The dark sector in mid-2024

It's almost a year since I last posted here. This blog is devoted mostly to physical ideas that are empirically motivated, and within the bounds of theory in a broad sense (e.g. not just numerology, and not wanting to replace quantum field theory). And as far as I know, everything that deserves to be mentioned, pertains to the dark sector. In fact, the main problem is whether I can remember everything that deserves to be mentioned (and the secondary problem is weeding out any false alerts of new physics). But here is what I can think of, right now. 

Let's start with "dark energy". The main empirical problem here is the Hubble tension, a discrepancy between the Hubble constant for the early universe (as determined via CMB observations) and the Hubble constant for the late universe (as determined via distance measurements). I have no opinion on what physics could cause it, or even whether it's real (there are many opinions in the literature). 

During discussions with my collaborator T.L., we ran across a somewhat Machian concept of "Newtonian dark energy" that I find very intriguing, but I don't have a theoretical implementation of it yet. 

A video by "Tibees" brought Beatrice Tinsley to my attention. It turns out she coauthored a paper in favor of an accelerating universe in 1975 (where it is discussed in terms of a positive cosmological constant). But I don't know if any of the arguments in favor, are still valid. 

Then we have "dark matter" (or "modified gravity" explanations of "dark matter" phenomena). The key issue here is whether wide binaries show MOND effects or not. Three papers came out, two saying yes, and one saying no. Stacy McGaugh favors the "yes" paper that made the most cautious claims (by Hernandez et al). 

McGaugh has also just coauthored a paper claiming that the flattened rotation curves of galaxies extend perhaps twice as far as the radius of the usual dark matter halo, but this is very new and hasn't yet been discussed critically. 

Meanwhile, I have also learned of studies claiming that Cassini probe data and cometary data falsify MOND on solar-system scales. On the other hand, there is no serious challenge to MOND on galactic scales. We should be prepared to find that the apparent gravitational force law varies with different scales. 

For reference: a list of attempted refutations of MOND, and refutations of the refutations. 

Finally, I link to my commentary on Sabine Hossenfelder's "7 Strangest Coincidences" in nature. Some of these coincidences suggest that a unified explanation for dark matter and dark energy may be required.