. Is there a single, clearQmathematicalquestion that has emerged astheopen problem in General Relativity?

I ask this on the ~100^{th} anniversary of Einstein's (4-page!) 1915 paper,

"Die Feldgleichungen der Gravitation,"

Preussische Akademie der Wissenschaften, Sitzungsberichte, 1915 (part 2), 844–847. (Wikisource:The Field Equations of Gravitation.)

^{ Einstein's notebook, Riemann curvature tensor. }

It is difficult to surpass Willie Wong's thorough 2011 answer to the MSE question, "Open problems in General Relativity." But perhaps a core mathematical question has risen to the fore since?

Naked singularities? Beyond the Cauchy horizon?
"Electrodynamics of moving bodies?"
Florentin Smarandache's 2013 *Unsolved Problems
in Special and General
Relativity* (PDF download.)?

Or perhaps Willie's inventory cannot be condensed or sorted further at this point in time?

theproblem on everyone's minds. But, you'll be interested in the mathematical seminar paper of Penrose,Some Unsolved Problems in Classical General Relativity. $\endgroup$ – Chris Gerig Apr 4 '15 at 0:04Mathematical Problems in General Relativity. $\endgroup$ – Joseph O'Rourke Apr 4 '15 at 16:24