Status  Confirmed 
Seminar Series  SEMGRECO 
Subjects  grqc 
Date  Monday 2 November 2020 
Time  11:00 
Institute  IAP 
Seminar Room  zoom details: contact pitrou@iap.fr 
Speaker's Last Name  Anson 
Speaker's First Name  Timothy 
Speaker's Email Address  timothy [dot] anson [at] th [dot] upsud [dot] fr 
Speaker's Institution  LPT Orsay 
Title  Disforming the Kerr metric 
Abstract  Starting from a recently constructed stealth Kerr solution of higher order scalar tensor theory involving scalar hair, we analytically construct disformal versions of the Kerr spacetime with a constant degree of disformality and a regular scalar field. While the disformed metric has only a ring singularity and asymptotically is quite similar to Kerr, it is found to be neither Ricci flat nor circular. Noncircularity has far reaching consequences on the structure of the solution. As we approach the rotating compact object from asymptotic infinity we find a static limit ergosurface similar to the Kerr spacetime with an enclosed ergoregion. However, the stationary limit of infalling observers is found to be a timelike hypersurface. A candidate event horizon is found in the interior of this stationary limit surface. It is a null hypersurface generated by a null congruence of light rays which are no longer Killing vectors. Under a mild regularity assumption, we find that the candidate surface is indeed an event horizon and the disformed Kerr metric is therefore a black hole quite distinct from the Kerr solution. 
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