The Standard Models
It is possible that most cosmologists would claim that there is only one restriction on any model, i.e. that it is supported by observations. However, in practice, there are only a few restrictive caveats to limit the number of models that 'might' be considered acceptable within the definition of mainstream of cosmology. However, any concordance model must still align to a list of caveats, which may restrict the degrees of freedom that any given component model is allowed.
Today, there is almost an implicit caveat that any model must conform to the generally accepted idea of the `Big Bang’ as opposed to a `Steady-State` model, even though there are some who might still challenge even this assumption. While bearing these compliance requirements in mind, the intention is to consider the implications of the 2 main variant models of what might be called the 'Standard Big Bang Model':
Within these 2 models, the universe undergoes expansion with time and complies with the additional restrictions imposed by the cosmological principle. In this context, the overall scope of these 2 models encompass an initial inflationary model followed by a subsequent expansive energy-density model, which in combination form the main elements of mainstream cosmology, at least, from a general perspective.
So does general acceptance mean that this 2-phase model have to be right?
Probably most cosmologists would accept that the status of these models is simply the best general description that aligns to observations, at this point in time, without necessarily implying that they represent the final word on this subject. Of course. while there are some who may understandably defend a lifetime's work with what can appear to be something more akin to religious zeal, most would still accept the need to question any aspect of these models. One of the key grey areas of the overall model, which we have been discussing, concerns the cause of expansion. Within the development timeline of cosmology in the 20th century, the `Energy Density Model` was originally presented as the 'Big Bang Model' in isolation of any initial inflationary process. If so, we might query how this model was thought to explain the evolution of the universe without inflation. However, given that such questions would be primarily retrospective in nature, we shall try to focus our questions on the composite standard model:
Did inflation cause and maintain the expansion of the universe?
At first glance, some may query the why the expression `cause and maintain` has been used, when expansion might simply be the `effect` of some `initial cause` such as inflation. The issue being highlighted in the question above lies in the implied nature of the Big Bang, not as an explosive expansion, but rather as the uniform expansion of each unit volume of space as a function of time. In these terms, it can be difficult to imagine how the expansion of space persisted for 7 billion years, as calculated by the energy-density model, given only a minuscule period of inflation within the first second of existence without some `cause` being `maintained`. However, while the component energy densities of the ΛCDM model do not appear to account for any expansion of the universe prior to +7 billion years, there is the suggestion that space continued to expand due to the inertia of its mass-energy. So, having now raised this key issue, we shall reiterate another question that has been previously outlined:
Is the universe finite or infinite in spacetime?
Often, on first exposure to the Big Bang model, there is the natural assumption that an expanding universe, which has a finite age of ~13.7 billion years, must also have a finite size. However, the issue of the size of the universe turns out to be problematic and only leads to the definition of a number of cosmological horizons, which although already outlined will be quantified further within the results section of the ΛCDM model. At this point, we might also highlight another area of ambiguity, which has already been outlined:
Does our perception of the universe include all of space and time, such that nothing exists outside the universe or before its creation?
Initially, almost by definition, it was generally assumed that `nothing` existed outside the universe, although our ability to perceive ‘absolute nothing’ seems as limited as our ability to imagine the infinite. Today, science is possibly more open to a slightly more philosophical line of questioning concerning the meaning of ‘nothing’ and ‘infinity’, such that cosmology may have become a little more circumspect about the possibility that the current model may not, as yet, even come close to describing the totality or the complexity of the universe. As a result, future developments may throw up additional caveats concerning both the age and size of the universe, which might be taken as a healthy sign that the current models should only be viewed as work-in-progress. If so, the scope of the universe, how it started, and how it works, are still valid questions that are open to debate, even though certain degrees of speculative freedom are restricted by observation.