasars.

The rate of receding of galaxies is proportional to the distance to them

The distant-galaxy speed relative to the Earth is the sum of the speed of a closer galaxy and the relative velocity of the two galaxies.

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