Are Galaxies Moving Away from Us?

I've come here today to give a dumb down version of how we are able to identify that most galaxies are moving away from us. To start off I'll give some needed info then I'll jump into the meat and jelly.

What is Light?

Light acts as both wave and a particle. (There are ways to detect this look up double slit experiment if you're interested but you'll just have to accept this from me) Because of this property you can think of light traveling through space the exact same way sound waves travel through our air.

The light waves look somewhat like the wave depicted in this picture. Each wave has a specific amplitude (the height) and wavelength (length horizontal that it takes the wave to reach the top, bottom and then back to the center...look at the diagram it'll be eaiser).

Credit to Khan Academy for this

In short: Amplitude affects the brightness of the light. Wavelength affects the color of the light.

Now with all that out of the way...here is the meat and jelly

The Doppler effect. This is an effect predominantly known for being discussed along side emergency vehicles. I.E. When a firetruck whizzes towards you with it's sirens blaring, the sound emitted from the sirens is louder. As soon as the firetruck passes you the siren sound drops in volume. Another way to look at this is when the firetruck comes towards you the sound waves have more energy and when it moves away they have less energy.

Light is also affected by the Doppler effect because it's a wave.

There is a visible light spectrum that looks like this picture on the right. The red colored (light) waves have less energy and the violet have more energy.

The light that scientists view from distant galaxies is predicted to look like the vertical visible spectrum on the left side (look up absorption spectrum to see why there are black lines) instead the light observed looks like the spectrum next to it. This model is called red shifted. Pay attention to where the black lines end up...they all got shifted up towards the red section.

Now think back to the Doppler effect. When waves were moving away from you, you perceived them with less energy. The light being red shifted has less energy as it is more in the red spectrum of colors. Therefore the light of the galaxies are moving away from you as the light waves (kinda) have less energy then what they should have.

Now. What would it mean for light to be blue shifted?




Cheers.