  Some theoretic results related to the cosmological constant, obtained in the
80's and 90's of last century, are reviewed. These results exhibit some
interesting underlying pattern when viewed from a category-theoretic
perspective. In doing this, we illustrate how Baez and Dolan's Periodic Table
of $k$-tuply monoidal $n$-categories can serve as a basic framework of a
quantum spacetime structure. Explicitly, we show how Einstein gravity (with the
cosmological constant turned on) emerges when we de-categorify certain
$2$-tuply monoidal $2$-categories properly, as Crane proposed a long time ago.
In particular, we find that some recent work on Vertex Operator Algebras and
4-manifolds fits nicely into this framework.
