Observations in the neighbourhood of our Solar system and Galaxy
show an irregular distribution of matter.
However, if we investigate the Universe on larger and larger scales,
we observe clusters of galaxies in which the structures of individual
stars disappear and eventually also the galaxy structures merge into a
homogeneous and isotropic density distribution.
Consequently, the large scale Universe can be described as a cosmic fluid
and we are part of it.
Following the principles of Einstein's general relativity theory,
the space-time structure of this cosmic fluid is derived and described
by the so-called Robertson-Walker metric.
In doing so, only symmetry arguments of 3D-space and an extension to 4D
space-time are used, to avoid an elaborate treatment of tensor calculus and
make the subject also accessible to bachelor and university college students.
Using classical arguments, the dynamics of such a homogeneous and isotropic
distribution are investigated, resulting in the Friedmann-Lemaitre equation
which describes the dynamical evolution of the Universe.
This provides various cosmological models, including ingredients like
critical density, dark energy and the cosmological constant.
Introduction of the concept of cosmological redshift allows interpretation
of the observations concerning very distant objects (e.g. Gamma Ray Bursts),
from which the (astro)physical messengers are significantly affected by the
evolution of the Universe on their journey to Earth.
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