In this insert, we discuss Newtonian cosmology from Isaac Newton (1643--1727) to modern Newtonian cosmology (1934--).
A physically consistent model
is one with NO violations of
the assumed physical laws.
The boundary conditions
and initial conditions
of a physically consistent model may or may NOT be required to be physically real.
Of course, a physically consistent model
may be completely WRONG with respect to
reality.
The best you can usually hope for is that it approximates
reality to some degree.
Remarkably, this theory
was NOT widely accepted by
astronomers
from 18th century
to circa 1920s.
Many astronomers believed
the Milky Way was the
whole universe, finite or otherwise,
and was STATIC on average
(IAL 26: The Discovery of Galaxies: The Discovery
of Galaxies: An Example of the Process of the Scientific Method;
Bo-75).
However, this STATIC universe
was NEVER made into a consistent
Newtonian cosmology
(Bo-75).
The begining of general relativistic cosmology
was with the
Einstein universe
discovered in 1917 by
by Albert Einstein (1879--1955).
But Einstein made the discovery
the hard way WITHOUT the
Friedmann equation (1922)
which we discuss below.
The key component of
general relativistic cosmology is
the Friedmann equation (1922)
discovered by Alexander Friedmann (1888--1925).
We will NOT give NOR
explicate this
derivation.
In practice, when doing
celestial mechanics
those 18th century
and 19th century
astronomers
effectively made this replacement all the time.
But they did NOT make the mental leap that it should be done in principle.
The mental leap was done by
Albert Einstein (1879--1955)
in deriving general relativity (1915).
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