Luminosity Density
	• So what are observed mass-to-light ratios? – Much lower than critical, and now MAP seems to be confirming baryons 4% of critical, consistent with Big Bang Nucleosynthesis (yes, we will get to this stuff!). Dark matter helps, but only up to about 30%. • Handy numbers – Mean galaxy luminosity is 1.25 L* = 1.55x1010 h-2 Lsun – Typical number density of 0.01 h-3 Mpc-3 indicates a typical galaxy separation of 5 h-1 Mpc, but of course on those scales galaxies do cluster (Ch. 2)