  We consider a test of the Copernican Principle through observations of the
large-scale structures, and for this purpose we study the self-gravitating
system in a relativistic huge void universe model which does not invoke the
Copernican Principle. If we focus on the the weakly self-gravitating and slowly
evolving system whose spatial extent is much smaller than the scale of the
cosmological horizon in the homogeneous and isotropic background universe
model, the cosmological Newtonian approximation is available. Also in the huge
void universe model, the same kind of approximation as the cosmological
Newtonian approximation is available for the analysis of the perturbations
contained in a region whose spatial size is much smaller than the scale of the
huge void: the effects of the huge void are taken into account in a
perturbative manner by using the Fermi-normal coordinates. By using this
approximation, we derive the equations of motion for the weakly
self-gravitating perturbations whose elements have relative velocities much
smaller than the speed of light, and show the derived equations can be
significantly different from those in the homogeneous and isotropic universe
model, due to the anisotropic volume expansion in the huge void. We linearize
the derived equations of motion and solve them. The solutions show that the
behaviors of linear density perturbations are very different from those in the
homogeneous and isotropic universe model.
