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Everybody has been taught at one point, "oh the universe expands, but that doesn't mean that everything is expanding uniformly, since that means we can't detect the expansion, but only that huge galaxies are moving away from each other".

But I'm rather confused. Can't the expanding universe simply be thought as a coordinate axis that expands? The definition of "1" on the coordinate axis constantly expands? After all, spacetime itself is expanding, and not only "average distance of galaxies".

So our Planck length would expand, light wavelengths expand, we expand, etc. But clearly this isn't happening. Of course, the handwavy explanation is "electromagnetism/gravity overcomes expansion at small scales", but the electromagnetic and gravity forces are defined in terms of constants relative to our "coordinate intervals", so if spacetime itself is expanding "under the feet" of gravity, shouldn't gravity not be able to do anything about it?

We would also be unable to measure the expansion of spacetime since all of our measurement benchmarks and tools are also expanding.

Clearly, this isn't happening. What is the precise reason?

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  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/2110/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Feb 17, 2014 at 15:14
  • $\begingroup$ yes, seems like exactly the same question... $\endgroup$
    – BjornW
    Commented Feb 17, 2014 at 16:35
  • $\begingroup$ Why do you think that "The definition of "1" on the coordinate axis constantly expands"? Meter is defined as a distance travelled by light in certain time. The speed of light is locally independent of the global spacetime geometry, or its evolution, so meter does not change. Thus Planck length does not change either. $\endgroup$
    – mpv
    Commented Feb 18, 2014 at 7:05

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You can see the expansion with the Doppler Effect. Nothing else is expanding in the Universe except from the Universe. The Planck length is the same because it's a constant length. The stars and the Galaxies are getting pushed away from each other due to the red shifts.

Here's a video on redshifts: http://www.youtube.com/watch?v=TLwiOToY79I The objects in the Universe doesn't just simply expand because the Universe expands. Spacetime can't change the shape of matter. It just simply can't. http://www.youtube.com/watch?v=9N6l1E-X5jQ

Hope it helped!

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The molecules do not expand, because they are kept together by the electromagnetic interaction. The same applies to hadrons (strong force). These interactions have their own coupling constants, independent of gravity. So space expanding in an atom is like trying to pull the atom apart very slowly by a force which does not hold the atom together. The electromagnetic force just does not care. Any attempt to pull the atom apart is cancelled by electromagnetic attraction which immediately returns the distances back to what it should be according to the quantum electrodynamics. It is like pulling 2 bricks apart when they are connected by a string and each brick is on a wheel and the 2 wheels rotate in opposite directions to pull the bricks. The bricks are not stuck to the wheels, also the particles are not stuck to the space. Particles can move in space and bricks can slide on the rotating wheels. That is in a nutshell the reason behind. In simplified terms of course.

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    $\begingroup$ I am tired of all these handwavy analogies. I want an actual explanation; I just gave a handwavy analogy that "shows" that everything expands together in my post. $\endgroup$
    – ithisa
    Commented Feb 17, 2014 at 17:05

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