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Let's say I go out into deep space and set up two observation stations a long ways apart. I want to find an absolute rest frame for each one.

I decide that if the microwave background radiation is at the exact same redshift in all directions then I must be standing absolutely still. If it seems a bit blue-er in any direction, I presume I am moving in that direction and I use my retro-rockets to slow myself down. So, I visit each station and survey the surrounding microwave background radiation and bring each station "to rest" by making sure the background is the exact same color in all directions.

I then pick one station and bounce beams of light off the other station and measure how long the round trips take. I suspect they will take longer as the universe expands and my two "at rest" stations become more distant.

In theory, is this a valid way to measure the expansion of the universe?

safesphere
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Paul Young
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1 Answers1

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Your procedure works, but what you are determining is not an absolute rest frame, it's just the frame that's at rest with respect to the matter and radiation. People sometimes refer to this as the frame of the Hubble flow.

When we talk about an absolute rest frame, we mean something different. It would be a frame in which the laws of physics have some special, simple, or preferred form, as with aether theories.