For example, the further a galaxy is from us, the more red it is.
In: Planetary Science
Say you’re releasing a pulse of light every 10 seconds. If you’re stationary, I’ll see a pulse of light every ten seconds – but if you’re moving away from me at 10% of the speed of light I’ll see a pulse every 11 seconds, because each pulse has to travel a little further to reach me than the one before it.
The colour of light is based on how often it peaks, with more common peaks being more blue, and less common being more red – it’s not exactly pulses, it’s a wave, but you can think of the peaks of the waves as pulses and get a reasonable understanding for this issue – so if a galaxy is moving away from us, the colour gets redder.
As it turns out, the further away from us a galaxy is the faster it’s moving away from us – so further away galaxies have all their light redshifted.
Space is expanding. You can think of this as the universe is basically stretching to get larger. One of the consequences of this is that it also stretches the wavelengths of light traveling through space.
The color of light is tied to its wavelength. So as the wavelength gets longer, it shifts down towards the red side of the spectrum, so visible light gets more red. Hence, redshift.
Redshift generally happens when an object moving away from the observer emits light towards the observer. The wavelength of the light increases – This is called the dopper effect. We know the amount of redshift that has happened by comparing known spectral lines against observed spectral lines.
The reason we see Galaxies redshifted is because the universe is expanding, and thus, galaxies appear to be moving away from us at some high velocity. Think of spacetime as the surface of a balloon and two Galaxies as 2 random points on the surface of the balloon. As the balloon inflates, the distance between the two points increases, too. Relative to one point, the other point would appear to move away at some velocity. That’s a simplified 2d analogy of the expansion of the universe.
*Notice that space is basically being created in between the 2 points in the balloon, that is true for Galaxies as well.*
Now picture a photon of 10 units wavelength traveling from a distant galaxy to you. As the universe expands, the photon also expands. As it crosses light years and light years and reaches you, all the amount of expansion that the universe had undergone is “imprinted” into that photon’s wavelength. Viz. The wavelength of the photon might end up as 87 units. Now we can isolate a factor of expansion here *z*, which is defined as *z*+1=wavelength observed/original wavelength, in this case, *z* being 7.7. This factor *z* is known as cosmological redshift, which is a crude indicator of distance traveled.
As you can easily see, the farther the galaxy is, the more time photons from that galaxy get to “expand”, and thus the redshift on those photons get higher.
Edit:Grammar and formatting
So, my understanding is that it’s less about distance, and more about velocity. Assuming a galaxy is emitting “yellow light” from around the middle of the visible spectrum, the fact that it’s moving away from us causes it to get strung out – so it arrives at a longer wavelength ( red shifted ). If however, it’s moving towards us, then the light waves arrive slightly compressed, at a shorter wavelength ( blue shifted ) – the process behind the change is the relativism Doppler effect
None of these are ELI5 so I’ll have a go.
Your friend throws you a ball every second. You catch it every second.
But now they start to walk backwards away from you. They are throwing a ball every second but because they are moving away, you don’t catch one every second any more, it’s one every two seconds.
The same thing is happening with light from stars. Light that arrives less often than it was sent out looks to us more red than it would if the object wasn’t moving away.
Think of it like a moving car. The closer you are to it the louder it is: you can hear the tires, the engine, the exhaust backfire, etc. As the car drives father away the less you hear. At the farthest distance you can hear the car it is just a drone in the distance. (ELIadult The sound wave is busy closer to you but flat farther away)
Light works in the exact same way. The closer you are to something the more complex things you can see, the farther away you are the less you can see. At the extreme side of it the only color we can see is red. Thus, the red galaxies are the ones furthest away. (ELIadult Light works like sounds as a wave: the closer you are the more waves and the more colors you see, but when it is flat the only light is red)
This is the Doppler Effect. We experience it with sound every day (if you ever watch Formula 1, listen to how the engine noise from the cars changes as they approach, pass and move away from the camera and microphones; frequency increases as they get closer and decreases as they pass – we hear the same with sirens from emergency vehicles as they approach and pass us). Basically if something approaches you, waves it emits get bunched together: they become more frequent i.e. higher frequency. Waves from an object moving away from you are spread out – less frequent, lower frequency.
We can experience this because sound moves comparatively slowly (around 330 metres per second) so a vehicle moving at 10 metres per second will cause a noticeable change in frequency of sound emitted. Light moves at three hundred million metres per second; way too fast for movement on earth to affect frequencies – though I did get a chuckle at a bumper sticker with red lettering saying “If this appears blue then you’re travelling too fast”.
Blue light has a higher frequency than red light so looking at light from an object moving towards you, it will appear “blue shifted”, shorter wavelength. An object moving away from you is conversely “red shifted”. We can use this phenomenon to show the universe is expanding and that more distant objects are more red shifted – the further away they are, the faster they’re moving away from us.
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