How Do Scientists Know It's Space Expanding Not Galaxies Moving?

[Alex K]

Interesting! Aren't there also measurements of the background temperature of high redshift galaxies based on some atomic excitations?

[Jehanne]

Interesting! Aren't there also measurements of the background temperature of high redshift galaxies based on some atomic excitations?

I copied the last part of your sentence "measurements of the background temperature of high redshift galaxies based on some atomic excitations" into Google:

https://arxiv.org/pdf/1701.08720.pdf

[Jackalope]

Yeah, but that's just one supernova.  I am not sure that is a standard candle type of supernova.  It pins down distance to the megellanic cloud, but doesn't tell much of anything else.

Type Ia supernovae are standard candles. I thought SN 1987A was a type II though.

ETA: Oh shit, I misread your post, you knew this already.

[Alex K]

Interesting! Aren't there also measurements of the background temperature of high redshift galaxies based on some atomic excitations?

I copied the last part of your sentence "measurements of the background temperature of high redshift galaxies based on some atomic excitations" into Google:

https://arxiv.org/pdf/1701.08720.pdf

Ah yes, references 77, 78 therein

[Rhondazvous]

How do we know that rather than space expanding, matter isn't contracting, the idea being that light is exempt from shrinkage?  What, for example, if one of the fundamental four forces is actually in flux?

This would also lead to a red shift, no?

Absolutely, because the receiving atoms would be smaller, the received light would look more long-waved to them.

If you take the equations of matter and spacetime in General Relativity, both interpretations are possible. One can interpret the change of the space metric as an increase in distance by changing the coordinates, or one can model it by rescaling all the masses and strengths of forces, which would lead you to a picture where the mass of the electron rises making the atoms smaller. This should be equally consistent, but one still has to change the definition of time (which comes natural though because all the physical clocks are ticking differently as well).  The space expansion interpretation of the equations is much simpler though.

I see no change in time in the space expansion interpretation, since time is measured by the movements of celestial bodies not by the movement of space. Units of time such as days, months or years are based on movement of stars and planets within galaxies, while space expansion occurs between galaxies. To my knowledge humans do not base any calendar on intergalactic movement. But give us time (pun intended).

[Alex K]

Absolutely, because the receiving atoms would be smaller, the received light would look more long-waved to them.

If you take the equations of matter and spacetime in General Relativity, both interpretations are possible. One can interpret the change of the space metric as an increase in distance by changing the coordinates, or one can model it by rescaling all the masses and strengths of forces, which would lead you to a picture where the mass of the electron rises making the atoms smaller. This should be equally consistent, but one still has to change the definition of time (which comes natural though because all the physical clocks are ticking differently as well).  The space expansion interpretation of the equations is much simpler though.

I see no change in time in the space expansion interpretation, since time is measured by the movements of celestial bodies not by the movement of space. Units of time such as days, months or years are based on movement of stars and planets within galaxies, while space expansion occurs between galaxies. To my knowledge humans do not base any calendar on intergalactic movement. But give us time (pun intended).

The perception and measurement of time is always based on the speed at which physical processes occur, right? In the scheme where everything shrinks, everything becomes faster. For example, how long does it take for a beam of light to cross a hydrogen atom? This is a basis of time and if the hydrogen atom shrinks, this time becomes shorter. So, in order to maintain formally the same laws of physics throughout the "expansion of space", now shrinking of all scales, the notion of how long is one second has to be adjusted to be shorter and shorter. I think...

[CapnAwesome]

Short answer, because all the galaxies are moving away from us. If they were just moving about, some would be getting closer and some farther away. It really only makes sense with an expanding universe.

[Rhondazvous]

I see no change in time in the space expansion interpretation, since time is measured by the movements of celestial bodies not by the movement of space.  Units of time such as days, months or years are based on movement of stars and planets within galaxies, while space expansion occurs between galaxies. To my knowledge humans do not base any calendar on intergalactic movement. But give us time (pun intended).

The perception and measurement of time is always based on the speed at which physical processes occur, right? In the scheme where everything shrinks, everything becomes faster. For example, how long does it take for a beam of light to cross a hydrogen atom? This is a basis of time and if the hydrogen atom shrinks, this time becomes shorter. So, in order to maintain formally the same laws of physics throughout the "expansion of space", now shrinking of all scales, the notion of how long is one second has to be adjusted to be shorter and shorter. I think...

As space expands, matter shrinks? That goes a long way toward answering my op question even without the calculus. BTW, I'm not afraid of calculus. I'm a mathemagician. I can take four 8s and turn them into three 1s. 888 / 8 = 111. So there.

Then again, if the galaxies are shrinking, then they may not necessarily be moving away from us but just be appearing to do so. will every galaxy eventually shrink into its own little black hole, then burst into billions of Big Bangs? Wish I had a camera, that would be so kewl.

[Alex K]

The perception and measurement of time is always based on the speed at which physical processes occur, right? In the scheme where everything shrinks, everything becomes faster. For example, how long does it take for a beam of light to cross a hydrogen atom? This is a basis of time and if the hydrogen atom shrinks, this time becomes shorter. So, in order to maintain formally the same laws of physics throughout the "expansion of space", now shrinking of all scales, the notion of how long is one second has to be adjusted to be shorter and shorter. I think...

As space expands, matter shrinks? That goes a long way toward answering my op question even without the calculus. BTW, I'm not afraid of calculus. I'm a mathemagician. I can take four 8s and turn them into three 1s. 888 / 8 = 111. So there.

Then again, if the galaxies are shrinking, then they may not necessarily be moving away from us but just be appearing to do so. will every galaxy eventually shrink into its own little black hole, then burst into billions of Big Bangs? Wish I had a camera, that would be so kewl.

Well, not really both - we are talking about two separate empirically equivalent interpretations of the same theory. Either space can be taken to expand, or, interestingly, stuff can be thought to shrink to create the same *observed* effect.
The shrinking in this picture would not as far as I can tell cause stuff to collapse into a black hole. The shrinking is, mathematically, only the outcome, the effect of all dimensionful quantities changing: particle masses become larger thus shrinking all atoms whose extent is inverse proportional to the mass, but the gravitational constant gets smaller in this scheme, and so collapse into a black hole is prevented.

[Rhondazvous]

Well, not really both - we are talking about two separate empirically equivalent interpretations of the same theory. Either space can be taken to expand, or, interestingly, stuff can be thought to shrink to create the same *observed* effect.
The shrinking in this picture would not as far as I can tell cause stuff to collapse into a black hole. The shrinking is, mathematically, only the outcome, the effect of all dimensionful quantities changing: particle masses become larger thus shrinking all atoms whose extent is inverse proportional to the mass, but the gravitational constant gets smaller in this scheme, and so collapse into a black hole is prevented.

Mass is getting larger but gravity is getting smaller? Do you mean the gravity is getting weaker (which goes against my understanding of gravity vis a vis mass) or that the range of the gravitational pull is getting shorter?