With Miguel Faria, an undergraduate student at U.Coimbra, and Fabrizio Rompineve we showed here that due to inflationary diffusion, the QCD axion can form some extended objects called domain walls which end up forming very (very) massive primordial black holes.
The long story…
The search for the QCD axion continues in multiple forms. This time, we asked the question: if axions are present during the period of cosmic inflation and the inflationary diffusion is large, what can happen?
Inflationary Diffusion– Gravitational effects during inflation are known to diffuse light (or massless) scalar fields such as the axion, a process that has been famously described by Starobinsky as stochastic. Crucially, and in contrast with other scalar fields, axions have periodic properties which can be more formally described as discrete symmetries. Normally, we just talk about the axion in the “first” period. But what happens then if diffusion can move parts of the field to the “second” period? Well, during inflation, nothing special really happens. However, after inflation, as the universe cools down and the expansion rate decreases, those diffused axions start oscillating. Quickly, they will realize that in the regions of space where the axion was diffused to the second period, the oscillations take place around a minimum that is different from the rest of the universe.
Domain walls– Dividing those regions, there are what physicists call domain walls, extended and dense objects that store a significant amount of scalar field energy. Domain walls redshift more slowly with the expansion, their relative energy density can increase significantly. That was the important feature that we found with respect to some previous work.
Particle Dark Matter or Primordial Black Holes– Those domain walls are essentially frozen until their physical radius becomes comparable to the cosmological horizon, the Hubble scale. At that point, they typically collapse into the dark matter axion particles unless… they are so massive that their would be Schwarzschild radius is larger than the cosmological horizon. In such case, the walls collapse into primordial black holes (PBHs).
Super Duper PBHs– Primordial Black Holes are already somewhat exotic, but these ones have an extra layer of strangeness: they are very (very) massive. So massive that their mass is larger than the (TOTAL!) mass of galaxies and they have been called stupendeously large PBHs. Incredibly, and to our great surprise, as of April 20205 this hypothetical population of PBHs has not been excluded observationally!
So, one could ask: Can these giant dark monsters be the messengers of the axion kingdom?