X17 News

The viable parameter space for the hypothetic X17 particle (with a mass of about 17 MeV) proposed to explain some unexpected nuclear physics is very nearly null.

In recent years, the ATOMKI collaboration has performed a series of measurements of excited nuclei, observing a resonant excess of electron-positron pairs at large opening angles compared to the Standard Model prediction. 

The excess has been hypothesized to be due to the production of a new spin-1 or spin-0 particle, X17, with a mass of about 17 MeV. 

Recently, the PADME experiment has reported an excess in the e+e− cross section at center-of-mass energies near 17 MeV, perhaps further hinting at the existence of a new state. Studies of the spin-1 case have hitherto focused on either vector or axial-vector couplings to quarks and leptons, whereas UV theories more naturally produce both vector and axial-vector i.e. chiral couplings, analogous to the Standard Model weak interactions. 

We consider the ATOMKI anomalies in the context of an X with chiral couplings to quarks and explore the parameter space that can explain the ATOMKI anomalies, contrasting them with experimental constraints. 

We find that it is possible to accommodate the reported ATOMKI signals. However, the 99% CL region is in tension with null results from searches for atomic parity violation and direct searches for new low mass physics coupled to electrons. This tension is found to be driven by the magnitude of the reported excess in the transition of 12C(17.23), which drives the best-fit region towards excluded couplings.

Max H. Fieg, Toni Mäkelä, Tim M.P. Tait, Miša Toman, "The X17 with Chiral Couplings" arXiv:2602.11263 (February 11, 2026).

A Provocative Emergent Gravity Theory

This essay argues that gravity emerges from the running of physical constants with energy scale (called the Renormalization Group flow), and that this viewpoint can guide us to a viable theory of quantum gravity. It explains why this approach is not ruled out by "no go" theorems in the quantum gravity field, and what the existing paradigm for trying to develop a theory of quantum gravity may ge futile.

It's only ten pages long and more readable than many papers on the topic, so give it a read.

In this essay and utilizing the holographic Renormalization Group (RG) flow, we demonstrate how the effective action of a non-gravitating quantum field theory in the ultraviolet (UV) develops an Einstein-Hilbert term in the infrared (IR). That is, gravity is induced by the RG flow. 

An inherent outcome of holography that plays a crucial role in our analysis is the RG flow of boundary conditions: the rigid Dirichlet conditions on the background metric in the UV become an admixture of Dirichlet and Neumann as we flow to the IR, thereby ``unfreezing'' the metric and transforming it from a non-dynamical background into a dynamical field. 

This mechanism, which is a conceptually new addition to the standard Wilsonian RG flow, also provides the mechanism to evade the Weinberg-Witten no-go theorem. 

Within the GR from RG picture outlined here, the search for a quantum theory of gravity by treating the metric as a fundamental field may be a hunt for a phantom -- akin to seeking the atomic structure of water by quantizing the equations of hydrodynamics.

M.M. Sheikh-Jabbari, V. Taghiloo, "GR from RG: Gravity Is Induced From Renormalization Group Flow In The Infrared" arXiv:2602.11806 (February 12, 2026) (Essay written for the Gravity Research Foundation 2026 Awards for Essays on Gravitation).

The S8 Tension

The parameter S(8) quantifies how homogeneous the entire Universe is in terms of matter density, with lower values being more homogeneous than larger values. At higher values, matter is more concentrated in clumps and webs of high matter density, while comparative cosmic voids are bigger and more deep. At lower values, the amount of matter in a volume of space doesn't vary as much across the universe.

S(8) appears to vary between the early-universe and late universe, even though in the paradigmatic ΛCDM model of cosmology, which has been battered by numerous contradictions with astronomy observations, this parameter should remain the same. This tension has also been parallel to the Hubble tension, causing many astrophysicists to suspect that  they have a common cause.

The S8 tension between the early-universe and late universe, however, may be substantially a function of systemic measurement errors, rather than a real phenomena, as a new review article observes.

The parameter S(8)≡σ(8)*(Ωm/0.3)^0.5 quantifies the amplitude of matter density fluctuations. A persistent discrepancy exists between early-universe CMB observations and late-universe probes. 

This review assesses the ``S8 tension'' against a new 2026 baseline: a unified ``Combined CMB'' framework incorporating Planck, ACT DR6, and SPT-3G. This combined analysis yields S(8) = 0.836 + 0.012 − 0.013, providing a higher central value and reduced uncertainties compared to Planck alone. 

Compiling measurements from 2019-2026, we reveal a striking bifurcation: 

DES Year 6 results exhibit a statistically significant tension of 2.4σ--2.7σ (DESY6), whereas KiDS Legacy results demonstrate statistical consistency at <1σ (Wright2025). 

We examine systematic origins of this dichotomy, including photometric redshift calibration, intrinsic alignment modeling, and shear measurement pipelines. We further contextualize these findings with cluster counts (where eROSITA favors high values while SPT favors low), galaxy-galaxy lensing, and redshift-space distortions. The heterogeneous landscape suggests survey-specific systematic effects contribute substantially to observed discrepancies, though new physics beyond ΛCDM cannot be excluded.

Ioannis Pantos, Leandros Perivolaropoulos, "Status of the S8 Tension: A 2026 Review of Probe Discrepancies" arXiv:2602.12238 (February 12, 2026).

The Amazon Is Older Than The Andes

 The fascinating deep history of the Amazon River.

Experimental Bounds On Baryon And Lepton Number Non-Conservation

Baryon number (B) conservation means that the number of quarks minus the number of anti-quarks in any interaction remains constant. Lepton number (L) conservation means that the number of leptons (electrons, muons, tau leptons, and neutrinos) minus the number of anti-leptons in any interaction remains constant.

The Standard Model separately conserves B and L in all interactions except sphaleron interactions, which have been never observed and are theoretically confined to extremely high energy scales and mass-energy densities, which the Large Hadron Collider (LHC) (the most powerful particle collider of all time), cannot reach.

The conservation of baryon number and lepton number is established remarkably robustly in experiments.

Some of the main experimental searches that have not detected B and L non-conservation are the searches for neutrinoless double beta decay, the search for tree-level flavor changing neutral currents, and the search for proton decay. These non-detections have ruled out or tightly constrained many theories in physics including Majorana neutrino mass and most of the simpler grand unified theories (GUTs), such as SU(5).

Baryon number (B) conservation underlies the apparent stability of ordinary matter by forbidding the decay of nucleons, while lepton number (L) conservation plays a central role in the structure of lepton interactions and the possible origin of neutrino mass. 

In the Standard Model, B and L are accidental global symmetries rather than imposed fundamental principles. However, they are expected to be violated in many extensions of the theory, including frameworks of unification and processes in the early Universe. 

This review summarizes the status of experimental tests of B and L conservation and discusses them within a unified framework for interpreting current and future searches across different processes and experimental approaches, outlining historical and theoretical motivation, key physical processes, as well as their broader connections and complementarity to other searches.

Volodymyr Takhistov, "Experimental Tests of Baryon and Lepton Number Conservation" arXiv:2602.09097 (February 9, 2026).

A Catalog Of WISP Theories

A new preprint has a catalog (with references) of beyond the Standard Model theories that are "Weakly Interacting Slim Particle" (WISP) theories. Wikipedia explains the concept, which the abstract and introduction fail to do:

In particle physics, the acronym WISP refers to a largely hypothetical weakly interacting sub-eV particle, or weakly interacting slender particle, or weakly interacting slim particle – low-mass particles which rarely interact with conventional particles.

The term is used to generally categorize a type of dark matter candidate, and is essentially synonymous with axion-like particle (ALP). WISPs are generally hypothetical particles.

WISPs are the low-mass counterpart of weakly interacting massive particles (WIMPs).

The goal of the project is as follows: 

The search for physics beyond the Standard Model (SM) has led to the proposal of a vast landscape of theoretical frameworks. Among them, the family of Weakly Interacting Slim Particles (WISPs) has emerged as a particularly rich and versatile class of candidates, capable of addressing open questions in cosmology, astrophysics and particle physics. 

These particles, ranging from axions and axion-like particles to hidden photons, scalars, pseudoscalars, sterile neutrinos and spin-2 particles, illustrate the growing diversity of ideas within the field.  

The WISPedia is motivated by the need for a unified and systematic reference that organises this rapidly expanding model space. While numerous reviews exist on specific Weakly Interacting Slim Particle (WISP) candidates or experimental searches, the goal of this work is different: to provide a concise, model-oriented encyclopedia that outlines the essential ingredients of each framework– its particle content, interactions and phenomenological role, while pointing the reader toward the original literature and key complementary resources. Rather than serving as an exhaustive review, the WISPedia aims to serve as a quick, structured gateway into the theory landscape of light, weakly coupled particles. It also provides some information on bounds for each of them in a succinct way.

It's top level categorization is by the spin (a.k.a. intrinsic angular momentum a.k.a. "J") and parity of each kind of the Beyond the Standard Model (BSM) particles. 

It also has a very cute table of contents that summarizes some of the high points of each model. It uses emojis to annotate it. This cute legend is what inspired me to make this post, even though, like any catalog of BSM theories, the vast majority of theories discussed don't reflect reality and are "garbage theories" (not in the sense of being technically unsound, but in the sense of being ill-motivated and improbable).

This list of models currently in the catalog, envisioned as a Wikipedia-like or Particle Data Group-like encyclopedia of BSM particle theories that fit the (ill-defined) WISP paradigm, is as follows:

MOND Better In Clusters Than Previously Believed

The discovery that there is more non-stellar ordinary mass in galaxies and galaxy clusters than previously known makes MOND perform better than than previously believed.

In the framework of Milgromian dynamics (MOND), galaxy clusters are known to exhibit a residual missing mass problem, with the baryonic mass falling short of the dynamical mass by about a factor of two. 

The baryon content of clusters is dominated by the intracluster medium (ICM), while the stellar contribution depends sensitively on the assumed stellar initial mass function (IMF). 

We re-evaluate the stellar and remnant masses in galaxy clusters by adopting the integrated galaxy-wide initial mass function (IGIMF) theory, which accounts for the dependence of the IMF on galaxy properties and star formation histories. Massive elliptical galaxies, characterized by high metallicities and short formation timescales, are inferred to form with top-heavy IMFs, leading to a substantial population of stellar remnants. 

Using observational data from WINGS and 2MASS for 46 nearby (z < 0.1) galaxy clusters, we compute stellar, remnant, and intracluster light masses and combine them with previously derived ICM masses. The resulting total baryonic masses are compared to MOND dynamical masses inferred from hydrostatic equilibrium. 

We find that the baryonic mass in stars, remnants and the ICM accounts for at least 88+5+2−4−1% of the MOND dynamical mass. This constrains the kick velocities of the remnants and substantially alleviates the missing mass problem for galaxy clusters in MOND.

Dong Zhang, Akram Hasani Zonoozi, Pavel Kroupa, "Revisiting the missing mass problem in MOND for nearby galaxy clusters" arXiv:2602.06082 (February 4, 2026) (accepted by PDR).

My name is Bond. Molecular Bond.

Ian Fleming*, "Molecular Orbitals and Organic Chemical Reactions" (2009).

*not the James Bond author.

The Physicists And Mr. Epstein

Not my usual fare, but it belongs here, from Matt Strassler's blog:

The Physicists and Mr. Epstein

Mr. Epstein was not only a world-class child abuser, he was a big fan of theoretical high-energy physics and of theoretical physicists. Some of my colleagues, unfortunately, got to know him. A number who were famous and/or had John Brockman as a book agent were even invited to a physics conference on Epstein's private island, well before he was first arrested.... 

More Evidence That The Standard Model Still Works

The ATLAS Paper

Once again, a search for beyond the Standard Model particles comes up empty and places strict limits on the parameter space of such particles. Also, the author list for this 27 page long paper is 17 pages long.

A model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb−1, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018.

The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. 

The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces.

ATLAS Collaboration, "Search for dimuon resonance in the 35 to 75 GeV mass range using 140 fb−1 of 13 TeV pp collisions with the ATLAS detector" arXiv:2601.21361 (February 2, 2026) (44 pages in total, author list starting page 27, 9 figures, 3 tables, submitted to JHEP).

The introduction to the body text of the paper notes that:

Searches for low-mass dimuon resonances have been performed by the CMS and LHCb Collaborations, covering mass ranges of 1.1–7.9 GeV, 11.5–75 GeV and 110–200 GeV for CMS, and 30.214–70 GeV for LHCb.

Those searches also came up empty. 

The trickiest mass range to study of those already studied is the 1.1-7.9 GeV mass range which has lots of different hadron resonances that decay in a great many different ways, with each decay having its own probably of occurring, generating substantial background noise, even though the backgrounds are well understood.

The range from 11.5-75 GeV has very little background noise, because it exceeds all but the heaviest hadron resonance masses (with most hadrons predicted to have masses above 11.5 GeV having never been definitively observed even in the numerous and extremely high energy collisions of the LHC), but it is comfortably less than the W boson mass (roughly 80.4 GeV), the Z boson mass (roughly 91.2 GeV), or the Standard Model Higgs boson mass (roughly 125.1 GeV).

In the 110-200 GeV mass range, the only significant backgrounds that can have dimuon decays are single Standard Model Higgs bosons (roughly 125.1 GeV), W boson pairs (roughly 160.8 GeV), and Z boson pairs (roughly 182.4 GeV). 

So, the total observations of dimuon resonances should have three very precisely predictable bumps and can often be confirmed to be background events because additional decay products in addition to the dimuons are observed. The decays of these background processes to particles other than dimuon pairs can also be used to calibrate the expected number of background dimuons for each of these three resonances.

For example, you can estimate the total Higgs boson production from the number of b quark pair decays that are observed by using that to determine how many dimuon decays from Higgs bosons should be expected, since the ratio of the b quark pair branching fraction of Higgs boson decays to the dimuon branching fraction of Higgs boson decays can be theoretically predicted to high precision. And, when you know how many background dimuon events you expect to see from Higgs boson decays, you can subtract that background from the observed number of dimuon events to determine if there is any beyond the Standard Model particle decay signal in the vicinity of the 125.1 GeV Higgs boson mass. 

You can do something similar for W boson pair decays and Z boson pair decays.

The 110-200 GeV mass range range is far more massive than any predicted Standard Model hadrons or any single W or Z boson, however. But, it is also far less than than the combined mass of a Higgs boson pair (roughly 250.2 GeV) or a top quark-antitop quark pair (which is roughly 345 GeV) or a Toponium meson (which is just a bit more massive that an unbound pair of oppositely charged top quarks).

Charged leptons, like those found in dimuon decays (which also only decay in turn to electrons quite slowly compared to other conceivable decay products with their roughly one microsecond mean lifetime, that turns out to be longer from an outside observer's perspective due to special relativity) are easy for the detectors at the LHC to see, so there are few false negatives. 

The rest mass of a dimuon pair is about 0.21 GeV, so even with an invariant mass of 1.1 GeV, the special relativistic kinetic energy of the dimuon pair is about four times its rest mass, so the pair of charged leptons will be traveling at very close to the speed of light, and the closer to the speed of light that the muons are traveling at is, the slower time passes in their rest frame relative to the rest frame of an outside observer. So, from the perspective of an outside observer, the dimuon pair takes much more than the microsecond of a muon at rest to decay.

Furthermore, a lot of false positive dimuon decays would be accompanied by additional detectible decay products that could distinguish those events from the pure dimuon decay signal that the experimenters were looking for in this paper and its companion papers over other invariant mass ranges.

So, these measurements can be quite precise, and can rule out even quite small beyond the Standard Model signals in these mass ranges.

The CMS Paper

In another recent paper, not only does the data on W boson pair production at the LHC confirm the Standard Model, it also strongly suggests that both the experimental and theoretical uncertainties are highly conservative estimates that greatly overstate the true uncertainty (something that is commonly seen in measurements of electroweak phenomena in high energy physics experiments). 

The first measurement is 0.08 sigma away from the predicted value, and the second is 0.10 sigma away from the predicted value. If a prediction and experiment were repeatedly conducted at random and had those uncertainties, the difference would average 1 sigma. So, the results are 10-12 times closer to each other than would be predicted by random chance given the stated uncertainties. It is very unlikely that this particular experiment is such a statistical fluke. 

This particularly unlikely given that this unexpected closeness between the experimentally measured value and the predicted value is seen in a large share of all high energy physics experiments involving electroweak phenomena but not the strong force, which prevents this seemingly fluke result being due to look elsewhere effects that would undermine their global statistical significance.

So, while the stated experimental uncertainties in the total production cross-section are on the order of ± 12%, the actual uncertainties are closer to being on the order of ± 1%. And, while the stated experimental uncertainties in the fiducial production cross-section are on the order of 20%, the actual uncertainties are closer to being on the order of ± 2%.

This analysis presents an observation of the photon-fusion production of W boson pairs using the CMS detector at the LHC. The total cross section of the W+W− production in photon fusion is measured using proton-proton collision data with an integrated luminosity of 138 fb−1 collected with the CMS detector in 2016−2018 at a center-of-mass energy of s√ = 13 TeV. Events are selected in the final state with one isolated electron and one isolated muon, and no additional tracks associated with the electron-muon production vertex. 

The total and fiducial production cross sections are 643 +82 −78 fb and 3.96 +0.53 −0.51 fb, respectively, in agreement with the standard model predictions of 631 ± 126 fb and 3.87 ± 0.77 fb. 

This agreement enables stringent constraints to be imposed on anomalous quartic gauge couplings within a dimension-8 effective field theory framework.

CMS Collaboration, "Measurement and effective field theory interpretation of the photon-fusion production cross section of a pair of W bosons in proton-proton collisions at s√ = 13 TeV" arXiv:2601.21574 (January 29, 2026).

Gravity Anomalies Seen In Wide Binary Stars Systems

No time to write. The paper is here.

Salient ET impacts, volcanic eruptions and climate events

Version one of this post is from memory. I plan to add links and confirm details later.

ET Impacts:

1. The Tunguska event. Russia, June 30, 1908.

2. The meteor strike (an airburst actually) that destroyed Sodom and Gomorra. Levant, 1650 BCE.

3. The biggest meteor impact on Earth in the last 10,000 years struck far Western India around 4955 BCE. The crater it left behind is known as the Luna structure. It didn't have obvious cultural or civilizational impact in Neolithic South Asia.

4. The Young Dryas impact. North America, ca. 12,900 years ago in North America.

5. The Southeast Asian ET impact. 790,000 years ago near Laos. Close in time to the emergence of a common ancestor of modern humans, Neanderthals, and Denisovans, and close in time to a hominin population bottleneck apparent in our DNA.

6. The ET impact that killed the dinosaurs. ca. 66 million years ago.

Volcanic eruptions:

1. Mount Tambora. Indonesia. 1815 CE (as a comment notes, the impact of this event on climate that impacted horses may have spurred the invention of a workable bicycle).

2. Volcanic eruption. 1345 CE. Place uncertain but probably a near tropical event in the Northern Hemisphere. Led to the Little Ice Age and a black plague outbreak in Europe.

3. The eruptions that led to the Justinian plague. ca. 536 CE.

4. Pompei. Mount Vesuvius. 76 CE. Italy. Honestly, not all that exceptional an eruption in the greater span of history, but notable because it was well attested and created a time capsule of that time period that has been archaeologically important.

5. The Santorini (Thera) eruption, occurring around 1600 BCE, in what is now Greece, dealt a serious blow to Minoan civilization, even though most residents of the island fled to safety before it occurred.

5. The volcanic eruptions at the Upper Paleolithic boundary in Europe that probable drove modern human Cro-Magnon replacement of Neanderthals. Europe. ca. 40,000 years ago.

6. The Toba eruption ca. 74,000 years ago in Indonesia. This coincides with behavioral modernity in modern humans including technologies like the bow and arrow (even though anatomically modern humans date to about 300,000 years ago), and to the first expansion of modern humans past India to Southeast Asia. Possibly a cause of the extinction of Homo erectus in Asia (the youngest attested confidently classified H. erectus remains are from ca. 100,000 years ago, but remains are scarce, and new finds could fill the gap).

7.  Yellowstone's last big eruption ca. 630,000 years ago.

Climate events:

1.  The European Little Ice Age. ca. 1300-1850 CE.

2.  The drought in the American SE that ended ancient Puebloan culture. 

3.  The drought that took down the Mayans. A century or two before Y1K.

4. The horrible year: 536 CE. This was the start of the "Late Antique Little Ice Age," which lasted about 150 years and was probably volcanic in origin.

5.  The Bronze Age collapse event. ca. 1200 BCE.

6.  The aridity event that preceded Indo-European expansion and led to collapses in civilizations in Europe, the Middle East, West Asia, and India. ca. 4,000 years ago.

7.  The Green Sahara and its end. ca. 15,000 to 5,000 years ago.

8.  The Younger Dryas. ca. 12,900 years ago. Delayed the Neolthic revolution by about 3,000 years. Suddenly ended the North American Clovis culture.

9.  The Last Glacial Maximum ca. 20,000 years ago.

Another Dark Matter Particle Model Fail And Other Gravity Papers

Another (fairly byzantine) self-interacting dark matter particle model fails to reproduce the empirically observed baryonic Tully-Fischer relation, which MOND and several other gravity based explanations for dark matter phenomena naturally produce. 

This is a generic problem with the lion's share of all dark matter particle models that do not have ultralight bosons with masses of the same order of magnitude as the mass-energy of hypothetical typical gravitons as their dark matter particles.

But, some geometrical gravity based explanations of dark matter and dark energy phenomena have their own deep problems.

More optimistically, a new, theoretically observable and well-defined quantity to determine if gravity is quantum or classical in nature in future observations has been devised.

A Parts Per Thousand Measurement Of The Electroweak Mixing Angle

The weak mixing angle or Weinberg angleis a parameter in the Weinberg–Salam theory (by Steven Weinberg and Abdus Salam) of the electroweak interaction, part of the Standard Model of particle physics, and is usually denoted as θ(W). It is the angle by which spontaneous symmetry breaking rotates the original W(0) and B(0) vector boson plane, producing as a result the Z(0) boson, and the photon. Its measured value is slightly below 30°, but also varies, very slightly increasing, depending on how high the relative momentum of the particles involved in the interaction is that the angle is used for.

- Wikipedia.

In the Standard Model of Particle Physics, the electroweak mixing angle is a function of the ratio of the W boson mass to the Z boson mass, and is also a function a simple formulas that have the electromagnetic coupling constant and the weak force coupling constant as inputs.

The electroweak mixing angle is of mostly theoretical interest as a key derived parameter in the electroweak force unification (i.e. it can be calculated from other Standard Model fundamental constants) that was a key breakthrough in the development of the Standard Model of Particle Physics. 

A part per thousand measurement honestly isn't all that precise for electroweak physics (some physical constants in electroweak physics are known to parts per million levels or better), but since it doesn't have many direct engineering applications, its measurement is mostly a consistency check on the electroweak portion of the Standard Model as a whole, that provides a fairly tight global constraint on the magnitude of beyond the Standard Model physics of many varieties that can be consistent with the experimental data (in much the same way as muon g-2 measurements do). 

But, unlike muon g-2, at least at the precisions at which we can measure it, the electroweak mixing angle only receives electromagnetic force and weak force contributions, and does not receive QCD strong force contributions.

The measurement of this physical constant described in the paper below is made at the momentum scale of the Z boson pole mass, about 91.19 GeV/c^2, which in an energy range known as the electroweak scale.

This energy scale is considerably greater than the mass-energies of first and second generation quarks, the electrons, muons, tau leptons, protons, neutrons, and the light mesons that bind protons and neutrons in atomic nuclei. But, it is considerably less the the maximum momentum scales that can be reached at the Large Hadron Collider (LHC), which is the highest energy particle collider. 

The energy scale at which this measurement is made is about three orders of magnitude higher in energy scale than the energy scale at which the anomalous magnetic moment of the muon (i.e. muon g-2) is measured, which is about 0.10566 GeV/c^2.

This contribution presents a overview of a recent CMS-based determination of the effective leptonic weak mixing angle, sin2θℓeff, derived from forward-backward asymmetry measurements in Drell-Yan events at 13 TeV. Although the CMS analysis achieved a major reduction in uncertainties, its overall precision is ultimately limited by residual parton distribution function (PDF) uncertainties. 

This proceeding highlights the role of complementary CMS observables, which probe distinct parton-density combinations and provide additional constraints beyond those obtained from the original asymmetry measurement alone. 

The improved analysis yields a substantially reduced total uncertainty, resulting in sin2θℓeff = 0.23156 ± 0.00024. This result is consistent with the Standard Model prediction and represents the highest precision achieved so far in an individual determination of this parameter.

Arie Bodek, Hyon-San Seo, Un-Ki Yang, "Summary of the Precision Measurements of the Electroweak Mixing Angle in the Region of the Z pole" arXiv:2601.20717 (January 28, 2026).

The value measured by the CMS experiment at the LHC is about 0.00005 lower than the Standard Model prediction (which is about 0.2 sigma and indicates that the uncertainty in the measurement is probably overstated with conservative assumptions about its accuracy).

This result, more clearly than past experimental results, favors the Standard Model of Particle Physics over the beyond the Standard Model "two Higgs doublet" model, which which there are four extra Higgs bosons, two charged Higgs bosons (positive and negative), one odd parity Higgs boson, and one heavy even parity Higgs boson. 

Earlier LHC measurements (in blue), Tevatron measurements (in green), and pre-Tevatron measurements from LEP and SLD (in black), were collectively inconclusively in their relative preferences for the Standard Model compared to a two Higgs doublet model. The CDF M(W) value below is an outlier that has never been taken very seriously, and probably the product of some sort of subtle analysis error.

Cosmology Evidence For A Normal Neutrino Hierarchy

Cosmology data increasingly favors, even under dynamical dark energy models, a normal neutrino mass hierarchy over an inverted neutrino mass hierarchy, although still not at the five sigma "discovery" level.

Constraints from direct measurements of the neutrino's absolute masses are much less constraining, although neutrino oscillation data also favors a normal neutrino mass hierarchy, in a completely independent measurement, to a similar degree.

We present cosmological parameters measurements from the full combination of DESI DR1 galaxy clustering data described with large-scale structure effective field theory. By incorporating additional datasets (photometric galaxies and CMB lensing cross-correlations) and extending the bispectrum likelihood to smaller scales using a consistent one-loop theory computation, we achieve substantial gains in constraining power relative to previous analyses. 

Combining with the latest DESI baryon acoustic oscillation data and using cosmic microwave background (CMB) priors on the power spectrum tilt and baryon density, we obtain tight constraints on the ΛCDM model, finding the Hubble constant H0=69.08±0.37 kms−1Mpc−1, the matter density fraction Ωm=0.2973±0.0050, and the mass fluctuation amplitude σ8=0.815±0.016 (or the lensing parameter S8≡σ8Ωm/0.3‾‾‾‾‾‾‾√=0.811±0.016), corresponding to 0.6%, 1.7%, and 2% precision respectively. Adding the Pantheon+ supernova sample (SNe), we find a preference of 2.6σ for the w0wa dynamical dark energy model from low-redshift data alone, which increases to 2.8σ when exchanging the SNe with Planck CMB data. 

Combining full-shape data with BAO, CMB, and SNe likelihoods, we improve the dark energy figure-of-merit by 18% and bound the sum of the neutrino masses to Mν<0.057 eV in ΛCDM and Mν<0.095 eV in the w0wa dynamical dark energy model (both at 95\% CL). 

This represents an improvement of 25% over the background expansion constraints and the strongest bound on neutrino masses in w0waCDM to date. Our results suggest that the preference for the normal ordering of neutrino mass states holds regardless of the cosmological background model, and is robust in light of tensions between cosmological datasets.

Mikhail M. Ivanov, et al., "Reanalyzing DESI DR1: 4. Percent-Level Cosmological Constraints from Combined Probes and Robust Evidence for the Normal Neutrino Mass Hierarchy" arXiv:2601.16165 (January 22, 2026).

CDM Fails Again

I'm not surprised, but again and again and again, the evidence against cold dark matter theories piles up. 

The properties of substructure in galaxy clusters, exquisitely probed by gravitational lensing, offer a stringent test of dark matter models. Combining strong and weak lensing data for massive clusters, we map their total mass--dominated by dark matter--over the dynamic range needed to confront small-scale predictions for collisionless cold dark matter (CDM). Using state-of-the-art lens models, we extract four key subhalo properties: the mass function, projected radial distribution, internal density profile, and tidal truncation radius. 

We find that the subhalo mass function and truncation radii are consistent with CDM expectations. In contrast, the inner density profiles and radial distribution of subhalos are strongly discrepant with CDM. The incidence of galaxy-galaxy strong lensing (GGSL) from subhalo cores exceeds CDM predictions by nearly an order of magnitude, requiring inner density slopes as steep as γ≳2.5 within r≲0.01R200 consistent with core-collapsed self-interacting dark matter (SIDM), while the same subhalos behave as collisionless in their outskirts. Additionally, the observed radial distribution of subhalos hosting bright cluster member galaxies, explicitly modeled in the lens reconstructions, remains incompatible with CDM. Together, these small-scale stress tests reveal an intriguing paradox and challenge the dark matter microphysics of purely collisionless CDM and motivate hybrid scenarios, such as a dual-component model with both CDM and SIDM, or entirely new classes of dark matter theories.

Priyamvada Natarajan, Barry T. Chiang, Isaque Dutra, "New CDM Crisis Revealed by Multi-Scale Cluster Lensing" arXiv:2601.07909 (January 12, 2026).

Lava Worlds

Until most of my posts, this isn't notable because it sheds light on any deeper laws of physics. It is just amazing that worlds like this exist.

Lava worlds are rocky planets with dayside skins made molten by stellar irradiation. Tidal heating on these shortest-period planets is more than skin deep. We show how orbital eccentricities of just a few percent (within current observed bounds and maintained secularly by exterior companions) can create deep magma oceans. ``Lava tidal waves'' slosh across these oceans; we compute the multi-modal response of the ocean to tidal forcing, subject to a coastline at the day-night terminator and a parameterized viscous drag. Wave interference produces a dayside heat map that is spatially irregular and highly time-variable; hotspots can wander both east and west of the substellar point, and thermal light curves can vary and spike aperiodically, from orbit to orbit and within an orbit. Heat deposited by tides is removed in steady state by a combination of fluid, mushy, and solid-state convection in the mantle. For Earth-sized planets with sub-day periods, the entire mantle may be tidally liquified.

Mohammad Farhat, Eugene Chiang, "Magma Ocean Waves and Thermal Variability on Lava Worlds" arXiv:2601.07080 (January 11, 2026) (Submitted to AAS Journals).

Baryonic Feedback

One of the ways to overcome the discrepancies between dark matter particle theories and what we observe is to attribute the discrepancies to baryonic feedback effects that are not terribly well understood. An ambitious new paper with many co-authors examines feedback effects in multiple cosmology simulations. The trouble is that the feedback seems to aggravate the discrepancies between what of observed and what simulations predict, rather than resolving them. 

Galaxy cores behave more or less like galaxies without dark matter phenomena, while the dynamics of galactic fringes are dominated by dark matter phenomena. And, more massive galaxies are less proportionately dark matter phenomena driven than less massive galaxies. Yet, these are just the opposite of the effects of baryonic feedback in the simulations considered.

Baryonic processes such as radiative cooling and feedback from massive stars and active galactic nuclei (AGN) directly redistribute baryons in the Universe but also indirectly redistribute dark matter due to changes in the gravitational potential. In this work, we investigate this "back-reaction" of baryons on dark matter using thousands of cosmological hydrodynamic simulations from the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project, including parameter variations in the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE galaxy formation models. 

Matching haloes to corresponding N-body (dark matter-only) simulations, we find that virial masses decrease owing to the ejection of baryons by feedback. Relative to N-body simulations, halo profiles show an increased dark matter density in the center (due to radiative cooling) and a decrease in density farther out (due to feedback), with both effects being strongest in SIMBA (> 450% increase at r < 0.01 Rvir). The clustering of dark matter strongly responds to changes in baryonic physics, with dark matter power spectra in some simulations from each model showing as much as 20% suppression or increase in power at k ~ 10 h/Mpc relative to N-body simulations. 

We find that the dark matter back-reaction depends intrinsically on cosmology (Omega_m and sigma_8) at fixed baryonic physics, and varies strongly with the details of the feedback implementation. These results emphasize the need for marginalizing over uncertainties in baryonic physics to extract cosmological information from weak lensing surveys as well as their potential to constrain feedback models in galaxy evolution.

Matthew Gebhardt, et al., "Cosmological back-reaction of baryons on dark matter in the CAMELS simulations" arXiv:2601.06258 (January 9, 2026).

A new paper suggesting an interacting dark energy model is also intriguing.

Recent DESI baryon acoustic oscillation data reveal deviations from ΛCDM cosmology, conventionally attributed to dynamical dark energy (DE). We demonstrate that these deviations are equally, if not better, explained by interactions between dark matter and dark energy (IDE), without requiring a time-varying DE equation of state. Using a unified framework, we analyze two IDE models--coupled quintessence and coupled fluid--against the latest CMB (Planck, ACT, SPT), DESI BAO, and SN (including DES-Dovekie recalibrated) data. Both IDE scenarios show robust evidence for non-vanishing interactions at the 3-5σ level, with marginalized constraints significantly deviating from the ΛCDM limit. This preference persists even under DES-Dovekie SN recalibration, which weakens dynamical DE evidence. Crucially, for the same number of free parameters, IDE models provide fits to low- and high-redshift data that match or exceed the performance of the CPL dynamical DE parametrization. Our results establish IDE as a physically motivated alternative to dynamical DE, highlighting the necessity of future cosmological perturbation measurements (e.g., weak lensing, galaxy clustering) to distinguish between these paradigms.

Tian-Nuo Li, et al., "Strong Evidence for Dark Sector Interactions" arXiv:2601.07361 (January 11, 2026).

See also a new paper exploring Moffat's modified gravity approach, and a new paper examining the warm dark matter hypothesis.

Stacy McGaugh On Thin Galaxies

Astrophysicist Stacy McGaugh, at his Triton Station blog, observes that there are far more thin spiral galaxies than expected from cold dark matter halo explanations of galactic rotation curves. MOND does much better in this respect.

This is notable because MOND wasn't designed to produce this data point, and because once again, MOND is predictive while the LambdaCDM model of cosmology is not.

He is mostly highlighting results from a December 2025 paper by Benavides et al., that had escaped my notice in the daily flood of new astronomy papers. Some key illustrations from that paper:

q is a mathematical measurement of how thin a galaxy is relative to its diameter (roughly speaking, thickness divided by diameter). The chart above demonstrates how measurement effects driven by the angle of inclination at which we see galaxies make a world with many thin galaxies look more evenly spread.

This chart illustrates that LambdaCDM simulations dramatically underestimate the proportion of thin galaxies at all but the highest masses (and that many models don't even manage that match to reality at any point).

A final illustration is from one of McGaugh's own papers in 1998 and shows that MOND tends to produce flatter galaxies than Newtonian physics does (even though, unlike Deur's model, MOND is spherically symmetric, rather than relying, in part, on  the shape of a galaxy to demonstrate the dark matter replacing gravitational effect).

Neutrino Oscillations Disfavor Dark Dimensions And Right Handed Neutrinos

Two moderately popular beyond the Standard Model neutrino physics models are strongly disfavored by empirical data.

Right-handed neutrinos are naturally induced by dark extra dimension models and play an essential role in neutrino oscillations. The model parameters can be examined by the long-baseline neutrino oscillation experiments. In this work, we compute the predicted neutrino oscillation spectra within/without extra dimension models and compare them with the experimental data. We find that the neutrino data in the T2K and NOvA experiments are compatible with the standard neutrino oscillation hypothesis. The results set the stringent exclusion limit on the extra dimension model parameters at a high confidence level. The derived constraints on dark dimension right-handed neutrinos are complementary to those results from the collider experiments and cosmological observations.

Ai-Yu Bai, Auttakit Chatrabhuti, Yin-Yuan Huang, Hiroshi Isono, Jian Tang, "Dark Dimension Right-handed Neutrinos Confronted with Long-Baseline Oscillation Experiments" arXiv:2601.00790 (January 2, 2026).

An Alternative To MOND and Dark Matter

This deserves further attention. 

We present a new empirical model for galaxy rotation curves that introduces a velocity correction term omega, derived from observed stellar motion and anchored to Keplerian baselines. Unlike parametric halo models or modified gravity theories, this approach does not alter Newtonian dynamics or invoke dark matter distributions. Instead, it identifies a repeatable kinematic offset that aligns with observed rotation profiles across a wide range of galaxies. Using SPARC data [1], we demonstrate that this model consistently achieves high fidelity fits, often outperforming MOND and CDM halo models in RMSE and R-squared metrics without parametric tuning. The method is reproducible, minimally dependent on mass modeling, and offers a streamlined alternative for characterizing galactic dynamics. While the velocity correction omega lacks a definitive physical interpretation, its empirical success invites further exploration. We position this model as a local kinematic tool rather than a cosmological framework, and we welcome dialogue on its implications for galactic structure and gravitational theory. Appendix B presents RMSE and R2 comparisons showing that this method consistently outperforms MOND and CDM halo models across a representative galaxy sample.

David C. Flynn, Jim Cannaliato, "A New Empirical Fit to Galaxy Rotation Curves" arXiv:2601.00522 (January 2, 2026) (published at 12 Front. Astron. Space Sci. 1680387 (2025)).

Fairies And Fungi

* Fairy rings are a fungal phenomena.

* Fairies are often depicted as chthonic with underground halls, and eating fairy food traps you in their world forever. Fungi are one of the few living things that can survive and thrive underground without light.

* The Santa Claus myth and Christmas tree ornaments are deeply tied to hallucinogenic mushroom use by shamans in places where reindeer roam.

* Fungi are pervasively present in temperate forests which are seen as a natural habitat of fairies and were places feared in medieval times.

* Fairies are associated with glamours and deception, and many fungi, such as ergot, cause hallucinations and a sense of distortion of time..

* Fungal infections can make insects and small plants look and behave weirdly in ways that could cause them to be called fairies.

* The bane of fairies, iron and to a lesser degree salt, are inorganic while fungi are organic.

* Curious children eating mushrooms that cause their death or cause them to act abnormally could be associated with the changling myth.

* Fungi come in many varieties that are hard to distinguish from each other like fungi.

* Fungi have properties that distinguish them from "normal" biological things like plants and animals.

* Could "fairy dust" be spores or yeast?

* Mushrooms are of a scale often associated with fairies.

* There is a forests, fairies, and fungi sticker book anthology.

* Fairies are often depicted as amoral or having fundamentally different motivations than humans, which is a fit to fungi and its effects on mankind.

Signs That What You Are Reading Does Not Describe Reality

Look out for these key words in scientific claims that make it almost certain that what you are reading about don't describe reality, or has been misunderstood by a science journalist:

* Tachyons.

* Traversable wormholes.

* Anything that could make faster than light communication technologies possible (key point: quantum entanglement cannot be used to send faster than light messages).

* String theory.

* Supersymmetry.

* WIMPs (or even claims that WIMPs are well-motivated).

* Models with sterile dark matter of MeV particle mass or more.

* Claims that dark matter distributions in galaxies usually or typically have an NFW distribution.

* Negative mass or mass-energy.

* Claims that antimatter gravitates differently than matter.

* Claims that the Lambda CDM cosmology model is fully consistent with astronomy observations.

* Claims that understanding CP violation in the Standard Model could explain the baryon asymmetry of the universe (i.e. why matter is so much more common than antimatter).

* Tired light.

* Claims that scientists have created black holes on Earth.

* Perpetual motion machines.

* Claims that any widely used vaccine does more harm than good.

* Claims that vaccines cause autism.

* Homeopathy.

* Claims that autism doesn't have a large genetic component.

* Chem trails.

* Claims that human activity has not caused significant global warming.

* Young Earth creationism.

* Claims that a global flood really happened.

* Intelligent design.

* Gender ideology.

* Intelligent extraterrestrial life on Earth.

* Claims that people in pre-modern societies were less violent than modern societies.

* Claims that genocide didn't happen prior to the modern era.

* An Anatolian origin for the Indo-European languages.

* A South Asian origin for the Indo-European languages.

* A Neolithic origin for the Indo-European languages.

* Technologically advanced civilizations prior to the Last Glacial Maximum (ca. 20,000 years ago).

* The Solutrean Hypothesis.

* Claims that modern humans evolved outside Africa (although modern humans did experience admixture with other hominin species in small amounts outside of Africa).

* Claims that hominins as a clade evolved outside of Africa.

* Claims that ancient artisans were "impossibly advanced."

There are other claims that, while not impossible or more or less definitively disproven should be viewed with great skepticism:

* Explanations for phenomena that rely on new, beyond the Standard Model particles or forces (except gravity).

* Sterile neutrino theories.

* Claims that discrepancies between inclusive and exclusive measurements of something point to new physics.

* Any claim motivated by the muon g-2 anomaly (which does not exist).

* Claims of any baryon number violating process, or any lepton number violating process (other than sphalerons).

* Claims of Lorentz symmetry violations.

* Claims of charged leptons have any properties that differ from each other, other than mass (sometimes called "lepton universality violations").

* Claims that someone has seen dark matter annihilation signatures.

* Claims of CP violation or time-symmetry violation that don't involve W boson mediated phenomena.

* Claims of CPT symmetry violation.

A Neolitic Collapse

Around 5000 BCE, the Linear Pottery Culture (LBK) of first European farmers experienced a meltdown. After meany centuries of peace, suddenly war broke out between settlements and there are traces of mass slaughter of whole villages. We know when it happened and what happened to a great extent, but we don't know why peace suddenly collapsed all across the LBK cultural region.

This appears to have been an internal Neolithic era event. It preceded the Bronze and Copper Ages, and preceded Indo-European expansion.

Dark Matter Still Hasn't Been Directly Observed

"Dark matter phenomena" are real, based upon the consensus interpretation of astronomers and astrophysicists looking at astronomy evidence.

Overwhelming evidence shows the existence of "dark matter phenomena" evidenced primarily by the dynamics of galaxies and galaxy clusters, by the gap between all discernible sources of ordinary matter and the amount of matter inferred using a Newtonian approximation of gravity, and by the gap between all discernible sources of ordinary matter and the amount of matter inferred from gravitational lensing of light using a weak field approximation of General Relativity. Cosmic background radiation patterns also support the conclusion that dark matter phenomena are real.

But Stacy McGaugh in his latest blog post at Triton Station, reminds us that there have been no credible and reliable detections of dark matter itself in the Milky Way, even though many hypotheses have been searched for with a variety of means.

Direct detection experiments have come up empty and have probed dark matter particle mass ranges from a bit below 1 GeV to about 1000 GeV. 

Macroscopic dark matter candidates like primordial black holes and MACHOs have been ruled out. 

No missing momentum signals in collider experiments up to the 13 TeV energies of the Large Hadron Collider have revealed any anomalies that are good dark matter candidates produced in these reactions.

Astronomy searches for dark matter annihilation signatures have come up empty and where there have been anomalies have other explanations that don't require new physics or dark matter particles.

The dynamics of dark matter also, generally speaking, rule out heavy dark matter candidates with particle masses in excess of 1 TeV, or for that matter, in excess of a KeV mass.

The lack of direct detections or detections of decay products doesn't in and of itself rule out the dark matter hypothesis. It just tightens the parameter space for dark matter to something that doesn't interact via Standard Model forces and is stable on a time frame of many billions of years of mean lifetime or more.

But, the problem you get when you impose those conditions is that you can't explain why dark matter isn't observed to have the NFW halo distribution that dark matter like that should have. A self-interaction of dark matter with dark matter only (SIDM) could partially remedy that problem, although efforts to model that and fit parameters for that self-interaction have largely been unsuccessful.

Some dark matter models are ruled out by evidence from Big Bang Nucelosynthesis.

Most importantly, you can't explain why dark matter phenomena can be accurately predicted from the distribution of ordinary matter in a system in a very tight correlation if it has no non-gravitational interactions with ordinary matter.

This is why I strongly favor gravity or fifth force explanations for dark matter phenomena. An extremely light bosonic dark matter particle candidate, however, starts to blur the line between a fifth force and a dark matter particle explanation. 

In all other aspects of physics, forces are carried by bosons (i.e. particles with integer intrinsic angular momentums like 0, 1, 2, etc.) that we sometimes simplify to think of as force fields, while the stuff that we think of as matter in a plain English sense of the word, is made up of fermions (i.e. particles with intrinsic angular momentum of 1/2, 3/2, etc.).

DES Reduces S8 Tension

One of the persistent tensions in cosmology measurements, that has attracted less attention from the general public than the Hubble tension, is the value of a parameter called S8 (which measures "clustering amplitude" at a cosmological level) estimated from the cosmic background radiation measurements and the measurement from other means. New data from the Dark Energy Survey (DES) weakens that tension.

Cosmology from weak gravitational lensing has been limited by astrophysical uncertainties in baryonic feedback and intrinsic alignments. 

By calibrating these effects using external data, we recover non-linear information, achieving a 2% constraint on the clustering amplitude, S8, resulting in a factor of two improvement on the ΛCDM constraints relative to the fiducial Dark Energy Survey Year 3 model. The posterior, S8 = 0.832+0.013−0.017, shifts by 1.5σ to higher values, in closer agreement with the cosmic microwave background result for the standard six-parameter ΛCDM cosmology. 

Our approach uses a star-forming 'blue' galaxy sample with intrinsic alignment model parameters calibrated by direct spectroscopic measurements, together with a baryonic feedback model informed by observations of X-ray gas fractions and kinematic Sunyaev-Zel'dovich effect profiles that span a wide range in halo mass and redshift. Our results provide a blueprint for next-generation surveys: leveraging galaxy properties to control intrinsic alignments and external gas probes to calibrate feedback, unlocking a substantial improvement in the precision of weak lensing surveys.

Leah Bigwood, et al., "Confronting cosmic shear astrophysical uncertainties: DES Year 3 revisited" arXiv:2512.04209 (December 3, 2025).

New astronomy observations also strongly constrains multi-field cosmological inflation models. And, another study combining data from multiple collaborations, strongly disfavors cosmological "inflation models preferred by Planck alone, such as Higgs, Starobinsky, and exponential α-attractors, in favor of other models, such as polynomial α-attractors," based upon its new measurements of cosmological parameter n(s) (the primordial power spectrum).

Finally, there were several new preprints today exploring the WIMP dark matter hypothesis, which is irritating because the WIMP dark matter hypothesis has been almost completely ruled out by a variety of independent means.

Volcano Driven Famine Brought Black Plague Bearing Fleas In Grain Shipments To Europe In 1347

Volcanic eruptions in 1345 CE, lead to crop failures from 1345 to 1347 in the Mediterranean. This led Italians to import grain from Mongols near the sea of Azov region (currently between Ukraine and Russia) in 1347, where a black plague infestation was already present, spreading the plague to Europe. The black plague then ran rampant across Europe from 1347 to 1353 killing an immense share of the population of Europe (up to 60% of some towns and villages). 

Several years of famine also probably weakened the immune systems of most Europeans, impairing their ability to fight to black plague bacteria and making its lethality rate greater.

This black plague pandemic actually started in "the arid foothills of the Tien Shan mountains west of Lake Issyk-Kul in modern-day Kyrgyzstan" in 1338, but it took nine more years for it to reach Europe. While the volcano induced famines in Europe sped its spread, arguably its eventual arrival in Europe, sooner or later, was almost inevitable.

Human history is pockmarked with periods of death and destruction on unimaginable scales. Of these calamitous epochs, one stands out: The Black Death. The mid 14th century scourge killed tens of millions of people in Europe, Asia, and Africa and changed the course of history—marking the tail end of the Middle Ages and ushering in the cultural reawakening of the Renaissance by disrupting society, the feudal system, and economies across the continent. 

Researchers have long known the Black Death’s central villain: the bacterium Yersinia pestis, which caused the bubonic plague that swept through towns and villages with a mortality rate of up to 60 percent. Experts also know this microbial agent was spread by fleas, borne on the backs of rodent pests and maybe domestic animals, and passed between humans through the air and bodily fluids. But historians have had a tougher time recreating the sequence of events that initially started the devastating pandemic. 

Now, a pair of scientists have found new clues hidden in tree rings. By looking at these rings in the Spanish Pyrenees—as well as details in historical accounts of the time—they suggest that heightened volcanic activity sometime around 1345 may have sparked a famine, kicking off the sequence of events that eventually led to the Black Death raging through Eurasia from 1347 and 1353. They published their findings today in Communications Earth & Environment. . . . 

Here is the model Bauch and his colleague Ulf Büntgen, a dendrochronologist at Cambridge University, propose. As yet unknown volcanic eruptions ejected huge amounts of ash and gases into the atmosphere around 1345, causing drops in annual temperatures that persisted for several years. The cross sections from living and relic trees that the researchers studied had “blue rings,” denoting abnormally cold and wet summer growth seasons, in 1345, 1346, and 1347. Additional accounts from the time considered by Bauch and Büntgen tell of abnormal cloudiness and dark lunar eclipses, further hints of volcanic activity. This sustained cooling could have caused widespread crop failure across the Mediterranean. 

The resulting food shortages drove merchants in the maritime republics of Venice, Genoa, and Pisa to increase imports of grain from the Mongols living around the sea of Azov in 1347. Along with shipments of grain coursing across established trade routes came plague-infested fleas. Once Y. pestis and the fleas that carried it landed in Europe, the pathogen jumped to rats, mice, and perhaps domesticated animals. Eventually the disease hopped to humans, and people began transmitting it in densely packed population centers. The rest is a dark part of history. 

“For more than a century, these powerful Italian city states had established long-distance trade routes across the Mediterranean and the Black Sea, allowing them to activate a highly efficient system to prevent starvation,” said Bauch. “But ultimately, these would inadvertently lead to a far bigger catastrophe.” 

From a Facebook post by Nautilus Magazine. 

The introduction to the published paper states:

Recent advances in paleogenetic research now demonstrate that the Black Death was caused by the bacterium Yersinia pestis, which is likely to persist in different forms in natural reservoirs, including wildlife rodent populations. Investigations of great gerbil (Rhombomys opimus) populations in Kazakhstan, for instance, have outlined how the bacterium can be transmitted from one mammalian host to another by hematophagous insect vectors, such as fleas. The zoonotic disease, however, only occasionally spills over to domestic mammals and humans, and so far three pandemics have been documented: The Justinianic plague from circa 541 to the second half of the 8th century CE; the second pandemic starting around 1338 CE in central Asia and later outbreaks in the Mediterranean region and Europe until the early 19th century CE; and the third plague pandemic that had its origin in the 1770s in China and is arguably still prevalent in endemic rodent populations in different parts of the world.

A combination of archaeological, historical and ancient genomic data proposes that the causal agent of the second plague pandemic most likely originated from the arid foothills of the Tien Shan mountains west of Lake Issyk-Kul in modern-day Kyrgyzstan. A genetically distinct strain of the bacterium was then transmitted along ancient trade routes and entered Europe via the northern Black Sea region in the early 1340s. While changes in long-distance maritime grain trade have been introduced as a possible explanation for the import of plague-infected fleas to Venice and other Mediterranean harbour towns in 1347 CE, this chain of arguments excludes alternative transmission pathways, such as human-to-human infection or the transport of rodents and goods. Intriguingly, the role climatic changes and associated environmental factors may have played in the onset and establishment of the Black Death remains controversial amongst scholars from the natural and social sciences and the humanities.

Despite an ever-growing understanding of the evolution, origin and transmission of Yersinia pestis during the second plague pandemic, it is still unclear if the bacterium was frequently re-introduced into Europe or if natural reservoirs of the bacterium ever existed there. Recent insights into plague ecology include aspects of prolonged flea survival without human and/or rodent hosts but feeding opportunities on grain dust during long-term food shipments. Empirical evidence from around 1900 CE may therefore be considered as a possible explanation of how Yersinia pestis could have arrived in medieval Italy. While there is so far no convincing argument to pre-date the beginning of the second plague pandemic into the 13th century CE, changes in socio-economic structures, political institutions and trade networks since the second half of the 13th century possibly impacted the course of the second plague pandemic.

Here, we show that interdisciplinary investigations into the entanglements between weather, climate, ecology and society well before the Black Death are essential to understand the exceptional level of spread and virulence that made the first wave of the second plague pandemic so deadly. Based on annually resolved and absolutely dated reconstructions of volcanically forced cooling, transregional famine, and changes in long-distance maritime grain trade from 1345–1347 CE, we argue that the onset of the Black Death most likely resulted from a complex interplay of natural and societal factors and processes. Although this unique spatiotemporal coincidence of many influences seems rare, our findings emphasise the increased likelihood of zoonotic infectious diseases to suddenly emerge and rapidly translate into pandemics in both, a globalised and warmer world with COVID-19 just being the latest warning sign.