The Doppler Effect: How Do We Know the Universe Is Expanding?
By Heather Morrison
In the mid-1920s Edwin Hubble, an astronomer working at Mount Wilson in California, proved that other galaxies existed besides the Milky Way. By taking photos and comparing the luminosity of various stars in the Andromeda nebula, he was able to estimate that the nebula was situated nearly 900,000 light years away from the Milky Way and therefore had to be its own galaxy. It was later proven to be almost 2.48 million light years away and renamed the Andromeda Galaxy. This led onto his research in measuring the light shift in galaxies in comparison to the Earth. His research was published in 1929 which theorises that galaxies are moving away from each other due to redshift in the galaxies’ light emissions, therefore evaluating that the Earth must be expanding. I will go on to explain what redshift is and how he drew this conclusion about the universe expanding through the physics all time classic: The Doppler Effect.
Daisy the Duck and the Doppler Effect:
Imagine that Daisy the Duck is in the middle of a pond. Every so often she decides to kick her feet on the surface of the water, the ripples which emerge from these kicks will travel out from the point her foot touched the surface. This point is called the origin as it is where the ripples originated.
Now Daisy decides to test her stamina. She grabs a branch so that she does not move and kicks her foot three times a second with the same force each time. Say we are standing on the bank at point B and the origin is at point A. The ripples will travel in all directions at the same speed. This is illustrated in Figure 1.
In a second exercise she lets go of her branch. Continuing to kick her foot three times a second, she now moves towards point B. The ripples in front of her are closer together because she is moving towards them, whereas the ripples behind her are further apart because she is moving away from them.
The Doppler Effect With Sound Waves:
The Doppler Effect can be apparent with any type of wave, most often recognised is its effect on sound waves. When an ambulance siren approaches you, you may notice the pitch become slightly higher. This is due to the smaller wavelength (distance between two adjacent troughs on a wave) at the front of the vehicle. Each wavelength decreases as the ambulance travels towards the wavefronts. This means that the frequency is higher because frequency = waves speed / wavelength. Therefore, if the wavelength decreases, the frequency increases because more waves can fit into a given period of time. With the same logic, the waves at the back of the ambulance will have a lower pitch because there is a greater wavelength and therefore the frequency is lower. This is illustrated in Figure 4: the man standing in front of the ambulance will hear a higher pitched sound than the woman at the back of the ambulance.
The Doppler Effect in Astronomy:
By comparing the light from different stars measured through an electronic camera to the different wavelengths of visible light, we can see if a galaxy is moving further away or coming towards us. When a galaxy is moving towards us, the wavelength is longer and therefore the light is shifted towards the red end of the spectrum, astronomers call this redshift. If a galaxy is moving away from us, the light is shifted towards the blue end of the spectrum, blueshift. Further, by measuring the change in shift, astronomers can tell how fast an object is moving. From this information, astronomers have proved that the universe is not only expanding, but expanding at an accelerated rate.
