214 Dark Matter
The shocking conclusion is that the predominant form of matter in the Universe
is nonbaryonic, and we do not even know what it is composed of! Thus we are our-
selves made of some minor pollutant, a discovery which may well be called the fourth
breakdown of the anthropocentric view. The first three were already accounted for in
Chapter 1.
Black Holes. Primordial black holes could be good candidates because they evade
the nucleosynthesis bound, they are not luminous, they (almost) do not radiate, and if
they are big enough they have a long lifetime, as we saw in Equation (5.82). They are
believed to sit at the center of every galaxy and have masses exceeding 100푀⊙.The
mass range 0.3–30푀⊙is excluded by the nonobservation of MACHOs in the galactic
halo (cf. Section 5.4). Various astrophysical considerations limit their mass to around
104 푀⊙. But black holes cannot be the solution to the galactic rotation curves nor to
merging clusters.
Exotica. At higher scales extra dimensions could exist. the usual(3+1) spacetime
could be abraneembedded in a higher dimensionalbulk. Standard model fields would
be confined on the brane while gravity propagates in the extra dimension. In this kind
of models the hierarchy problem is solved in various ways for example the higher
dimensions are compactified on different topologies of scale푅with the effect of low-
ering the Planck scale energy closer to the electroweak scale. Compactification of extra
dimensions gives rise to a quantization of momentum,푝^2 ≃ 1 ∕푅^2 of the fields prop-
agating in the bulk, and the apparition of a set of Fourier expanded modes [Kaluza
Klein (KK) states], for each bulk field. Particles moving in extra dimensions appear
as heavy particles with masses푚푛∕푅. The new states have the same quantum num-
bers (e.g., charge, color, etc). Another way to reach the same goal is to introduce extra
dimensions with large curvature.
If extra dimensions are compactified around a circle or torus, the extra-dimensional
momentum conservation implies conservation of the KK number푛, and the lightest
first level KK state is stable. Theories with compact extra dimensions can be written
as theories in ordinary four dimensions by performing a KK reduction.
Among further exotica aresolitons, which are nontopological scalar-field quanta
with conserved global charge푄(Q-balls) or baryonic charge퐵(B-balls).
Mirror Matter[18] implies reintroducing. all the known fields with the same cou-
pling constants, but with opposite parities. The most natural way to do so is to
add to the existing Lagrangian its parity-symmetric counterpart, so that the whole
Lagrangian is invariant under the parity transformation, each part transforming
into the other. We then end up with a new sector of particles, called mirror sector,
which is an exact duplicate of the ordinary sector, but where ordinary particles have
left-handed interactions, mirror particles have right-handed interactions.
As a consequence, the three gauge interactions act separately in each sector, the
only link between them being gravity. Because mirror baryons, just like their ordi-
nary counterparts, are stable and can be felt only through their gravitational effects,
the mirror matter scenario provides an ideal interpretation of dark matter. Its partic-
ularity is that it is a self-interacting candidate, but without any new parameter at the