# Tag Archives: backreaction

## Acceleration paper published

Hooray! The acceleration paper was published in Phys. Rev. D a couple of days ago.

I was quite pleased with how this one turned out – it’s a nice clarification, I think. Tim had the idea of using the blueshift in collapsing regions to mimic acceleration in the Hubble diagram, which is pretty cool in itself. It was also good to find a concrete example of the link between acceleration of the average and acceleration in the Hubble diagram that Syksy Rasanen has discussed in a couple of papers (see our discussion for references).

Of course, we’re not claiming that “dark energy is backreaction” or anything nearly as strong as that, but I think it does extend the backreaction debate a little. The papers by Ishibashi, Green, and Wald, which seem to show that inhomogeneities on small scales don’t affect the background evolution much, suggest that backreaction effects can’t have any bearing on dark energy. I suppose our paper responds to theirs by saying “yes, perhaps they can’t dynamically, but what about non-linear optical effects?”

Philip Bull, Timothy Clifton (2012). Local and non-local measures of acceleration in cosmology. Phys. Rev. D: 10.1103/PhysRevD.85.103512

As it turns out: No. I’ll have to keep trying. But it did lead to what I thought was an interesting little result: the Buchert averaged hypersurface deceleration parameter, usually written $q_\mathcal{D}$, is invariant under $\Theta \mapsto - \Theta$, where $\Theta$ is the expansion scalar for the dust congruence. This means that it doesn’t care whether your structures are collapsing or expanding, as long as the density profile and variance of the Hubble rates are the same. This is pretty trivial by inspection of the general form of the expression for $q_\mathcal{D}$, but it hadn’t crossed my mind before.