probset7a - COSMOLOGY Problem Set 7 May 10, 2010 Problem 1...

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Unformatted text preview: COSMOLOGY Problem Set 7 May 10, 2010 Problem 1 : Another Sterile Neutrino Suppose there is a new, very weakly interacting, sterile neutrino s which does not mix with the usual (active) neutrinos. The s are light but not massless (0 negationslash = m s 1 MeV). a) If these sterile neutrinos decouple at a temperature of 10 GeV, find their contribu- tion to N s / , the effective number of equivalent neutrinos at T 1 MeV. [10] b) Assume that these s are non-relativistic at present. In terms of their mass m s , find the present value of their density parameter s . (You may adopt H = 70 km/s/Mpc and T = 2 . 725 K). If observations require that the upper bound to their present density is s . 23, find the upper bound to the s mass. [10] Problem 2 : Early Universe Neutron Proton Ratio In the early Universe the charged-current weak interactions keep neutrons and protons in equilibrium (chemical equilibrium) until (approximately) the weak rate (...
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This note was uploaded on 07/28/2011 for the course PHY 880.20 taught by Professor Steigman during the Spring '10 term at Ohio State.

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