24 Dark Universe
183
problem, whereas the axion particle was postulated to solve the so-called strong
CP problem of the Quantum Chromodynamic theory of strong interactions.
The strong CP problem uses a symmetry, first invented by Roberto Peccei and
Helen Quinn. Then there are particles from models with extra dimensions,
which also qualify as dark matter. The dark matter particles are the lightest
particles of the corresponding theories. The particles heavier than the lightest
particle in these theories will eventually decay to the lightest particle and
cannot therefore be dark matter. The masses of the dark matter particle
are often not given by the theory and they can have masses ranging from
thousands of times the proton mass to an extremely tiny mass which is 10
−32
times the proton mass, depending on the theory. Most of these particles have
properties that make them observable in experiments using familiar matter. So
far however, they all have evaded detection.
This lack of dark matter signal in currently running experiments has encouraged speculation that, possibly, dark matter has no or very little interaction
with matter, as would be the case if dark matter came from a mirror sector
of the universe. In most versions of the mirror universe, dark matter typically
has very little to no interaction with known matter. This will be discussed in
a subsequent subsection.
There is also the strong possibility that dark matter interacts with itself.
This property is not shared by many of the above dark matter candidates [97].
One reason to suspect self-interaction is that the density profile of dark matter
in galaxies such as ours has been calculated using computer models for the
evolution of the universe [79]. It is found that if the dark matter has no selfinteraction, the density will peak sharply at the center of a galaxy, unlike what
is observed. What is observed is that the density of matter flattens out at the
center. If the dark matter particles interact with themselves, they will push each
other away due to self-interaction if they get too close. That way, the galactic
center will not be sharply peaked in density (or cuspy) [95].
The possibility of self-interaction and lack of signal in underground searches
together make a strong case for a model based on the mirror universe idea,
with the lightest mirror baryon being the dark matter candidate. It is not only
invisible to our light, but it also has self-interaction arising from the mirror
strong force to satisfy other properties. As we will see below, such candidates
will have very feeble to no non-gravitational interaction with familiar matter,
so they cannot be detected using normal detectors.
An interesting possibility arises if the dark matter has only strong interaction
but no electromagnetic interaction. In that case, the dark particles will form
large globs floating in the sky and depending on the strength of the dark
183
problem, whereas the axion particle was postulated to solve the so-called strong
CP problem of the Quantum Chromodynamic theory of strong interactions.
The strong CP problem uses a symmetry, first invented by Roberto Peccei and
Helen Quinn. Then there are particles from models with extra dimensions,
which also qualify as dark matter. The dark matter particles are the lightest
particles of the corresponding theories. The particles heavier than the lightest
particle in these theories will eventually decay to the lightest particle and
cannot therefore be dark matter. The masses of the dark matter particle
are often not given by the theory and they can have masses ranging from
thousands of times the proton mass to an extremely tiny mass which is 10
−32
times the proton mass, depending on the theory. Most of these particles have
properties that make them observable in experiments using familiar matter. So
far however, they all have evaded detection.
This lack of dark matter signal in currently running experiments has encouraged speculation that, possibly, dark matter has no or very little interaction
with matter, as would be the case if dark matter came from a mirror sector
of the universe. In most versions of the mirror universe, dark matter typically
has very little to no interaction with known matter. This will be discussed in
a subsequent subsection.
There is also the strong possibility that dark matter interacts with itself.
This property is not shared by many of the above dark matter candidates [97].
One reason to suspect self-interaction is that the density profile of dark matter
in galaxies such as ours has been calculated using computer models for the
evolution of the universe [79]. It is found that if the dark matter has no selfinteraction, the density will peak sharply at the center of a galaxy, unlike what
is observed. What is observed is that the density of matter flattens out at the
center. If the dark matter particles interact with themselves, they will push each
other away due to self-interaction if they get too close. That way, the galactic
center will not be sharply peaked in density (or cuspy) [95].
The possibility of self-interaction and lack of signal in underground searches
together make a strong case for a model based on the mirror universe idea,
with the lightest mirror baryon being the dark matter candidate. It is not only
invisible to our light, but it also has self-interaction arising from the mirror
strong force to satisfy other properties. As we will see below, such candidates
will have very feeble to no non-gravitational interaction with familiar matter,
so they cannot be detected using normal detectors.
An interesting possibility arises if the dark matter has only strong interaction
but no electromagnetic interaction. In that case, the dark particles will form
large globs floating in the sky and depending on the strength of the dark