Abstract
We present a new formalism, together with efficient numerical methods, to directly calculate the cosmic microwave background (CMB) bispectrum today from a given primordial bispectrum using the full linear radiation transfer functions. Unlike previous analyses which have assumed simple separable Ans\atze for the bispectrum, this work applies to a primordial bispectrum of almost arbitrary functional form, for which there may have been both horizon-crossing and superhorizon contributions. We employ adaptive methods on a hierarchical triangular grid and we establish their accuracy by direct comparison with an exact analytic solution, valid on large angular scales. We demonstrate that we can calculate the full CMB bispectrum to greater than 1% precision out to multipoles $ll1800$ on reasonable computational time scales. We plot the bispectrum for both the superhorizon (``local'') and horizon-crossing (``equilateral'') asymptotic limits, illustrating its oscillatory nature which is analogous to the CMB power spectrum.
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