Collisional dynamics of macroscopic dark matter
The dark matter could be primordial black holes. These behave differently from particle dark matter for two main reasons:
- They are independently uniformly distributed in the initial conditions, which makes random Poisson density variations. Consequently, primordial black hole clusters begin to form around the time of matter-radiation equality (\(52\,000\) years after the Big Bang), whereas halos of particle dark matter are not expected to form until around 100 million years after the Big Bang.
- Gravitational dynamics between individual black holes are important.
The following movie shows a comparison between cosmic structure formation with particle dark matter (left), with primordial black holes if the interparticle dynamics are neglected (middle), and with primordial black holes if these “collisional” dynamics are accounted for.
Collisional heating
N-body dynamics between the black holes dynamically heat them, making a component of hot dark matter that could suppress structure formation on galaxy scales. Often, this can be viewed as resulting from an interaction between a tight, very rapidly orbiting binary, and a third black hole. The following movies show examples.
Sometimes there are so many black holes that the picture is more complicated, as these movies show.
In the following movie, we see a cluster of about 6 black holes dissove due to these “collisional” dynamics.
Relativistic heating
Black holes can also merge together in a gravitational-wave inspiral. This process makes significantly gravitational waves, and it is also important for structure formation because the gravitational radiation can be emitted directionally, giving the merger remnant a recoil kick. These movies show examples: