The Code for Anisotropies in the Microwave Background [CAMB] is an application (written in Python and Fortran) that computes cosmic microwave background (CMB) spectra given a set of input cosmological parameters. The package was written by Antony Lewis and Anthony Challinor and may be downloaded from It is based on CMBFAST by U. Seljak and M. Zaldarriaga.

LAMBDA developed a web-based server-side interface to CAMB that allows users to enter parameters in a web form and run the program on our server (see below). The results are displayed graphically and the spectrum files are made available for downloading. Most of the configuration documentation is provided in the sample parameter file provided with the application. The HEALPix synfast application has been linked into the online tool so that simulated sky maps may be generated using the spectra produced by the CAMB run.

The forms and scripts for this web interface are available in the file camb_interface.tar.gz. A description of this tarball is provided in its Readme.txt file.

Web Interface

Most of the configuration documentation is provided in the sample parameter file provided with the application.

CAMB Version

The underlying camb program used was built from the Fortran code included in the version downloaded from the git repository in January 2024. information can be found at

Actions to Perform

  • Vector Cl's are incompatible with Scalar and Tensor Cl's. The Transfer functions require Scalar and/or Tensor Cl's.
Maximum Multipoles and k*eta


  • Tensor limits should be less than or equal to the scalar limits.
  • Note that C_ls near l_max are inaccurate (about 5%), go to 50 more than you need
  • Lensed power spectra are computed to l_max_scalar-100
  • To get accurate lensed BB need to have l_max_scalar>2000, k_eta_max_scalar>10000
  • To get accurate lensing potential you also need k_eta_max_scalar > 10000
  • Otherwise k_eta_max_scalar=2*l_max_scalar usually suffices, or don't set to use default
Cosmological Parameters

Neutrino mass splittings

  • The Equation of State entry is the effective equation of state parameter for dark energy and is assumed constant.
  • The Comoving Sound Speed parameter is the constant comoving sound speed of the dark energy; 1=quintessence.
  • Setting Degeneracies to zero sets the mass degeneracies parameter to massive neutrinos. Otherwise this should be a space separated list of values, one per eigenstate.
  • Fractions should be a space separated list indicating the fraction of Ωνh2 accounted for by each eigenstate.

Power Spectrum


Transfer Function Settings

  • The ratio is that of the initial tensor/scalar power spectrum amplitudes. The vector modes use the scalar settings.
  • Supply 'Number' values in each after the first, separated by spaces.
  • To get μK2, set a realistic initial scalar amplitude (e.g., 2.3e-9) and the output scale factor to ~7.43e12; for dimensionles transfer functions, set scalar amp to 1 and the output scale factor to 1.<.li>
  • kmax=0.5 is enough for sigma_8. Each redshift must be supplied, separated by spaces. They will be sorted into descending order.
Initial Scalar Perturbation Mode


Bispectrum Parameters

  • Bispectrum: Primordial is currently only a local model (fnl=1) and can take several minutes to run. Lensing is fairly quick.
  • Separate the Bispectrum delta values with spaces.
Performance and Reporting Parameters

  • Massive Neutrino Approximation: The series velocity weight option: switch to this approximation once non-relativistic. The fast approximation scheme is a CMB-only option, accurate for light neutrinos.
  • Accurate EE: Set to yes if you are bothered about percent accuracy on EE from reionization.
  • Tensor Neutrinos: Indicates whether or not to include neutrinos in the tensor evolution equations.
  • Truncate late radiation heirarchies: Indicates whether or not to turn off small-scale late tume radiation heirarchies. This is a time versus accuracy tradeoff.
  • Accuracy Boosts: Higher numbers improve accuracy by reducing time steps.

A service of the HEASARC and of the Astrophysics Science Division at NASA/GSFC
Goddard Space Flight Center, National Aeronautics and Space Administration
HEASARC Director: Dr. Andrew F. Ptak
LAMBDA Director: Dr. Thomas M. Essinger-Hileman
NASA Official: Dr. Thomas M. Essinger-Hileman
Web Curator: Mr. Michael R. Greason