19.4 The Physical Nature of Dark Energy
291
History provides an analogy. In the sixteenth century Johannes Kepler attempted
to calculate the relative sizes of the orbits of the planets according to ideas popular at
the time, that is in terms of Euclidean geometry and Platonic solids. It only became
clear many years later that his attempt was doomed since we now know there are
many stellar planetary systems in which the relative sizes of the planetary orbits are
different than in our solar system; the orbital ratios are accidents of initial conditions
and complex dynamics. Some theorists have used such historical facts in support of
the multiverse idea, that there are many universes in which the cosmological constant
can have diverse values; the value is an accident of initial conditions (Susskind 2005).
We have focused on the cosmological constant numerical value in the above
paragraphs, but it is clear that the idea of a multiverse is also relevant to the values of
the other fundamental constants in our universe, in particular the parameters of the
standard model of particle physics, such as particle charges and masses and mixing
parameters: these might all be accidents and not calculable.
The idea of a multiverse has produced much controversy and objections, the
deepest of which is the question of whether it can make testable predictions about
the one universe we actually observe, and thus whether it is in any way relevant to
science. There is now a large literature and diverse opinions on the subject (Vaas
2010).
19.5 The Physical Nature of Dark Matter
At present all the relevant observations of dark matter concern its gravitational effects.
The physical nature of dark matter is unknown. The theory and observational search
for dark matter are one of the most active research areas in physics. Of the many
theoretical guesses as to its nature we will only mention a few of the most popular
or interesting (Randall 2018).
Large macroscopic bodies such as burnt out stars and stellar mass black holes
were some of the first things considered as dark matter candidates. They have been
searched for and not found. There is a further problem with any dark matter candidate
composed of ordinary baryons, that nucleosynthesis theory and observation suggest
that there cannot be large numbers of such objects (Misner 1973; Drees 2018).
Particles of various kinds that have not yet been seen in the lab are natural candidates for dark matter. One of the most popular types is the so-called weakly interacting
massive particle, the WIMP; such particles are particularly attractive to some theorists since they are a natural part of supersymmetric (SUSY) quantum field theory.
However for decades there have been many active searches for WIMPS in the laboratory, all with negative results. Moreover there is as yet no experimental evidence for
SUSY. The possibility remains that WIMPS with large enough mass to have eluded
detection could be the dark matter, and the search continues (Drees 2018).
For some general relativity theorists there is a dark matter candidate that is particularly interesting. As we discussed previously black holes of sufficiently small mass
should emit Hawking radiation and grow yet smaller and thus radiate faster. It can
291
History provides an analogy. In the sixteenth century Johannes Kepler attempted
to calculate the relative sizes of the orbits of the planets according to ideas popular at
the time, that is in terms of Euclidean geometry and Platonic solids. It only became
clear many years later that his attempt was doomed since we now know there are
many stellar planetary systems in which the relative sizes of the planetary orbits are
different than in our solar system; the orbital ratios are accidents of initial conditions
and complex dynamics. Some theorists have used such historical facts in support of
the multiverse idea, that there are many universes in which the cosmological constant
can have diverse values; the value is an accident of initial conditions (Susskind 2005).
We have focused on the cosmological constant numerical value in the above
paragraphs, but it is clear that the idea of a multiverse is also relevant to the values of
the other fundamental constants in our universe, in particular the parameters of the
standard model of particle physics, such as particle charges and masses and mixing
parameters: these might all be accidents and not calculable.
The idea of a multiverse has produced much controversy and objections, the
deepest of which is the question of whether it can make testable predictions about
the one universe we actually observe, and thus whether it is in any way relevant to
science. There is now a large literature and diverse opinions on the subject (Vaas
2010).
19.5 The Physical Nature of Dark Matter
At present all the relevant observations of dark matter concern its gravitational effects.
The physical nature of dark matter is unknown. The theory and observational search
for dark matter are one of the most active research areas in physics. Of the many
theoretical guesses as to its nature we will only mention a few of the most popular
or interesting (Randall 2018).
Large macroscopic bodies such as burnt out stars and stellar mass black holes
were some of the first things considered as dark matter candidates. They have been
searched for and not found. There is a further problem with any dark matter candidate
composed of ordinary baryons, that nucleosynthesis theory and observation suggest
that there cannot be large numbers of such objects (Misner 1973; Drees 2018).
Particles of various kinds that have not yet been seen in the lab are natural candidates for dark matter. One of the most popular types is the so-called weakly interacting
massive particle, the WIMP; such particles are particularly attractive to some theorists since they are a natural part of supersymmetric (SUSY) quantum field theory.
However for decades there have been many active searches for WIMPS in the laboratory, all with negative results. Moreover there is as yet no experimental evidence for
SUSY. The possibility remains that WIMPS with large enough mass to have eluded
detection could be the dark matter, and the search continues (Drees 2018).
For some general relativity theorists there is a dark matter candidate that is particularly interesting. As we discussed previously black holes of sufficiently small mass
should emit Hawking radiation and grow yet smaller and thus radiate faster. It can