I previously talked about a thought experiment which would supposedly show that "expansion of the universe" is fundamentally different from just "everything moving away from each other". However, discussion at the Physics Forum has convinced me that light would not be redshifted over time in a perfectly reflecting mirrored box despite the light being stretched along with the expansion of space.
In the comments for another blog post about cosmological distances, I suggested a thought experiment that would highlight how the redshift from distant galaxies is not due to a Doppler shift, that is, light from very distant galaxies is not redshifted because of the movement of the galaxies, but rather because the intervening space itself is expanding:
If someone managed to trap light when the universe first became transparent, about 300,000 years after the Big Bang, inside a perfectly reflecting mirrored box such that the light is contained in perpetuity, what would have happened to the wavelength of the trapped light by now?
Scientists from NASA have found that a large region of our universe is moving in a particular direction using observations of the cosmic microwave background and X-ray clusters. It seems that the local region of our universe, at least a couple of billion light years across, is moving towards a particular patch of the sky relative to the cosmic microwave background.
Gravitational models of the observable universe are not able to account for such a large-scale "dark flow", so they are speculating that the motion might have been due to the pre-inflation gravitational attraction from mass outside the observable universe. From what I can gather, it seems that an uneven distribution of mass and energy throughout the universe set our particular region of the universe moving slightly towards a specific direction. This would have been before the inflationary phase of the Big Bang. After going through inflation, the masses responsible for the gravitational attraction would have moved too far away for any gravity from them to reach us. And yet our local region of the universe would still maintain the motion due to inertia.
I could have misunderstood what the scientists were saying, though, so any corrections would be welcome. In any case, if there really is such a "dark flow", which should be confirmed with further astronomical observations, then it's mind-boggling in that it might give some of the first hints of what the outside of our observable universe looks like.
I'm a sop for articles about fundamental physics. The October issue of Scientific American has an article about how loop quantum gravity might indicate that the universe didn't start with the Big Bang; instead, the Big Bang might actually be a Big Bounce, where the universe started expanding again after a collapse which erased all trace of its previous existence. It appears that at extremely high energy densities, gravity would no longer be an attractive force and instead becomes a repulsive force.