Chapter 4: Hubble Advances
Edwin Hubble noticed a slight decline in the frequency of light from distant galaxies. This light had slightly longer wavelengths than expected and was shifted toward light’s red spectrum.
Hubble realized that this variance was caused by the distant galaxies moving away from us, similar to the Doppler Effect applied to light rather than sound.
The example of an approaching or receding police siren explains the Doppler Effect. As the siren approaches, each successive sound wave is emitted from a location closer to the listener than the previous wave, resulting in higher frequency or pitch. These sound waves are compressed. As the siren recedes, successive sound waves are emitted from more distant locations, causing the waves to spread out with reduced frequency or lower pitch.
When Hubble applied this concept to the light of galaxies, he realized that only the light of local galaxies had higher frequencies then known values. These nearby galaxies were approaching us because of gravity.
All distant galaxies emitted light with lower than expected frequencies. Since red light is at the lowfrequency end of visible light’s spectrum, Hubble described this light as “red-shifted.” Hubble correctly concluded that all distant galaxies (outside of our gravity’s range) were receding and spreading out.
Hubble also observed that the light from galaxies which appeared fainter and smaller (more distant) was shifted further toward red than nearer galaxies. This discovery proved that galaxies which were further away were also moving away faster.
Hubble further showed that “recessional velocity” was directly proportional to distance and proposed “Hubble’s constant” to estimate distances. Modern improved telescopes made more precise measurements which caused a revision of Hubble’s constant, but the idea remained correct: a galaxy’s recessional velocity divided by Hubble’s constant equals its distance from us.
In summary, Hubble proved that galaxies are separating unless gravity binds them together.Galaxies recede at velocities directly proportional to their distance from the observer. He also provided a tool for determining those distances.Hubble’s work made him the greatest modern astronomer, whose name is worthy of the first space based telescope.