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Unformatted text preview: Phys/Astro 228: Extragalactic Astronomy and Cosmology September 29 2006 C.P. Ma Problem Set 4 Due 5pm Friday October 6 in grader Brad Hagans mailbox, Campbell Hall 6th floor mailroom. Readings: (1) Ch 3 of Dodelson; (2) Sec 19.3 of Big-Bang Cosmology (Note that they set c = h = k B = 1 and didnt know 90 32 is 45 16 .) (3) All of Big-Bang Nucleosynthesis (linked at course website). 1. in Photon Background Radiation The present temperature of the cosmic background photons is known to exquisite precision: T , = 2 . 725 . 001(1 ) Kelvin (Mather et al 1999). (a) Calculate the present number density of photons (in cm- 3 ) in this radiation. (Dont worry about the error bars.) (b) Calculate , , the present ratio of the mass-energy density in photons to the critical density. (Express your answer in terms of the Hubble parameter h .) Is the universe today radiation or matter dominated?...
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- Fall '06