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Gravitational Instabilities of Uniform Black Strings in AdS

Locally AdS$_{d-1}\times\mathbb{R}$ uniform black strings (UBS) in the presence of a massless scalar field are believed to avoid the onset of the Gregory-Laflamme (GL) instability in $d\ge4$ as no tachyonic modes exist in the spectrum of the Laplace-Beltrami operator. We present analytic and numerical evidence of GL modes in the Lichnerowicz spectrum indicating that AdS$_{d-1}$ UBSs are classically and thermodynamically unstable at the linear level in $d>4$. In $d=4$, we confirm that uniform BTZ$_3$ strings are indeed stable as previously suggested. We propose that linear instabilities of black strings are triggered if and only if a tachyonic mode exists in the Lichnerowicz spectrum. At the end state of the instability, AdS$_{d-1}$ UBSs of finite length may tunnel to a SAdS$_d$ black hole or converge onto a novel non-uniform AdS$_d$ black string. We conjecture that weak cosmic censorship is violated if the non-uniform solution is an exact AdS$_d$ black funnel and compute entropy estimates in $d>4$ as evidence.