A fascinating idea is presented by National Geographic News this week, discussing A Measurement of Large-Scale Peculiar Velocities of Clusters of Galaxies: Results and Cosmological Implications, an article published in the Astrophysical Journal Letters on October 20th.
The idea is this: Because galaxy clusters in vastly separated parts of the universe have been observed moving at a uniform velocity in a uniform direction, it is possible that cause of this movement is located outside of our universe itself.
Think of the universe as a basin of water in which the water is all flowing at the same speed in the same direction. The cause of that flow couldn’t be in the basin itself, or there would not be the uniformity. Instead, the flow may be caused by something like a tide, the gravitational pull of the moon, affecting the entire basin uniformly all at once, but coming from far outside of the basin.
What would the equivalent the cause of our universe’s tide be? It would have to be pretty darned big – a lot bigger than our universe – if big is even the right word.