(December 25, 2022 at 8:29 pm)LinuxGal Wrote:(December 25, 2022 at 7:55 pm)polymath257 Wrote: Without a good theory of quantum gravity, we simply don't even have a good explanation for the cosmological constant at all. It's been said that the prediction for the value was 'only' off by 120 *orders of magnitude*.
In other words, nobody has any real idea.
The toolkit for quantizing classical forces doesn't work with gravitons because they are not perturbatively renormalizable (I know you know that). Superstring theory gave it a shot but we're not seeing even the lightest SUSY particles at the LHC. Loop Quantum Gravity made a prediction that never materialized experimentally (c being to a small degree dependent on photon wavelength). The best hope for even demonstrating gravity is quantum at all is by looking at big molecules in superposition but we're not there yet.
All that said, why do we seek an "explanation" for a constant? No one explained the magnitude of G or c or h bar, they just read it off the dial.
Much care is needed here. There be dragons.
Yes, we frequently explain constants: first, the constant decay rate of a neutron, or the mass of a proton. The problem is *which* constants to explain and which to consider fundamental.
Second, G,c,h,, etc are actually NOT the fundamental constants! We know because their values are unit dependent. Instead, there are constructs like the fine-structure constant, e^2/hc, which are the same in every (rational) system of units.
Third, there is a very good reason to expect an explanation for the cosmological constant. As an 'energy density of a vacuum', it is easy to guess it should be the value of the 'zero point energy' of all the different particle types. That would be the same for every unit of volume and qualify as an energy density of space.
The problem is that when this calculation is done, it gives a result that is 120 orders of magnitude off the actual result. It has been called the worst theoretical prediction ever made. Even more, it would be easier to 'explain' a cosmological constant that is equal to 0. Anything else requires amazing cancellation to happen: almost, but not quite complete.
And that is why a quantum theory of gravity is thought to be required to explain it. But yes, at this point, it is 'read off the dial' and put into the equations. Well, except for those models that allow it (require it) to change over time.
