Speaker
Description
In this talk, “Bottom-up” and “top-down” approaches are presented for the formation and evolution of the nonlinear dark matter structures in different eras. Results strongly suggest a superheavy dark matter scenario with a critical particle mass of $10^{12}$GeV. Superheavy right-handed neutrinos of this mass can be a very promising candidate. The sterile neutrinos of mass $10^{12}$GeV can account for neutrino oscillations, dark matter, and baryon asymmetry at the same time and potentially stabilize the electroweak vacuum. Particles of this mass can have a free streaming mass comparable to the particle mass and form the smallest structure among all particles of any mass. In the "bottom-up" approach, particles of this critical mass can form the smallest haloes as early as $10^{-6}$s with a density ratio of $32\pi^2$ from the spherical collapse model. The halo mass increases rapidly to $10^8M_{\odot}$ at the matter-radiation equality to allow for an early and rapid galaxy formation and to $10^{13}M_{\odot}$ that matches observations at $z=0$. In the "top-down" approach, the mass and energy cascades are identified for hierarchical structure formation with a scale-independent constant rate of the energy cascade $\varepsilon_u \approx 10^{-7}m^2/s^3$. This leads to universal scaling laws on relevant scales $r$, that is, a two-thirds law for the kinetic energy ($v_r^2\propto \varepsilon_u^{2/3}r^{2/3}$) and a four-thirds law for the halo inner density ($\rho_r\propto\varepsilon_u^{2/3}G^{-1}r^{-4/3}$). These scaling laws can be confirmed by both Illustris simulations and rotation curves. By extending these scaling down to the smallest structure scale, we can estimate a particle mass $m_X=(\varepsilon_u\hbar^5G^{-4})^{1/9}=10^{12}$GeV (consistent with the critical mass in the “bottom-up” approach), size $l_X=(\varepsilon_u^{-1}\hbar G)^{1/3}=10^{-13}$m, and a characteristic time $\tau_X=c^2/\varepsilon_u=10^{16}$ years. Here, $\hbar$ is the Planck constant, and $c$ is the speed of light. The binding energy $E_X=(\varepsilon_u^5\hbar^7G^{-2})^{1/9}=10^{-9}$eV suggests a dark radiation field associated with the formation and evolution of haloes. If exists, axion-like dark radiation should be produced around $t_X=(\varepsilon_u^{-5}\hbar^2G^2)^{1/9}=10^{-6}$s (QCD phase transition) with a mass of $E_X=10^{-9}$eV, a GUT scale decay constant $10^{16}$ GeV, or an effective axion-photon coupling $10^{-18}$GeV$^{-1}$. The energy density of dark radiation is estimated to be about 1\% of the CMB photons. This work suggests a heavy dark matter scenario along with a light axion-like dark radiation. Superheavy right-handed neutrinos can be a very promising candidate. More details can be found in arXiv:2202.07240.
