Daily Archives: September 26, 2008

Dark Flow, the Speed of Gravity, and the CMB

Kashlinsky, Atrio-Barandela, Kocevski, and Ebeling have just put out a preprint on the peculiar motions of galactic clusters: A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications. In short, they claim that all galactic clusters appear to have a motion with respect to the cosmic microwave background (CMB). The motion of a galactic cluster slightly effects the energy of the microwave radiation that travels through it, so they use the temperature map of the CMB to determine the velocity of those galactic clusters.

And the result is that the whole (observable) universe appears to be moving with respect to the CMB. This was not expected because the observable universe is approximately isotropic and so shouldn’t be going anywhere. They write (in the abstract):

This flow is difficult to explain by gravitational evolution within the framework of the concordance LCDM model and may be indicative of the tilt exerted across the entire current horizon by far-away pre-inflationary inhomogeneities.

However, the tilt is easy to explain when you assume that the speed of gravity is larger than C: If gravitational interactions travel faster than light, you will automatically be able to feel the gravitational attraction of matter even if it is too far away for you to see.
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