# The Weight of Authority

In the first section of this discussion, we have simply undertaken
a basic ‘*duty of inquiry*’ into
some of the basic concepts and implications underpinning the standard
cosmological model. While humanity has always had an interest in the
stars, we might trace the roots of '*modern*' cosmology back
to the publication of Einstein’s *general
theory of relativity* in 1916. At this point, the model began
to change from an infinite and static universe to one that was finite
and dynamic.

Today, the consensus or weight of authority appears to support the
model of a finite and dynamic universe, at least, as understood within
the scope of the *Big Bang*
model; although a number of caveats and modifications
have been made to this model along the way.

- In principle, general relativity is said to explain how
the gravitational effects of the energy-density of the universe
results in the spacetime geometry of the universe.
- At the largest scale, the cosmological principle assumes the
energy-density of the universe to be both homogeneous and isotropic.
- Einstein originally thought the universe was essentially a static
system, which required a constant to be added to the model in order
maintain the static state, which became known as the cosmological
constant [Λ].
- Einstein later accepted that a static and homogeneous distribution
of galaxies would be inherently unstable, i.e. the universe as a
*thermodynamic system*must either expand or contract. - Today, the cosmological constant [Λ] is still in used, but it
is now often interpreted in the context of a vacuum energy density,
which is sometime thought to manifest itself as
*dark energy*.

- Contrary to it name, the
*Big Bang*does not describe the expansion of the universe in terms of something that can be liken to an explosion. Therefore, it is difficult to understand how the continued expansion of the universe was initially explained.

So, in brief, this is the starting point for the next stage of the
discussion. As indicated, Einstein’s matter-dominated universe required
the cosmological constant [Λ] in order to maintain a static universe.
However, this model was only metastable, as any perturbation would cause
either collapse or expansion. In contrast, another model known as the
‘*deSitter *
*model'* considered a conceptual universe with no matter,
only the cosmological constant [Λ], which led to a run-away exponential
expansion. Finally, in the same basic timeframe as Einstein and deSitter,
Alexander Friedmann found a solution within general relativity, which
suggested a model inclusive of matter and radiation that could be extrapolated
back in time towards a singularity of infinite density. As alluded to
by *Olbers’ Paradox*, a homogeneous, but unchanging universe would
cause every line of sight to end on a star and result in the night sky
being as bright as the surface of the Sun. As such, the idea of a
static universe was rejected in favour of an expanding universe, e.g.
de Sitter and/or Friedman models, which were thought better able to
explain the observed dark night sky.

*But why is reference to the ‘deSitter’ model
required in a universe that is known to contain matter? *

Although, much of the next section of the discussion will focus on
the *Friedmann model*, the mass-energy
density of an expanding universe has led to the idea that normal matter
now only accounts for 4% of the observable universe. Equally, if the
universe continues to expand, the figure of 4% will continue to fall
towards zero, see *cosmic calculator*
for details, such that the dominant energy density of the universe will
be eventually defined by only the cosmological constant [Λ] or
what is also known as dark energy. Therefore, such a universe would
approach the description of the deSitter model. However, before we become
too attached to any of these models, we need to consider the weight
of authority, and evidence, which underpins the current accepted model
of cosmology.