.. _physics-correlationFunctionTwoPoint:

Two-point Correlation Functions
===============================

Class providing two-point correlation functions :math:`\xi(r,t)`---the excess probability above Poisson of finding a pair of dark matter particles or galaxies separated by comoving distance :math:`r` at cosmic time :math:`t`. The correlation function is the Fourier transform of the matter power spectrum :math:`P(k)`, and both its volume-averaged form :math:`\bar{\xi}(r)` and point form are used in analyses of large-scale structure clustering. It is used, for example, in computing the survey window functions required for two-point statistics of galaxy samples.

**Default implementation:** ``correlationFunctionTwoPointPowerSpectrumTransform``

Methods
-------

``correlation`` → ``double precision``
   Return the two-point correlation function for :math:`r=`\ ``separation`` [Mpc].

   * ``double precision, intent(in ) :: separation, time``

``correlationVolumeAveraged`` → ``double precision``
   Return the volume-averaged two-point correlation function for :math:`r=`\ ``separation`` [Mpc].

   * ``double precision, intent(in ) :: separation, time``

.. _physics-correlationFunctionTwoPointPowerSpectrumTransform:

``correlationFunctionTwoPointPowerSpectrumTransform``
-----------------------------------------------------

Provides a two-point correlation function class in which the correlation function is found by Fourier transforming a power spectrum.

**(Default implementation)**