[cygx]

> If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there

I wouldn't go that far: Rather, adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around. However, in the absence of observational evidence to constrain its value, it's also justified to start any investigation with its value assumed 0...

[pdonis]

> adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around

Yes, that's what I was trying to say. I didn't mean that including it mathematically in the equations necessarily requires one to adopt a non-zero value for it; you are quite correct that one shouldn't do that unless one has observational evidence to back it up (and cosmologists in fact didn't adopt a non-zero value until observational evidence required it).