# Thread: acceleration due to gravity and red shift

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## acceleration due to gravity and red shift

I was looking at the following Wikipedia article https://en.wikipedia.org/wiki/Redshift about red shift. It has the following equation.

I'm wondering what the evidence is that the earth isn't in a higher gravitational field than most other parts of the universe. Is there a way to rule that out?

2. I don't see how that could could cause the red-shift to increase linearly with distance in all directions.

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Originally Posted by Strange
I don't see how that could could cause the red-shift to increase linearly with distance in all directions.
Are you saying the equation is wrong?

4. If the Earth was in higher gravitational shift than that would mean the entire solar system would be in higher gravitational shift because the moons orbit around the their planets in a manor in accordance to Earth moon system plus all our probes go to these planets work. Plus the entire galaxy would be in a higher gravitational shift because we can figure out the orbits of binary stars and exoplanets. So it looks like the formula might be right and it is probably your expectations that are wrong.

5. Originally Posted by Copernicus
Are you saying the equation is wrong?
I assume not but I haven't seen that form before.

However, if you think of gravitational potential as a hill, why would we be at the top of it with the rest of the universe falling away all round. You may be able to come up with a function for gravitational potential that reproduces what we see but I can't imagine it would be physically meaningful.

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Actually, when we look into the distance, we are in effect on a hill like that, but it's not because of spatial variation it is because of expansion with time coupled with the fact that seeing farther away means seeing further back in time. This is the "cosmological principle," which we don't know is true but it's the simplest model that fits, and underpins all of our assumptions about the history of the universe. Note all this means one possible interpretation of cosmological redshift is that the expansion of the universe is weakening its gravity with the effect that time itself is speeding up as the universe ages. That interpretation means that when you see a redshift, you are looking at an age when time was just going by more slowly, akin to the kind of language used in the "twin paradox" of special relativity. Note this interpretation is not "either/or" relative to the common one, it's just a coordinate choice.

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Originally Posted by astrotimer
If the Earth was in higher gravitational shift than that would mean the entire solar system would be in higher gravitational shift because the moons orbit around the their planets in a manor in accordance to Earth moon system plus all our probes go to these planets work. Plus the entire galaxy would be in a higher gravitational shift because we can figure out the orbits of binary stars and exoplanets. So it looks like the formula might be right and it is probably your expectations that are wrong.
I'm not sure if you are not over extending what I was saying. I was suggesting ruling out the whole milky way and are local super clusters being in a higher gravitational field than more distant super clusters etc. I think the whole idea would be too convoluted because it might suggest our location would be at the edge of the universe, within 100 million light-years, depending on the model.

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Originally Posted by Ken G
Actually, when we look into the distance, we are in effect on a hill like that, but it's not because of spatial variation it is because of expansion with time coupled with the fact that seeing farther away means seeing further back in time. This is the "cosmological principle," which we don't know is true but it's the simplest model that fits, and underpins all of our assumptions about the history of the universe. Note all this means one possible interpretation of cosmological redshift is that the expansion of the universe is weakening its gravity with the effect that time itself is speeding up as the universe ages. That interpretation means that when you see a redshift, you are looking at an age when time was just going by more slowly, akin to the kind of language used in the "twin paradox" of special relativity. Note this interpretation is not "either/or" relative to the common one, it's just a coordinate choice.
Makes sense.

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Originally Posted by Strange
I assume not but I haven't seen that form before.

However, if you think of gravitational potential as a hill, why would we be at the top of it with the rest of the universe falling away all round. You may be able to come up with a function for gravitational potential that reproduces what we see but I can't imagine it would be physically meaningful.
Probably not meaningful.

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How does one add z. Lets say the z value for light from some red shift from expansion is 10, now that light is moving towards a gravity source. Are z's multiplied added or what?

11. Originally Posted by Copernicus
I'm not sure if you are not over extending what I was saying. I was suggesting ruling out the whole milky way and are local super clusters being in a higher gravitational field than more distant super clusters etc. I think the whole idea would be too convoluted because it might suggest our location would be at the edge of the universe, within 100 million light-years, depending on the model.
I was just showing the silliness of it.

12. Originally Posted by Copernicus
How does one add z. Lets say the z value for light from some red shift from expansion is 10, now that light is moving towards a gravity source. Are z's multiplied added or what?

What you would get is a wavelength and then figure the change in wavelength of the source.

Z=

then you have the formula of
Z=
and then move over the one and square and rearrange you get

to find how fast an object is moving away.

Sorry for the delay had to look up a few things.

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Originally Posted by astrotimer
What you would get is a wavelength and then figure the change in wavelength of the source.

Z=

then you have the formula of
Z=
and then move over the one and square and rearrange you get

to find how fast an object is moving away.

Sorry for the delay had to look up a few things.
Thanks, I'll check it out.

14. Originally Posted by Ken G
Actually, when we look into the distance, we are in effect on a hill like that, but it's not because of spatial variation it is because of expansion with time coupled with the fact that seeing farther away means seeing further back in time. This is the "cosmological principle," which we don't know is true but it's the simplest model that fits, and underpins all of our assumptions about the history of the universe. Note all this means one possible interpretation of cosmological redshift is that the expansion of the universe is weakening its gravity with the effect that time itself is speeding up as the universe ages. That interpretation means that when you see a redshift, you are looking at an age when time was just going by more slowly, akin to the kind of language used in the "twin paradox" of special relativity. Note this interpretation is not "either/or" relative to the common one, it's just a coordinate choice.
I like it.

Originally Posted by Copernicus
I was looking at the following Wikipedia article https://en.wikipedia.org/wiki/Redshift about red shift. It has the following equation.

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