Here's how his work unfolded:
1. Observations of Cepheid Variable Stars: Hubble used Cepheid variable stars as "standard candles" to measure the distances to galaxies. These stars have a predictable relationship between their pulsation period and their luminosity, allowing astronomers to determine their absolute brightness. By comparing their absolute and apparent brightness, Hubble could calculate their distances.
2. The Redshift: He then observed the light from these distant galaxies using spectroscopy. He noticed that the spectral lines of these galaxies were shifted towards the red end of the spectrum compared to their counterparts on Earth. This "redshift" was a clear indication that the galaxies were moving away from us.
3. Hubble's Law: He further observed a direct relationship between the redshift of a galaxy and its distance from us. This relationship, known as Hubble's Law, states that the farther away a galaxy is, the faster it is moving away from us. This was a groundbreaking discovery that led to the understanding of the expansion of the universe.
Hubble's explanation: He did not propose a specific *mechanism* for the redshift. He simply presented the observed relationship between distance and redshift. Later, physicists like Georges Lemaître and Albert Einstein realized that Hubble's observations supported the theory of an expanding universe, originally proposed by Lemaître.
In short, Hubble's contribution was the empirical evidence that confirmed the universe is expanding, leading to a paradigm shift in our understanding of the cosmos.
