Is there proof that backreaction of inhomogeneities is irrelevant in cosmology?
Abstract
No. In a number of papers, Green and Wald argue that the standard FLRW model approximates our Universe extremely well on all scales, except close to strongfield astrophysical objects. In particular, they argue that the effect of inhomogeneities on average properties of the Universe (backreaction) is irrelevant. We show that this latter claim is not valid. Specifically, we demonstrate, referring to their recent review paper, that (i) their twodimensional example used to illustrate the fitting problem differs from the actual problem in important respects, and it assumes what is to be proven; (ii) the proof of the tracefree property of backreaction is unphysical and the theorem about it fails to be a mathematically general statement; (iii) the scheme that underlies the tracefree theorem does not involve averaging and therefore does not capture crucial nonlocal effects; (iv) their arguments are to a large extent coordinatedependent, and (v) many of their criticisms of backreaction frameworks do not apply to the published definitions of these frameworks. It is therefore incorrect to infer that Green and Wald have proven a general result that addresses the essential physical questions of backreaction in cosmology.
 Publication:

Classical and Quantum Gravity
 Pub Date:
 November 2015
 DOI:
 10.1088/02649381/32/21/215021
 arXiv:
 arXiv:1505.07800
 Bibcode:
 2015CQGra..32u5021B
 Keywords:

 relativistic cosmology;
 inhomogeneity effects;
 weaklimits;
 04.20.q;
 04.20.Cv;
 95.30.k;
 95.36.+x;
 98.80.Jk;
 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Theory
 EPrint:
 44 pages