Explore advanced concepts in general relativity through this graduate-level lecture from the Warsaw4PhD and GeoPlanet PhD schools. Delve into the Riemann tensor in Rindler's accelerating spacetime, followed by an in-depth examination of the geodesic deviation equation. Investigate the Einstein equations, including their derivation and implications. Learn about the cosmological constant and its role in modern cosmology. Conclude with an exploration of the variational principle for Einstein equations and an introduction to linearized gravity. This comprehensive lecture, delivered by Mikołaj Korzyński from the Center for Theoretical Physics, Polish Academy of Sciences, provides a rigorous mathematical treatment of key topics in theoretical physics and cosmology.