kelly0303 said:
Hello! Are there any areas in physics where we have observational evidence of a phenomena but no solid/widely accepted theoretical explanation, something similar to atomic spectra, photo electric effect, mercury precession (for example) in the beginning of the 20th century? Thank you!
Yes, but not all very definitive or statistically significant ones. Thirty-two of the more notable are the following:
Astronomy and Gravity and Dark Matter and Dark Energy
* The phenomena attributed to "dark matter" absolutely exist and are inconsistent with core theory that is restricted to general relativity and the Standard Model. But
why? Either GR is wrong in weak fields (perhaps due to a quantum gravity effect), or there are particles not found in the Standard Model that exist, or there are forces beyond the canonical four that interact with dark matter, or some combination of those explanations. No extant theory of any variety explains this phenomena at all scales and in all circumstances, although some theories of each type do better than others.
* The observational evidence is not inconsistent with the correct version of General Relativity having a cosmological constant (the lambda in the lambda CDM model of cosmology a.k.a. the "Standard Model of Cosmology"), but there are
other interpretation of "dark energy" phenomena that are not inconsistent with the data and may even fit it a little bit better, and the cosmological constant is one of the harder things to analogize from classical GR to a theory of quantum gravity.
* General relativity is
inconsistent at a theoretical level with the Standard Model, although this inconsistency is rarely a concern as a practical matter because the domains of applicability of the respective theories are so different.
* There are ultra diffuse galaxies that contrary to the usual case (in which the galaxies appear dark matter dominated) have
no apparent dark matter, something that is not easily explained with dark matter particle theories, and with modified gravity theories that do not have what is called an "
external field effect."
* The inferred distribution of dark matter in galactic dark matter halos based upon the dynamics of stars observed in galaxies is
different in shape and density than the NFW (Navarro-Frank-White) distribution which would be expected from theory with cold dark matter particles.
* The
correlation between inferred dark matter effects and the distribution of ordinary matter in galaxies and galaxy clusters is tighter than would be expected from a simple cold dark matter theory.
*
Wide binary stars show dynamics inconsistent with simple GR plus dark matter particles.
* There is no good explanation in lambda CDM for the fact that
elliptical galaxies that are more perfectly spherical have less inferred dark matter than elliptical galaxies are are less perfectly spherical.
* Cold dark matter theory predicts
too many satellite galaxies, relative to what is observed.
* There are https://www.skyandtelescope.com/astronomy-news/tension-continues-hubble-constant/ by different means. It isn't clear why and it is possible that the Hubble constant itself is not a conceptually sound physical constant in the way it is often assumed to be in lambda CDM for example.
* Measurements similar to the cosmic background radiation measurements
made at the 21 centimeter wave length naively appear to be inconsistent with the existence of dark matter at the end of the "radiation era".
*
Galaxies form earlier than would be naively predicted in the lambda CDM model.
* There are indications from the dynamics of solar system objects strongly hints at
the existence of an as yet undiscovered planet similar in size to the Planet Neptune which has not yet been observed.
* Gravitational wave evidence has demonstrated that there are far more
intermediate sized black holes in existence than previously inferred from other data, but we don't know how many there are or what the overall distribution in size, frequency or location is of intermediate sized black holes. Also, why didn't we infer their existence before?
* The Bullet cluster and a couple of other colliding galaxies that have been observed are
inconsistent with plain vanilla cold dark matter theory (the velocities are too high and there seems to be significant self-interaction in the dark matter component) and also with one of the leading modified gravity theories (MOND), but is explained by other modified gravity theories designed to explain dark matter phenomena (e.g. MOG, Conformal Gravity, and Alexandre Deur's work).
*
Galactic clusters have lots of inferred dark matter relative to luminous matter (much more than galaxies). Why is this so?
* There is some evidence that supports a phenomena known as cosmological inflation, but it isn't very conclusive and there are
literally hundreds of significant variations on this theory. Recent Planck cosmic microwave background observations have ruled out many of those possibilities but not ruled out others. What data can be collected to pin this down?
* For the most part, Big Bang Nucleosynthesis accurately predicts the relatively abundance of various kinds of periodic table elements in the early universe. But,
it overestimates the amount of Lithium-7 present in the early universe. Why?
Particle and High Energy Physics
* The
lifetime of the free neutron is longer with one set of experiments, using a particular type of measurement, than another set of experiments making a measurement in a different manner, for reasons not easily explained.
* There is a discrepancy between the proton charge radius observed in ordinary hydrogen and muonic hydrogen (a proton with a muon rotating around it) that has not been adequately resolved. This is the subject on
ongoing investigation. There are strong indications that this is on the verge of being solved.
* The anomalous magnetic moment of the muon (a.k.a.
muon g-2) as measured experimentally is in strong tension with the theoretically calculated value based on the Standard Model (about three and a half standard deviations different, but still consistent with each other to nine significant digits).
* There are notable indications that "lepton flavor universality" (i.e. that electrons, muons and tau leptons have exactly the same properties except mass) is
violated in certain kinds of B meson decays.
* The structure and mass/quantum number spectrum of
scalar mesons and axial vector mesons is not well understood and is the subject of many conflicting explanations.
* Free glueballs (composite particles made up of gluons without quarks bound by the strong force) a.k.a. gluonium are
well understood theoretical in quantum chromodynamics a.k.a. QCD (the part of the Standard Model pertinent to the strong force), and predicted to exist, but have never been definitively observed experimentally.
* The exact mechanism by which the
residual strong force binding protons and neutrons in atomic nuclei is a matter of ongoing disagreement and uncertainty and we use phenomenological models inspired by QCD but not derived rigorously from it, in practice.
* We have not come up with a way to determine the PDFs (
parton distribution functions) of QCD from first principles and instead have to measure them experimentally, even though in theory, they can be derived from the equations and physical constants of the Standard Model. There is a pretty clear pattern but it is hard to derive from first principles.
*
Koide's rule, relating the masses of the electron, the muon and the tau lepton hold true to high precision for no reason within the Standard Model but seems to be a functional relationship that has some sort of non-random or coincidental cause.
* The sum of the square of the masses of the fundamental fermions of the Standard Model is
equal to well within the limits of experimental uncertainty the square of the Higgs vacuum expectation value even though there is no reason within the Standard Model for this relationship to exist. Almost all of the uncertainty in this measurement comes from uncertainty in measuring the masses
Neutrino Physics
* We don't know and there are competing mechanisms to explain the nature of neutrino masses and neutrino oscillation even though we do know that there are either two or three non-zero neutrino masses and the parameters of the oscillations are
increasingly well measured.
* There is disputed evidence regarding whether there is a fourth type of neutrino that oscillates with the three know neutrinos but does not interact via the weak force and hence is called a "
sterile neutrino" (as opposed to an "active neutrino that interacts via the weak force). This is sometimes called the "reactor anomaly".
*
ICE Cube has seen very high energy cosmic rays, some possibly involving neutrinos, whose source and nature are not well understood.
* There are good theoretical reasons to think that
neutrinoless beta decay might exist (even though it is prohibited in the Standard Model), but observations are so far mostly null except for anomalous observations in a Moscow experiment that has not been replicated or explained.
(The links are a convenience sample and there are many articles and even many leading articles on each of these. I have favored review articles and secondary sources in the links for the purposes of familiarizing someone not acquainted with the literature about the basic nature of the issue in each case.)
