Solving the curvature and Hubble parameter inconsistencies through
  structure formation-induced curvature
  Recently it has been noted by Di Valentino, Melchiorri and Silk (2019) that
the enhanced lensing signal relative to that expected in the spatially flat
$\Lambda$CDM model poses a possible crisis for the
Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) class of models usually used to
interpret cosmological data. The 'crisis' amounts to inconsistencies between
cosmological datasets arising when the FLRW curvature parameter $\Omega_{k0}$
is determined from the data rather than constrained to be zero a priori.
Moreover, the already substantial discrepancy between the Hubble parameter as
determined by Planck and local observations increases to the level of
$5\sigma$. While such inconsistencies might arise from systematic effects of
astrophysical origin affecting the Planck Cosmic Microwave Background (CMB)
power spectra at small angular scales, it is an option that the inconsistencies
are due to the failure of the FLRW assumption. In this paper we recall how the
FLRW curvature ansatz is expected to be violated for generic relativistic
spacetimes. We explain how the FLRW conservation equation for volume-averaged
spatial curvature is modified through structure formation, and we illustrate in
a simple framework how the curvature tension in a FLRW spacetime can be
resolved -- and is even expected to occur -- from the point of view of general
relativity. Requiring early-time convergence towards a Friedmannian model with
a spatial curvature parameter $\Omega_{k0}$ equal to that preferred from the
Planck power spectra resolves the Hubble tension within our dark energy-free
model.
