I got a question about my last post. How can you calculate the number of atoms in the universe ? Looking around a bit I found two different ways of calulating (or estimate) it on wikipedia.
1. The horizon size of our universe is about 14 thousand million light years. If we neglect space curvature effects, the volume of visible space represents 4/3 π R3 = 8.8 × 1083 cubic centimeters. The critical density of the universe for this value of the Hubble constant is 3 H2/8 π G, which works out to be 1×10−29 grams/cubic centimeter or about 5×10−6 atoms of hydrogen/cc. It is believed that only 4 percent of the critical density is in the form of normal atoms, so this leaves 5×10−6 × 4×10−2 = 2×10−7 hydrogen atoms/cc. Multiplying this by the volume of the visible universe, you get about 1.7 × 1077 hydrogen atoms.
2. A typical star weighs about 2×1033 grams, which is about 1×1057 atoms of hydrogen per star. A typical galaxy has about 400 thousand million stars so that means each galaxy has 1×1057 × 4×1011 = 4×1068 hydrogen atoms. There are possibly 80 thousand million galaxies in the Universe, so that means that there are about 4×1068 × 8×1010 = 3×1079 hydrogen atoms in the Universe. But this is definitely a lower limit calculation, and ignores many possible atom sources.
So the first approach makes use of what we estimate the size to be and what we estimate the density of the universe to be, while the second makes use of the numbers of stars we estimate to exist and how many atoms a star consists of. Nothing is exact of course and both calculations are probably too low, but it gives you an idea about the numbers of atoms. Or does it ? Because there is no way anyone can really comprehend how large these numbers really are.