Slides of 5 lectures at XV Brazilian School on Cosmology and ...

Backreaction in Buchert averagingKinem<strong>at</strong>ic backreaction term can also be writtenQ = 2 3 〈(δθ)2 〉 − 2〈σ 2 〉i.e., combines variance <strong>of</strong> expansion, and shear.Eq. (6) is required to ensure (3) is an integral <strong>of</strong> (4).Buchert equ<strong>at</strong>ions look deceptively like Friedmannequ<strong>at</strong>ions, but deal with st<strong>at</strong>istical quantitiesThe extent to which the back–reaction, Q, can lead toapparent cosmic acceler<strong>at</strong>ion or not has been thesubject <strong>of</strong> much deb<strong>at</strong>e (e.g., Ishibashi & Wald 2006):How do st<strong>at</strong>istical quantities rel<strong>at</strong>e to observables?Wh<strong>at</strong> about the time slicing?How big is Q given reasonable initial conditions?15th <strong>Brazilian</strong> <strong>School</strong> on Cosmology and Gravit<strong>at</strong>ion, August 2012 – p.36/143