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Energy in fluctuations

We will now use equation (21) to evaluate the fluctuation energy at different spatial scales assuming that individual particles are uncorrelated. The particle wakes do in fact give rise to correlations but this is of higher order in 1/N in the BBGKY expansion (cf. Gilbert 1969) than the lowest-order effect we will consider here.

This leaves us with individual particles reacting coherently to the effect of their own wakes. Because the particles are uncorrelated, the number density of particles at tex2html_wrap_inline1150 at time 0 and at tex2html_wrap_inline1154 at time t is
 eqnarray404
where tex2html_wrap_inline1158 is the equilibrium particle distribution with
equation424
Direct substitution demonstrates that equation (23) solves the Liouville equation with the initial condition tex2html_wrap_inline1160 and tex2html_wrap_inline1162 at t=0. Similarly, integrating equation (23) over all coordinates gives N.

For a given harmonic lm, the fluctuation energy is then
 eqnarray435
where the expectation value of some quantity tex2html_wrap_inline1170 is defined by
 eqnarray451

Applying equations (21) and (26) to equation (25) gives
 eqnarray469
Gathering terms, this can be simplified as follows:
 eqnarray497
Note that each term in the fluctuation energy, equation (28), is negative definite as expected. The contribution for each triple in the angle expansion, tex2html_wrap_inline1024, and each term in the basis expansion k may be tabulated separately.

We may compute the fluctuation energy in the absence of gravity by returning to equation (25) and evaluating tex2html_wrap_inline1176 without any dynamics. For N particles, the sample value for tex2html_wrap_inline1180 is tex2html_wrap_inline1182 Using the expectation defined by equation (26) one finds:
 equation527
This is identical to equation (28) with out the particle dressing: tex2html_wrap_inline1184.



Martin D. Weinberg
Wed Jul 16 10:06:31 EDT 1997