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# Space Expansion, wavelength and energy density

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How exactly does the expansion of space result in the lengthening of the wavelength of a photon?  So you have a tiny photon in superposition with regards to its position and momentum travelling through spacetime for 13 billion years.  An excitation propagating through the EM quantum field.  Presumably the field is stretched by expansion, but the photon at any moment is a point.  So how do the properties of the photon get stretched when it is just a point in the field?  Unless the photon isnt a point, and is a line? And if it is a line, then space expansion doesnt occur at any instant but rather over a period?

If a volume experiences space expansion, how do you measure the increase in volume from inside the volume?  I'm guessing you cant because any ruler you have will expand with the volume.  I'm guessing from inside the volume, there is no measurable increase in volume.

If a volume experiences space expansion, how do you measure the increase in volume from outside the volume?  Assuming, for any observer outside the volume, in order to be able to measure a redshifted photon that exceeds the speed of light you have to be sufficiently far away in spacetime such that the observer and the volume do not share a valid local reference frame.  If there is no valid local frame of reference, how do we measure its volume?

If there is no increase in volume locally, and you cannot measure the volume from outside, how does space expansion increase volume and result in lower average energy density?

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8 minutes ago, AbstractDreamer said:

ruler you have will expand with the volume

The ruler does not expand. The expansion is present on the distances of hundreds Mpc's.

Expansion of space is a feature of homogenous isotropic space. But the physical space is not homogenous and isotropic. It has local areas of various densities of energy and momentum. They are sources of gravity, and they cause the spacetime to curve this way and that way, to shrink and to expand here and there with various rates. Only when all these local effects are averaged on the distances of hundreds Mpc's, the net effect of them becomes a uniform expansion of space.

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14 minutes ago, Genady said:

The ruler does not expand. The expansion is present on the distances of hundreds Mpc's.

-1  for irrelevance.  You quoted half a line and took it out of context to troll a response that has no context to my original text, with no intent other than to derail my topic with graffiti.

I mean if you read my post you'd realise the expansion is present in the wavelength of a photon.   And if the photon wavelength is the ruler, and the wavelength has expanded, then the ruler has expanded.

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15 minutes ago, AbstractDreamer said:

-1  for irrelevance.  You quoted half a line and took it out of context to troll a response that has no context to my original text, with no intent other than to derail my topic with graffiti.

I mean if you read my post you'd realise the expansion is present in the wavelength of a photon.   And if the photon wavelength is the ruler, and the wavelength has expanded, then the ruler has expanded.

But the ruler is not a wavelength of a travelling photon. The ruler is defined locally. For example, like this:

Such rulers do not expand.

PS. I've picked that half a line because I saw it a pivot for the rest. It is relevant.

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42 minutes ago, Genady said:

But the ruler is not a wavelength of a travelling photon. The ruler is defined locally. For example, like this:

Such rulers do not expand.

PS. I've picked that half a line because I saw it a pivot for the rest. It is relevant.

So rulers do not expand.  Great.  So what?

Edited by AbstractDreamer
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5 minutes ago, AbstractDreamer said:

So rulers do not expand.  Great.  So what?

So, you can "measure the increase in volume from inside the volume."

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3 hours ago, AbstractDreamer said:

An excitation propagating through the EM quantum field.  Presumably the field is stretched by expansion, but the photon at any moment is a point.  So how do the properties of the photon get stretched when it is just a point in the field?  Unless the photon isnt a point, and is a line?

?????? We need EM concept reloaded to make wave-particles nature of a photon more clearer.

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• 7 months later...

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