What if the cosmological constant (from which, if I understand correctly, the Hubble constant is derived) is not constant? Could a changing comological "constant" explain the discrepancy between the various methods of calculating the Hubble constant?
From Wikipedia [0]: The cosmological constant "was revived and reinterpreted as the energy density of space, or vacuum energy, that arises in quantum mechanics. It is closely associated with the concept of dark energy."
Do vacuum energy and dark energy have to be constant?
Dark energy is a constant in the standard model (equivalent to the cosmological constant) and all observations I'm aware of besides the Hubble tension are consistent with it being constant. But nothing stops you from going beyond the standard model, and people have done that and proposed models with a dynamic dark energy density, for example quintessence models. They'd solve some theoretical issues with the standard model, but so far no observations have been precise enough to differentiate between those models and the standard model.
That is, besides a whole family of potential other solutions:
Dark energy in extended parameter spaces [289] Early Dark Energy [235] Early Dark Energy [229]
Dynamical Dark Energy [309] Phantom Dark Energy [11] Decaying Warm DM [474]
Metastable Dark Energy [314] Dynamical Dark Energy [11, 281, 309] Neutrino-DM Interaction [506]
PEDE [392, 394] GEDE [397] Interacting dark radiation [517]
Elaborated Vacuum Metamorphosis [400–402] Vacuum Metamorphosis [402] Self-Interacting Neutrinos [700, 701]
IDE [314, 636, 637, 639, 652, 657, 661–663] IDE [314, 653, 656, 661, 663, 670] IDE [656]
Self-interacting sterile neutrinos [711] Critically Emergent Dark Energy [997] Unified Cosmologies [747]
Generalized Chaplygin gas model [744] f (T ) gravity [814] Scalar-tensor gravity [856]
Galileon gravity [876, 882] ¨Uber-gravity [59] Modified recombination [986]
Power Law Inflation [966] Reconstructed PPS [978] Super ΛCDM [1007]
f (T ) [818] Coupled Dark Energy [650]
The Hubble constant is not necessarily derived from the cosmological constant. To be clear, it's not even a constant either, it's a proportionality factor between the recessional velocity of distant objects and their distance from us. Though in some usages it refers to the value of this factor at the current time, which would in fact make it a constant. Regardless, even without a cosmological constant, after the Big Bang you'd still have an expanding universe, possibly collapsing or endlessly expanding, and I like to think of the Hubble constant/parameter in this case as representing the "momentum" the matter in the universe has left from the Big Bang.
The cosmological constant does in fact have to be constant within the constraints of general relativity. The mathematical machinery of GR only allows two parameters: Newton's constant G, representing the coupling of matter to gravity, and the cosmological constant. Both have to be true constants, numbers with a unit.
However, this is only true of the most basic theory of dark energy, where you directly add a constant to the general relativistic lagrangian. More complicated theories, like, for example, quintessence, involve adding new dynamical fields to the theory. The "effective cosmological constant" associated to such theories, quantifying the effect these fields are having on the expansion of the universe, can dynamically change over time, and some of these theories are proposed to solve the Hubble tension. To be clear, although none of these theories are fringe or pseudoscience, they haven't been accepted as a final explanation either, resolving these issues is still a work in progress, at this point they're all simply interesting hypotheses.
What if the cosmological constant (from which, if I understand correctly, the Hubble constant is derived) is not constant? Could a changing comological "constant" explain the discrepancy between the various methods of calculating the Hubble constant?
From Wikipedia [0]: The cosmological constant "was revived and reinterpreted as the energy density of space, or vacuum energy, that arises in quantum mechanics. It is closely associated with the concept of dark energy."
Do vacuum energy and dark energy have to be constant?
[0] https://en.wikipedia.org/wiki/Cosmological_constant