The Annotated Lagrangians of Fundamental Physics

Via Backreaction.

A Stable Sexaquark?

A new preprint proposes that there is a stable (i.e. half-life longer than the age of the universe) hadron that could exist called a sexaquark, a bound state of up, down and strange quarks, which would be a dark matter candidate.

It is proposed that the neutral, B=2, flavor singlet sexaquark (S) composed of uuddss quarks, has mass m_S <~ 2 GeV. If m_S < 2 (m_p + m_e), it is absolutely stable, while for m_S < m_p+m_e + m_Lambda, its lifetime can be greater than the age of the Universe. Lattice gauge theory cannot yet predict m_S, but indirect evidence supports the hypothesis of stability. A stable S is consistent with QCD theory and would have eluded detection in accelerator and non-accelerator experiments. If it exists, the S is a good Dark Matter candidate. Analyses of existing Upsilon decay and LHC data can be used to discover it and measure its mass.

Glennys R. Farrar, "Stable Sexaquark" (August 29, 2017).

While the premise that a sexaquark that was lighter than two time the mass of a proton would be absolutely stable is sound, I have no confidence whatsoever that Farrar's heuristic argument that the not yet calculated mass of a sexaquark is less than twice the proton mass has any validity whatsoever. indeed, the fact of its non-detection at colliders so far strongly militates against this hypothesis.

It would be a convenient dark matter candidate that would explain the lack of fundamental dark matter particles, but the cross-section of interaction would be too high and the gravitational dynamics would probably be wrong as well.

The Case Against Humans In California 130,000 Years Ago

In April, I reported on an article in Nature claiming evidence of a human presence in California around 130,000 years ago. Here is a rebuttal:

In a controversial study published in Nature, Holen et al. (2017) claim that hominins fractured mastodon bones and teeth with stone cobbles in California ∼130,000 years ago. Their claim implies a human colonization of the New World more than 110,000 years earlier than the oldest widely accepted archaeological sites in the Americas. It is also at odds with genetic and fossil evidence for the dispersal of anatomically modern humans (Homo sapiens) out of Africa and around the world. 

Recognizing the incompatibility of their claim with extant knowledge, the authors suggest that the Cerutti Mastodon locality might have been created by an as-yet unidentified archaic hominin, for which no fossil, archaeological, or genomic evidence currently exists in northeast Asia or the Americas. 

We assess Holen et al.’s (2017) supporting evidence and argue that such extraordinary claims require extraordinary evidence, which their paper and supporting materials fail to provide.

Todd J. Braje, et al., "Were Hominins in California ∼130,000 Years Ago?" 3 PaleoAmerica 200-202 (July 17, 2017)  dx.doi.org/10.1080/20555563.2017.1348091

The rebuttal paper largely appears to recap the points that I made in my post at the time, but it is closed access, so it isn't easy to tell.

A Modern Physics Recap

A Recap Of The Core Theory

For all of those of you who haven't been paying attention to physics for the last century or so, here is a quick recap of the the current situation from the introduction to a recent pre-print on quantum gravity:

The past few hundreds of years of development have combined precision experiments with abstract theoretical reasoning, and our current understanding of the basic building blocks of nature is as follows. 

The universe contains matter, which is acted upon by forces. 

All observed forces are consequences of only four fundamental forces. 

Three of these - electromagnetism, the weak nuclear force, and the strong nuclear force - are described by the Standard Model (SM) of particle physics, which also lists all known matter particles – quarks and leptons – that combine to make the various composite particles observed in nature. 

All forces and matter types in the SM are described by fields filling all of spacetime. The equations describing these fields include the effects of quantum mechanics and relativity, so that the SM is an example of a quantum field theory (QFT). 

Indeed, the particles themselves emerge as quanta of the fields, with the canonical example being the photon (a quantum of the electromagnetic field). Furthermore, the fields have certain abstract mathematical symmetries, so that this type of QFT is also called a (non)-Abelian gauge theory.

There. Got it. The Standard Model has been in place more or less since the 1970s, and general relativity is more or less unchanged from its state a hundred years ago.

Recent Standard Model Developments - The Higgs Boson and Neutrino Physics

The only really significant developments in the Standard Model since the 1970s have been the discovery of various Standard Model predicted particles (the last of which was the Higgs bosons discovered in 2012 completing the set) and some properties of neutrinos. 

We've learned that neutrinos are massive and oscillate between mass eigenstates according to a three by three complex matrix with four parameters known as the PMNS matrix. We know the difference between the masses of the three mass eigenstates, but not their absolute values (although we have capped the maximum value of the sum of the three masses) or the order of the masses, and we know three of the four PMNS matrix parameters, although not as precisely as we'd like and not the fourth CP violating parameter. We also don't know which of two mechanisms generate the neutrino masses. 

Dark Matter and Dark Energy

On the gravitational front, we've learned that we need a cosmological constant or dark energy or the equivalent, and that we need either dark matter or a tweak to the behavior of gravity in weak gravitational fields to explain a lot of astronomy observations. Unlike the remaining aspects of the Standard Model which require us to simply set a few parameters, the dark matter/dark energy problems are truly unsolved.

Quantum Gravity

It seems like it ought to be possible to formula General Relativity as a quantum theory (it is currently a classical theory like Newtonian physics) and that quantum theory might even resolve some other loose ends. But, we haven't managed to do that yet and our two "Core Theories" are theoretically inconsistent without it. This may or may not be related to the dark matter and dark energy problems.

The First Moments After The Big Bang - An Optional Problem

There is also some lack of clarity regarding just what happened in the first few moments after the Big Bang - primarily baryogenesis, leptogenesis, cosmological inflation, and matter-antimatter asymmetry. But, since these things happened at basically unreproducible energy scales, there is really no practical or functional reason that we must know the answers to any of these questions which together conspire to create initial conditions a very short period of time after the Big Bang, after which conventional physics takes over.

Within The Standard Model - An Optional Problem

We would also prefer to condense the somewhat parameters heavy, multi-equation "Core Theory" to an even deeper "within the Standard Model" theory or even a "Theory of Everything", but ultimately that would be simply an academic exercise once finished and wouldn't have observable differences from the Standard Model and General Relativity with the parameters it predicts.

Non-Fundamental Physics

Step back one more step and there are some more practical, probably not fundamental, issues left.

QCD Issues

We still don't really understand how scalar mesons and axial vector mesons form or take the mass spectrum that they do. We are still trying to better grasp the physics of glueballs and hadrons with more than three constituent quarks. QCD calculations are very hard to do precisely, taking lots of time, expertise and computational capacity to conduct. There are quite a few Standard Model and General Relativity constants that aren't measured terribly precisely and we could do better.

Experimental Anomalies

There are also a few relatively minor glitches in comparing experiment and theory. Most notably: there are some indications of a violation of charged lepton universality, a measurement called muon g-2 does not quite square with the predicted value, the charge radius of muonic hydrogen isn't quite right, and the predicted abundance of Lithium-7 in the universe isn't quite right.

Dreamers hope these are signs of new physics. Cynics think this is probably due to a combination of measurement errors and flawed calculations of the theoretically predicted values in subtle ways.

Astronomy

We would like to understand black holes, galaxy formation and galactic cluster formation better.

Outlook

Lots of these problems will take more experiments, observations and calculations to solve, but are in principle eminently solvable. A handful of mostly optional problems may be a bit more intractable.

Who Are The Lebbo' People Of Borneo?

Why care about the Lebbo' people of Borneo?

Eurogenes reports that in his efforts to explain the autosomal genetic origins of South Asians that the Lebbo' people of the Indonesian part of the island of Borneo were a better fit to the Ancestral South Asian component of autosomal genetic diversity of South Asians (especially Dravidian speakers) than the Onge people of the Andaman islands who have often been used for that purpose.

This puts front and center the question:

Who are the Lebbo' people of Borneo?

Anthropology and Archaeology

Two Lebbo' men from this source.

The source for the picture of these two men notes their probably longstanding, pre-Neolithic, presence in the region:

Previous surveys have led to the unexpected discovery of more than 150 caves with unique rock art paintings (mostly hand stencils), dating back to at least the Early Holocene (9900 years BP; Plagnes et al. 2003) and presenting some similarities to those from eastern Indonesian Islands, West Papua and Australia (Fage et al. 2010). These discoveries confirmed for the first time the early human occupation of eastern Borneo. Liang Abu is located 130km north-west of the shore of the Makassar Strait, within the rainforest of a mountainous karstic area. Hundreds of cliff caves and shelters are present, where human activity occurred on three levels: human settlement at the base, burial in the middle and rock art at the top (Chazine 2005). . . . The site is located 6km from the remote Lebbo' village of Merabu. The Lebbo' are an isolated indigenous population of hunter-gatherers and are the only ethnic group which have lived in this inner region for a long time. Ethno-linguistic studies of their oral traditions and cultural practices, to document their former use of caves in relation to the proto-historic and pre-historical periods in the region, are currently under way. Some of the Lebbo's representations can be directly related to the prehistoric rock art found in nearby caves. All these characteristics make Liang Abu a site well suited for the investigation of links between late Pleistocene human occupation, rock art, the communities represented by them as well as the communities of their potential descendants.

Geography

The image above is Figure 1 in this article in Nature. The Lebbo' are locating in the Northeast portion of Indonesian Borneo. The Andaman islands, which are home to the Onge are in the chain of Islands extending to the south from Bangladesh to the island of Sumatra (to the west of Myanmar f.k.a. Burma).

The Lebbo' peoples historic range may have been more expansive than it is today.

Autosomal Genetics

The ancestry components of the Lebbo' people based upon a fourteen population admixture analysis (shown in Figure 2 of from the same article, below in bottom panel of the far left) are quite heterogenous, with a plurality component that is most common in Sumatra and Peninsular Malaysia (labeled C13 in the figure below), a significant share of a component that is almost pure in the H'tin people of Thailand (labeled C12), and a notable shares associated with the Ma'anyan people of Borneo (labeled C8, who are linguistically the closest match to the people of Madagascar) and with the people of Sulawesi and the Lesser Sunda islands (labeled C8), but almost none of a Philippine Negrito component (labeled C2), a Hmong component (labeled C7), a component found in the nearby Bidayuh people of Borneo (labeled C9), the Temuan people of Peninsula Malaysia (C10), the Miabei people of Thailand (labeled C5), a West Eurasian component (labeled C6), or an African component (labeled C1).

Their lack of Papuan or Philippine Negrito affinity, their lack of Denisovan admixture, and the apparent archaeological case for their arrival in Borneo ca. 35,000 years ago would be consistent with the deep ancestors of the Lebbo' people being second wave hunter-gatherers of Southeast Asia, rather than first wave hunter-gatherers (like Papuans and Aboriginal Australians and Philippine Negrito populations) in the region. It is this wave of hunter-gatherer people, rather than the first one, to whom ancestral South Indians apparently have a strong genetic affinity.

On the other hand, uniparental ancestry seems to indicate that some first wave hunter-gatherers were incorporated into this population.

The same article from Nature also concludes based upon another statistical analysis that mainland Southeast Asia "gene flows reached the west of Borneo, but not the east." But the Lebbo' people are in eastern Borneo (Borneo was, in historic times, divided by a shallow gulf that divided the western and eastern part of the island). An f3-test analysis "showing shared genetic history with Austronesian groups (represented by indigenous Formosan) compared to Mainland Southeast Asian groups (represented by the H’tin)" places the Lebbo' people in an almost overlapping position with the Ma'anyan people (the closest linguistic ancestors of the people of Madagascar), despite their quite different mixes of ancestry components in an Admixture analysis. 

Another analysis, however, puts the Lebbo' equidistant with the Formosan indigenous people from the Ma'anyan people.

In a PCA analysis (Figure 5 in the Nature paper), the Lebbo' people (light gray triangles in a central position on the PCA figure overlapping the green blue and red boxes) are a near perfect match for the Asian component of the ancestors of the people of Madagascar, while the closest linguistic match, the Ma'anyan are clearly not a good match.

The disparity between the genetically Asian component of the people of Madagascar and the people most linguistically similar to the people of Madagascar suggests either that the Lebbo' people may be a relict population of one which once included people with advance maritime skills who spoke a language similar to the Ma'anyan people of today, or that the Lebbo' people coincidentally have similarity on a PCA chart to the end result of free admixture of Eastern Indonesia, Bornean and mainland Southeast Asian people (much like my own half-Korean, half-European children look like Uighars on a PCA chart).

There is passing discussion of the Lebbo' who are noted for having some unexpected Austro-Asiatic ancestry in another paper on the current population genetics of the region, and there is more analysis of the genetic data (mostly also discussed in the Nature article) here.

Uniparental Genetics

Y-DNA

In terms of uniparental genetics, the Lebbo' have the following Y-DNA haplogroups:

C* a.k.a. C-M130 (13.33%), K* a.k.a. K-M9 (6.67%), KxLT a.k.a. K2 (13.33%), O2a1 a.k.a. O-M88 (33.33%), O1a2 a.k.a. O-M50 (26.67%), and O3a2 a.k.a. O-M164 (6.67%). This is a subset of the diversity of the Ma'anyan people with one exception understandable given the small sample size of just 12 Lebbo' men, two-thirds of whom have Y-DNA haplogroup O which is rare in South Asia. 

Five of the twelve Y-DNA clades are associated with first wave hunter-gatherers in Southeast Asia, and six are often associated with the Austronesian founding population, and one is associated with Austro-Asiatic mainland Southeast Asia.

Y-DNA C* is found in Australian Aborigines, India, Sri Lanka, Southeast Asia, in Europe among males with the surname Llach originating from Garrotxa, Catalonia, Spain, and in prehistoric remains, dating from 34,000 years BP, found in Russia and known as "Kostenki 14". 

Y-DNA C, in general, "attains its highest frequencies among the indigenous populations of Mongolia, the Russian Far East, Polynesia, Australia, and at moderate frequency in Korea and Manchu people, it displays its highest diversity among modern populations of India. It is therefore hypothesized that . . . [it] either originated or underwent its longest period of evolution within India or the greater South Asian coastal region. The highest diversity is observed in Southeast Asia, and its northward expansion in East Asia started approximately 40,000 years ago."

Y-DNA K* is derived from Y-DNA F, which is a brother clade to Y-DNA C (through several intermediate steps), and Y-DNA KxLT a.k.a. K2 is derived from Y-DNA K*. "The basal paragroup K* is exceptionally rare, although it has been reported at low frequencies in various parts of Eurasia, Oceania and Africa." 

Y-DNA KxLT in common in Australian Aborigines, Papuans, and is present as a minor clade in island Southeast Asians. Two individuals with Y-DNA KxLT have been found in a South Asian specific clade, K2e (K-M147) and two individuals with Y-DNA KxLT have been found in a clade with one Telegu man (i.e. Dravidian India) and one ethnically Malaysian man, K2a1 (K-M2313). Y-DNA KxLT is ancestral to Y-DNA N, Y-DNA O, and Y-DNA Q, and Y-DNA Q, in turn, is ancestral to Y-DNA R. 

Y-DNA O-M88 is found frequently among Hani, She people, Tai peoples, Cambodians, and Vietnamese, with a moderate distribution among Qiang, Yi, Hlai, Miao, Yao, Taiwanese aborigines, and Han Chinese. 

Y-DNA O-M50 occurs among Austronesian peoples of Taiwan, the Philippines, Indonesia, Melanesia, Micronesia, and Madagascar as well as among some populations of continental Southeast Asia and among Bantu peoples of the Comoros. 

Y-DNA O-M164 is most common in Cambodia and Laos.

mtDNA

The Lebbo' have the following mtDNA haplogroups:

B4a (21.05%), B5a (15.79%), M20 (15.79%), M71a2 (15.79%), R9b1a1a (10.53%),  and E1a (21.05%) out of a sample size of 19 Lebbo' people. This is about a 50% overlap with the diversity of the Ma'anyan people. 

mtDNA B4a, B5a, M71a, and R9 are all also found in aboriginal populations in Southern Laos, while mtDNA E1a is typically Austronesian haplogroup. 

mtDNA M20 is typical of Southern China and mainland Southeast Asia.

The mtDNA M71a2 is a rare haplogroup associated with mainland Asia. mtDNA M71 is found in Indonesia, Laos, Vietnam, the Philippines, and China. 

mtDNA E is found especially in Taiwan (aborigines), Maritime Southeast Asia, and the Mariana Islands. mtDNA E is a descendant of mtDNA M9 which is found in East Asia and Central Asia, especially in Tibet.  

mtDNA B is a descendant of mtDNA R which, in turn, is derived from mtDNA N. mtDNA B4a is found among Polynesians including the Malagasy people of Madagascar, among Koreans and among some Native Americans. 

mtDNA B5a is found, among other places, in Southern China such as in Hubei province. 

mtDNA R9b is typical of Southeast Asia and mtDNA R is derived from mtDNA N. 

Of course, both mtDNA m and mtDNA N are derived from mtDNA L3, an African mtDNA clade.

South Asia Compared

Neither of these sets of uniparental markers significantly overlap with the uniparental markers of South Asia.

Culture and Language

One of the few good references directly discussing the Lebbo' people is a closed access chapter in book of conference proceedings entitled "The Lebbo’ Language and Culture: A Window on Borneo’s Ancient Past" (January 2015), by Antonio Guerreiro, in the book: 12-ICAL Bali, Proceedings volume I, Publisher: ANU, Editors: Wayan Arka, I, pp.149-177. This paper is, however, discussed in a secondary source which recounts that abstract:

The Lebbo’ are former hunter-gatherers/horticulturists; they are localized in remote places in Eastern Borneo, in the regencies of East Kutai and Berau (Province of East Kalimantan). 

Linguistically, they are distinct from the neighbouring Dayak, and Punan (Pnaan) belonging to various branches of the Kayanic and Kenyahic linguistic groups. While the other languages spoken in the region are the Kutai and Berau Malay dialects, besides Bajau and Bugis (on the coastal areas). 

The paper considers the main features and the lexical/phonological aspects of the two Lebbo’ isolects, Lebbo’ Aso’ and Lebbo’ Isi, which amount to less than 1,000 speakers. Both isolects have evolved from the so-called “Basap” language (known also as “Bahasa Sanimban” in Dutch sources). I will relate the Lebbo’ to the other components of this scattered linguistic and cultural group, such as the Ulun Darat (Basap Selatan) in Mangkalihat, the Ulun Latti in Northern Berau and related Basap in Talisayan. Until now the Lebbo’ language has not been recorded and described systematically. 

In central Borneo, the ancient links of the Lebbo’ are to be found with the proposed Rejang-Sajau linguistic group (Hudson 1978). These links, spread across the Island, point to other languages from East to Western Borneo. The field research was carried from 2009 to 2011 in the frame of the French-Indonesian pluridisciplinary programme, and supplemented by the study of archival materials.

Another story (in Spanish) advocating for designation of parts of their homeland as a UNESCO World Heritage site notes (via Google translation of the Spanish) that:

In karst, thick tropical forests conceal the worn limestone towers, among which the team discovered 35,000-year-old bones and charcoal, the oldest test of human occupation found so far in Kalimantan. "These remains are interesting because for a long time Kalimantan was excluded from the history of human evolution." . . .

The Franco-Indonesian team is working in an area where ancient peoples left impressions of their hands in caverns 10,000 years ago or more. In addition to ancient bones, they found hundreds of rock paintings in orange to brown pigments showing figures of tapirs (now extinct in Borneo), wild cattle and some creatures unknown to us today. 

The wonders of the Sangkulirang-Mangkalihat karst are not limited to the human footprint: the southeastern limestone hills and valleys of Southeast Asia have been called "biodiversity coffers" in danger. The region abounds in rare species restricted to limestone. "I saw blind freshwater fish, rare bats, black and white swifts, innumerable animals. It is also a refuge for orangutans fleeing forest fires during the El Niño years," explains biologist Rondang Siregar of the University of Indonesia. 

The diversity of species of bats is greater than in any point of Southeast Asia. Also discovered cockroaches of a species endemic to a cave and that with its 10 centimeters is the largest in the world. 

About 2,000 Lebbo live in the region, the only ethnic group that inhabits this inland part of eastern Borneo. The Lebbo incorporate to their narrations elements of the cave paintings. One of his myths speaks of a young shaman and his eight wives who took possession of the Karst territory marking the caves with paintings of their hands, as was done in the caves millennia ago. 

The Lebbo have been a great help since they know the karst and the forest and collaborate in the location of the shelters and interpret the paintings.

A DVD recounting of the story of the Shaman is available. See also this description of those materials:

A Lebbo' Shaman's Tale 

During the first part of the interview, Pak Kuling, a Lebbo' shaman (belian) from the village of Pana'an (aged about 50), who is currently the kepala adat, tells of the feats of Belian Danyam. Belian Danyam, literally, "The Young Shaman," is a culture hero of the Lebbo' people of the Lesan River area of East Kalimantan. Among the villagers the hero's legacy remains alive to this day. Several important aspects of this narrative are stressed. Generally, the narrative focuses on the idea of territorial origins and shamanic powers. Belian Danyam was miraculously born as a shaman among human beings on earth. However, who his parents were is not known. As a child he was ready to practice rituals at once and did not have to study first with an elder shaman as is usually the case. During his travels and adventures in the Karst region, the hero expelled evil spirits from the earth, thus making it a suitable place for humans to live. Because he was jealous of his eight wives' behavior, he broke the taboo against mocking animals and so had to cope with the anger of the powerful Thunder God, Meruaa Laut, the protector of adat. Eventually, in his flight the hero vanished into a mountain named Batu Lujep located at the headwater boundary of the upper Lesan and Tabalar Rivers. After his disappearance into the rock his seven successors, known by their shamanic titles, continued to perform in the shamanic tradition (adat belian) he instituted. 

It was Belian Danyam's eight wives who made the stencils of their hands and other marks in the caves as a sign of "being there" and of taking possession of the surrounding area. The hero thus produced a social space in this remote Karstic region. Perhaps, this aspect of the narrative can be related to the holistic character of Lebbo' shamanistic representations, involving the forested landscape and the plentiful reproduction of natural resources (fruits, honey, game). Conversely, a feature that is prevalent in the tale is the notion of bounded territories ascribed to Lebbo' local groups. Spatial boundaries are well-known and relate directly to natural resource exploitation within local group territories. 

In the second part of the interview, Pak Ruling describes ritual ideas and practices at the core of Lebbo' shamanism, i.e. the Tuak harvest festival and the Nobet curative/ purification rites.

Not much is known about their languages, as explained in the link to a presentation describing an effort to document these languages:

The project is focussed on documenting the oral traditions and compiling bilingual vocabularies of some former hunters-gatherers’ languages in the Province of East Kalimantan. Until now these languages, mainly the Punan Tubu’ and the Ulun Lebbo’ and Ulun Darat are poorly known and scattered in different Regencies. The topics addressed in this documentation project are: 

• oral traditions (songs, epics, stories, myths, proverbs); 

• linguistic interactions and borrowings, phenomenon of diglossia with the National language (Bahasa Indonesia), and other, regional, languages; 

• lexicology and dialectal variations. 

• Very little information on the oral traditions of these people is available. 

• The main kind of narrative genre is mbui. 15 mbui have been collected and 8 have been transcribed and translated. A dictionary is being prepared and will be published together with the stories and their translations in Indonesian. 

Many languages from multiple only modestly related language families are spoken in the region, although all (including their's) are apparently Austronesian languages, albeit with substrate influences probably from Austro-Asiatic languages which are common in mainland Southeast Asia.

The 2017 Solar Eclipse In Denver

The 93% solar eclipse in Denver was still surprisingly bright (like a heavily overcast day, much more than moonlight), but discernibly cooler than normal, despite the high fraction of the sun obscured as seen with eclipse glasses.

The relevant physics have been known since the 1600s, and eclipses were predicted by cruder means even back in the late 1400s. So, there was nothing new to learn on that front.

In short, slightly better than meh. It was a fun minor social event.

CMB Constraints On Axion Dark Matter

There is no evidence of axion dark matter in its most natural mass range, which implies that axion dark matter is not a plurality contributor to dark matter and probably makes up only a small percentage of it, if it exists at all. 

The cosmic microwave background (CMB) places strong constraints on models of dark matter (DM) that deviate from standard cold DM (CDM), and on initial conditions beyond the scalar adiabatic mode. Here, the full Planck data set (including temperature, E-mode polarisation, and lensing deflection) is used to test the possibility that some fraction of the DM is composed of ultralight axions (ULAs). This represents the first use of CMB lensing to test the ULA model. We find no evidence for a ULA component in the mass range 10−33≤ma≤10−24 eV. We put percent-level constraints on the ULA contribution to the DM, improving by up to a factor of two compared to the case with temperature anisotropies alone. 

Axion DM also provides a low-energy window onto the high-energy physics of inflation through the interplay between the vacuum misalignment production of axions and isocurvature perturbations. We perform the first systematic investigation into the parameter space of ULA isocurvature, using an accurate isocurvature transfer function at all ma values. We precisely identify a "window of co-existence" for 10−25 eV≤ma≤10−24 eV where the data allow, simultaneously, a ∼10% contribution of ULAs to the DM, and ∼1% contributions of isocurvature and tensors to the CMB power. ULAs in this window (and all lighter ULAs) are shown to be consistent with a large inflationary Hubble parameter, HI∼1014 GeV. The window of co-existence will be fully probed by proposed CMB-S4 observations with increased accuracy in the high-ℓ lensing power and low-ℓ E and B-mode polarisation. If ULAs in the window exist, this could allow for two independent measurements of HI in the CMB using the axion DM content and isocurvature, and the tensor contribution to B-modes.

Renée Hlozek, David J. E. Marsh, Daniel Grin "Using the Full Power of the Cosmic Microwave Background to Probe Axion Dark Matter" (August 18, 2017).

From the introduction in the body text:

A central role in the standard cosmological model is played by Cold Dark Matter (CDM). Using a combination of temperature (T), E-mode polarisation, and weak gravitational lensing power spectra of the CMB, the CDM density is measured to be

Ωch^2 = 0.1199 ± 0.0022 (Planck Collaboration et al. 2016b). CDM is the only form of Dark Matter (DM) required by the data, providing the backbone of the “cosmic web” of large scale structure on linear scales.  

One well-motivated theoretical possibility for the CDM is the axion, a new hypothetical particle motivated by the charge-parity conjugation problem of quantum chromodynamics (QCD) (Peccei & Quinn 1977; Weinberg 1978; Wilczek 1978; Abbott & Sikivie 1983; Dine & Fischler 1983; Preskill et al. 1983). On scales of relevance to cosmology, QCD axions with mass ma < 10^−4 eV are produced non-thermally, have the correct cosmological relic density, and behave as CDM.  

There are very few direct constraints on the QCD axion CDM parameter space (e.g. Asztalos et al. 2010). The lightest supersymmetric particle also provides a canonical CDM candidate (Jungman et al. 1996), though limits on “natural” regions of its parameter space are strong (e.g. Akerib et al. 2017).  

The CMB places stringent limits on a variety of theoretical models for DM beyond CDM, including limits on the mass of standard model neutrinos (Planck Collaboration et al. 2016b), thermal axions (Archidiacono et al. 2013), DM interactions (Wilkinson et al. 2014b; Wilkinson et al. 2014a), and generalized models (Cyr-Racine et al. 2016; Thomas et al. 2016).  

A particularly interesting bound comes from constraints on non-thermally produced ultralight axions (ULAs), as this establishes an absolute lower-bound on the DM particle mass from linear observables, ma > 10^−24 eV (Hlozek et al. 2015). The constraints of Hlozek et al. (2015) (hereafter, H15) not  only bound the particle mass of the dominant component of DM, but also place stringent limits on the axion DM density over many orders of magnitude in particle mass (10^−33 eV < ma < 10^−24 eV) for models in which the DM is a mixture of “standard” CDM and ULAs.  

Cosmology thus probes not only the abundance but also the composition of the dark sector. ULAs are expected to be ubiquitous in string theory (e.g. Svrcek & Witten 2006; Conlon 2006; Arvanitaki et al. 2010) and provide candidates for dark matter (if ma > 10^−27 eV) or dark energy (if ma > 10^−27 eV) components, depending on their mass. 

For fairly natural values of the axion initial field value in string models, φ¯ i ∼ fa < 10^17 GeV, where fa is the axion “decay constant”, ULAs contribute at the percent level to the cosmic critical density (Marsh 2016; Hui et al. 2017).[1]  

[1] The boundary between the two regimes is a matter of convention. Here it has been chosen as approximately the inverse Hubble parameter at matter radiation equality.

Within the range of particle mass (10^−33 eV < ma < 10^−24 eV) the axion ULA percentage of all mass-energy in the universe in the mid-range is most likely about one part per thousand and definitely not more than one part per hundred. It could potentially be almost 10% at the high end of 10^-24 eV with the high end of the error bars (less than half that at 10^25 eV), where it almost reaches a CDM threshold, and potentially even more at the extreme 10^-33 eV low end (almost totally absent at 10^-32 eV) where there is degeneracy between dark matter and dark energy contributions allow it to hit 20.3% of the total mass-energy of the universe.

The bottom line is that there are strict experimental constraints from the CMB on axion dark matter, and it is basically ruled out for many orders of magnitude of particle mass as the sole component of dark matter. Another recent study also disfavored axion dark matter. More constraints on axion dark matter from cosmology observations are here. QCD evidence suggests that if axions exist that they should be heavier than the ULA mass range (see also here).

This matters, of course, because experimental and observational evidence is increasingly ruling out dark matter particle candidates.