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How did Edwin Hubble provide evidence for an expanding universe?

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American astronomer Edwin Hubble provided the first observational evidence for an expanding universe by establishing a linear relationship between the distance of galaxies and their redshift.123

To reach this conclusion, Hubble collaborated with astronomer Milton Humason to analyze the spectra of distant galaxies. While Humason measured the spectral shifts, Hubble focused on determining the distances to these galaxies, utilizing the period-luminosity relationship of Cepheid variable stars—a method pioneered by Henrietta Swan Leavitt.45

In 1929, Hubble published his findings, demonstrating that the more distant a galaxy is from Earth, the faster it appears to be receding, a principle now known as Hubble’s law. This relationship, expressed as velocity = Hubble constant × distance, indicated that the spectral shifts were a consequence of the motion of galaxies moving away from one another, providing the foundational evidence that the universe is not static but is instead undergoing continuous expansion.1326