Tuesday, May 14, 2013

Support for dark energy cosmologies added to Galacticus

Galacticus has recently (v0.9.2 revision 1809:c3577bea7b39) added support for more general dark energy cosmologies. It's always supported dark energy in the form of a cosmological constant, but since we don't know what dark energy is, it's useful to be able to explore more general equations of state.

Specifically, Galacticus can now compute models in which the equation of state of dark energy has the form:

w(a) = w0+w1a(1-a)

where a is the cosmological expansion factor, and w0 and w1 are adjustable parameters ("darkEnergyEquationOfStateW0" and "darkEnergyEquationOfStateW1" in a Galacticus parameter file).

The effects of dark energy are accounted for in computing the expansion history of the universe, the critical overdensity for collapse of spherical perturbations, and the density contrast of virialized dark matter halos (see the nice review by Percival 2005 for details).

To add dark energy to a model, simply add the following to an input file:

  <parameter>
    <name>cosmologyMethod</name>
    <value>matter-darkEnergy</value>
  </parameter>
  <parameter>
    <name>darkEnergyEquationOfStateW0</name>
    <value>-0.8</value>
  </parameter>

  <parameter>
    <name>darkEnergyEquationOfStateW1</name>
    <value>0.0</value>
  </parameter>

  <parameter>
    <name>criticalOverdensityMethod</name>
    <value>sphericalTopHatDarkEnergy</value>
  </parameter>
  <parameter>
    <name>virialDensityContrastMethod</name>
    <value>sphericalTopHatDarkEnergy</value>
  </parameter>


The present day density in dark energy is still set by the usual Omega_DE parameter.

The effects of the dark energy equation of state are small, but noticeable. This figure shows the stellar mass function of galaxies at z=0.07 for three different dark energy models1. For the highest mass galaxies there are small differences as the dark energy equation of state is varied.



1 The model isn't a perfect match to the data (from Li & White 2009). I'm working on that......

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