Friday, November 17, 2017

Yet Another Dark Matter Parameter Space Constraint

Simple cold dark matter models still don't work, although theorists don't have good alternatives:

A detection of wobbling Brightest Cluster Galaxies within massive galaxy clusters

A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, Aw, in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the CDM paradigm of Aw<2kpc at the 95% confidence limit. We carry out the same test on the data finding a non-zero amplitude of Aw=11.82+7.33.0kpc, with the observations dis-favouring Aw=0 at the 3σ confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large scale halo. While our small sample of galaxy clusters already indicates a non-zero Aw, with larger surveys, e.g. Euclid, we will be able to not only to confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.
I explained this in the following way to someone asking about this paper at the Physics Forums:

The really core point, from the abstract is that:
[The] Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. . . . This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters.
Ten years ago, this would have been a really big deal since it contradicts the cold dark matter (CDM) hypothesis as an explanation for dark matter phenomena. But, at this point, it is really just piling onto an abundant collection of evidence showing contradictions between the CDM hypothesis and observation.

One of these contradictions (there are several of them) is known as the cusp-core problem, which is that CDM theories, generically, predict that dark matter halos should have a cuspy density profile, but inferences about the distribution of dark matter from the dynamics of visible matter in galaxies and gravitational lensing observations demonstrate that this is not actually the shape of inferred dark matter distributions in galaxies. Instead, inferred dark matter halos distributions have what is known as an "isothermal" distribution of dark matter within the dark matter halo around a galaxy.

So, this result really just confirms in a novel way something that was widely known from other evidence. This is still important, because it makes the conclusion that there really is a cusp-core problem that is not just an artifact of a flaw in some particular methodology that provides the evidence for the cusp-core problem much more robust. But, it doesn't really change the bottom line from existing data.

To prevent a cuspy density profile from emerging in a halo you need some kind of feedback either between dark matter particles or between ordinary matter and dark matter that spreads it out when it gets too dense.

But, that contradicts the assumption made in early cold dark matter theories that dark matter should be collisionless, which has strong support from the failure to direct dark matter detection experiments to see it, from the absence of a strong dark matter annihilation signal, and from the non-detection of dark matter at the Large Hadron Collider (LHC), and is consistent with the success of the lamdaCDM model of cosmology at scales much larger than galaxies and galaxy clusters, although these methods would often miss detect interactions between dark matter and other dark matter that does not result in annihilation of the interacting dark matter particles and take place at relative small distances relative to those important for cosmology.

Warm dark matter proponents have suggested a quantum effect that only kicks in at masses of dark matter particles on the order of 2 keV/c2 or less. Others have proposed self-interacting dark matter (SIDM) models to address the issue. But, those models have their own problems beyond the scope of this discussion.

Wednesday, November 15, 2017

Most Dark Matter-Neutrino Interactions Can Be Ruled Out

The following new paper places significant new boundaries of the parameter space of particle dark matter candidates that interact with neutrinos to some extent.
Dark matter and neutrinos provide the two most compelling pieces of evidence for new physics beyond the Standard Model of Particle Physics but they are often treated as two different sectors. The aim of this paper is to determine whether there are viable particle physics frameworks in which dark matter can be coupled to active neutrinos. 
We use a simplified model approach to determine all possible renormalizable scenarios where there is such a coupling, and study their astrophysical and cosmological signatures. 
We find that dark matter-neutrino interactions have an impact on structure formation and lead to indirect detection signatures when the coupling between dark matter and neutrinos is sufficiently large. This can be used to exclude a large fraction of the parameter space. 
In most cases, dark matter masses up to a few MeV and mediator masses up to a few GeV are ruled out. The exclusion region can be further extended when dark matter is coupled to a spin-1 mediator or when the dark matter particle and the mediator are degenerate in mass if the mediator is a spin-0 or spin-1/2 particle.
Andres Olivares-Del Campo, et al., "Dark matter-neutrino interactions through the lens of their cosmological implications" (November 14 2017).

Meanwhile, the parameter space of axion-like dark matter theories has once again been greatly reduced. 

LHC Measures Electroweak Mixing Angle

One of the physical constants in the Standard Model of Particle Physics is called the electroweak mixing angle also known as the Weinberg angle.  This is a function of two more fundamental physical constants in the Standard Model. Crudely speaking, the coupling constants of the electromagnetic force and the weak force (g and g'), which more precisely speaking are the

  and  couplings (weak isospin g and weak hypercharge g', respectively).

The cosine of the weak mixing angle is also equal to the mass of the W boson divided by the mass of the Z boson.

Measuring it is complicated somewhat because one has to defined the weak mixing angle at a particular energy scale to produce a numerical value for it since it runs with energy scale, in order to make a consistent measurement.

The CMS experiment at the Large Hadron Collider (LHC) has measured the electroweak mixing angle with twice the precision of any previous LHC measurement. The new measurement is as follows (combining all sources of error into a single uncertainty figure):


Put another way, the plus or minus one standard deviation range that flows from this measurement is 0.23049 to 0.23153.

The central value of the previous state of the art LHC measurement (via the first link in this post) was 0.23142, which is consistent with, but at the high end, of the new measurement and only have a margin of error of about ± 0.00100.

A 2004 global average from a variety of experimental sources (from the same link) was 0.23120 ± 0.00015 which is also consistent with this result.

The CODATA 2014 determination based upon W and Z boson mass measurements (from the same link) was 0.2223(21) implying a one sigma range of 0.22010 to 0.22440, which is below the value of the new measurement and below the 2004 global average at the one standard deviation level, and is just barely consistent at a two sigma level with the new LHC measurement.

The new paper notes that:
The most precise previous measurements of sin2 θ lept eff are reported by LEP and SLD experiments However, the two most precise measurements differ by more than 3 standard deviations. Measurements of sin2 θ lept eff are also reported by LHC and Tevatron experiments. . . . The results are consistent with the most precise LEP and SLD measurements [ed. when they are combined]. 
Meta Footnote

This post brings the number of posts at this blog for the year to an all time record high for a yearly number of posts at the blog, of 223, with 46 days left to go in 2017.

Tuesday, November 14, 2017

We Know Precisely How People From Madagascar Ended Up As Slaves In The U.S.

In the study population genetics and prehistory, the question of how people with particular genes ended up one place from a distant destination frequently have answer that are only vague surmises because we have no historical accounts and limited archaeological data. Every once and a while some bit of data is so singular that we can do better. But, that is the exception and not the rule, and still leaves myriad details of the process fuzzy.

Once historical and business records are present, however, detailed accounts can be pieced together from business records, journals and news accounts, supplemented by the usual evidence from genetics and physical anthropology.

One of the things we know is that thousands of people from Madagascar were brought to the New World as slaves.

We know this with exquisite detail that recounts:

* the exact points of departure and points of arrival,
* the precise dates of departure and points of arrival (sometimes to the hour),
* the routes taken by the ships, 
* the name of each of the ships and who owned and insured those ships, 
* the names of the crew on those ships,
* the exact number of people and their gender and approximate age on each ship, 
* how many people died en route on each individual voyage and the cause of each death,
* the purchase prices and sale prices for the slaves, and
* the manner in which the people sold came to become slaves.

Often it is even reasonably possible to determine where these people were taken as slaves after they arrived, and with DNA evidence, it is frequently possible to detect the signature of ancestry from Madagascar which is distinctive because it contains an Indonesian component.

It isn't easy reading, but a precise understanding of how historical events have actually played out is critical to developing a more accurate intuition about the narratives that underlie less well documented gene exchange events.

This component is present in a surprisingly large share of African-Americans whose New World roots are in places where people from Madagascar were delivered as slaves. This is because all contributions to a gene pool are soon found in most members of the total population after a few generations under conditions approximating panmixia which are close enough to reality in this context to be a good model of reality, even though the Madagascar component was not a huge share of the total population of slaves imported to the New World.

Monday, November 13, 2017

Nature, the Magazine, Acknowledges Failure Of WIMP Dark Matter Paradigm

The journal Nature has run a short "news" article acknowledging the continued and serious failure to the WIMP paradigm to explain dark matter phenomena. Axion dark matter and modified gravity theories, or a completely sterile dark sector, are suggested as alternatives.

The article focuses on the failures of direct detection experiments and the Fermi satellite to detect dark matter, without even reviewing the ample and growing astronomy evidence against dark matter particles as the mechanism for the (undeniably real) phenomena attributed to dark matter.

Sunday, November 12, 2017

There Was A Rice Domestication In The Amazon

Rice was independently domesticated in Asia, Africa and South America. In South America wild rice was domesticated around 9,000 years ago and was abandoned when Europeans arrived. Pre-Columbian farmers in the Amazon farmed both corn and New World rice.

The detailed agricultural history pieced together with 320 remnants of rice plants from a dig in the Amazon corroborates earlier archaeological evidence of civilization in the Amazon with cities as large as 50,000 during the Middle Ages.

Thursday, November 9, 2017

Fire Use By Archaic Hominins

Neanderthals clearly made regular controlled use of fire. The timing is less clear and the evidence is less definitive for the immediate archaic hominin predecessor of Neanderthals, but it isn't entirely clear where to draw that line. 

The evidence for Homo Erectus is much patchier. There are two or three instances that suggest isolated sites where fire was used on a controlled basis, but the evidence is not nearly so clear or pervasive in H. Erectus sites as it is in Neanderthal sites.

Wednesday, November 8, 2017

Top Quark Continues To Behave As Expected

New data from the Large Hadron Collider, reported by the CMS experiment in pre-prints here and here, continues to show that top quarks behave as expected in the Standard Model, tightly constraining a variety of beyond the Standard Model physics parameters. If this sounds familiar, it is because it should as in the following posts in the last couple of years:

* Top quark width and W boson helicity as expected.

* Top quark width constrains BSM physics.

* Top quark width in relatable units.

Pole mass v. MS mass conversion factor for top quarks converges quickly.

* Hadronic measurements of top quark mass confirm other measurements of it, ATLAS and CMS top quark mass measurements are consistent with each other, ATLAS top quark mass measurements, CMS measures top quark mass, and more top quark mass measurements, and yet more top quark mass measurements, and Tevatron top quark mass measurements.

* The evidence against the existence of a t' quark and along the same lines here.

UPDATE November 13, 2017:

* Top quark charge asymmetry is also observed to be tightly in line with Standard Model predictions.

Evidence Of Ancient Butchering May Actually Be Due To Crocodiles

There are a few instances in the archaeological record of anomalously early apparent usage of stone tools by hominins where hominins weren't expected to have such tools.  A new study may explain these anomalies. According to Science News:
Recent reports of African and North American animal fossils bearing stone-tool marks from being butchered a remarkably long time ago may be a crock. Make that a croc. 
Crocodile bites damage animal bones in virtually the same ways that stone tools do, say paleoanthropologist Yonatan Sahle of the University of Tübingen in Germany and his colleagues. Animal bones allegedly cut up for meat around 3.4 million years ago in East Africa (SN: 9/11/10, p. 8) and around 130,000 years ago in what’s now California (SN: 5/27/17, p. 7) come from lakeside and coastal areas. Those are places where crocodiles could have wreaked damage now mistaken for butchery, the scientists report online the week of November 6 in the Proceedings of the National Academy of Sciences.
It cites the following papers for its report:

* Y. Sahle et al. Hominid butchers and biting crocodiles in the African Plio-Pliocene. Proceedings of the National Academy of Sciences. Published online the week of November 6, 2017. doi: 01.1073/pnas.1716317114.

* S.R. Holen et al. A 130,000-year-old archaeological site in southern California, USA. Nature. Vol. 544, April 27, 2017, p. 479. doi:10.1038/nature22065.

* S.P. McPherron et al. Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature. Vol. 466, August 12, 2010, p. 857. doi:10.1038/nature09248.

* J. Njau. The relevance of crocodiles to Olduwan hominin paleoecology at Olduvai Gorge, Tanzania. Ph.D. dissertation, Rutgers University, The State University of New Jersey, New Brunswick, N.J., May 2006.

The Early History of Agriculture In Eastern Asia

This is a paradigm consolidating and summarizing paper, rather than a paradigm shifting one.
Millets and rice were important for the demographic history of China. This review draws on current archaeobotanical evidence for rice and millets across China, Korea, eastern Russia, Taiwan, Mainland southeast Asia, and Japan, taking a critical approach to dating evidence, evidence for cultivation, and morphological domestication. There is no evidence to suggest that millets and rice were domesticated simultaneously within a single region. Instead, 5 regions of north China are candidates for independent early cultivation of millets that led to domestication, and 3 regions of the Yangtze basin are candidates for separate rice domestication trajectories. The integration of rice and millet into a single agricultural system took place ca. 4000 BC, and after this the spread of agricultural systems and population growth are in evidence. The most striking evidence for agricultural dispersal and population growth took place between 3000 and 2500 BC, which has implications for major language dispersals.
Chris J. Steven and Dorian Q. Fuller, "The spread of agriculture in eastern Asia: Archaeological bases for hypothetical farmer/language dispersals" 7(2) Language Dynamics and Change 152-186 (2017). DOI: 10.1163/22105832-00702001

Finding Lost Cities With Economics

In a stunningly original paper Gojko Barjamovic, Thomas Chaney, Kerem A. Coşar, and Ali Hortaçsu use the gravity model of trade to infer the location of lost cities from Bronze age Assyria! The simplest gravity model makes predictions about trade flows based on the sizes of cities and the distances between them. More complicated models add costs based on geographic barriers. The authors have data from ancient texts on trade flows between all the cities, they know the locations of some of the cities, and they know the geography of the region. Using this data they can invert the gravity model and, triangulating from the known cities, find the lost cities that would best “fit” the model. In other words, by assuming the model is true the authors can predict where the lost cities should be located. To test the idea the authors pretend that some known cities are lost and amazingly the model is able to accurately rediscover those cities.
From Marginal Revolution.

Earlier papers (some by the same authors) have validated the gravity model of trade in ancient West Asia using ancient commercial records, but this this application goes a step further by making predictions using that model.

The paper is closed access and the accounts I've read so far don't identify which lost cities they purport to have found.

The paper and its abstract are:
We analyze a large dataset of commercial records produced by Assyrian merchants in the 19th Century BCE. Using the information collected from these records, we estimate a structural gravity model of long-distance trade in the Bronze Age. We use our structural gravity model to locate lost ancient cities. In many instances, our structural estimates confirm the conjectures of historians who follow different methodologies. In some instances, our estimates confirm one conjecture against others. Confronting our structural estimates for ancient city sizes to modern data on population, income, and regional trade, we document persistent patterns in the distribution of city sizes across four millennia, even after controlling for time-invariant geographic attributes such as agricultural suitability. Finally, we offer evidence in support of the hypothesis that large cities tend to emerge at the intersections of natural transport routes, as dictated by topography.
Gojko Barjamovic, Thomas Chaney, Kerem A. Coşar, Ali Hortaçsu, "Trade, Merchants, and the Lost Cities of the Bronze Age" NBER Working Paper No. 23992 (November 2017).

Thursday, November 2, 2017

A New Species Of Great Apes

There are now eight recognized species of great apes, including humans, that are not extinct. Humans, three species of orangutans from Indonesia, two species of gorillas in Africa, and the chimpanzee/bonobo clade in Africa. Humans share the chimpanzee/bonobo clade vis-a-vis orangutans and gorillas, and are a sister clade to both chimpanzees and bonobos rather than being derived from one or the other of those species.
Six extant species of non-human great apes are currently recognized: Sumatran and Bornean orangutans, eastern and western gorillas, and chimpanzees and bonobos. However, large gaps remain in our knowledge of fine-scale variation in hominoid morphology, behavior, and genetics, and aspects of great ape taxonomy remain in flux. This is particularly true for orangutans (genus: Pongo), the only Asian great apes and phylogenetically our most distant relatives among extant hominids. Designation of Bornean and Sumatran orangutans, P. pygmaeus (Linnaeus 1760) and P. abelii (Lesson 1827), as distinct species occurred in 2001. Here, we show that an isolated population from Batang Toru, at the southernmost range limit of extant Sumatran orangutans south of Lake Toba, is distinct from other northern Sumatran and Bornean populations. By comparing cranio-mandibular and dental characters of an orangutan killed in a human-animal conflict to those of 33 adult male orangutans of a similar developmental stage, we found consistent differences between the Batang Toru individual and other extant Ponginae. Our analyses of 37 orangutan genomes provided a second line of evidence. Model-based approaches revealed that the deepest split in the evolutionary history of extant orangutans occurred ∼3.38 mya between the Batang Toru population and those to the north of Lake Toba, whereas both currently recognized species separated much later, about 674 kya. Our combined analyses support a new classification of orangutans into three extant species. The new species, Pongo tapanuliensis, encompasses the Batang Toru population, of which fewer than 800 individuals survive.
From Alexander Nater, et al., "Morphometric, Behavioral, and Genomic Evidence for a New Orangutan Species", Current Biology (2017).

Meanwhile, a 9.7 million year old great ape tooth has reportedly been found in Germany, outside the range of any previously known great ape population.

Wednesday, November 1, 2017

The Berdache Role In Native American Culture

Many Native American cultures have had historically and often still do have a gender role somewhat similar to that of a male to female transgender identity traditionally called a "berdache" in the anthropological literature. A lengthy post at Gambler's House explores what role this gender identity played in Native American cultures.

Sunday, October 29, 2017

One In Ten Vascular Plants In Madagascar Have Asian Origins

Figure S3 from the paper cited below.

Ancient evidence of Asian crops in Madagascar provide another piece of evidence for discerning the details of this long distance mass migration of Austronesian people to Africa.
The prehistoric settlement of Madagascar by people from distant Southeast Asia has long captured both scholarly and public imagination, but on the ground evidence for this colonization has eluded archaeologists for decades. Our study provides the first, to our knowledge, archaeological evidence for an early Southeast Asian presence in Madagascar and reveals that this settlement extended to the Comoros. Our findings point to a complex Malagasy settlement history and open new research avenues for linguists, geneticists, and archaeologists to further study the timing and process of this population movement. They also provide insight into early processes of Indian Ocean biological exchange and in particular, Madagascar’s floral introductions, which account for one-tenth of its current vascular plant species diversity.
The Austronesian settlement of the remote island of Madagascar remains one of the great puzzles of Indo-Pacific prehistory. Although linguistic, ethnographic, and genetic evidence points clearly to a colonization of Madagascar by Austronesian language-speaking people from Island Southeast Asia, decades of archaeological research have failed to locate evidence for a Southeast Asian signature in the island’s early material record. Here, we present new archaeobotanical data that show that Southeast Asian settlers brought Asian crops with them when they settled in Africa. These crops provide the first, to our knowledge, reliable archaeological window into the Southeast Asian colonization of Madagascar. They additionally suggest that initial Southeast Asian settlement in Africa was not limited to Madagascar, but also extended to the Comoros. Archaeobotanical data may support a model of indirect Austronesian colonization of Madagascar from the Comoros and/or elsewhere in eastern Africa.
Alison Crowther, et al., "Ancient crops provide first archaeological signature of the westward Austronesian expansion" 113(24) PNAS 6635-6640 (2017) doi: 10.1073/pnas.1522714113

From the introductory body text:
One line of evidence that has been largely overlooked in archaeological investigations of Madagascar and, indeed, eastern Africa more broadly is ancient plants. However, it is estimated that some 10% of Madagascar’s flora was introduced from elsewhere, and plant introductions include a significant number of staple crops, spices, and arable weeds of Asian origin. Historically or currently important crops on Madagascar, like banana (Musa spp.), yam (Dioscorea alata), taro (Colocasia esculenta), and coconut (Cocos nucifera), are Southeast Asian cultivars. Asian rice (Oryza sativa), which was domesticated separately in East and South Asia but is the basis of traditional agriculture across much of Madagascar today, was also widely grown in Southeast Asia by the first millennium CE. Other Asian crops, like mung bean (Vigna radiata) and Asian cotton (Gossypium arboreum), are also cultivated on Madagascar. The fact that early crop introductions to Madagascar may have arrived with Austronesian settlers seems particularly feasible given that Austronesian expansion into the Pacific was linked to the spread of a similar suite of cultivars. 
To directly explore early cultivated plants on Madagascar and their potential to inform on its colonization history, we collected new archaeobotanical data from the island as well as contemporaneous sites on the African mainland coast (Kenya and Tanzania) and nearshore islands (Pemba, Zanzibar, and Mafia) and the Comoros. These data were collected from 18 sites in total, dating between approximately 650 and 1200 calibrated years (cal) CE. The archaeobotanical datasets derive primarily from recent excavations at 16 sites, during which systematic sampling for charred macrobotanical remains at high stratigraphic resolution was conducted. They are supplemented by existing records from one of the sites (Sima) as well as data from previous excavations at two other sites in the Comoros. The combined dataset includes 2,443 identified crop remains recovered from >7,430 L sediment across the sites and is supported by 48 accelerator MS (AMS) radiocarbon dates, 43 of which were obtained directly on crop seeds.
From the conclusion:
Although the presence of Asian crops that likely originate from Southeast Asia on early sites in Madagascar corresponds well with linguistic, genetic, and ethnographic evidence for a prehistoric migration of people from this region, the finding that these crops also dominate early assemblages on the Comoros is rather unexpected. In particular, the presence of Asian crops at sites in the Comoros earlier than at sites on Madagascar is of significant interest, and although sampling and preservation biases cannot be discounted, may reflect Austronesian colonization of the Comoros before Madagascar. As noted, however, Comorians today speak Bantu languages, and in addition, preliminary molecular genetic studies suggest that they possess only a small proportion of Southeast Asian ancestry. Nonetheless, the population of the Comoros is small and has been historically subject to significant population bottlenecks and Bantu input as a result of slave raiding and trading over many centuries. Thus, it is possible that the Comoros were settled at an early date by a Southeast Asian population that was later genetically and linguistically swamped. 
Direct colonization from Southeast Asia is common to many models of Madagascar’s Austronesian settlement, particularly those put forward by archaeologists and geneticists. However, linguistics have offered another perspective, with some linguists taking the view that the remarkable unity of Bantu loanwords and grammatical features throughout Malagasy dialects can only be explained through initial Austronesian settlement on the African mainland and/or the Comoros. Early Southeast Asian presence or influence on the Comoros has also been suggested on the basis of the apparent presence of several 10th or 11th century “Austronesian-type” furnaces on Mayotte as well as findings of shell-impressed pottery at early sites on the islands. These suggested Austronesian linkages, however, have been both limited and contentious. This study suggests that they deserve reinvestigation together with the argument that the Comoros may have served as a key base for Southeast Asian commercial activity in the western Indian Ocean, including an alternative slave-trading corridor. Independent linguistic, genetic, and archaeological studies are required to examine the role of the Comoros in early Indian Ocean population movements and commercial trade.

Friday, October 27, 2017

Another Problem For Particle Dark Matter Theories

The galactic cluster BCG wobble is essentially the galactic cluster scale analog to the long standing core-cusp problem with Cold Dark Matter at the galaxy scale. The press release accompanying this new paper suggests that self-interacting dark matter is the only solution, but other studies broadly rule out all astrophysically relevant parameters of SIDM models.
A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, Aw, in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the CDM paradigm of Aw<2kpc at the 95% confidence limit. We carry out the same test on the data finding a non-zero amplitude of Aw=11.82+7.33.0kpc, with the observations dis-favouring Aw=0 at the 3σ confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large scale halo. While our small sample of galaxy clusters already indicates a non-zero Aw, with larger surveys, e.g. Euclid, we will be able to not only to confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.
David Harvey, F. Courbin, J. P. Kneib, Ian G. McCarthy. "A detection of wobbling brightest cluster galaxies within massive galaxy clusters." 472(2) Monthly Notices of the Royal Astronomical Society 1972 (2017).
Using the NASA/ESA Hubble Space Telescope, astronomers have discovered that the brightest galaxies within galaxy clusters "wobble" relative to the cluster's centre of mass. This unexpected result is inconsistent with predictions made by the current standard model of dark matter. . . . 
[C]lusters have very dense cores, each containing a massive galaxy called the "brightest cluster galaxy" (BCG). The standard model of dark matter (cold dark matter model) predicts that once a galaxy cluster has returned to a "relaxed" state after experiencing the turbulence of a merging event, the BCG does not move from the cluster's centre. It is held in place by the enormous gravitational influence of dark matter. But now, a team of Swiss, French, and British astronomers have analysed ten galaxy clusters observed with the NASA/ESA Hubble Space Telescope, and found that their BCGs are not fixed at the centre as expected. 
The Hubble data indicate that they are "wobbling" around the centre of mass of each cluster long after the galaxy cluster has returned to a relaxed state following a merger. In other words, the centre of the visible parts of each galaxy cluster and the centre of the total mass of the cluster -- including its dark matter halo -- are offset, by as much as 40,000 light-years. "We found that the BCGs wobble around centre of the halos," explains David Harvey, astronomer at EPFL, Switzerland, and lead author of the paper. "This indicates that, rather than a dense region in the centre of the galaxy cluster, as predicted by the cold dark matter model, there is a much shallower central density. This is a striking signal of exotic forms of dark matter right at the heart of galaxy clusters." 
The wobbling of the BCGs could only be analysed as the galaxy clusters studied also act as gravitational lenses. . . . This effect, called strong gravitational lensing, can be used to make a map of the dark matter associated with the cluster, enabling astronomers to work out the exact position of the centre of mass and then measure the offset of the BCG from this centre. 
If this "wobbling" is not an unknown astrophysical phenomenon and in fact the result of the behaviour of dark matter, then it is inconsistent with the standard model of dark matter and can only be explained if dark matter particles can interact with each other -- a strong contradiction to the current understanding of dark matter.
 From the press release is here. Hat tip wolram.