Truncation in equations.f90

[Olga Mena]

Hi!

Sorry for the naive question. I would like to know where exactly in CAMB are computed the Cl's up to l=lmax. It seems to me that in the equations.f90 file
the photon moment expansion is truncated at a much lower l...am I missing something obvious?
If so, sorry for my ignorance...
Thanks a lot in advance,

Olga

[Ben Gold]

Those \ell aren't quite the same thing as the \ell in the C_\ell that we observe. There's a projection that comes later. What more or less happens is that the code computes what the photons look like locally up to the time of last scattering, and then projects those perturbations onto a far-away sphere to see what they look like to us. So when doing the calculation during the early universe, the code only needs to keep track of a few multipole moments --- this makes things much faster!

There's no reason this should be obvious at first glance... after all, it apparently wasn't obvious to Bertschinger when he wrote the ancestor to CAMB :)

(though to be fair I don't think there was much interest in high multipoles back then)

[Olga Mena]

Those \ell aren't quite the same thing as the \ell in the C_\ell that we observe. There's a projection that comes later. What more or less happens is that the code computes what the photons look like locally up to the time of last scattering, and then projects those perturbations onto a far-away sphere to see what they look like to us. So when doing the calculation during the early universe, the code only needs to keep track of a few multipole moments --- this makes things much faster!

There's no reason this should be obvious at first glance... after all, it apparently wasn't obvious to Bertschinger when he wrote the ancestor to CAMB :)

(though to be fair I don't think there was much interest in high multipoles back then)

Hi!

Thanks a lot, a lot. Yes! Now is more or less clear. I got really confused...
thanks a lot again!