This question already has an answer here:
- Why does space expansion not expand matter? 12 answers
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?