Luminous matter density

Spergel et al. 2007), but greatly exceeds the smoothed-out cosmological density of luminous matter deduced from galaxy redshift surveys, which give a luminosity density in blue light... 

Luminous matter has revealed dark matter, but the new substance remains obscure. What is it made from Is it perhaps composed of known forms of matter Only partly Is dark matter made up of microscopic particles If the answer is affirmative, we may suppose that this unknown form of energy penetrates and permeates the galaxies, the Solar System and even our own bodies, just as neutrinos pass through us every second without affecting us in any way. And like the neutrinos, these unknown particles would hardly interact at all with ordinary matter made from atoms. To absorb its own neutrinos, a star with the same density as the Sun would have to measure a billion solar radii in diameter. Luminous and radiating matter is a mere glimmer to dark matter. 

An inventory of the Universe Critical density = 10 gcm density of luminous matter/critical density = 0.005% density of gravitating matter/critical density = 10-30% density of nuclear matter/critical density = 2-5% total density of the Universe/critical density = 1. 

Note that the density of nuclear matter is greater than the density of luminous matter, but less than the density of gravitationally active matter. Note also that it is very much smaller than the critical density. We may therefore deduce the existence of dark nuclear matter and dark (or even invisible) non-nuclear matter. 

X-ray observations detect the dominant baryonic component in clusters. While clusters were first detected optically, the dominant baryonic component in clusters is the X-ray emitting gas. Despite its low mean density, (10-2 to 10-3 cm-3), the gas mass in a rich cluster exceeds the optically luminous matter in the galaxies by a factor of 3-5. Thus, to see the bulk of the baryons that have temperatures of 10-100 million degrees, X-ray observations provide the only direct approach. 

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 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. 

The density of cerebral capillaries, especially in the cortical grey matter, is very high with mean distances of 40 /xm. The capillary network has a total length of 600-650 km, the mean velocity of the blood flow is below 0.1 cm/s, and the luminal surface extends to 15-30 m2. Thus the blood-brain barrier represents an important surface for potential drug delivery besides gut (30CM100 m2), lung (70-120 m2), or skin (1.8 m2) [24-26, 33-37],... 

Clusters of galaxies are the most massive collapsed systems in the Universe. A typical luminous cluster (e.g., Coma cluster) is filled with a hot, 100 million degree, low density (10-3 cm-3) gas. In addition to the optically luminous galaxies and diffuse X-ray gas, clusters are dominated by dark matter. The X-ray gas, relaxing on the relatively short sound crossing time, tmpc = 6.6 x 108(T/108) yrs(wherer = D/cs and cj. = fP/ p) is an effective tracer of this unseen dark matter. 

By the end of 1970s most objections against the dark matter hypothesis were rejected. In particular, luminous populations of galaxies have found to have lower mass-to-luminosity ratio than expected previously, thus the presence of extra dark matter both in galaxies and clusters has been confirmed. However, the nature of dark matter and its purpose was not yet clear. Also it was not clear how to explain the Big Bang nucleosynthesis constraint on the low density of matter, and the smoothness of the Hubble flow. 

Reconciliation (or perhaps clearer contradictions ) of the details of structures made by x-CDM simulations with those in the real world. The standard problems are generally described as missing satellites (the expectation of more substructure in dark matter halos than we see in the luminous stuff) and core/cusp (the steeper rise in central density of the simulations than we see in centers of galaxies and clusters). There are perhaps some other issues, like the pair-wise velocity dispersion. All occur on length scales where feedback from what the baryons are doing must be important and has not yet been fully included in the calculations. 

Figure 10. The various contributions to the present universal mass/energy density, as a fraction of the critical density (Q), as a function of the Hubble parameter (Ho). The curve labelled Luminous Baryons is an estimate of the upper bound to those baryons seen at present (z ( 1) either in emission or absorption (see the text). The band labelled BBN represents the D-predicted SBBN baryon density. The band labelled by M (Om = 0.3 0.1) is an estimate of the current mass density in nonrelativistic particles ( Dark Matter ).

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