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. 

MOND galore and the CMB dipole

There were some new developments in astronomical and cosmological physics. 

The James Webb Space Telescope detected fully formed galaxies much closer in time to the Big Bang than had been expected. But this was actually predicted in a 1997 MOND paper by Bob Sanders, "Cosmology with Modified Newtonian Dynamics". Stacy McGaugh remarks that he also 

correctly anticipated the size of the largest structures collapsing today (things like the local supercluster Laniakea) and the scale of homogeneity (a few hundred Mpc if there is a cosmological constant)

"Aether scalar tensor theory" is considered a promising new framework for MOND. 

At the conference "MOND at 40", widely separated binary systems were a hot topic, with researchers divided on whether or not they show evidence of the MOND force law. 

It's not new but I only just heard about it: there's an attempt to explain the Bullet Cluster in terms of MOND plus sterile neutrinos; see section 7.1 here. 

And finally, something that's not MOND: a proposed explanation for the "CMB dipole", as resulting from gigantic flows in the early universe. 

Mysteries and a martyr

I have a number of physics blogs, with different themes. This one might be regarded as a "phenomenology" blog - it's the most empirically driven, in the sense that here I muse about how to account for things like the dark sector, neutrino properties, and the latest particle physics anomalies. Elsewhere, I focus on more theoretical topics, on standard-model numerology, and so on. 

At the moment there's a sufficient build-up of new empirical issues, that I feel the need to talk about it here. Muon magnetic moment and B-meson decay anomalies have made news lately, and an elementary review of dark matter evidence reminded me that there's more than galactic rotation curves and the third CMB peak to account for. (While I think of them, let's also remember baryogenesis and the cosmological lithium problem.) 

The particle physics anomalies may eventually go away entirely - the B-meson decays might be an error or a fluctuation, and as for muon magnetic moment, maybe we were just doing the theory calculations wrong. If one views the anomalies as real and seeks a neo-minimalist explanation, maybe a leptoquark scalar or two would be best. And if I were seeking explanations that match my theoretical proclivities, perhaps I'd look for flavor gauge bosons related to the Sumino mechanism and to the embedding of the weak force in holographic QCD. 

But while I think things through, I have to note the terrible loss of Marni Sheppeard. She has been a constant presence in my thinking about physics - I cited her in the post just before this one! In a better world, she would still be here, and suggesting her unique perspectives on these new measurements. 

The eV scale IV

Unless I have forgotten something, I do not know a good explanation for where a new, 1 eV scale of physics (e.g. as the mass of a sterile neutrino) would come from. Sheppeard has an idea but it relies on a particular mass spectrum for the active neutrinos, that I don't find compelling.

Reality of a 1 eV neutrino seems to require baroque interactions. They would also be needed to satisfy cosmological constraints (section 3).

Dual purpose Higgs

Someone at a philosophy club asked me about the Higgs boson, and I was saying the usual stuff about how it's the Higgs field which does the real work, and 'the Higgs boson' is actually just a leftover, the only one of four Higgs field excitations which doesn't even get absorbed into a gauge boson... but the Higgs mass does turn out to be important and mysterious since it indicates our vacuum is critically tuned by unknown means and for unknown reasons. Oh, and there's also a theory that the Higgs is the inflaton.

Afterwards I was thinking, isn't it strange that the leftover Higgs boson should happen to be the messenger of criticality? Is there some theoretical framework, in which this neat division of labor between seemingly disparate tasks - electroweak goldstones, and messenger of criticality - makes sense, is to be expected, is the product of a deeper unity? That's the question for today.

Some intuition makes me think of conformal symmetry breaking. As if the Higgs is a top condensate that spontaneously breaks conformal symmetry and electroweak symmetry at the same time, and the mechanics of the process is responsible for giving it a critically tuned mass. But I do not know a mechanism that works this way.

Desert denial

There is lately a debate about whether a new generation of even more powerful particle accelerators would be scientifically warranted. I have said a few times that the LHC's most important discovery may be, not just confirmation of the Higgs boson's existence, but determination of its mass as being around 125 GeV, because that places the standard model vacuum on the edge of instability.

Light scalars have a finetuning problem, and the theoretical expectation was that there are other particles (such as superpartners) that render physics "natural" through quantum cancellations. The failure of such particles to appear, along with the discovery that the Higgs mass is tuned to a critical value, might lead one to suppose that whatever mechanism is responsible for this critical tuning, is also responsible for stabilizing it.

As it happens, the Higgs mass was more or less predicted by Shaposhnikov and Wetterich in 2009, using an alternative quantum gravity paradigm. Even given the attractions of string theory, I always thought their work was mysteriously neglected. But now it occurs to me: maybe it's because their theory is premised on a 'desert' - no new particles to discover at higher energies - so it has no instrumental value when making the case for new colliders.

The eV scale III

It seems unlikely, but since some say MiniBoone is confirming LSND's evidence for a 1 eV sterile neutrino, let me state again the idea of obtaining MOND phonons (as in Berezhiani & Khoury) from a plasma of sterile neutrinos.