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R. Barrett and P. P. Delsanto
Let us turn now to the remaining CDM part of the CDM model. The
letters CDM stand for Cold Dark Matter. We have seen in an earlier Section
of this Chapter that dark matter was introduced to explain the observed
effects of gravity in large-scale clusters of matter. The various hypotheses
proposed by theorists about the nature of dark matter may be grouped into
three categories: cold, warm and hot dark matter. The three categories are
related to the free-streaming length 1 of the particles (or to their speed):
cold means slow, i.e. non-relativistic; warm means weakly relativistic and hot
means relativistic. None of the three types is expected to interact strongly
with ordinary matter, but each has a different influence on the size of the
structures (galaxies, galaxy clusters, and superclusters) that are observed in
the universe. Of the three types, Cold Dark Matter provides the best description of the distribution and size of observed structures, and as a consequence
it has been included in the Concordance Model.
Below, we summarise the evolution of the universe described by the
Concordance Model, making use of a graphic provided by NASA (see
Fig. 11.4), where space–time appears as a bell-shaped hyper-surface.
In Fig. 11.4, the three dimensional surface is depicted as a two dimensional
expanding envelope. This is an example of the usual compromise forced on
mapmakers when representing a three dimensional surface (e.g. the earth) on
a plane sheet of paper. The third dimension of Fig. 11.4 is time. The universe
comes into existence on the left with the sudden appearance of the Big Bang
singularity. Immediately, the limitations of such a depiction become apparent
when one asks what occurs in the region shown to the left of the singularity.
The question cannot be answered. This region of the figure has no meaning
because time (as well as space) is created at the instant of the singularity.
Following the Big Bang, and close to it, events are dominated by the
interaction of strong gravity with quantum fluctuations. As we have indicated earlier, the laws of physics in this region are not well understood. Then
follows an inflationary exponential expansion, and in the minutest fraction of
a second, the universe explodes in scale from the microscopic to the cosmic.
With the burgeoning expansion, the temperature of the content of the
universe decreases, and the synthesis of nuclear particles begins. The nuclear
plasma continues to cool and the expansion rate of space slows down until the
Recombination Era is reached at a cosmic age of 380,000 years, and neutral
atoms are formed.
1 The free-streaming length is the average distance a particle can travel without interacting with matter
scattered along the way. The weaker the coupling with ordinary matter, the longer the free-streaming
length.
R. Barrett and P. P. Delsanto
Let us turn now to the remaining CDM part of the CDM model. The
letters CDM stand for Cold Dark Matter. We have seen in an earlier Section
of this Chapter that dark matter was introduced to explain the observed
effects of gravity in large-scale clusters of matter. The various hypotheses
proposed by theorists about the nature of dark matter may be grouped into
three categories: cold, warm and hot dark matter. The three categories are
related to the free-streaming length 1 of the particles (or to their speed):
cold means slow, i.e. non-relativistic; warm means weakly relativistic and hot
means relativistic. None of the three types is expected to interact strongly
with ordinary matter, but each has a different influence on the size of the
structures (galaxies, galaxy clusters, and superclusters) that are observed in
the universe. Of the three types, Cold Dark Matter provides the best description of the distribution and size of observed structures, and as a consequence
it has been included in the Concordance Model.
Below, we summarise the evolution of the universe described by the
Concordance Model, making use of a graphic provided by NASA (see
Fig. 11.4), where space–time appears as a bell-shaped hyper-surface.
In Fig. 11.4, the three dimensional surface is depicted as a two dimensional
expanding envelope. This is an example of the usual compromise forced on
mapmakers when representing a three dimensional surface (e.g. the earth) on
a plane sheet of paper. The third dimension of Fig. 11.4 is time. The universe
comes into existence on the left with the sudden appearance of the Big Bang
singularity. Immediately, the limitations of such a depiction become apparent
when one asks what occurs in the region shown to the left of the singularity.
The question cannot be answered. This region of the figure has no meaning
because time (as well as space) is created at the instant of the singularity.
Following the Big Bang, and close to it, events are dominated by the
interaction of strong gravity with quantum fluctuations. As we have indicated earlier, the laws of physics in this region are not well understood. Then
follows an inflationary exponential expansion, and in the minutest fraction of
a second, the universe explodes in scale from the microscopic to the cosmic.
With the burgeoning expansion, the temperature of the content of the
universe decreases, and the synthesis of nuclear particles begins. The nuclear
plasma continues to cool and the expansion rate of space slows down until the
Recombination Era is reached at a cosmic age of 380,000 years, and neutral
atoms are formed.
1 The free-streaming length is the average distance a particle can travel without interacting with matter
scattered along the way. The weaker the coupling with ordinary matter, the longer the free-streaming
length.