The Nature of the Universe
The cosmological principle infers that the universe, on the very large scale, is both homogeneous and isotropic. The primary evidence underpinning this position is the uniformity of the temperature of the Cosmic Microwave Background (CMB), which is discussed in more detail in later sections, but suggests uniformity across the universe to a few parts in 105. In addition to the general 'cosmological principle', measurements of the redshift of galaxies, quasars and galaxy clusters all appear to support the idea of an expanding universe that is encapsulated in the Hubble parameter [H].
So what is the scope of the observational evidence?
Historically, evidence supporting the current cosmological model has been accumulating for hundreds of years, although the beginning of the 20th century saw a number of significant changes, which in the modern idiom, we might describe in terms of the ‘paradigm shifts’ associated with relativity and quantum mechanics. However, the observational evidence in just the last few decades has also been a critical factor in establishing the ‘current’ standard model of cosmology. However, at this point, it is worth highlighting that the standard cosmology model does not really align with the known physics of the standard particle model, as the former requires the introduction of non-standard, and essentially hypothetical, concepts known as dark energy and dark matter. With this note of caution raised, it should also be highlighted that the weight of authority now appears to firmly support the current cosmological model, which the following list is simply a representative example:
- Hubble’s law based on a wealth of redshift measurements against
distance, which appears to confirm that the universe is expanding.
However, the cause of this expansion is another matter.
- The uniformity of the Cosmic Microwave Background (CMB) has
been measured with increasing accuracy. The latest being linked
to the Wilkinson Microwave Anisotropy Probe program.
- Recent measurement of the redshift associated with Type-Ia a
supernova seems suggest that the universe might now be accelerating
rather than deceleration under expansion. This observation is one
of the primary reasons supporting the introduction of a gravitationally
repulsive dark energy.
- The observed abundances of light elements is said to be consistent
with the predictions of
nucleosynthesis within the Big Bang model. However,
for this model to be consistent, baryonic matter can only account
for about 4% of the energy-density, which aligns to current measurements
of the CMB.
- The clustering of matter, when including both normal and dark
matter, seems to explain a power spectrum that is in good agreement
with the standard model.
- Without dark energy,
the age of some stars would be older than the universe itself. However,
it is not clear that this can be class as observational evidence
as it may appear to simply be a justification of the present model.
- The evidence for dark matter
has been formulated from the sizes and velocities of various structural
objects within the universe, e.g.
- The rotation curves of spiral galaxies.
- The temperature and distribution of x-ray gas in elliptical galaxies.
- The temperature and distribution of x-ray gas in clusters of galaxies.
- Gravitational lensing by clusters of galaxies.
While there are many other instances and possibly better examples, which support the current model, the intention has only to been to generally highlight the considerable weight of authority that is now said to support the current model.
So what is the scope of the current model?
In a subsequent section entitled ‘The Big Bang Model’ , the evolution of the universe will be described in terms of its past, present and future phases. However, this description is anchored in the assumptions of present-day cosmology, some of which are highly speculative. Therefore, as a prelude to describing the accepted model, we shall initially reflect on how our understanding of the universe has transitioned from what was originally assumed to have a ‘material nature’ to one that has also had to take into account its ‘quantum nature’. It is also a model that is predicted on humanity’s hands-on experience of the universe only extending out as far as the Moon, which as a fraction of the visible radius of the universe amounts to only 1/1018. Of course, the last 30 years has seen astronomy gain the ability to ‘peer’ much, much further into the depth of space, which also implies an ability to look further back in time. However, this said, we should not forget that the perceived nature of the universe is becoming increasing dependent on mathematical models, which are in-turn predicated on a myriad of assumptions, of which some have only been subject to limited verification at this time.