Matter content The observable universe contains about 3 to 100 × 10 22 stars (30 sextillion to a septillion stars),     organized in more than 80 billion galaxies , which themselves form clusters and superclusters .  Two approximate calculations give the number of atoms in the observable universe to be close to 10 80 . 1. Observations of the cosmic microwave background from the Wilkinson Microwave Anisotropy Probe suggest that the spatial curvature of the universe is very close to zero, which in current cosmological models implies that the value of the density parameter must be very close to a certain critical value. A NASA page gives this density, which includes dark energy, dark matter and ordinary matter all lumped together, as 9.9×10 −27 kg/m 3 ,  although the figure has not been updated since 2005  and a number of new estimates of the Hubble parameter have been made since then. The present value of the Hubble parameter H0 is important because it is related
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This note was uploaded on 12/15/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.