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This question already has an answer here:

When physicists talk about the expanding universe they often say that the distant galaxies are not really "moving" away but instead the space itself between us and them is expanding. If this is true then the expansion should apply to every region of space proportionally. Coming from a different direction, we all know from particle physics that atoms are mostly empty space. So doesn't the expansion theory imply that the space inside the atoms is also expanding and, as a result, every single object (excluding the unusual dark stuff) in the universe?

If this is the case then how do measure the difference? It's not so obvious since our meter sticks (or any other apparatus we use to measure space) are also expanding. If the length of my car was 3 meters (say) when I bought it a few years ago it would be more now but I wouldn't be able to tell since the definition of a meter has now changed. Just for clarification, I am talking about the length when the car is at rest with respect to me so relativity doesn't come into equation.

However, there is a way. Measure the distance light travels in a given period of time (off course synchronize your clocks and all that). I am assuming that the speed of light is unaffected by the expansion of space because if this wasn't the case, we would be able to see the whole universe and there would be no such thing as the "observable universe".

So apparently we all are expanding over time but the speed of light is not. Shouldn't the relative speed of light (relative to our measuring techniques) decrease over time as a result? Shouldn't we measure a different speed of light now than a hundred years ago?

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marked as duplicate by John Rennie, Kyle Kanos, ACuriousMind, Danu, Jim Aug 29 '14 at 15:54

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ @John Rennie, Yes the two posts are based on the same topic but don't suppose the questions asked are the same. The other post implies that we "see" the universe expanding. I know this is not the case. We don't see the galaxies moving, we just measure the amount by which light from them is redshifted. Besides, my question in not about the expansion itself but rather about the speed of light. (I can change the wordings if you feel appropriate). $\endgroup$ – Rahat Aug 29 '14 at 12:49
  • $\begingroup$ The answer John pointed to, and also my answer is concerned with your premise that our meter sticks expand - They do not. $\endgroup$ – Mr.WorshipMe Aug 29 '14 at 13:12
  • $\begingroup$ Rahat: you start your question by asking So doesn't the expansion theory imply that the space inside the atoms is also expanding and, as a result, every single object (excluding the unusual dark stuff) in the universe? and the question I linked explains why the answer is "no". $\endgroup$ – John Rennie Aug 29 '14 at 15:44
  • $\begingroup$ @John Rennie, yes that part was wrong I admit. btw, I actually searched for the topic before posting my question but couldn't find that other post. Is there a way I can verify if my post is going to be a duplicate before submitting it? $\endgroup$ – Rahat Aug 30 '14 at 2:24
  • $\begingroup$ It can be very hard to tell if a question has been asked before, because it's hard to know what to search for. Those of us that have been around a long time tend to remember popular topics, which is how I knew the subject had already been explored. I wouldn't worry too much about it. $\endgroup$ – John Rennie Aug 30 '14 at 5:02
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Space is indeed expanding everywhere, and not only between galaxies. The reason we don't grow with it, is that the attraction between the electrons and the protons is strong enough to keep them bounded. You can look at it as if they always re-adjust their position to counter the expansion of space. This also applies to our solar system, our galaxy, and even out local galaxy cluster. The expansions of space comes into effect only for very long distances.

Let's say the expansion rate is 1% per 140 million years (this is the observed expansion rate right now). a meter would expand in a year less than an atom's diameter, this is hardly a perturbation for the atom. But the distance between galaxy clusters is several hundred million light years - this same rate would cause it to expand by more than $10^{10}$ km per year. And considering that the gravitation is very weak between these clusters because of their distance, they accelerate away from each other in a tremendous speed.

So our meter sticks do not grow, and we should measure the same speed of light over time.

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  • $\begingroup$ Thanks. This explains why our meter sticks don't grow but I am still finding it difficult to understand the gravity part. If we take the general theory of relativity to be true then gravity is not an attractive force. Then how can it pull the stars back when the space between them is expanding? I can sort of understand the spacetime curvature thing in the case of a planet moving around a star. But how can two stars, stationary with respect to each other, keep their distance constant over time? $\endgroup$ – Rahat Aug 29 '14 at 13:38
  • $\begingroup$ @Rahat stars in the same galaxy are not stationary, they orbit the galaxy's center of mass. and galaxies that share a cluster orbit each other, or even "collide". $\endgroup$ – Mr.WorshipMe Aug 29 '14 at 16:53
  • $\begingroup$ Sorry I meant to say this in my previous comment but had to cut it short because of the character constraint. Yes the stars are orbiting the center but I thought stars at similar distance from the center move at the same speed and thus remain stationary with respect to each other as I often hear scientists say that a certain star is this many light years away and so on. $\endgroup$ – Rahat Aug 30 '14 at 2:36
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On your first question the answer should be here http://en.m.wikipedia.org/wiki/Metric_expansion_of_space, after the big bang only the distance between space is expanding but you should be able to understand on the link i have included, yes the speed of light remains constant but the expansion is causing it to have longer time than expected to arrive it is the red shift i guess, and there is also a theory if the expansion of universe becomes more faster than light it is the big rip which coul or would result to a deep chill or freeze in the universe and just an add on, the universe is still accelerating on its expansion..

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  • $\begingroup$ Umm.. so what is your main point? $\endgroup$ – Chris Ger Aug 29 '14 at 13:01
  • $\begingroup$ Sorry im adding my comment here instead on yours, i have a problem clicking on the add comment on top $\endgroup$ – Chris Ger Aug 29 '14 at 13:04
  • $\begingroup$ Thanks Chris Ger, your answer was helpful. I am not sure I understand your second comment though. $\endgroup$ – Rahat Aug 29 '14 at 13:51
  • $\begingroup$ oh.. on which part $\endgroup$ – Chris Ger Aug 29 '14 at 13:57
  • $\begingroup$ "Umm.. so what is your main point?" Were you talking about the question? $\endgroup$ – Rahat Aug 29 '14 at 14:00

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