Black hole shadow to probe modified gravity
  We study the black hole's shadow for Schwarzschild - de Sitter and Kerr - de
Sitter metrics with the contribution of the cosmological constant \Lambda.
Based on the reported parameters of the M87* black hole shadow we obtain
constraints for the $\Lambda$ and show the agreement with the cosmological
data. It is shown that, the coupling of the \Lambda-term with the spin
parameter reveals peculiarities for the photon spheres and hence for the
shadows. Within the parametrized post-Newtonian formalism the constraint for
the corresponding \Lambda-determined parameter is obtained.
