There are various unresolved questions in physics about which I have an opinion. I'm not 100% sure of any of them, but more sure of some than others.
In this post, I give my subjective probabilities for various possibilities, in numbers rounded to avoid spurious accuracy and to increments not less than 1% (even if the true probability expressed as 1% is a bit less than 0.5%):
1. Dark matter phenomena:
* Dark matter phenomena are explained by general relativity or subtle modifications or quantum gravity, that only discernible in weak gravitational fields: 90%
* Dark matter phenomena are explained by a 5th force or a singlet ultralight dark matter boson: 6%
* Dark matter phenomena are explained by dark matter particles of micro-eV to TeV mass: 3%
* Dark matter phenomena are explained by dark matter particles of greater than TeV mass (including composite dark matter candidates such as MACHOs, primordial black holes, and stable heavy hadrons in addition to heavy fundamental particles): 1%
2. Dark energy phenomena:
* Dark energy phenomena are an emergent result of the same gravitational effects that give rise to dark matter phenomena (and do not violate mass-energy conservation): 60%
* Dark energy phenomena are equivalent to the cosmological constant of general relativity: 15%
* Dark energy phenomena exist and are fundamental and not just a side effect of dark matter phenomena, but dark energy is not a constant: 15%
* Dark energy phenomena are a result of flawed astronomy methods and don't really exist: 10%
3. The Lambda CDM model:
* The Lambda CDM model is deeply flawed (even though it may be a useful crude first order approximation): 95%
* The Lambda CDM model is basically correct (although it may omit some minor factors like neutrino masses): 5%
4. Cosmological inflation:
* Cosmological inflation did not happen: 85%
* Some form of cosmological inflation happened: 15%
5. Quantum gravity:
* Gravity is fundamentally a quantum phenomena involving gravitons in Minkowski space: 65%
* Gravity arises from a discrete or quantum space-time (whether or not it also has gravitons): 15%
* Gravity is emergent from Standard Model forces: 10%
* Gravity is fundamentally a classical and deterministic phenomena: 10%
6. Universe scale asymmetry:
* The universe is not homogeneous and isotropic at the largest possible scales: 65%
* At the largest possible scales, the universe is homogeneous and isotropic: 35%
7. Maximum density:
* There is no physical constraint on maximum mass-energy density: 65%
* There is a maximum mass-energy density greater than the mass-energy density of a minimum mass stellar black hole (such as a Planck scale limitation): 20%
* There is a maximum mass-energy density close to the mass-energy density of a minimum mass stellar black hole: 15%
8. Supersymmetry:
* There is no version of supersymmetry that exists: 99%
* Some version of supersymmetry exists: 1%
9. String Theory:
* Reality is not fundamentally described by string theory: 98%
* Reality is fundamentally described by string theory: 2%
10. Fundamental fermions:
* The Standard Model includes all of the fundamental particles that are fermions: 95%
* The Standard Model omits up to five fundamental fermions (none of which are additional generations of existing Standard Model fundamental fermions) such as a dark matter particle(s) or right handed neutrinos or supersymmetric partners of Standard Model bosons: 4%
* The Standard Model omits at least one additional generation of Standard Model fermions, and/or omits more than five additional fundamental fermions: 1%
11. Fundamental bosons:
* The Standard Model includes all of the fundamental particles that are bosons other than a possible massless spin-2 graviton: 85%
* The Standard Model omits additional fundamental particles that are bosons beyond a massless spin-2 graviton (e.g. additional Higgs bosons, dark matter bosons, dark matter self-interaction bosons, X17 bosons, bosons involved in neutrino mass generation, bosons involved in cosmological inflation and/or dark energy, fifth force carrying bosons, scalar or vector gravitons, massive gravitons, leptoquarks, supersymmetric partners of Standard Model fermions): 15%
12. Sphalerons:
* Sphaleron interactions are physically possible: 50%
* Sphaleron interactions are not physically possible: 50%
13. Stable heavy hadrons:
* There are no stable or metastable hadrons other than the proton and neutron: 95%
* There are stable or metastable hadrons other than the proton and neutron: 5%
14. Stable heavy elements:
* There are no chemical elements with an atomic number in excess of 118 with a half-life of more than 30 seconds: 65%
* There are chemical elements in "islands of stability" with an atomic number in excess of 118 with a half-life of more than 30 seconds: 35%
15. Neutrino mass:
* Neutrinos have Majorana mass: 10%
* Neutrino mass arises from a see-saw mechanism with one or more heavy right handed neutrinos: 5%
* Neutrino mass arises from some other mechanism not yet widely considered: 85%
16. Sterile neutrinos:
* Right handed sterile neutrinos with the same mass as left handed neutrinos exist: 1%
* One or more sterile neutrinos that oscillate or interact with left handed neutrinos, and masses not identical to left handed neutrinos, exist: 2%
* The three left handed neutrinos of the Standard Model are the only neutrinos that exist: 97%
17. Neutrino mass hierarchy:
* The neutrino masses have a "normal" hierarchy: 95%
* The neutrino masses have an "inverted" hierarchy: 5%
18. CP Violation by neutrinos:
* The PMNS matrix exhibits maximal CP violation: 8%
* The PMNS matrix exhibits near maximal CP violation: 85%
* The PMNS matrix exhibits low levels of CP violation: 5%
* The PMNS matrix does not allow for CP violation in neutrino oscillation: 2%
19. Non-standard neutrino interactions:
* There are no non-standard neutrino interactions (i.e. interactions beyond neutrino oscillations and weak force interactions) to be discovered: 90%
* There are some non-standard neutrino interactions: 10%
20. Lepton number and baryon number violation:
* Lepton number and baryon number are always conserved: 50%
* Lepton number and baryon number are only violated in sphaleron interactions: 45%
* Lepton number and baryon number are violated in non-sphaleron interactions (such as neutrinoless double beta decay, proton decay, flavor changing neutral currents, etc.): 5%
21. LP & C:
* The sum of the squares of correctly defined masses of the fundamental particles is equal to the sum of the Higgs vacuum expectation value: 60%
* The sum of the squares of correctly defined masses of the fundamental particles is not equal to the sum of the Higgs vacuum expectation value: 40%
22. Koide's Rule:
* Koide's rule for the masses of charged leptons is true to at least one part per 100,000: 90%
* Koide's rule for the masses of charged leptons is violated by more than one part per 100,000: 10%
23. An extended Koide's rule for quarks:
* The quark masses obey some extended version of Koide's rule: 70%
* The quark masses do not obey some extended version of Koide's rule: 30%
24. The physics desert:
* There are no beyond the Standard Model high energy physics to be discovered between the highest energy scale reached by the Large Hadron Collider (about 10^4 GeV), and energy scales a billion times greater than the highest energy scale reached by the Large Hadron Collider (about 10^13 GeV): 85%
* There are new high energy physics to be discovered between the highest energy scale reached by the Large Hadron Collider (about 10^4 GeV), and energy scales a billion times greater than the highest energy scale reached by the Large Hadron Collider (about 10^13 GeV): 15%
25. Planet Nine:
* Planet Nine exists: 65%
* Planet Nine does not exist: 35%