Slashdot Mirror
"Dark Flow" Outside Observable Universe
DynaSoar writes "NASA astrophysicists have discovered what they claim is something outside the observable universe exerting an effect on the observable. The material is pulling clusters of galaxies towards a region of space known not to contain sufficient matter to create the effect. They can only speculate on what the material is and how space might differ there: 'In these regions, space-time might be very different, and likely doesn't contain stars and galaxies (which only formed because of the particular density pattern of mass in our bubble). It could include giant, massive structures much larger than anything in our own observable universe. These structures are what researchers suspect are tugging on the galaxy clusters, causing the dark flow.'"
Since no one reads TFA anyway, and since you clearly didn't:
The universe is thought to have formed about 13.7 billion years ago. So even if light started travelling toward us immediately after the Big Bang, the farthest it could ever get is 13.7 billion light-years in distance. There may be parts of the universe that are farther away (we can't know how big the whole universe is), but we can't see farther than light could travel over the entire age of the universe.
And then:
A theory called inflation posits that the universe we see is just a small bubble of space-time that got rapidly expanded after the Big Bang. There could be other parts of the cosmos beyond this bubble that we cannot see. In these regions, space-time might be very different, and likely doesn't contain stars and galaxies (which only formed because of the particular density pattern of mass in our bubble). It could include giant, massive structures much larger than anything in our own observable universe. These structures are what researchers suspect are tugging on the galaxy clusters, causing the dark flow.
Finally, on a side note, years of watching slashdot paid off in a truly interesting story!
An old-timer with old-timey ideas.
At cosmological scales, metric expansion of space becomes very important. Light that left 13.7 billion years ago will actually travel 47 billion lightyears because of metric expansion. Since metric expansion implies space-time is curved (at cosmological scales, locally it is flat, like the earth is flat locally), general relativity comes into play. This means the normal causality described by special relativity is no longer applicable.
Imagine points A-B-C to be gravitationally bound. Because of metric expansion, space between A-B and B-C expands. This can cause A to move away from C at larger than lightspeed. Since space between B-C only expanded half of A--C, B will be withing light distance from C and thus visible by observers on C. Light from A can reach B, but it will never reach C. By the time it would, space between B and C will have expanded so much that observers from C will no longer see B.