Doppler Effect & Redshift

Fig. 1: Redshift displayed in a distant galaxy

If you have taken or take high-school physics, you have probably heard of the doppler effect. This effect has many practical applications in the physics world, from ambulances to even calculating the rate of expansion of the universe.

What is it? The doppler effect is the change in the frequency and wavelength of a wave, caused by a change in distance of the object emitting those waves. There are four cases of the doppler effect, of which we are going to focus on two, which can help us calculate redshift. I

n the first case, the viewer is stationary, but the object emitting waves is moving towards the viewer. Here, the frequency increases and the wavelength of the wave decreases: for example, when a police siren approaches you when you are stationary, you hear an increase in pitch, which corresponds to a higher frequency. When something has high frequency, it has a lower wavelength and vice versa. This can be seen in the physics equation v = fλ. 

In the next case, the viewer is stationary once again, but this time around, the emitter is moving away. Once again, imagine a police siren. It has zoomed past you, and you now hear it in the distance; the pitch has decreased! This is because there was a decrease in the frequency of the wave, and hence a reduction in the wavelength of the sound wave.

But what does this have to do with space? This method can be applied to astronomy to calculate the rate of the expansion of the universe and discover the rate at which other galaxies are moving closer to us or moving away from us. All galaxies emit electromagnetic waves in various forms, such as visible light. As all these galaxies are very far away, the light they emit takes a long time to reach us, and so we can observe the shift in the frequency of the visible light over time. If you remember from school, in the visible light spectrum in the electromagnetic wave spectrum, the colors from the lowest frequency to the highest frequency are red, orange, yellow, green, blue, indigo, and violet.

What this means is that if a galaxy is moving towards us, case 1 mentioned before is applicable. The frequency of the waves emitted increases, and so the frequency of the visible light emitted increases. So, the visible light shifts to the colors that correspond to higher frequencies. So, the galaxy emits light that is blue in color and is called blueshift.

Likewise,  if a galaxy is moving away from us, the frequency at which we detect the visible light it emits is lower, and so the color is red. This is called redshift.

Using these methods, we can see how galaxies and other bodies move around in the universe and are how we found that the universe expands! We saw that the redshift of galaxies was more and more, which meant they were accelerating away from us at a high speed.

Pretty cool, right?