4gravitons make an interesting analysis of four different kinds of experiments and their respective likelihoods of discovering something interesting.

Some experiments are almost sure to discover something new because we know nothing about something new except that it is out there to measure. Some experiments are strongly expected to reproduce the status quo and are just dotting i's and crossing t's to more fully establish the range of the status quo with the off chance that an unexpected anomaly is discovered.

In between are theory testing experiments. These can be powerful or merely paradigm confirming, unless a theory can be easily modified to adapt to any set of results, or there are many similar theories, not all of which can be falsified simultaneously.

## 7 comments:

to continue from before,

the most minimal and well motivated extension of the SM includes sterile neutrinos, which can explain leptogenesis and neutrino mass, and axions, which solves strong QCD problem, and both could be dark matter candidates. add on a MOND modification of GR. MOND for galaxy rotation, sterile neutrinos and axions for the other issues like CMB, galaxy clusters, large scale structure.

re Deur, at the very least, it requires gravitons to exist. if gravity is curved spacetime, I'm not clear if the particle associated with curved propagating spacetime is a spin-2 boson.

or to put it another way,

astrophysicists think MOND and dark matter are mutually exclusive,

but why can't they both be correct?

They aren't necessarily mutually exclusive, but t seems unlikely.

Scientists favor solutions that replicate what is observed with the fewest number of new moving parts. Tweaking both gravity and the particle content of the universe adds more parts than just one or the other. Deur's approach, if it works, while adding one part (the graviton) actually takes one away from GR (the cosmological constant).

If you can explain everything that is unexplained with a net zero additional parameters relative to the SM and GR, then that is very attractive.

Also

the SM includes sterile neutrinos, which can explain leptogenesis and neutrino mass, and axions, which solves strong QCD problem.

The strong QCD problem is not a problem so axions aren't necessary to fix it, and neither really is leptogenesis, which can be just the initial conditions of the universe. It would e nice to understand neutrino mass, but there is no good indication that sterile neutrinos exist.

@Andrew

Scientists favor solutions that replicate what is observed with the fewest number of new moving parts.How sure are we that the universe shares that sense of aesthetics?

. It would e nice to understand neutrino mass, but there is no good indication that sterile neutrinos exist.

physicists have 2 hypothesis on its origin, sterile neutrinos and see-saw mechanism, or majorana fermions, which implies neutrinoless beta decay.

thus far neither has been observed, so unless there is a third unknown theory of neutrino mass, neutrino mass is in itself a good indication one of 2 hypothesis is correct

FWIW, I am very comfortable that sterile neutrinos, the see-saw mechanism, or majorana neutrino mass are all incorrect.

I suspect that neutrinos, like all other fermions, have Dirac mass, although not necessarily via the Higgs mechanism.

"How sure are we that the universe shares that sense of aesthetics?"

All of the experimentally measured (as opposed to theoretically derived from experimentally measured) constants of core theory (SM + GR) has a specific experimental basis. It is always possible to create infinitely many models with additional parameters that aren't needed.

It isn't really a sense of aesthetics of the universe, it is a human desire to describe the universe in as compact a manner as is feasible.

If it turns out that we need more parameters, so be it. But, from the perspective of us as users, the fewer parameters we can manage to describe everything with, the better. Our goal as physicists is to develop the minimal model needed to describe everything.

Sometimes that does mean more parameters. The famous quip when the muon was discovered was "who ordered that?" It turns out that we are currently unable to describe everything in the universe without twelve fermion masses, four CKM matrix parameters, four PMNS matrix parameters, four coupling constants (including Newton's constant), Planck's constant, the speed of light, and the cosmological constant, plus some parameters that are not experimentally measured like the number of QCD colors, the number of generations of fermions, the EM charges of the fundamental particles, their parities, and their spins.

Like seemed a lot easier when we had protons, neutrons, electrons, Maxwell's equations, Newtonian gravity, and Newton's laws of motion. But, we had no choice to increase complexity because it was absolutely necessary to explain what we observed.

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