Kavli Affiliate: Ulrich Sperhake

| First 5 Authors: Ulrich Sperhake, William Cook, Diandian Wang, ,

| Summary:

We compute the gravitational wave energy $E_{rm rad}$ radiated in head-on

collisions of equal-mass, nonspinning black holes in up to $D=8$ dimensional

asymptotically flat spacetimes for boost velocities $v$ up to about $90,%$ of

the speed of light. We identify two main regimes: Weak radiation at velocities

up to about $40,%$ of the speed of light, and exponential growth of $E_{rm

rad}$ with $v$ at larger velocities. Extrapolation to the speed of light

predicts a limit of $12.9,%$ $(10.1,~7.7,~5.5,~4.5),%$. of the total mass

that is lost in gravitational waves in $D=4$ $(5,,6,,7,,8)$ spacetime

dimensions. In agreement with perturbative calculations, we observe that the

radiation is minimal for small but finite velocities, rather than for

collisions starting from rest. Our computations support the identification of

regimes with super Planckian curvature outside the black-hole horizons reported

by Okawa, Nakao, and Shibata [Phys.~Rev.~D {bf 83} 121501(R) (2011)].

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