Primordial Black Holes Could Kick Out Stars and Replace Them.

Primordial black holes formed during the earliest stages of the evolution of the universe. Their immense gravity may be playing havoc in stellar systems. They can transfer energy into wide binary systems disrupting their orbits. Like celestial bullies their disruption might lead to extreme outcomes though like the ejection of a star, only to be replaced by the black hole itself! A new paper studies the interactions of systems like these and looks at ways we might be able to detect them. 

It’s been theorised that during the earliest moments after the Big Bang, black holes may have formed. They are not the result of supermassive stars having collapsed but instead have formed out of fluctuations in the density of matter. Regions with great density would simply collapse under their own gravitational influence forming what have been dubbed primordial black holes (PBHs). They are thought to vary in size from subatomic to some that are more massive than the Sun. 

Whether primordial black holes really do account for dark matter in the universe is still up for debate. Among the astronomical community it is generally accepted that they cannot account for all dark matter but probably account for up to 10% of dark matter in the planetary mass range (10^-7 to 10^-3 solar masses.) Whether this is PBHs account for any of the dark matter in the universe requires further analysis.

If large scale is taken into account then PBHs are indistinguishable from a background of particle dark matter. At small scales the distribution of PBHs is not uniform across the universe relative to the background of particle dark matter and so we are forced to look for a unique and new theory. Observing PBHs to understand how close the model is to reality is difficult but it is possible to study their interactions with star systems. 

In a paper published by Badal Bhalla from the University of Oklahoma and a team of astronomers they explore the way PBHs can lose energy when interacting with stellar binary systems. These interactions can result in any one of 5 possible outcomes; 

1: Hardening – the two bound objects lose energy to the third free object causing their separation to decrease;

2: Softening – the free body transfers energy to the bound system causing their separation to increase but remain bound;

3: Disruption – the free body transfers enough energy to the bound system that the components become unbound and all objects continue unbound;

4: Capture – the bound objects capture the free object;

5: Exchange – the free object transfers enough energy to unbind one of the bound objects and in doing so loses sufficient energy to become bound to the remaining one. 

Previous studies have explored softening and disruption in PBH and binary interactions as has the capture model. The team propose that hardening is also unlikely and so explore the possibility of the exchange model. They find that the exchange model should lead to a population of PBH binaries in the Milky Way and indeed some observations hint that they may exist. The team also suggest it may be possible to detect PBHs in binary systems with a sub-solar mass PBH by the properties of the system. Observations are now needed to validate the model. The discovery of black holes in a binary system may be detectable and go some way to support the findings.

Source : Dancing with invisible partners: Three-body exchanges with primordial black holes