The Universe ________________________________________________________________ 12. How do astronomers know the universe is expanding?
In 1924, Edwin Hubble showed that nebulae were, in fact, other galaxies, separated by vast distances.
In 1929, Hubble showed that these other galaxies were moving away from one another with speeds proportional to their distance. The expanding Universe was born, and our idea of teh Universe changed forever.
Figure 10.7 WavelegthDoppler effect
The Doppler effect is a change in some properties of waves, wavelength and frecuency (light and sound are waves). It is caused by the change in distance between the thing creating the wave (causer) and whatever is measuring seeing or hearing the wave (watcher or observer).
Wavelength is the measured distance between two identical points on two back-to-back waves . Different wavelengths of light appear to our eyes as different colours.
Shorter wavelengths appear blue or violet, and longer wavelengths appear red.