Non-Constant Cosmo-Constant: Controversy in GR?

[ChrisVer]

I think that in the Hilbert action for the General Relativity /gravity, one can add a constant scalar quantity, which we call the Cosmological Constant.
I am wondering though, how can someone suggest ( after introducing it as a constant ) that this quantity could in fact vary?

 

[Ben Niehoff]

I see varying cosmological constants all the time. This comes straightforwardly out of Kaluza-Klein reductions from higher-dimensional theories, or you can just put it in by hand.

This is something you typically do if you want the cosmological constant to be an emergent phenomenon, as follows:

Put some scalar fields in your Lagrangian with the usual kinetic term (or you can put in a sigma model kinetic term with some target space), and also throw in a potential term. For the right choice of potential, your scalars will want to asymptotically approach nonzero constants, thus dynamically generating your cosmological constant.

Scalars are very flexible and can turn up anywhere in the Lagrangian, including as coupling constants for gauge fields.