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Phys/Astro 228: Extragalactic Astronomy and Cosmology
October 27 2006
C.–P. Ma
Problem Set 6
Due Friday November 3
Readings: (1) James Grahms notes on “Dynamics of Ideal Fluids” linked at course
website; (2) Ch 7.1 of Dodelson.
1.
Validity of NonInteracting Electron Gas
No one in class has questioned why we can treat the electrons in the early universe as a noninteracting
thermal gas despite their Coulomb interactions. Give a heuristic argument (i.e. don’t worry about
numerical factors of order 1) for why this is okay. (
Hint
: Compare the relevant energies.)
2.
Sound Waves and Jeans Length
At typical sealevel conditions, the density of air is 1
.
23
×
10

3
g cm

3
and the speed of sound is
3
.
4
×
10
4
cm sec

1
. Find (a) the Jeans length and comment on how it compares with the thickness of
the atmosphere and if you expect Jeans instabilities to occur; (b) the fractional change in frequency
due to the selfgravity
of the air, for a sound wave with wavelength 1 meter.
3.
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This note was uploaded on 08/01/2008 for the course ASTRO 228 taught by Professor Chungpeima during the Fall '06 term at University of California, Berkeley.
 Fall '06
 CHUNGPEIMA
 Cosmology

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