Black Hole Perturbation Theory
Black hole perturbation theory is a technique for modeling black hole spacetimes which are a small perturbation away from a known exact black hole solutions of Einstein’s theory of General Relativity. This approach has many uses including modeling compact binary coalescences when one object is much more massive than the other, or (for all mass ratios) modeling the aftermath of a black hole binary coalescence (the so-called ‘ringdown’).
Careful modeling of these systems allows accurate models of the emitted gravitational waves to be constructed. The waveform templates theoreticians predict are used by data analysts studying the output from large physics experiments to detect and understand astrophysical gravitational wave sources. Ringdown models are used in searches with currently running ground-based detectors such as LIGO and Virgo. Small mass-ratio binaries are key sources for the future space-based LISA detector.
Small mass-ratio binaries
There are a number of different classes of small mass-ratio binaries. To date most attention has been devoted to so-called extreme mass-ratio inspirals (EMRIs). These are thought to occur in galatic nuclei when a massive black hole (with mass a few million times that of the sun) captures a compact object of a few solar masses, such as a neutron star or black hole. These are key sources for the space-based LISA detector. Studying the gravitational waves from these binary systems will allow for a census of the properties of massive black holes, giving us unprecedentedly accurate measurements of their masses and spins. This in turn will inform us about structure formation in the universe as, e.g., rapidly spinning black holes are expected to have grown from gas accretion whereas more slowly slow spinning massive black holes are thought to have grown through mergers. Detecting EMRIs will also allow for exquisit tests of Einstein theory of gravity, General Relativity.
There might also intermediate mass-ratio inspirals (IMRIs). These come in two types, both involving an intermediate mass black hole (IMBH) with a few thousand solar masses. If an IMBH inspirals into a massive black hole this will be a very loud source for LISA. If a stellar mass compact object inspirals into an IMBH then it is possible that ground-based detectors would observe their merger and ringdown (LISA might also be able to see their inspiral). As yet there is not clear confirmation that IMBHs exist but if they do they will be very exciting sources (gravitational waves may prove to be the best way to confirm the existence of such objects).
The below video by Steve Drasco depicts an example of a complicated motion of a neutron star (with mass about one and half times that of our sun) about an intermediate mass black hole (mass about 3,000 times our sun). The motion is shown at its natural speed, is drawn accurately, and has a thirty second tail.
Information to come soon