The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 3.

Brief Survey of Dark Matter Candidates

There is no shortage of ideas as to what the dark matter could be. In

fact, the problem is the opposite. Serious candidates have been

proposed with masses ranging from eV = kg

(axions) up to (black holes). That's a range

of masses of over 75 orders of magnitude! It should be clear that no

one search technique could be used for all dark matter candidates.

Even finding a consistent categorization scheme is difficult, so we

will try a few. First, as discussed above, is the baryonic vs

non-baryonic distinction. The main baryonic candidates are the

Massive Compact Halo Object (Macho) class of candidates. These

include brown dwarf stars, jupiters, and 100 black holes. Brown

dwarfs are spheres of H and He with masses below 0.08 , so they

never begin nuclear fusion of hydrogen. Jupiters are similar but with

masses near 0.001 . Black holes with masses near 100 could

be the remnants of an early generation of stars which were massive

enough so that not many heavy elements were dispersed when they

underwent their supernova explosions. Other, less popular, baryonic

possibilities include fractal or specially placed clouds of molecular

hydrogen [16]. The non-baryonic candidates are basically

elementary particles which are either not yet discovered or have

non-standard properties.

Outside the baryonic/non-baryonic categories are two other

possibilities which don't get much attention, but which I think

should be kept in mind until the nature of the dark matter is

discovered. The first is non-Newtonian gravity. See Begeman et al.

[17] for a provocative discussion of this possibility, but watch for

results from gravitational lensing which may place very strong

constraints. Second, we shouldn't ignore the ``none-of-the-above"

possibility which has surprised the Physics/Astronomy community

several times in the past.

Among the non-baryonic candidates there are several classes of

particles which are distinguished by how they came to exist in large

quantity during the Early Universe, and also how they are most

easily detected. The axion (Section 5) is mentioned as a possible

solution to the strong CP problem and is in a class by itself. The

largest class is the Weakly Interacting Massive Particle (Wimp)

class (Sections 4 and 6), which consists of literally hundreds of

suggested particles. The most popular of these Wimps is the

neutralino from supersymmetry (Section 6). Finally, if the tau

and/or muon neutrinos had a mass in the 2 eV to 100 eV range, they

could make up all or a portion of the dark matter. This possibility

will be discussed by Masiero and also Klypin [68, 69].

Another important categorization scheme is the ``hot" vs ``cold"

classification. A dark matter candidate is called ``hot" if it was

moving at relativistic speeds at the time when galaxies could just

start to form (when the horizon first contained about ). It is

called ``cold" if it was moving non-relativistically at that time. This

categorization has important ramifications for structure formation,

and there is a chance of determining whether the dark matter is hot

or cold from studies of galaxy formation. Hot dark matter cannot

cluster on galaxy scales until it has cooled to non-relativistic speeds,

and so gives rise to a considerably different primordial fluctuation

spectrum [69]. Of the above candidates only the light neutrinos

would be hot; all the others would be cold.

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