Beyond standard model cosmologies
- Authors:
Shikhar Mittal and Prakhar Bansal
- Paper:
Interacting dark matter model
ECHO21 can be used to study a special class of interacting dark matter (IDM) model: Coulomb-like DM. We assume that all DM particles are Coulomb-like and that they can interact with all baryons. The interaction cross-section for such a system is given by \(\sigma(v_{\mathrm{b}\chi}) = \sigma_0(v_{\mathrm{b}\chi}/c)^{-4}\), where \(v_{\mathrm{b}\chi}\) is the baryon-DM relative velocity. In addition to cooling, IDM leads to a suppression and delay in star formation.
IDM requires two additional parameters, namely \(m_{\chi}\) (mass of Coulomb-like DM) and \(\sigma_{45}\) (dimensionless cross-section, expressed as \(\sigma_0/10^{-45}\,\mathrm{m^2}\)). If you want work with IDM, supply both these parameters in your cosmo dictionary. Note that \(m_{\chi}\) should be provided in units of GeV. The cosmo dictionary now looks like
cosmo = {'Ho':67.4,'Om_m':0.315,'Om_b':0.049,'sig8':0.811,'ns':0.965,'Tcmbo':2.725,'Yp':0.245, 'mx_gev':1.0,'sigma45':1.0}
ECHO21 interfaces with a modified version of CLASS to get the (linear) matter power spectrum for IDM (at \(z=0\)) which is then fed into COLOSSUS to obtain variance \(\sigma(M)\). We translate the power spectrum and variance to an arbitrary redshift linearly with a scale-independent growth factor. Accordingly, we can obtain HMF at an arbitrary redshift.
A few points to note on which the latest version of IDM implementation of ECHO21 differs from the one described in Mittal et al (2026).
User-defined value of \(\sigma_8\) in case of IDM simulation is not used directly. Rather, a corresponding CDM simulation is done to get the primordial spectrum amplitude \(A_\mathrm{s}\). This \(A_\mathrm{s}\) is then used for IDM simulation. Such an approach ensures that CDM and IDM had a common primordial state.
We adopt a top-hat filter to smoothen the matter density field for the calculation of variance \(\sigma(M)\). Previously, a sharp-k filter was used. (Top-hat makes the simulation a bit faster and is more commonly used in the literature.)
Previously, only Tinker et al (2008) HMF was available for IDM simulations. Now, the user can choose any standard HMF. Moreover, the halo mass definition was restricted to \(M_{500\mathrm{c}}\) (mass enclosed within a radius where the mean density is 500 times the critical matter density). Now, the user can choose any halo mass definition supported by COLOSSUS.