We know galaxies are moving apart from each other due to the ability to look at the color shift of those galaxies.

The fact that galaxies are moving apart seems to be most-widely attributed to the idea that the universe itself is expanding or, that a Big Bang (explosion) occurred, propelling matter in every direction.

According to this question, a body could be propelled by emitting photons, (and, from my understanding this expands to other EM waves as well). It seems to go with it that being hit with photons and radiation would push us as well. (but correct me if I am wrong - because it is a basis for the rest)

If you imagine a roughly spherical shape of points, which aren't close enough to affect each other with gravity, all emitting photons or radiation, it seems to suggest that the points would start to move away from each other. The reasoning behind this is that the sum of being hit by all the other points' radiation would push a point in the direction opposite of where most of the other points are located.

Since the universe has existed for, well, a bit of an understatement: a long time, this could have built up momentum photon by photon over all these years between the galaxies?

Are there arguments which go against this possibility? Does this theory already exist and does it have a name?

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    $\begingroup$ Right off the bat, I'll note that there would have to be a large asymmetry in whichever direction the photons were emitted for this to have any effect. $\endgroup$
    – HDE 226868
    Nov 5, 2015 at 20:12
  • $\begingroup$ @HDE226868 I don't quite follow. I'm thinking each point emits radiation equally in all directions. But since more points are in the general direction of the other side of the sphere, you are getting hit with their radiation more on that side, causing you to accelerate away from the majority. (You would be "catching" their photons, rather than "throwing" your own for propulsion) $\endgroup$ Nov 5, 2015 at 20:14
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    $\begingroup$ The universe is thought to be infinite in size, so you can't think of it as a sphere. There's no preferred center or direction. $\endgroup$
    – HDE 226868
    Nov 5, 2015 at 20:18
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    $\begingroup$ I'll put it like this: There isn't one direction on which there are more galaxies than any other, so all pushing from photons will cancel out. There's no net push. $\endgroup$
    – HDE 226868
    Nov 5, 2015 at 20:25

1 Answer 1


The universe, as far as we can see it, is fairly uniform, with no edge and no favored direction (Axis of Evil being a possible counterexample, but not here relevant). So, each galaxy has galaxies on all sides of it, all radiating energy towards it. The reverse is also true: each galaxy radiates energy in all directions.

The net result is that the radiation causes no net force, either generated within or imposed from without, and plays no significant part in the expansion of the universe.


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