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From what I can see in a physics textbook page (on redshift), it seems to imply that all planets/galaxies emit red, rather than blue waves (for red-shift).

Wouldn't this mean that everything is moving away from us and that we're the centre of the Big Bang? (See quote from the book below)

On a similar note, does everything move at the same speed away from the centre of the Universe? - If not, then wouldn't that mean that we'd see a few planets/galaxies that emit blue, as they are traveling with a greater velocity from the centre than we are (with it being closer to the centre) and that they are 'catching up' with us?

It's not a big deal and I don't need to know it, but I would like to know, if possible.

~StratusGold

Quoting the textbook: "When we look at light from distant galaxies, we find that the wavelength has increased. The wavelengths are longer than they should be - they're shifted towards the red end of the spectrum. This is called red-shift. (...omitted...) Measurements of the red-shift indicate that these galaxies are moving away from us (receding) very quickly - and it's the same result whichever direction you look in" - CGP GCSE AQA 9-1 Physics Revision Guide, Topic 8 'Space Physics', Pg 102

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    $\begingroup$ I don't have time for a full answer, but the quick version is this. Not all galaxies are moving away, there are a couple of small ones orbiting the Milky Way, and Andromeda is on a collision course straight for us. Red shift means that the frequencies are stretched out, blue shift means they are compressed. It does not mean exclusively red or blue light. Also, as weird as it sounds, everywhere was the "center" of the big bang. Its complicated... In general everything isn't just moving away from us, everything is moving away from everything else (with some exceptions, see Andromeda). $\endgroup$
    – Cody
    Feb 2, 2017 at 20:10
  • $\begingroup$ Ok, thanks @Cody I think I understand it a bit more now :) $\endgroup$
    – user15682
    Feb 2, 2017 at 20:12
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    $\begingroup$ This answer explains why every galaxy can be said to be in the center. This answer explains why Big Bang wasn't a point-like explosion in space. $\endgroup$
    – pela
    Feb 2, 2017 at 21:00

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No, not all of them. Only the ones very far away.

When you walk your dog, the dog is tied to you via the leash. It does not recede into the distance.

Planets in the Solar System are all tied up to the Sun via gravity. They don't recede into the distance.

All stars in our galaxy are bound together in a big clump (the galaxy) and so they stay together.

Finally, all galaxies in our neighborhood are also bound by gravity, so they stay together.

Beyond that range, things are not bound by gravity, so they keep running away from each other.

Gravity is like a leash. If things are close enough, they stay together.


Keep in mind that the universe does not have a center. It's infinite (as far as we can tell), and infinite things don't have centers. Rather, if you take any two distant galaxies, they all recede from each other. But the expansion does not go back in time to a common center.

Here's a comparison. Imagine you're a bi-dimensional creature; instead of having 3 dimensions, you only have 2. Like a stick figure drawn on a paper. You only exist in the 2 dimensions of the paper, and can't even imagine the 3rd dimension.

Now imagine a balloon. You draw a bunch of stick figures on it. Now blow air into the balloon. All stick figures recede from each other, but for them there's no center for that expansion - because all they know is the surface of the balloon. They don't know anything that's not on the surface. So they all see each other running away, but there doesn't seem to be a center for that expansion.

It's just an analogy, because the universe is not a balloon, but it will get you started.


Finally, red-shifted galaxies don't emit only red light. They emit the usual radiation like everything else. But because us and them are running away from each other, radiation from one side reaches the other side a little bit stretched out. For the visible part of the spectrum, it's like taking the spectrum and shifting it towards the red part.

That's all there is to it. It's still the usual radiation, but all wavelengths are a little bit blown up.

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