Non-baryonic

The other kind of dark matter must be non-baryonic (NDM) and is thought to consist of some kind of particles envisaged in extensions of the Standard Model ... 

Fig. 4.1. Schematic thermal history of the Universe showing some of the major episodes envisaged in the standard model. GUTs is short for grand unification theories and MWB is short for (the last scattering of) the microwave background radiation. The Universe is dominated by radiation and relativistic particles up to a time a little before that of MWB and by matter (including non-baryonic matter) thereafter, with dark energy eventually taking over.

The existence of dark matter (either baryonic or non-baryonic) is inferred from its gravitational effects on galactic rotation curves, the velocity dispersions and hydrostatic equilibrium of hot (X-ray) gas in clusters and groups of galaxies, gravitational lensing and departures from the smooth Hubble flow described by Eq. (4.1). This dark matter resides at least partly in the halos of galaxies such as our... 

In the standard case there are four variables to be calculated the total system mass M (not counting non-baryonic dark matter), the mass of gas g, the mass existing in the form of stars (including compact remnants) s and the abundance Z of the element(s) of interest, assuming certain initial conditions and laws governing the SFR and flows of material into and out of the system. 

The fact that the Sun is in the plasma state means that it has the flexibility of a gas. This flexibility in turn ensures its longevity. Indeed, the size of the particles making it up, i.e. separate nuclei and electrons, is much less than the mean distance separating them, and this allows us to identify it with a gas. (Non-baryonic dark matter, with no electrical properties, cannot be ionised. There is no such thing as a dark matter plasma.)... 

Only those scraps of matter more concentrated than the medium in which they have accumulated can foretell the emergence of cosmic forms. And these embryonic galaxies can only be random concentrations of the undifferentiated magma comprising photons, baryons and non-baryonic particles. 

Basic models of massive non-baryonic matter (of neutralino type) suggest a hierarchical formation of structures. They work on the hypothesis that... 

This method is based on the measurement of the baryonic fraction in clusters, consisting mainly of the hot gas seen in X-rays. The X-ray image of a cluster allows one to measure the mass of this X-ray gas. The knowledge of the X-ray temperature allows one to estimate the total mass Mit. It possible therefore to estimate the baryon fraction in clusters (the contribution of stars, around 1% for h = 0.5 is often neglected to first order) assuming that the remaining dark matter is non-baryonic, which can be related to Do ... 

Already in 1970s suggestions have been made that some sort of non-baryonic elementary particles may serve as candidates for dark matter particles. Gunn et al. (1978) considered heavy stable neutral leptons as possible candidates for dark matter particles, however in a later study Tremaine Gunn (1979)... 

The new paradigm wins when its theoretical foundation is established. In the case of the dark matter this was done by Blumenthal et al. (1984) with the non-baryonic cold dark matter hypothesis. Also the need for non-baryonic dark matter was clarified otherwise the main constituents of the universe -galaxies, clusters and filamentary superclusters - cannot form. 

To conclude we can say that the story of dark matter is not over yet - we still do not know of what non-baryonic particles the dark matter is made of. 

Nothing is known about the nature of the energy component, which goes under the name of dark energy. Of the matter component, less than 2% is luminous, and no more than 20% is made of ordinary matter like protons, neutrons, and electrons. The rest of the matter component, more than 80% of the matter, is of an unknown form which we call non-baryonic. Finding the nature of non-baryonic matter is referred to as the non-baryonic dark matter problem. 

Figure 16.1. The concordance cosmology and the need for non-baryonic dark matter. Current cosmological measurements of the matter density fim and energy density Qa give the value marked with a cross at fim 0.27, Qa — 0.73. The baryon density does not exceed 0.05 (black vertical band). The rest of the matter is non-baryonic. (Figure adapted from Verde et al.(2002).)...

A new kind of elementary particle has been the dominant (exclusive ) candidate for non-baryonic dark matter. 

The three active neutrinos are our only known particle candidates for non-baryonic dark matter. Since they fail to be cold dark matter, we are lead to consider hypothetical particles. 

We need another non-baryonic candidate for cold dark matter. 

In the Type II category we put all hypothetical cold dark matter candidates that are neither Type la nor Type lb. Some of these candidates have been proposed for no other reason than to solve the dark matter problem. Others are examples of beautiful ideas and clever mechanisms that can provide good possibilities for non-baryonic dark matter, but in some way or another lack the completeness of the theoretical particle physics models of Types la and lb. Although Type II candidates are not studied as deeply as others, it may well be that eventually the question of the nature of cold dark matter might find its answer among them. 

The second part of these lectures is an introduction to several methods to detect non-baryonic dark matter. We will use the lightest neutralino as our guinea pig, because of the variety of techniques that can be employed to detect it, but the discussion is more general and can be applied to a generic WIMP. Thus in this second part we assume that non-baryonic dark matter is made of WIMPs (in particular, neutralinos), and we examine several observational ways to test our assumption. 

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