14.3 Observed Dark Matter and Dark Energy Densities
227
although this correspondence should not be taken to be definitive since it implies an
infinite total mass for the halo. See Exercise 14.5.
The existence of dark matter was first suggested by Zwicky in the 1930s; Zwicky
studied clusters of galaxies and from their random velocities estimated their mass.
See Exercise 14.6 (Zwicky 1933; Wiki DM).
The physical nature of the dark matter is not evident from observation. It could be
almost anything that does not interact with light. For example, some of the dark matter
could be small nonluminous stars called brown dwarfs, or black holes, or substellar
lumps of matter, or interstellar gas and dust etc. More exotic possibilities are heavy
elementary particles not yet seen in the laboratory such as supersymmetric particles,
nonzero mass neutrinos, speculative light elementary particles called axions etc. The
field is open to speculation (Schutz 2009; Randall 2018).
For the purpose of cosmology one important characteristic of the unseen material
is the ratio of pressure to energy density, what we have called w. For ordinary
matter and heavy elementary particles the ratio is very small, whereas for very light
elementary particles it is about 1/3, characteristic of a hot gas. For light the ratio is
exactly 1/3. The first case is referred to as cold dark matter, and the latter case is
referred to as hot dark matter. The currently prevalent opinion is that the dark matter
is probably cold.
The search for the physical nature of dark matter using laboratory detectors has
been long and intense and unsuccessful, and is still a very active field. At present we
have only the evidence of astronomical observations (Randall 2018, Wiki DM).
Concerning the nature and density of dark energy we will say more about this in
Chap. 15, but we note here that it is now generally believed to be the cosmological
constant and constitutes a large fraction of the total density in the universe, as we
will discuss below.
Concerning the magnitude of the various densities we note at this point that the
favored values, consistent with present observations are that the dark energy density
is about 70% of the critical density, the dark matter is about 25% of critical, visible
matter is only about 5% and the total density is equal to the critical density; thus
the universe looks to be spatially flat with k = 0. In terms of the present fractional
densities V = 0.70, , = CDM = 0.25, vis = 0.05. Remarkably it thus appears
that the dominant constituents in the cosmic fluid, dark matter and dark energy, are
not directly visible and the fundamental nature of the dark matter is not understood.
We have a reasonable understanding of only about 5% of the stuff of our universe.
This could be taken as a demand for modesty concerning our success in our overall
understanding of nature.
14.4 Evolution of Cosmic Fluid Constituents
This section will deal with the behavior of the constituents of the cosmic fluid, such
as cold matter and radiation, during the evolution of the universe. We will first show
how energy is conserved in the expansion of the universe. Then we will obtain the
227
although this correspondence should not be taken to be definitive since it implies an
infinite total mass for the halo. See Exercise 14.5.
The existence of dark matter was first suggested by Zwicky in the 1930s; Zwicky
studied clusters of galaxies and from their random velocities estimated their mass.
See Exercise 14.6 (Zwicky 1933; Wiki DM).
The physical nature of the dark matter is not evident from observation. It could be
almost anything that does not interact with light. For example, some of the dark matter
could be small nonluminous stars called brown dwarfs, or black holes, or substellar
lumps of matter, or interstellar gas and dust etc. More exotic possibilities are heavy
elementary particles not yet seen in the laboratory such as supersymmetric particles,
nonzero mass neutrinos, speculative light elementary particles called axions etc. The
field is open to speculation (Schutz 2009; Randall 2018).
For the purpose of cosmology one important characteristic of the unseen material
is the ratio of pressure to energy density, what we have called w. For ordinary
matter and heavy elementary particles the ratio is very small, whereas for very light
elementary particles it is about 1/3, characteristic of a hot gas. For light the ratio is
exactly 1/3. The first case is referred to as cold dark matter, and the latter case is
referred to as hot dark matter. The currently prevalent opinion is that the dark matter
is probably cold.
The search for the physical nature of dark matter using laboratory detectors has
been long and intense and unsuccessful, and is still a very active field. At present we
have only the evidence of astronomical observations (Randall 2018, Wiki DM).
Concerning the nature and density of dark energy we will say more about this in
Chap. 15, but we note here that it is now generally believed to be the cosmological
constant and constitutes a large fraction of the total density in the universe, as we
will discuss below.
Concerning the magnitude of the various densities we note at this point that the
favored values, consistent with present observations are that the dark energy density
is about 70% of the critical density, the dark matter is about 25% of critical, visible
matter is only about 5% and the total density is equal to the critical density; thus
the universe looks to be spatially flat with k = 0. In terms of the present fractional
densities V = 0.70, , = CDM = 0.25, vis = 0.05. Remarkably it thus appears
that the dominant constituents in the cosmic fluid, dark matter and dark energy, are
not directly visible and the fundamental nature of the dark matter is not understood.
We have a reasonable understanding of only about 5% of the stuff of our universe.
This could be taken as a demand for modesty concerning our success in our overall
understanding of nature.
14.4 Evolution of Cosmic Fluid Constituents
This section will deal with the behavior of the constituents of the cosmic fluid, such
as cold matter and radiation, during the evolution of the universe. We will first show
how energy is conserved in the expansion of the universe. Then we will obtain the