The projection is obtained by separating the total angular dependence of
the mode from its decomposition in spherical coordinates: i.e. into radial
functions times spin harmonics .We discuss their explicit construction in Appendix B.
The full solution immediately follows by integrating the projected source
over the radial coordinate,
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The interpretation of these equations is also readily apparent from their form
and construction.
The decomposition of into radial and spherical parts
encapsulates the summation of spin and orbital angular momentum as well as
the geodesic deviation factors described in §IIIB.
The difference between the integral solution and the differential form
is that in the former case the coupling is performed in one step from the
source at time
and distance
to the present, while in the
latter the power is steadily transferred to higher
as the time advances.
Take the flat space case.
The intrinsic local angular momentum at the point is
but must be added to the orbital angular momentum
from the plane wave which can be expanded in terms of
.The result is a sum of
to
angular momentum states with
weights given by Clebsch-Gordan coefficients. Alternately a state of definite
angular momentum involves a sum over the same range in the spherical Bessel
function.
These linear combinations of Bessel functions are exactly the radial functions
in Eq. (41) for the flat limit [1].
For an open geometry, the same analysis follows save that the spherical
Bessel function must be replaced by a hyperspherical Bessel function
(also called ultra-spherical Bessel functions) in the manner described
in Appendix B. The qualitative aspect of this modification
is clear from considering the angular diameter distance arguments of
§IIIB.
The peak in the Bessel function picks out the angle which a scale
subtends at distance
.A spherical Bessel function peaks when its argument
or
in the small angle approximation.
The hyperspherical Bessel function peaks at
for
or
in the small angle approximation.
The main effect of spatial curvature is simply to shift features in
-space with the angular diameter distance, i.e. to higher
or smaller angles in open universes.
Similar arguments hold for closed geometries [16].
By virtue of this fact the division of polarization into E and B-modes
remains the same as that in flat space. More specifically, for a single
mode the ratio in power is given by
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(34) |
The integral solutions (41) are the basis of the ``line of sight''
method [5,13] for rapid numerical calculation of CMB spectra,
which has been employed in CMBFAST.
The numerical implementation of equations (41) requires an efficient
way of calculating the radial functions ().
This is best done acting the derivatives of the hyperspherical Bessel function
in the radial equations (B3)-(B5) and
(B11) on the sources through integration by parts.
The remaining integrals can be efficiently calculated with the techniques of
[4] for generating hyperspherical Bessel functions.
The tensor CMBFAST code has now been modified to use the formalism described
in this paper and the results have been cross-checked against solutions of
the Boltzmann hierarchy equations (33)-(34) with
very good agreement.