A Couldn't the cosmological constant also have some sort of "dynamics"?

[mad mathematician]

Why should we believe that the cosmological constant is indeed a constant, and not another parameter that changes with spacetime dynamics?

Obviously it would make the calculations even more intractable; but who said that life in the universe is easy?!

 

[PeterDonis]

Why should we believe that the cosmological constant is indeed a constant, and not another parameter that changes with spacetime dynamics?

Because a constant is the simplest model, and we have no evidence that it's not constant, so we use the simplest model that's consistent with the evidence.

 

[mad mathematician]

Because a constant is the simplest model, and we have no evidence that it's not constant, so we use the simplest model that's consistent with the evidence.

I wonder how difficult it would be to to write down a model where it changes with time.

 

[Ibix]

I wonder how difficult it would be to to write down a model where it changes with time.

We usually call it "dark energy" when it's not assumed to be perfectly constant.

 

[PeterDonis]

I wonder how difficult it would be to to write down a model where it changes with time.

It's straightforward to write down the model; indeed doing that is an exercise in some advanced GR textbooks. The usual way is to use a scalar field.

But when you compare the model with data, you find that the field never changes, so it ends up being equivalent to a cosmological constant that is constant. Which means all the extra work you put into your model got you nothing in terms of predictive power. At least, not with the data we have now.