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In Lawrence Krauss's excellent book "Universe from Nothing" and his related lectures, he cites findings of the BOOMERANG-instrument observations in Anarctica as used to measure the diameter of slightly warmer regions in the cosmic microwave radiation background. Dr. Krauss explains that we are able to measure this because these structures formed in the last scattering surface when the universe was only ~300K years old. Dr. Krauss presents the argument that since gravity cannot travel faster than the speed of light, objects farther apart would not have had time to interact gravitationally. Therefore, these clumps can be no more than ~300K light-years across.

How do we know that a perturbation in a gravitational field cannot travel faster than the speed of light?

I know that special relativity applies to electromagnetic energy, i.e. "light", but how do we know it applies to gravity as well. After all, gravity is related by general relativity to the curvature of space-time itself? Are perturbations in the curvature of space-time (gravitational waves) necessarily constrained by special relativity?

Qmechanic
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Lawrence I. Siden
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    http://physics.stackexchange.com/q/7041/ Check this out. – Omar Nagib Dec 26 '15 at 02:13
  • Thanks, Omar. This may not be a better answer, but it's one I can understand. http://physics.stackexchange.com/a/75124/102193 – Lawrence I. Siden Dec 26 '15 at 02:55
  • a direct evidence Radio Tests of GR with Jupiter and a quasar by Fomalont and Kopeikin –  Dec 26 '15 at 06:17
  • Just a quick comment! Lawrence Krauss' book and related lectures on a "Universe from Nothing" is NOT excellent by any means. It contains many, many errors, and none of the major ideas he talks about are supported by general relativity or quantum field theory. See: http://arxiv.org/abs/1405.6091 – Dr. Ikjyot Singh Kohli Feb 17 '16 at 17:28

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