We live in a time of great observational advances in cosmology, which have given us a consistent picture of the matter and energy content of our Universe. Here matter and energy (which special relativity tells us are equivalent) are distinguished by their different dependence on the cosmic volume: matter density decreases with the inverse of the volume, while energy density remains (approximately) constant.
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.
A summary of the current measurements of the matter density m and the energy density are shown in Figure 1 (adapted from Verde et al., 2002). Both are in units of the critical density crit = 3H02 / (8G), where G is the Newton's gravitational constant, and H0 is the present value of the Hubble constant. Three types of observations - supernova measurements of the recent expansion history of the Universe, cosmic microwave background measurements of the degree of spatial flatness, and measurements of the amount of matter in galaxy structures obtained through big galaxy redshift surveys - agree with each other in a region around the best current values of the matter and energy densities m 0.27 and 0.73 (cross in Figure 1). Measurements of the baryon density in the Universe using the cosmic microwave background spectrum and primordial nucleosynthesis constrain the baryon density b to a value less than ~ 0.05 (black vertical band in Figure 1). The difference m - b 0.22 must be in the form of non-baryonic dark matter. 1
Figure 1. The concordance cosmology and the need for non-baryonic dark matter. Current cosmological measurements of the matter density m and energy density give the value marked with a cross at m 0.27, 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 precise determination of the cosmological density parameters is able to give the matter and energy densities in physical units. For example, in units of 1.879 × 10-29 g/cm3 = 18.79 yg/m3, Spergel et al. (2003) have determined a total matter density
is in the form of neutrinos (to 95% confidence level),
is in the form of baryons (protons and nucleons in cosmological parlance), and
is in the form of cold dark matter (CDM), a non-baryonic component whose nature we are still trying to uncover. Some ideas of what it may be are presented in the next Section.
1 The red vertical band labeled `stars' in Figure 1 shows the density of luminous matter, corrected for all expected dim stars and gas (see, e.g., Fukugita, Hogan, & Peebles, 1998). The difference between the amount of luminous matter and the amount of baryons constitutes the dark baryon problem, which will not be addressed here (see, e.g., Silk, 2003). Back.