A Kerr black hole, probably the most common type of black hole, is thought to contain a ring shaped singularity within a ellipsoidal (genus 0) event horizon. (Interestingly, because information cannot traverse an event horizon, we can only write "thought to"; no experiment can ever directly confirm this fact.)
Can a black hole have so much angular momentum that the event horizon itself becomes torus shaped (genus 1 not genus 0)?
Inspired by thinking about accretion discs around supermassive black holes (possibly even regular black holes). Matter orbiting in the accretion disc will not fall in until it loses enough energy (angular momentum) through friction. This might take a while. In the meantime, matter keeps on piling up and up, perhaps millions of solar masses compressed into a flattened ring shaped accretion disc. What happens if the matter density of the disc exceeds the critical density where the escape velocity exceeds the speed of light?
The disc goes dark: a black ring around a black hole, unable to radiate energy due to friction, so no longer able to lose angular momentum this way.
It will probably be like Saturn's rings: a system of many black rings.
We now have enough toys to explain the jets often seen with astronomical accretion discs, e.g., quasars. I had often been confused how a point mass could create jets, but now we have more than just a point mass. We have a region of complicated gravitational fields with black rings, and regions of high gravitational gradient between the rings and the central hole. If a particle should enter such a region at an oblique angle to the plane of the ring system, we can imagine it being somehow accelerated in a polar direction, yielding jets. In a particle enters at a trajectory close to the ring plane, it merely joins the accretion disc.
"Theorems from the 1970s proved that 4D black holes can only adopt a spherical shape"
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My thesis focuses on whether or not there is an intrinsic change in the distribution of quasar jet lengths with redshift. This excludes changes from selection effects. Not the easiest task, but very interesting nonetheless. It would tell us if the quasars have been evolving over time, and this might confirm (or not) that the density of the intergalactic medium was higher at earlier epochs.
Quasar jets are strange: if the quasar energy being emitted from the "poles", then an additional observed jet must be emitted at a random nonpolar angle. How does it pick that angle? Why is it stable?
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