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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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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.

Mr.WorshipMe
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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..

Chris Ger
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