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.

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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.