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

[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...