Speaker
Description
A new theory is presented to estimate the mass, size, lifetime, and other properties of cold dark matter particles (CDM) within the ΛCDM cosmology. Using Illustris simulations, we demonstrate the existence of mass and energy cascade that facilitates the formation of hierarchical structures. A scale-independent rate of cascade $\varepsilon_u\approx 10^{-7}m^2/s^3$ can be identified. The energy cascade leads to universal scaling laws on relevant scales r, i.e. a two-thirds law for kinetic energy ($v_r^2\propto \varepsilon_u^{2/3} r^{2/3}$) and a four-thirds law for DM halo density ($\rho_r\propto \varepsilon_u^{2/3}G^{-1}r^{-4/3}$), where G is the gravitational constant. For cold and collisionless dark matter that interacts via gravity only, these scaling laws can be extended down to the smallest scale, that is, a free streaming scale. For standard WIMPs, that scale is about Earth's mass. For superheavy dark matter particles of mass $10^{12}$GeV, the free streaming mass can be comparable to the particle mass such that quantum effects can be important on that scale. Combined with the uncertainty principle and virial theorem, three constants ($\varepsilon_u$, $\hbar$, and G) dominate the physics on that scale, so that the properties of CDM can be estimated. We estimate a mass $m_X=(\varepsilon_u\hbar^5G^{-4})^{1/9}=10^{12}$GeV, a size $l_X=(\varepsilon_u^{-1}\hbar G)^{1/3}=10^{-13}$m, and a lifetime $\tau_X=c^2/\varepsilon_u=10^{16}$ years for CDM particles. Here, $\hbar$ is the Planck constant and c is the speed of light. The typical energy on that scale $E_X=(\varepsilon_u^5\hbar^7G^{-2})^{1/9}=10^{-9}$eV suggests a “dark radiation” field to provide a viable mechanism for the energy dissipation during gravitational collapsing of CDM. If existing, the “dark radiation" should be produced around $t_X=(\varepsilon_u^{-5}\hbar^2G^2)^{1/9}=10^{-6}$s (quark epoch) with mass of $10^{-9}$eV, a GUT scale decay constant $10^{16}$GeV, or an effective axion-photon coupling $10^{-18}$GeV$^{-1}$, such that the axion particle can be a very promising candidate for “dark radiation". The energy density of “dark radiation” is estimated to be about 1% of the cosmic microwave background (CMB). This work suggests a heavy dark matter scenario created during inflationary epoch along with a light axion-like dark radiation field. Potential extension to self-interacting dark matter is also presented. More details can be found at arXiv:2202.07240.
