  We start with the cosmic Friedmann equations, where we adopt a novel
perspective rooted in a Lagrangian formulation grounded in Newtonian mechanics
and the first law of thermodynamics. Our investigation operates under the
assumption that the universe is populated by either a perfect fluid or a scalar
field. By elucidating the intricate interplay between the Lagrangian
formulation and the cosmic Friedmann equations, we uncover the fundamental
principles governing the universe's dynamics within the framework of these
elemental constituents.
  In our concluding endeavor, we embark on the task of harmonizing the
classical equations - namely, the conservation, Euler, and Poisson equations -
with the principles of General Relativity. This undertaking seeks to extend
these foundational equations to encompass the gravitational effects delineated
by General Relativity, thus providing a comprehensive framework for
understanding the behavior of matter and spacetime in the cosmic context.
