Abstract
An outstanding prediction of general relativity is the fact that the angular momentum S of an isolated black hole with mass μ is limited by the Kerr bound, S≤Gμ2/c. Testing this cornerstone is challenging due to the difficulty in modeling spinning compact objects that violate this bound. We argue that precise, model-independent tests can be achieved by measuring gravitational waves from an extreme mass ratio inspiral around a supermassive object, one of the main targets of the future LISA mission. In the extreme mass ratio limit, the dynamics of the small compact object depends only on its multipole moments, which are free parameters. At variance with the comparable-mass case, accurate waveforms are valid also when the spin of the small object greatly exceeds the Kerr bound. By computing the orbital dephasing and the gravitational-wave signal emitted by a spinning point particle in circular, nonprecessing, equatorial motion around a Kerr black hole, we estimate that LISA will be able to measure the spin of the small compact object at the level of 10%. Together with mass measurements, this will allow for theory-agnostic, unprecedented constraints on string-theory inspired objects such as “superspinars”, almost in their entire parameter space.
Highlights
The dawn of black-hole (BH) physics can be arguably traced back to the seminal work by Penrose [1, 2], Wheeler [3], Hawking [4], Bekenstein [5, 6], Carter [7, 8], and many others during the first “golden age” of general relativity (GR) in the 1970s
The unique stationary solution to GR in vacuum is the Kerr metric, which is regular outside an event horizon only if the above “Kerr bound” is fulfilled
In this letter and in a companion technical paper [39], we show that many of the above issues can be resolved with tests based on extreme mass ratio inspirals (EMRIs), which are model independent to a large extend
Summary
The dawn of black-hole (BH) physics can be arguably traced back to the seminal work by Penrose [1, 2], Wheeler [3], Hawking [4], Bekenstein [5, 6], Carter [7, 8], and many others during the first “golden age” of general relativity (GR) in the 1970s. In order to test the Kerr bound (1) we can study the EMRI evolution in which the secondary is assumed to be either a Kerr BH, which fulfills the constraint |χ| ≤ 1, or an extreme compact object [19] that can violate such a bound, i.e. As a function of the orbital frequency (up to the ISCO) for different values of the spin a ≡ J/M 2 of the primary.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
