  We study how the filamentary pattern of the cosmic web changes if the true
gravity deviates from the general relativity (GR) on the large scale. The f(R)
gravity whose strength is controlled to satisfy the current observational
constraints on the cluster scale is adopted as our fiducial model and a large
N-body simulation of high-resolution is utilized for this study. By applying
the minimal spanning tree algorithm to the halo catalogs from the simulation at
various epochs, we identify the main stems of the rich superclusters located in
the most prominent filamentary section of the cosmic web and determine their
spatial extents per member cluster as the degree of their straightness. It is
found that the $f(R)$ gravity has an effect of significantly bending the
superclusters and that the effect becomes stronger as the universe evolves.
Even in the case where the deviation from GR is too small to be detectable by
any other observables, the degree of the supercluster straightness exhibits
conspicuous difference between the $f(R)$ and the GR models. Our results also
imply that the supercluster straightness could be a useful discriminator of
f(R) gravity from the coupled dark energy since it is shown to evolve
differently between the two models. As a final conclusion, the degree of the
straightness of the rich superclusters should provide a powerful cosmological
test of large scale gravity.
