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Journal Article

Is there proof that backreaction of inhomogeneities is irrelevant in cosmology?


Andersson,  L.
Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;

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Buchert, T., Carfora, M., Ellis, G. F. R., Kolb, E. W., MacCallum, M. A. H., Ostrowski, J. J., et al. (2015). Is there proof that backreaction of inhomogeneities is irrelevant in cosmology? Classical and quantum gravity, 32(21): 215021. doi:10.1088/0264-9381/32/21/215021.

Cite as: https://hdl.handle.net/11858/00-001M-0000-0027-A86D-7
No. In a number of papers Green and Wald argue that the standard FLRW model
approximates our Universe extremely well on all scales, except in the immediate
vicinity of very strong field astrophysical objects. In particular, they argue
that the effect of inhomogeneities on average properties of the Universe
(backreaction) is irrelevant. We show that their claims are not valid.
Specifically, we demonstrate, referring to their recent review paper, that (i)
their two-dimensional 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 trace-free property of backreaction is unphysical
and the theorem about it is mathematically flawed; (iii) the scheme that
underlies the trace-free theorem does not involve averaging and therefore does
not capture crucial non-local effects; (iv) their arguments are to a large
extent coordinate-dependent, and (v) many of their criticisms of backreaction
frameworks do not apply to the published definitions of these frameworks.