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R. Barrett and P. P. Delsanto
but in the real world they are spheres, and each of them corresponds to the
frontier of the visible universe.
The Concordance Model is currently the most popular model of the
universe among cosmologists. Its strength is in its relative simplicity and
internal consistency. However, the fact that in this model 95% of the universe
is “dark” (or unknown) presents a serious challenge to its credibility, and it is
not without its doubters, as we shall see in the next Section.
11.7 Alternative Cosmological Models
The conceptual difficulties with Dark Energy that we have outlined in the
last Section have spawned a vast literature describing attempts to find more
intuitive or better founded solutions to the problem. It is not within the scope
of this book to cover these endeavours in detail. However, some indication of
what is involved is appropriate.
Many theories begin by introducing a field, or fluid, whose properties are
tailored in an ad hoc fashion to reproduce the observed properties of the
universe. The names used for these quantities are sometimes whimsical, such
as “ghost energy” or “quintessence”. The latter is a reference to Aristotle’s
“fifth essence”, an ether (aka aether) that was supposed to pervade the skies.
A common approach in physics is to draw an analogy between one area
of physics and another, in the hope that the first will throw light on the
second. For instance, in nuclear physics it is of use in some instances to
compare the nucleus with a drop of liquid, and assign to it a “density” and
“surface tension”. In cosmology, dark energy is sometimes treated as arising
from the deformation of a four dimensional elastic continuum. This model
is the Strained State Cosmology. For an elastic material in three dimensional
space, the strain energy is related to the resistance of the material against an
imposed deformation. The accumulation of strain energy hinders the deformation and forces the material to return to the unstrained state. In a four
dimensional analogue, if we regard space–time as not just a mathematical artifice, but an entity with physical properties of its own, a strain energy density
can be associated with the presence of curvature, and drives an accelerated
expansion towards an asymptotic unstrained and completely flat state. It is
the analogue of a twisted piece of elastic material returning to its flat state.
This theory succeeds in explaining the observed accelerated expansion and
also the exponential expansion close to the origin.
Another class of theories involves modifications to Einstein’s General Relativity, or even its replacement. However, at this moment, the CDM model
R. Barrett and P. P. Delsanto
but in the real world they are spheres, and each of them corresponds to the
frontier of the visible universe.
The Concordance Model is currently the most popular model of the
universe among cosmologists. Its strength is in its relative simplicity and
internal consistency. However, the fact that in this model 95% of the universe
is “dark” (or unknown) presents a serious challenge to its credibility, and it is
not without its doubters, as we shall see in the next Section.
11.7 Alternative Cosmological Models
The conceptual difficulties with Dark Energy that we have outlined in the
last Section have spawned a vast literature describing attempts to find more
intuitive or better founded solutions to the problem. It is not within the scope
of this book to cover these endeavours in detail. However, some indication of
what is involved is appropriate.
Many theories begin by introducing a field, or fluid, whose properties are
tailored in an ad hoc fashion to reproduce the observed properties of the
universe. The names used for these quantities are sometimes whimsical, such
as “ghost energy” or “quintessence”. The latter is a reference to Aristotle’s
“fifth essence”, an ether (aka aether) that was supposed to pervade the skies.
A common approach in physics is to draw an analogy between one area
of physics and another, in the hope that the first will throw light on the
second. For instance, in nuclear physics it is of use in some instances to
compare the nucleus with a drop of liquid, and assign to it a “density” and
“surface tension”. In cosmology, dark energy is sometimes treated as arising
from the deformation of a four dimensional elastic continuum. This model
is the Strained State Cosmology. For an elastic material in three dimensional
space, the strain energy is related to the resistance of the material against an
imposed deformation. The accumulation of strain energy hinders the deformation and forces the material to return to the unstrained state. In a four
dimensional analogue, if we regard space–time as not just a mathematical artifice, but an entity with physical properties of its own, a strain energy density
can be associated with the presence of curvature, and drives an accelerated
expansion towards an asymptotic unstrained and completely flat state. It is
the analogue of a twisted piece of elastic material returning to its flat state.
This theory succeeds in explaining the observed accelerated expansion and
also the exponential expansion close to the origin.
Another class of theories involves modifications to Einstein’s General Relativity, or even its replacement. However, at this moment, the CDM model