Kirchberg's norm-ultrapower embedding problem
What the paper claims, in its own words
Constructs an explicit separable unital full group C∗-algebra that cannot embed unitally into the norm ultrapower of any fixed nonzero unital nuclear C∗-algebra, for any free ultrafilter on the natural numbers. Taking the target to be answers Kirchberg's norm-ultrapower embedding problem negatively.
The plain-language explainer for this entry is still coming.