# Quickstart¶

CCL is structured around Cosmology objects which hold the cosmological parameters and any tabulated data associated with a given cosmology. The library then provides functions to compute specific quantities of interest. See the full API documentation through the pyccl module and submodules for more details.

Further, CCL follows the following conventions:

• all units are non-h-inverse (e.g., Mpc as opposed to Mpc/h)
• the scale factor a is preferred over redshift z as a time coordinate.
• the Cosmology object always comes first in most function calls
• argument ordering for power spectra is (k, a)

This example computes the comoving distance and HALOFIT non-linear power spectrum using the BBKS transfer function:

>>> import pyccl
>>> cosmo = pyccl.Cosmology(Omega_c=0.25, Omega_b=0.05,
h=0.7, n_s=0.95, sigma8=0.8,
transfer_function='bbks')
>>> pyccl.sigma8(cosmo)  # get sigma8
0.7999999999999998
>>> z = 1
>>> pyccl.comoving_radial_distance(cosmo, 1./(1+z))  # comoving distance to z=1 in Mpc
3303.5261651302458
>>> pyccl.nonlin_matter_power(cosmo, k=1, a=0.5)  # HALOFIT P(k) at k,z = 1,1
143.6828250598087


See Notation, Models and Other Cosmological Conventions for more details on the supported models for various cosmological quantities (e.g., the power spectrum) and the specification of the cosmological parameters.

A comprehensive set of examples showcasing the different types of functionality implemented in CCL can be found here.