this post was submitted on 29 Sep 2024
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We can still measure the red/blue shift to find the star, but if you don't correct for it, it will be wrong.
Also I don't know enough about gravitational waves wo know how it would be effected by the expansion of the universe.
But remember when LIGO measures, it's not measuring absolute values that we would see drift in. It's all relative measurements from a short time period prior. It would follow in lockstep with the expansion.
Also gravitational waves arent particles. They're disturbances in the fabric of the universe. So they don't behave like standard waves do. They have their own wave mechanics that I haven't studied.
And light is having its wavelength stretched. Speed of is not proportional to frequency in a vacuum only the permittivity and permeability of free space. So it's wavelength is getting expanded without
But again. Space isn't expanding. Distance is.
Also that's not how informeters work.
They compare distance across two lines. They can only detect the differences between those lines. Because expansion is universal in all directions, it's not detectable on informeters.
Actually that's a good point about interferometers, the only detectable change whould be in the difference between each arm's length.
Gravitational waves do behave like EM waves, we've seen a neutron star merger simultaneously in gravity and light. If there was a difference, one observation would lag behind.
How exactly would we measure an absolute value of distance? The whole thing about general relativity is than everything is relative. If everything was scaled up such that the fine structure constant stayed the same, we wouldn't be able to measure a difference.
Which brings us back to the question I have with your model: How can a changing distance be measured by light to be the same (metre bar) but also different (redshift)? If light is scaling with the rest of the universe, it shouldn't get shifted. This in the crux of my confusion.
The answer is there's no such thing as absolute distance. Because there's no such thing as absolute position. Quantum garuntees inaccuracies in position.
And your right. We can't actually measure the expansion of the universe directly. It's actually because of the red shift we do.
The reason we can see the red shift is because the universe holds the speed of light in a vacuum constant.
So if the universe is expanding, and the speed of light is expanding with it, in-order for the speed of light to stay the same, it has to travel more distance in a time. Meaning it's stretching it's wavelength as it moves. Just like something moving away from us does. IIRC it's because of observations that everything is constantly moving further from us, the further out you go, the faster it's moving away.
But everything is moving from everything, including itself.
I do apologize if I'm a little muddy, I did my physics degree about a decade ago.
Edit as for why gravitational waves travel at the same as E&M waves is because "information" is what travels at the speed of light. For an electro magnetic wave that's disturbances in E&M. For gravity that's ripples in the fabric of space-time. For quantum there's experiments showing that entangled particles will collapse together, if sperated by distance, the lag time is also the speed of light.
EDIT 2:
The only thing faster than the speed of light, is actually the expansion of the universe beyond a certain distance. Don't remember what it is. But because distance istself is expanding, that's proportional to distance. So the expansion rate is actually faster than the speed of light far enough out. But no SINGLE point is expanding faster than the speed of light.