## CAMB: Matter power spectra and transfer functions; what gauge/slicing are they in?

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
T. P. Waterhouse
Posts: 2
Joined: September 27 2007
Affiliation: UBC Department of Physics and Astronomy

### CAMB: Matter power spectra and transfer functions; what gaug

I'm interested in the matter power spectrum before recombination. So I'd like to know, when I get CAMB to give me a power spectrum or a transfer function, what gauge and which time slicing is it working in?

More to the point, since a power spectrum is essentially a Fourier integral over a spacelike hypersurface, exactly what hypersurface (ie. time slice) is the integral taken over?

Antony Lewis
Posts: 1501
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: CAMB: Matter power spectra and transfer functions; what

CAMB works in the synchronous gauge (i.e. the surface orthogonal to the 4-vector velocity of the CDM).

T. P. Waterhouse
Posts: 2
Joined: September 27 2007
Affiliation: UBC Department of Physics and Astronomy

### CAMB: Matter power spectra and transfer functions; what gaug

Thanks for the quick reply, but I could use some clarification. I thought that surfaces of constant time coordinate in synchronous gauge are orthogonal to worldlines of constant spatial coordinate, not necessarily to the velocity of the matter. Also, the synchronous gauge condition gives a family of gauges with arbitrary time slicings!

So, do you mean the specific synchronous gauge whose time slicing is orthogonal to the CDM velocity? That's the comoving gauge, which is also a synchronous gauge during matter domination, right?