Non-Baryonic Matter

It is rather clear that if one believes in Big Bang Nucleosynthesis then there must be nonbaryonic material in the Universe. Whether it is 90 % or 97 % of the Universe is debatable, however, it is clear that it must be the majority of the Universe. The question is What is this stuff? I won't dwell on the detailed possbilities except for mentioning some particle candidates for the dark matter, in particular, neutrinos. However, I do want to make some comments about the general dynamical properties of the stuff. The distinctions have to do when the material de-coupled from the Universe.

In the early Universe,

before t = t(equipartition) ~ t(recombination), the Universe was radiation dominated and its dynamics were governed by a vast sea of thermal (but relativistic) particles. Depending upon the masses of hypothesized particles (because E = mc**2), different particles are pair produced during the various and sundry epochs and the Universe is dominated by different types of particles at different times. For example, protons and neutrons are pair-produced for T larger than ~ 10 trillion Kelvin.

Different theories for particle physics predict different sorts of particles and it is of interest from this point of view to pin from data what kinds of particles are consistent with our Univerese. Again, this may be an interesting way to probe physics on tiny scales by using observations of the largest known system, the Universe!

THERMAL RELICS OF THE EARLY UNIVERSE

If the particles in the early Universe were in initially in thermal equilibrium (e.g., p + anti-p <===> photon + photon, ... ) so that that there was a vast sea of particles and radiation all strongly coupled together, then we can consider a few important times:

• t < t(pairs), T higher than ~ 2 mc**2 ---> pair production and number of particles and photons are roughly the same.

• t > t(pairs), T < 2 mc**2 ---> pair production stops and particle density can go down as long as t(annihilation) < t(Hubble) = t(expansion)

  Particles can still maintain thermal equilibrium as long as other interactions coupling matter and radiation continue on sufficiently quickly.

• When the interactions become greater than t(expansion), the particles are said to freeze-out, like the neutron-to-proton ratio froze out in the nucleosynthesis process.
  • If the particles freeze-out when they are moving speeds close to the speed of light, c, they are said to be relativistic and are referred to as hot dark matter. In this case, the particles usually have abundances comparable to the photons.
  • If the particles freeze-out when they are moving at speeds much less than the speed of light, c, they are non-relativistic and are referred to as cold dark matter. In this case, because the annihilations must continue (in general) well beyond the pair production threshold in order for the particles to become non-relativistic, the particles are much less abundant than photons.

HOT vs. COLD DARK MATTER

Whether the dark matter is cold or hot is important for how structures form in the Universe. This is easy to understand simply from momentum arguments:

Force Law ===> dp/dt = -GMm/r**2

To cause a particle with large momentum (large speed) to change its motion, e.g., to clump and form a galaxy or some other structure, one needs to apply a large force. This means that a large amount of mass, M, is needed or that it is hard to form small structures.

To cuase a particle with small momentum to clump requires a much smaller force and thus a smaller clump. This means that it is easier to form small structures.