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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 BigBang Cosmology (Note that they set c = h = k B = 1 and didnt know 90 32 is 45 16 .) (3) All of BigBang 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 massenergy 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
 CHUNGPEIMA
 Cosmology

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