This lab will help you understand how helium is formed in the early universe, out of a "primordial soup" of neutrons and protons. In the very early universe when the temperature was very high, there were equal numbers of protons and neutrons. So, if helium (two protons plus two neutrons) was formed then, all of the mass in the universe would be in helium. However, only about 25% of the mass of the universe is helium, with the rest being hydrogen (protons). This is for two reasons. First, neutrons are slightly heavier than protons, so as the universe "cooled" with time, thermal equilibrium reactions left us with more protons than neutrons. Secondly, neutrons spontaneously decay to protons, further depleting their numbers. Both of these things happen until the universe is "cool" enough so that the "deuterium bottleneck" breaks and helium can form. The "bottleneck" comes about because of thermal photons with high enough energy which break up deuterium (one proton and one neutron) as soon as it is formed. Deuterium formation is a necessary intermediate step to form helium.

We will use the Particle Soup: Nucleosynthesis exercise from the University of Washington Astronomy Clearing House. The exercise includes a worksheet which you are to fill out. At one point, you are asked to let neutrons "decay" by flipping coins, but you can also do this by generating some random numbers on your computer (using Matlab or a similar program, for example) and counting the values above or below the middle of the range.

You are free to work in pairs. If you do so, please be sure to put both your names on the worksheet.