The Energy Density (ΛCDM) Model
While we have already covered many of the key details of this model, the intention is now to try to put some of the results of this model into some better context within the overarching development of the ‘concordance model’ over the last 100 years.
To start this process, we shall recap on the basic timeline of developments that have contributed to the standard cosmological model, which was originally anchored in Friedmann’s equations, dating back to the early 1920’s. However, by the late 1920’s, Lemaître’s basic idea of a 'creation' event from which the universe expanded was still the source of much debate. While this idea was to receive broad support from Hubble’s redshift observations, the next 30 years would see the debate between the Big Bang and Steady-State models continue without any real resolution of these two opposing positions. However, for many, this debate effectively ended with the discovery of the Cosmic Microwave Background (CMB) radiation by Penzias and Wilson in 1965. However, at this key point in the summary of developments, let us stop to consider the general status of cosmology as it stood on entering the 1970’s:
- While, today, the inflation
model is thought to address many of the problems originally
associated with the cosmological principle; the first speculative
proposals of inflation did not appear until the early 1980’s. Therefore,
even after the discovery of the CMB radiation, the early cosmological
models had to be masking a large number of problems.
- Equally, in the 1970’s, the standard energy-density model was
still essentially predicated on only ‘normal’ matter and
radiation, as the idea of cold
dark matter and dark
energy had not really been considered in detail, let alone
accepted, by mainstream cosmology.
- While the gravitational anomalies, now thought to be resolved
by dark matter, had been discussed since the 1930’s, the idea of
cold dark matter [CDM] did not really consolidate until the 1990’s
and even today is still subject to much speculation, as it exists
outside the accepted particle model of physics.
- While the history of dark energy might be said to go back to Einstein’s introduction of the cosmological constant [Λ], the origins of this constant might be described as more of a conceptual ‘fudge’ to maintain the idea of a static universe. As such, the forwarding of dark energy as a serious hypothesis did not really begin until the mid 1980’s and, for many; it is still a matter of open conjecture.
Of course, modern cosmology has benefited from many technological advances since the 1970’s, especially in the field of space-borne observations and measurements. So let us now start to focus on the assumptions of the present-day energy-density model, which is often referred to as the Lambda-Cold Dark Matter (Λ-CDM) model. Today, many will cite the inflation model as a prerequisite process of any energy-density model, which then helps explain many of the initial conditions of the universe, which are now part of the overall conclusions of the concordance model. However, it might be argued that there is still no seamless connection between the inflation and energy-density models as, in many ways, the latter model is essentially anchored to present-day observations of the universe, which is then run backwards in time towards the inflationary model. As a consequence, the inflationary model is required, as a matter of necessity, to end up at the reversed engineered starting point of the energy-density model.
So what are the key assumptions underpinning the current energy-density model?
One key assumption that appears to need some additional scrutiny relates to the issue of how space, as opposed to the universe, expands. Within the prerequisite inflation model, there is a general assumption that the negative pressure of the scalar field drove the expansion of space prior to the unraveling of the gravitational force in the very earliest moments of the universe. However, this process ended because the scalar field is required to decay to virtual non-existence during the exponential expansion space in the eary universe.
So what maintained the expansion of the universe in the absence of the scalar field?
While there seems to be some general acceptance of the idea that space simply continue to expand due to its inertia, it is unclear how this mechanism is thought to work. Therefore, this still considered to be an open question central to the overall concordance model. As such, we shall continue the line of questioning initially set out in the ‘Framework Model’ and the implications predicated on whether the universe is finite or infinite.