Scattering of dark matter and dark energy

We demonstrate how the two dominant constituents of the Universe, dark energy and dark matter, could possess a large scattering cross section without considerably impacting observations. Unlike interacting models which invoke energy exchange between the two fluids, the background cosmology remains unaltered, leaving fewer observational signatures. Following a brief review of the scattering cross sections between cosmologically significant particles, we explore the implications of an elastic interaction between dark matter and dark energy. The growth of large scale structure is suppressed, yet this effect is found to be weak due to the persistently low dark energy density. Thus we conclude that the dark matter–dark energy cross section may exceed the Thomson cross section by several orders of magnitude.

Figure 1

A collection of cross sections between cosmologically significant particles, in units of barns (${10}^{-24}{\mathrm{cm}}^2$). We assume a collisional energy associated with the era of recombination, 3000 K or equivalently $\sim 0.3\mathrm{eV}$. The dark matter particle is taken to have a mass of $10\mathrm{GeV}/c^2$, and the dark energy equation of state $w=-0.9$.

Figure 2

The logarithmic growth rate of linear dark matter perturbations, when subject to elastic scattering with the dark energy fluid. For this configuration the particle mass $m_D=10\mathrm{GeV}/c^2$, and $w=-0.9$. The solid line corresponds to a cross section of ${\sigma }_D=500\mathrm{b}$, showing a suppression of growth at late times compared to the dotted line with no scattering (${\sigma }_D=0$).

Figure 3

The evolution in the growth index as a function of the scale factor. Thick solid and dashed lines correspond to models of dark energy with $w=-0.9$ and $w=-0.99$, respectively. As with Fig. 2, the dark matter–dark energy cross section is taken to be 500 b. The dotted line represents the standard case of zero scattering. Below the dotted line, the thin solid and dashed lines correspond to $w=-1.1$ and $w=-1.01$ models.

Figure 4

The solid contours demonstrate the modification to the growth index induced by dark matter–dark energy scattering, with the cross section taken to be ${\sigma }_D=300\mathrm{b}$. The dashed contours provide an example of the bias which may be induced in the gravitational growth index $\gamma$ by the interacting model outlined in 7. The standard model is indicated by the black dot.