John Moffat's modification of gravity, one version of which is called MOG, is a leading alternative to dark matter theories. A recap at Backreaction
of a recent Perimeter Institute talk identifies an experimental test of his theory that may soon be possible to ascertain, namely, the size of a black hole's "shadow" (a name of the extreme lensing effects near its event horizon).
Is there a theory you find most likely/plausible for explaining this basket of unsolved questions? Or is this a problem of epicycles?
In Einstein's theory of general relativity, the energy associated with the gravitational field aka the curvature of space time, does not give rise to more gravity and is not localized. I think that this is an error and that the most likely solution to dark matter/dark energy is a correction to that theory, in which the mass-energy associated with a gravitational field generates its own gravitational field with a coupling identical to any other mass-energy. This axiom, correctly applied, is probably sufficient to abolish both dark matter and dark energy.
Now, keep in mind that this is merely an informed hypothesis that seems "most likely/plausible" and not an answer.
I would say that we are within a few decades of having enough data to force the right solution or something phenomenologically equivalent to it.
Runners up would be: (1) a modified gravity theory of which MOG is really the current front runner judged by performance, but for which there are several others, (2) scalar field dark matter, (3) a warm dark matter singlet that acts like as a truly sterile neutrino or gravitino (or a triplet with one dominant species), or (4) a simple fairly light sterile to the SM dark matter singlet with a massive boson giving rise to self-interaction of dark matter with itself.
In dark matter scenarios, even if the dark sector is actually more complex, but is dominated by either a single boson type, a single fermion type, or one boson and one fermion type.
Indeed, the GR scenario I propose can be viewed as tensor boson DM which phenomenologically is very similar to scalar boson DM, just as GR itself can be adequately approximated by Newtonian scalar gravitons modified to interact with mass-energy rather than just mass. The tensor character of GR gravity relative to Newtonian scalar gravity produces stunningly subtle phenomenological consequences in weak fields.
"I think that this is an error"
To be clear, what I mean is that Einstein's equations inaccurately describe Nature, not necessarily that Einstein's equations have been incorrectly interpreted mathematically to reach a result that is contrary to the actual predictions of that particular formulation of GR.
Any thoughts on brane cosmology?
And re: a complex dark sector - is there any reason to expect it to be dominated by one fermion or boson?
I think that brane cosmology is highly unlikely and mostly a device to use an easy out to explain the weakness of gravity.
The reason for dominance by one fermion or one boson is that dark matter simulations with a complex dark sector almost uniformly underperform simple dark matter models in replicating what we see in real life.
What sort of assumptions are they making when they have a complex dark sector?
I'd imagine if we were living in a "dark sector" universe, we'd have a lot of trouble guessing at all the structures that are formed from normal matter, no?
* I'm not sure that "assumptions" is the right word. Some of the more obvious and better analyzed complex cold dark matter sectors to test are:
1. Dark matter sectors with three generations of fermions (usually either spin-1/2 as in the Standard Model, or spin-3/2 like the gravitino of SUSY theories) often viewed as corresponding to right handed neutrinos, often with multiple dark sector bosons that are scalars (spin--0) or vector (spin-1), by analogy to the ordinary electro-weak bosons (photons, W and Z). Degenerate versions of this with two fermions, or with fewer dark bosons, have also been modeled.
2. Dark matter sectors with one to one correspondence (or nearly so) to the visible matter sector; for example, producing a "mirror matter" spectrum of particles similar to SUSY or anti-matter.
Of course, one can model the distribution and gravitational effects of regular matter in the universe can be very accurately with a model that assumes that everything in the universe is made of hydrogen and helium that either gather into stars or float as interstellar gas, ignores all other elements, lacks the weak nuclear force, and ignores all ordinary matter other than stars and interstellar gas. The rest of the Standard Model particles (fundamental and composite alike) can be safely ignored in that kind of model with little loss of accuracy. It isn't unreasonable to think that even if a dark sector is complex, that all we will ever be able to discern empirically via gravitational effects, for example, are the one or two dominant components of a far more complex phenomena.
There is no logical reason not to imagine dark matter sectors that are complex but completely different from the ordinary matter particle spectrums that are copied by analogy above, but physicists have only so much imagination that they can summon without some theoretical motivation for doing being imaginative.
Generally, what you do is devise a generalized dark matter simulator computer program that can assign various parameters to various particles, and test of wide range of parameters for every available configuration of particles in the simulation, and then compare the results to the universe that we see in real life to see if there are any similarities.
* Another analytical way to see that the dark sector must either be simple, or dominated by a small number of parameters is the fact that existing singlet or singlet fermion and singlet boson dark matter and modified gravity theories can make pretty decent approximations of what is observed with very few parameters that can be adjusted.
For example, if MOND can accurately describe the rotation curves of all individual galaxies from dwarf galaxies to ellipical galaxies and all phenomena smaller than galaxies with a single experimentally determined parameter (and it can, although it doesn't work for galactic cluster scale phenomena and has issues with stars outside a spiral galaxy's plane), a theory that requires four experimentally determined parameters to describe the same thing is probably artificially complex and Occam's Razor would disfavor it relative to a dark matter theory with just one or two parameters that must be experimentally fixed.
Seeing a shadow of black hole is the old Einstein’s prediction, needs no MOG.
Ethan Siegel is a great astrophysicist and wrote two great articles. I had some conversations with him on them.
One, about the cosmic-standard-candle (I discussed this at your site too), see https://medium.com/@Tienzen/ethan-siegel-this-is-potentially-a-very-very-big-deal-for-our-understanding-of-all-there-is-and-656e1c4a34aa
Two, about CMB, see https://medium.com/@Tienzen/the-unstable-matter-antimatter-pairs-would-annihilate-away-when-the-universe-cooled-below-the-910ad3d9b35
I think that these two conversations could be helpful on this MOG issue.
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