Axions as candidates for dark matter particlesThe existence of a dark, cold (i.e., nonrelativistic), and nearly collisionless matter component called "dark matter" is a meanwhile well established part of the standard model of cosmology, confirmed by independent probes ranging in scale from the Hubble length to galactic radii and in time from the first seconds to the last million years of cosmic evolution. Yet all attempts to identify the nature of dark matter, for instance in direct or indirect detection experiments or by direct production at the LHC, have so far been unsuccessful.
Axions are among the best motivated candidates for dark matter particles. They are a natural by-product of the Peccei-Quinn solution to the strong CP problem of quantum chromodynamics (QCD).
Axion miniclustersIf the Peccei-Quinn symmetry is broken after inflation, large axion isocurvature perturbations can collapse very early and form small, gravitationally bound structures called axion miniclusters. Predicting the their expected mass function and density profiles is important for astronomical and experimental axion searches.
We use cosmological N-body simulations to follow the gravitational collapse of axion miniclusters from realistic initial conditions, provided by simulations of the relativistic axion field, into the matter dominated epoch of the early universe.
String theory predicts the existence of axion-like particles whose masses can be so low that their de Broglie wavelength can reach several kiloparsecs. Below roughly this scale, the growth of cosmological structure is suppressed. Astronomical observations of the masses of distant galaxies and of the inner structure of dwarf galaxies help to determine whether dark matter consists of these so-called ultra-light axions.
Using three-dimensional simulations, we studied merging solitonic cores predicted to form in ultra-light axion dark matter halos. The classical, Newtonian equations of motion of self-gravitating scalar fields are described by the Schrödinger-Poisson equations. We investigated mergers of ground state (boson star) configurations with varying mass ratios, relative phases, orbital angular momenta and initial separation with the primary goal to understand the mass loss of the emerging core by gravitational cooling.
Volume rendered animations of runs conducted for our paper were uploaded to YouTube. A run with 13 merging cores is shown here .