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I keep hearing that universe expands. Since all is relative, the expansion must be relative to something. And since universe is all there is ... You got my point.

The only points of reference are inside the universe itself. Bonds forming, their length measured in wavelengths, etc. Red shifting of light tells us things fly away. So our point of reference is only light. (am I missing something?)

Here is my question. In a thought experiment, would it be valid to say that properties of light are somehow changing around us if we keep the size of the universe constant? If yes, what are the consequences of this line of thought? Would it give us any additional insight into what's going on?

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    $\begingroup$ Possibly helpful: physics.stackexchange.com/q/7359 $\endgroup$
    – DilithiumMatrix
    Commented Mar 4, 2017 at 1:38
  • $\begingroup$ No to your last question. And on expansion we just see all the far off galaxies going away from us, and from each other. $\endgroup$
    – Bob Bee
    Commented Mar 4, 2017 at 3:33
  • $\begingroup$ This is not a discussion site. It is a Q & A site. You have enough rep to take participate in the Chat Room, in which discussion is welcomed. $\endgroup$
    – sammy gerbil
    Commented Mar 4, 2017 at 3:53
  • $\begingroup$ @BobBee "See galaxies going away.." do you have any other means besides red shift? $\endgroup$
    – Lex Podgorny
    Commented Mar 4, 2017 at 3:55
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    $\begingroup$ Seems like you are describing something like tired light $\endgroup$
    – Kyle Kanos
    Commented Mar 4, 2017 at 11:22

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We measure more red shift as farther we look. How we decide how far we are looking - is based upon standard candles. So, more red shift for farther galaxies indicates faster expansion over larger distances. How would your line of thinking explain this observation? If properties were changing around us, how we would see more or less red shift? Also, it is difficult to explain the constant size - because, then, it has to be a contracting/slowing universe due to gravity. That was the initial expectation that led us to measure the red shift in the first place and the results were found opposite.

Not only that, the accelerated expansion is believed to have started ~5 billion years ago. Prior to that, it was a slowing expansion. So, if we look within 5 billion light years, we do not observe the red shift as per accelerated expansion. So, if things were changing around us, then why it would depend upon whether we look 5 billion light years, or beyond.

That said, it is good to think of different ways to explain. If nothing else, it would make you understand the current explanations in a better way.

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To answer your thought experiment, you would need to specify how light changes relative to something else. The speed of light is the same for all observers in all reference frames. That said, the rate of expansion of the universe is proportional or relative to the distance away from another location. This means redshifting galaxies that are further away from us are necessarily traveling away faster away from us than redshifting galaxies that are relatively closer to us.

To "test this" with an oversimplified experiment, you can use a sharpie to put dots on a balloon - watch the points move away from each other in an analogous way.

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  • $\begingroup$ You claim "The speed of light is the same for all observers in all reference frames" but how about the redshift in gravitational potential? $\endgroup$
    – HolgerFiedler
    Commented Mar 4, 2017 at 14:59
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Compared to the center and frontiers of the farthest something traveled the universe is expanding. For size it's a dumb idea to compare the expansion of the universe to the universe. I would bring up the multiverse, but that's for the Philosophy section.

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