How does red shift (Galaxies moving away at various speeds) tell that the universe is expanding?
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Red shift tells us that something is moving away from us. This is because that, as something moves away from us, it “stretches” the light out, shifting it toward the lower (red) end of the spectrum.
So how do we distinguish between something that is just moving away from us as a result of its own momentum vs something moving away from us because the universe is expanding?
When we look at many different things in the night sky, the degree to which they are moving from us is correlated with distance; things that are further away are moving away faster. This is highly indicative of universal expansion, as if it was simply objects moving under their own power, we wouldn’t expect there to be any correlation between them and distance.
Not to mention that everything* is moving away from everything else. If it was just random motion, we would expect some things moving away and some things moving toward us.
Lastly, without expansion, any movement away from us would gradually slow as a result of gravity. Yet not only is the motion not slowing down, it is actually speeding up! Why? Well, we don’t know that yet.
* – Not strictly “everything” is moving away from everything else. Within the scale of a few million light years, gravity is still strong enough to keep things gravitationally together and counteract the expansion of space. But outside that, everything is moving away from everything else.
It’s because, by a certain distance, they’re *all* moving away from us at a very predictable rate. Objects at opposite ends of the universe, but at the same distance away from us would have no other way to be moving at approximately the same speed away from us.
Red shift in light is similar (although not exactly the same as) the dopplar effect. The doppler effect is seen when an object which produces waves (sound, light, or even physical waves) moves through a medium, the waves are stretched or compressed when observed. This can easily be seen when looking at the bow of a boat or listening to a siren on a vehicle driving by at high speed. The waves in front of the object are compressed, while the ones behind the object are stretched.
Since light moves as a wave, when the source of the light is moving (or in this case, the space between the sources is changing size), the waves appear to the observer to be compressed (more blue) or stretched (more red).
Red-shift happens when the object being observed is becoming more distant. (I use this awkward phrasing because in this case the galaxies in question are not moving away, but rather the space between them is growing)
If we observed galaxies on one side of us with a red-shift, and ones on the other with a blue-shift, that would indicate that we are the ones moving through space.
However, this is not what we observe. What we actually find is that almost everything we look at has a red-shift, meaning that they are all getting further away. This means that instead of us moving, everything around us is expanding.
One thing to note is that this does not mean we are in the center of the expansion, but rather that space is expanding uniformly. This means that if we were to observe the universe from a different location, we should see the same effect. This detail is still under some debate btw, with some studies finding slight variances in the direction of expansion.
Because not only is everything moving away from us, everything is moving away from everyone. And further, the speed that everything is moving away isn’t uniform, but it’s predicable based on the distance between those two objects. So it’s not like an explosion where everything is moving away from us at a set speed. Each and every object that isn’t gravitationally bound with us is moving at speed based precisely on how far away they are from us, and this is true between any two objects (that aren’t gravitationally bound) in the universe. This is consistent with space, everywhere, expanding by very small amounts, but that builds up to large speeds as the distance between objects becomes very large.
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