My Confidence In Various Physics Hypotheses

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%

Predictions About Planet Nine

A new preprint sums up some of the expected properties of a hypothetical Planet Nine, which has been inferred from the orbits of other solar system object.

Evidence suggests the existence of a large planet in the outer Solar System, Planet Nine, with a predicted mass of 6.6 +2.6 / -1.7 Earth masses (Brown et al., 2024). Based on mass radius composition models, planet formation theory, and confirmed exoplanets with low mass and radius uncertainty and equilibrium temperature less than 600 K, we determine the most likely composition for Planet Nine is a mini-Neptune with a radius in the range 2.0 to 2.6 Earth radii and a H-He envelope fraction in the range of 0.6 percent to 3.5 percent by mass. Using albedo estimates for a mini-Neptune extrapolated from V-band data for the Solar Systems giant planets gives albedo values for Planet Nine in the range of 0.47 to 0.33. Using the most likely orbit and aphelion estimates from the Planet Nine Reference Population 3.0, we estimate Planet Nines absolute magnitude in the range of -6.1 to -5.2 and apparent magnitude in the range of +21.9 to +22.7. Finally, we estimate that, if the hypothetical Planet Nine exists and is detected by upcoming surveys, it will have a resolvable disk using some higher resolution world class telescopes.

David G. Russell, Terry L. White, "The Radius, Composition, Albedo, and Absolute Magnitude of Planet Nine Based on Exoplanets with Te(q) less than 600 K and the Planet Nine Reference Population 3.0" arXiv:2507.22297 (July 30, 2025).

Improving Top Quark Mass Measurements

Determining the top quark mass precisely is quite important to evaluating many theoretical proposals regarding the source of the experimentally measured mass constants in the Standard Model (equivalently, the pattern to the Higgs Yukawas). 

A new proposal would largely eliminate one of the main sources of systemic error in that measurement, which currently has a combined uncertainty from all sources in an inverse error weighted global average of ± 300 MeV or so. The W boson mass is known to about ± 12 MeV. And, the sources of uncertainty when measuring their masses in collider experiments are highly correlated. So, if the ratio of the top quark mass to the W boson mass can be determined precisely, then the uncertainty in the top quark mass measurement can be greatly reduced.

The top quark mass is a key parameter of the standard model, yet measuring it precisely at the Large Hadron Collider (LHC) is challenging. Inspired by the use of standard candles in cosmology, we propose a novel energy correlator-based observable, which directly accesses the dimensionless quantity 𝑚(𝑡)/𝑚(𝑊). We perform a Monte Carlo study to demonstrate the feasibility of the top mass extraction from Run 2, 3, and High-Luminosity LHC datasets. Our resulting 𝑚(𝑡) can be defined in a well-controlled short-distance mass scheme and exhibits remarkably small uncertainties from nonperturbative effects, as well as insensitivity to parton distribution functions, outlining a roadmap for a record precision measurement at the LHC.

Jack Holguin, et al., "Using the 𝑊 Boson as a Standard Candle to Reach the Top: Calibrating Energy-Correlator-Based Top Mass Measurements" Phys. Rev. Lett. 134, 231903 (13 June, 2025).

Minimal Gravitational Fields

Gravity is an infinite range force. In isolated circumstances, gravitational pulls from opposite directions can cancel out. But, the vast majority of the time, there is at least some small net gravitational pull in one direction or another.

Stacy McGaugh at Triton Station digs into this observation, in both a Newtonian approximation and MOND, to determine that the minimum gravitational acceleration in deep space in MOND (in light of new data about the percentage of baryons that are in deep space) is about 2% of Milgrom's constant a(0).

This is important in MOND in a way that it isn't in conventional general relativity, because "MOND breaks the strong equivalence principle (but not the weak or Einstein equivalence principle)" with its external field effect.

Can Gravity Help Explain Some Standard Model Constants?

An interesting short paper (five pages) argues that the difference between the CKM matrix parameters and those of the PMNS matrix can be explained with an asymptotically safe gravity extension of the Standard Model.

The quark mixing (CKM) matrix is near-diagonal, whereas the lepton mixing (PMNS) matrix is not. We learn that both observations can generically be explained within an ultraviolet completion of the Standard Model with gravity. 

We find that certain relations between CKM matrix elements should hold approximately because of asymptotically safe regimes, including |Vud|^2+|Vus|^2 ≈ 1 and |Vcd|^2+|Vcs|^2 ≈ 1. Theoretically, the accuracies of these relations determine the length of the asymptotically safe regimes. Experimental data confirms these relations with an accuracy of 10^−5 and 10^−3, respectively. This difference in accuracies is also expected, because the ultraviolet completion consists in a fixed-point cascade during which one relation is established already much deeper in the ultraviolet. This results in |Vub|^2 < |Vcb|^2 and translates into measurable properties of B-mesons. 

Similar results would hold for the PMNS matrix, if neutrino Yukawa couplings were large. The ultraviolet complete theory therefore must -- and in fact can -- avoid such an outcome. It contains a mechanism that dynamically limits the size of neutrino Yukawa couplings. Below an upper bound on the sum of Dirac neutrino masses, this allows the PMNS matrix to avoid a near-diagonal structure like the CKM matrix. Thus, large neutrino mixing is intimately tied to small Dirac neutrino masses, ∑mν ≲ (1) eV and a mass gap in the Standard Model fermion masses.

Astrid Eichhorn, Zois Gyftopoulos, Aaron Held, "Quark and lepton mixing in the asymptotically safe Standard Model" arXiv:2507.18304 (July 24, 2025).

Unsolved Physics Problems

 

I would add at least a couple more. 

There are false problems that ask "why doesn't the universe act like I think (for no good reason) that it should?" This includes the hierarchy problem, the strong CP problem, the baryon asymmetry of the universe, and all research invoking the concept of "naturalness."

And, there are contradictory data problems, where one asks why multiple measurements of the same thing (in your current theory) are producing irreconcilable results. These have included the proton radius puzzle, the data based calculation of muon g-2, the measurement of the mean lifetime of unbound neutrons, the reanalysis of CDF data to determine the W boson mass that produced an anomalous result, and the Hubble tension. Usually, in these cases, the answer is that somebody screwed up in one or both of the experiments (at a minimum by overstating the uncertainty in the result), or the theoretical analysis involved, but sometimes, the theory that said the measurements should be the same was wrong.

BSM Physics Constraints In Light Of Muon g-2

The confirmation that the Standard Model prediction for muon g-2 matches the experimental result greatly constrains beyond the Standard Model physics. But how much? 

A new preprint engages with that question.

We review the role of the anomalous magnetic moment of the muon a_mu as a powerful probe of physics beyond the Standard Model (BSM), taking advantage of the final result of the Fermilab g-2 experiment and the recently updated Standard Model value. This review provides both a comprehensive summary of the current status, as well as an accessible entry point for phenomenologists with interests in dark matter, Higgs and electroweak or neutrino and flavour physics in the context of a wide range of BSM scenarios. It begins with a qualitative overview of the field and a collection of key properties and typical results. It then focuses on model-independent, generic formulas and classifies types of BSM scenarios with or without chiral enhancements. A strong emphasis of the review are the connections to a large number of other observables -- ranging from the muon mass and the muon--Higgs coupling and related dipole observables to dark matter, neutrino masses and high-energy collider observables. Finally, we survey a number of well-motivated BSM scenarios such as dark photons, axion-like particles, the two-Higgs doublet model, supersymmetric models and models with leptoquarks, vector-like leptons or neutrino mass models. We discuss the impact of the updated Standard Model value for a_mu and of complementary constraints, exploring the phenomenology and identifying excluded and viable parameter regions.

Peter Athron, Kilian Möhling, Dominik Stöckinger, Hyejung Stöckinger-Kim, "The Muon Magnetic Moment and Physics Beyond the Standard Model" arXiv:2507.09289 (July 12, 2025) (Invited review for Progress in Particle and Nuclear Physics; 274 pages, 50 figures).

A Hubble Tension Recap

The Hubble tension has, for whatever reason, been treated as a more serious challenge to the LambdaCDM "standard model of cosmology", which contrary to the statement highlighted below in the abstract, actually has many other serious discrepancies with astronomy observations. A new preprint examines its implications for the model.

Differences in the values of the Hubble constant obtained from the local universe and the early universe have resulted in a significant tension. This tension signifies that our understanding of cosmology (physical processes and/or cosmological data) is incomplete. Some of the suggested solutions include physics of the early Universe. 

In this paper we aim to investigate common features of various early universe solutions to the Hubble constant tension. The physics of the early universe affects the size of the sound horizon which is probed with the Cosmic Microwave Background (CMB) data. Within the standard model, the size of the horizon (within limits of current measurements) is affected by processes that could occur between (approximately) 1 day after the Big Bang and the last scattering instant. We focus on simple extensions incorporating Early Dark Energy (EDE) and show how such a model affects the inferred values of the Hubble constant. We compare this model to LambdaCDM models using MCMC analysis, likelihoods over the parameter space and Bayesian evidence. The MCMC analysis shows that EDE leads to a decrease in the size of the sound horizon that is consistent with H0 = 73.56 km/s/Mpc but we also show that MCMC analysis favours increasing redshift and proportion of EDE. The Bayesian evidence favours our EDE model for very narrow, finely-tuned parameter space. 

The LambdaCDM model used for comparison has good evidence across a wide parameter space. We interpret this as an indication that more sophisticated models are required. We conclude that if the Hubble tension were to be related to the physics of the early universe, EDE could be used as a window to explore conditions of the early universe and extend our understanding of that era.

Gawain Simpson, Krzysztof Bolejko, Stephen Walters, "Beyond LambdaCDM: How the Hubble tension challenges early universe physics" arXiv:2507.08479 (July 11, 2025).

The History And Prehistory Of Human Disease

A new paper in Nature concludes from ancient DNA that while infectious diseases were common in humans since the hunter-gatherer era, that there was a real surge, not at the time of the Neolithic Revolution, but when steppe herders started to invade and conquerer farmers, and hunter-gatherers, possibly because they lived more closely with their animals and because the diseases that they carried helped facilitate their conquests. The New York Times also discusses the paper.

Infectious diseases have had devastating effects on human populations throughout history, but important questions about their origins and past dynamics remain. To create an archaeogenetic-based spatiotemporal map of human pathogens, we screened shotgun-sequencing data from 1,313 ancient humans covering 37,000 years of Eurasian history. We demonstrate the widespread presence of ancient bacterial, viral and parasite DNA, identifying 5,486 individual hits against 492 species from 136 genera. Among those hits, 3,384 involve known human pathogens, many of which had not previously been identified in ancient human remains. Grouping the ancient microbial species according to their likely reservoir and type of transmission, we find that most groups are identified throughout the entire sampling period. Zoonotic pathogens are only detected from around 6,500 years ago, peaking roughly 5,000 years ago, coinciding with the widespread domestication of livestock. Our findings provide direct evidence that this lifestyle change resulted in an increased infectious disease burden. They also indicate that the spread of these pathogens increased substantially during subsequent millennia, coinciding with the pastoralist migrations from the Eurasian Steppe

Martin Sikora, et al., "The spatiotemporal distribution of human pathogens in ancient Eurasia" Nature (July 9, 2025).

All the GUTs Worth Considering

A fairly short new paper (five pages plus seven pages of footnotes and an appendix) tries to list most or all of the possible Grand Unified Theories a.k.a. GUTs (i.e. theories the unify the three Lie groups of the Standard Model, but not gravity, into a single unified mathematical structure; unified theories that also include gravity are called Theories of Everything a.k.a. TOEs) that could include the Standard Model of Particle Physics, or an extension of it. 

There aren't all that many possibilities that are promising, and several decades of attempts to fit the Standard Model into one in a way that provides useful theoretical insight has not been very fruitful. While this line of inquiry isn't as troubled as supersymmetry (which is a dead man walking) or string theory (which is almost as troubled), it isn't very "hot" either.

Many potential GUTs, including the most minimal SU(5) GUT, would (1) imply violations of baryon number and/or lepton number conservation that aren't observed (e.g. proton decay, flavor changing neutral currents, and neutrinoless double beta decay), (2) lack some fundamental particles that are observed in the Standard Model, or (3) imply the existence of new fundamental particles beyond the Standard Model that haven't been observed (and in some cases, these particles have been ruled out to quite high energies). 

As a general rule, the bigger the Lie group of the unifying GUT, the more likely it is that it will imply far more new fundamental particles than there is any good reason to think that even a many particle dark sector should contain. Theoretical physicists prefer GUTs that imply as minimal an extension of the Standard Model as possible. Moreover, GUTs with certain kinds of new fundamental particles, such as those that imply more than three generations of fundamental Standard Model fermions, are strongly disfavored.

The experimental constraints on baryon number violating and lepton number violating processes (outside sphaleron interactions which are predicted in the Standard Model at extremely high energies but have not been observed) like proton decay, flavor changing neutral currents, and neutrinoless double beta decay are both very strict and very robust (i.e. they have been tested in multiple, independent ways). The exclusions of new fundamental particles are generally up to masses of several hundred to many thousands of GeVs, which is less strict, and the possibility of beyond the Standard Model fundamental particles is also strongly motivated (although not compelled) by the existence of dark matter phenomena. 

In the early days of GUT theories, a much sought after GUT property was that the three Standard Model forces unify at high enough energies in a manner that echos electroweak unification theory (which was one of the very attractive features of supersymmetry theory). But this has also been elusive. 

The Standard Model beta functions of the three Standard Model forces (electromagnetism, the weak force, and the strong force), which govern how the strength of these forces change with energy scale, extrapolated to arbitrarily high energy scales, based upon data all of the way up to the energy scales that can be reached by the Large Hadron Collider a.k.a. LHC (the highest energy scale high energy physics experiment every conducted), never unify. So, if a GUT the unifies the three Standard Model forces exists is some high energy scale, this must be due to new physics at energy scales above those that can be experimentally probed so far that is outside the domain of applicability of the Standard Model. 

Basically, given the energy scales that have already been reached by the LHC, energies at which the three Standard Model force could possibly unify haven't been present anywhere in the universe since some fraction of a second elapsed after the Big Bang. Of course, it is entirely possible that the three Standard Model forces simply don't unify at any energy scale that has ever existed or ever could exist.

Under a reasonable set of ab-initio assumptions, we define and chart the atlas of simple gauge theories with families of fermions whose masses are forbidden by gauge invariance. We propose a compass to navigate the atlas based on counting degrees of freedom. When searching for Grand-unification Theories with three matter generations, the free energy singles out the SU(5) Georgi-Glashow model as the minimal one, closely followed by SO(10) with spinorial matter. The atlas also defines the dryland of grand-unifiable gauge extensions of the standard model. We further provide examples relevant for gauge dual completions of the standard model as well as extensions by an additional SU(N) gauge symmetry.

Giacomo Cacciapaglia, Aldo Deandrea, Konstantinos Kollias, Francesco Sannino, "Grand-unification Theory Atlas: Standard Model and Beyond" arXiv:2507.06368 July 8, 2025).

The final paragraph of the conclusion of the main paper also enumerates some limitations on this paper serving as a truly comprehensive list of possibilities:

We have not considered yet scalar fields, as their mass cannot be prevented by any symmetry. Including spontaneous symmetry breaking of the gauge symmetry and generation of Yukawa couplings could imprint further constraints on the atlas, providing a phenomenological compass to navigate us towards the optimal high-energy theory. In our analysis, asymptotic freedom plays a crucial role in counting the degrees of freedom of each theory.

Non-Linear Cosmology Dynamics

Assuming the data has a Gaussian distribution (i.e. is distributed in a "normal" probability curve) is often reasonable, since this is what happens when data comes from independent simple percentage probability events. And, it is a convenient assumption when it works, because mathematically it is much easier to work with Gaussian distributions than most other probability distributions. But, sometimes reality is more complicated than that and this assumption isn't reasonable. 

The supernova data used to characterize dark energy phenomena isn't Gaussian. 

Trivially, this means that statistical uncertainty estimates based upon Gaussian distributions overestimate the statistical significance of observations in the fat tailed t-distribution. 

Non-trivially, this means that the underlying physics of dark matter phenomena are more mathematically complex than something like Newtonian gravity (often assumed for astronomy purposes as a reasonable approximation of general relativity) or a simple cosmological constant. Simple cosmology models don't match the data. 

This paper estimates dark energy parameters for more complex dark energy models that can fit the data.

Type Ia supernovae have provided fundamental observational data in the discovery of the late acceleration of the expansion of the Universe in cosmology. However, this analysis has relied on the assumption of a Gaussian distribution for the data, a hypothesis that can be challenged with the increasing volume and precision of available supernova data. 

In this work, we rigorously assess this Gaussianity hypothesis and analyze its impact on parameter estimation for dark energy cosmological models. We utilize the Pantheon+ dataset and perform a comprehensive statistical, analysis including the Lilliefors and Jarque-Bera tests, to assess the normality of both the data and model residuals. 

We find that the Gaussianity assumption is untenable and that the redshift distribution is more accurately described by a t-distribution, as indicated by the Kolmogorov Smirnov test. Parameters are estimated for a model incorporating a nonlinear cosmological interaction for the dark sector. The free parameters are estimated using multiple methods, and bootstrap confidence intervals are constructed for them.

Fabiola Arevalo, Luis Firinguetti, Marcos Peña, "On the Gaussian Assumption in the Estimation of Parameters for Dark Energy Models" arXiv:2507.05468 (July 7, 2025).

Steppe Ancestry In Italy

Blonde hair percentages, at a population statistics level, is a good proxy for Indo-European steppe ancestry levels (it's not as good as autosomal DNA, but the sample size and amount of fine grained geographic detail is much better). You'd need an estimate for the amount of steppe ancestry in Italians to calibrate this litmus test, however.

The first farmers of Europe had essentially 0% blonde hair, much like modern Sardinians, who are their closest genetic match. Blonde hair in Europe arrived more or less exclusively via steppe migration in late Neolithic to early Bronze Age from an ultimate homeland in the vicinity of modern Ukraine, although plenty of migration happened within Europe after this migration and not all steppe migrants had blonde hair. It is also possible to have very little steppe ancestry while still having the blonde hair gene. 

The chart shows the percentage of blond haired people in the regions shown on the map in (or near) Italy. Overall, about 8% of Italians are naturally blonde (another estimate suggests 15%). It suggests that Indo-European migration to Italy was largely north to south (with exceptions for urban centers) and reached southern Italy in far smaller proportions than northern Italy, although it is hard to know how much of the migration was modern, how much was medieval, how much was from the Roman era, and how much dates to pre-history.

Until the late 1870s, Italy was not a unified country, with Southern Europe belonging to the poorer Kingdom of the Two Sicilies with a more agricultural economy, and Northern Europe belonging to a number of smaller and more prosperous states with more mercantile economies, which could have impacted migration patterns by increasing migration from areas with more blonde people. 

From Reddit.

In the medieval era Northern Europeans, including the Normans and Vikings and Germanic tribes, had greater interactions with Northern Italy than with Southern Italy, as well. 

In the Roman era, migration to the Roman capital and its major cities from North Africa, Egypt, and the Levant might have diluted the percentage of people with steppe ancestry.

Shortly before the classical Roman era, there were a number of Greek colonies in Italy, which could be reflected in the purple regions on the map (about 4% of Greeks are naturally blonde), with some blurring out due to admixture with regions near former Greek colonies.

From Wikipedia.

A New Strong Force Coupling Constant Determination

The Particle Data Group value for the strong force coupling constant is 0.1180 ± 0.0009. This new determination, based upon earlier runs of LHC dijet data and lower energy HERA data, is consistent with the PDG value at the 0.1 sigma level. 

The strong force coupling constant is pervasively important in almost all high energy physics calculations, but it known much less precisely (with just one part per 131 parts precision) than most other Standard Model or fundamental physical constants. So, pinning this down more precisely is always big deal.

The beta function that describes how the strong force coupling constant runs with energy scale is an exact theoretical prediction of the Standard Model, with no experimental uncertainties. The conference presentation's confirmation that the strong force coupling constant runs with energy scale just as predicted in the Standard Model, over four orders of magnitude of energy scale, is arguably an even more important confirmation of the Standard Model, because there are fewer experimental confirmations of this in the literature.

In this talk we present a determination of the strong coupling constant αs and its energy-scale dependence based on a next-to-next-to-leading order (NNLO) QCD analysis of dijet production. 

Using the invariant mass of the dijet system to probe αs at different scales, we extract a value of αs(mZ) = 0.1178 ± 0.0022 from LHC dijet data. 

The combination of various LHC datasets significantly extends the precision and scale reach of the analysis, enabling the first determination of αs up to 7 TeV. By incorporating dijet cross sections from HERA, we further probe αs at smaller scales, covering a kinematic range of more than three orders of magnitude. Our results are in excellent agreement with QCD predictions based on the renormalization group equation, providing a stringent test of the running of the strong coupling across a wide energy range.

João Pires, "Precision determination of αs from Dijet Cross Sections in the Multi-TeV Range" arXiv:2507.01670 (July 2, 2025) (Contribution to the 2025 QCD session of the 59th Rencontres de Moriond).

A New Relativistic Generalization Of MOND (And More)

This six page article is just a conference paper summary of a much more involved modified gravity theory and its implications. The abstract is silent on how well it handles galaxy cluster physics, which deviate (in a quite systemic way) from simple toy-model MOND theories, or the Hubble tension.

We propose an alternative scalar-tensor theory based on the Khronon scalar field labeling a family of space-like three-dimensional hypersurfaces. This theory leads to modified Newtonian dynamics (MOND) at galactic scales for stationary systems, recovers GR plus a cosmological constant in the strong field regime, and is in agreement with the standard cosmological model and the observed cosmic microwave background anisotropies.

Luc Blanchet, Constantinos Skordis, "Khronon-Tensor theory reproducing MOND and the cosmological model" arXiv:2507.00912 (July 1, 2025) (Contribution to the 2025 Gravitation session of the 59th Rencontres de Moriond).

A fuller explanation of the theory can be found here.

Another lengthy paper by P. S. Bhupal Dev et al., examines the constraints dark matter-neutrino interactions which are very strict.

We present a comprehensive analysis of the interactions of neutrinos with the dark sector within the simplified model framework. We first derive the exact analytic formulas for the differential scattering cross sections of neutrinos with scalar, fermion, and vector dark matter (DM) for light dark sector models with mediators of different types. We then implement the full catalog of constraints on the parameter space of the neutrino-DM and neutrino-mediator couplings and masses, including cosmological and astrophysical bounds coming from Big Bang Nucleosynthesis, Cosmic Microwave Background, DM and neutrino self-interactions, DM collisional damping, and astrophysical neutrino sources, as well as laboratory constraints from 3-body meson decays and invisible Z decays. 

We find that most of the benchmarks in the DM mass-coupling plane adopted in previous studies to get an observable neutrino-DM interaction effect are actually ruled out by a combination of the above-mentioned constraints, especially the laboratory ones which are robust against astrophysical uncertainties and independent of the cosmological history. 

To illustrate the consequences of our new results, we take the galactic supernova neutrinos in the MeV energy range as a concrete example and highlight the difficulties in finding any observable effect of neutrino-DM interactions. 

Finally, we identify new benchmark points potentially promising for future observational prospects of the attenuation of the galactic supernova neutrino flux and comment on their implications for the detection prospects in future large-volume neutrino experiments such as JUNO, Hyper-K, and DUNE. We also comment on the ultraviolet-embedding of the effective neutrino-DM couplings.