Wednesday, June 19, 2019

Stacy McGaugh On Astronomy v. Astrophysics

There is more good stuff in the latest post at Triton Station, but this quote clears up important semantic issues and makes an important observation about the scientific effort to understand dark matter phenomena. 
When I say dark matter, I mean the vast diversity of observational evidence for a discrepancy between measurable probes of gravity (orbital speeds, gravitational lensing, equilibrium hydrostatic temperatures, etc.) and what is predicted by the gravity of the observed baryonic material – the stars and gas we can see. When a physicist says “dark matter,” he seems usually to mean the vast array of theoretical hypotheses for what new particle the dark matter might be. . . . 

To date, the evidence for dark matter to date is 100% astronomical in nature. That’s all of it. Despite enormous effort and progress, laboratory experiments provide 0%. Zero point zero zero zero. And before some fool points to the cosmic microwave background, that is not a laboratory experiment. It is astronomy as defined above: information gleaned from observation of the sky. That it is done with photons from the mm and microwave part of the spectrum instead of the optical part of the spectrum doesn’t make it fundamentally different: it is still an observation of the sky.
One could arguably slightly amend one sentence of the post to say instead: "To date, the positive empirical evidence for dark matter to date is 100% astronomical in nature." 

This is because while there is no positive empirical evidence for dark matter from any source other than observational evidence from astronomy, there are two other important means by which we better understand of dark matter phenomena.

One is computational work (both analytical and N-body) that looks at existing theories and select modifications of them to see what those theories predict and whether those theories are internally consistent and consistent with other laws of physics that are believed to be true.

The second is negative empirical evidence from laboratory-type experiments, such as particle collider experiments. Empirical evidence that rules out a possible explanation of something is still important empirical evidence, even though it can't provide us with an answer all by itself. Efforts to understand dark matter phenomena benefit greatly from negative empirical evidence that rules out a wide swath of dark matter particle theories including most of the parameter space for what was initially the most popular dark matter particle candidate: the supersymmetric WIMP.

Now, in fairness to the original language, negative empirical evidence is strictly speaking evidence "against dark matter", rather than "for it" even though it is still important evidence in conducting the overall scientific inquiry. And, arguably, the computational work is something you do with evidence, rather than evidence itself. But, the output of an analytic analysis or an N-body simulation are used in a manner very similar to that of observational evidence from astronomy and laboratory work, so maybe it is a distinction without a difference.


* "Brace for the oncoming deluge of dark matter detectors that won’t detect anything" with Twitter commentary.

* Dark matter interpretations of gamma ray excesses at the galactic center seen by the Fermi Gamma-Ray Space Telescope also doesn't look promising.

* This February 2019 conference on dark matter and modified gravity would have been great to attend.

Monday, June 17, 2019

CDM Fails Again In Describing Low Surface Brightness Galaxies

Empirically, low surface brightness galaxies (mostly, but with notable exceptions discussed in prior posts at this blog) have lots of "dark matter" effects which are apparent in their dynamics. The X-Ray emissions from low surface brightness galaxies should be high in low surface brightness galaxies with large halos and otherwise small. But, the low surface brightness galaxies that are observed are a poor fit to the CDM predictions to which they are compared in a new paper. They are, however, consistent with what a MOND-like theory would predict, where the apparently dark matter is due to dispersed matter distributions rather than halos creating "failed" spiral galaxies.

Constraining the dark matter halo mass of isolated low-surface-brightness galaxies

Recent advancements in the imaging of low-surface-brightness objects revealed numerous ultra-diffuse galaxies in the local Universe. These peculiar objects are unusually extended and faint: their effective radii are comparable to the Milky Way, but their surface brightnesses are lower than that of dwarf galaxies. Their ambiguous properties motivate two potential formation scenarios: the "failed" Milky Way and the dwarf galaxy scenario. In this paper, for the first time, we employ X-ray observations to test these formation scenarios on a sample of isolated, low-surface-brightness galaxies. Since hot gas X-ray luminosities correlate with the dark matter halo mass, "failed" Milky Way-type galaxies, which reside in massive dark matter halos, are expected to have significantly higher X-ray luminosities than dwarf galaxies, which reside in low-mass dark matter halos. We perform X-ray photometry on a subset of low-surface-brightness galaxies identified in the Hyper Suprime-Cam Subaru survey, utilizing the XMM-Newton XXL North survey. We find that none of the individual galaxies show significant X-ray emission. By co-adding the signal of individual galaxies, the stacked galaxies remain undetected and we set an X-ray luminosity upper limit of L0.31.2keV6.2×1037(d/65Mpc)2 erg s1 for an average isolated low-surface-brightness galaxy. This upper limit is about 40 times lower than that expected in a galaxy with a massive dark matter halo, implying that the majority of isolated low-surface-brightness galaxies reside in dwarf-size dark matter halos.
Comments:6 pages, 2 figures, accepted for publication to The Astrophysical Journal Letters
Subjects:Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)
Cite as:arXiv:1906.05867 [astro-ph.GA]
 (or arXiv:1906.05867v1 [astro-ph.GA] for this version)

Primordial Black Hole Dark Matter Not Quite Ruled Out

There is still a window of mass for which primordial black hole dark matter has not been ruled out by astronomy observation, although even if there are primordial black holes in the asteroid-mass size range, this still doesn't explain how these produce the halo distributions that are inferred that most popular variants of cold dark matter have been designed to address, so this is still probably a dead end.

Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates

As the only dark matter candidate that does not invoke a new particle that survives to the present day, primordial black holes (PBHs) have drawn increasing attention recently. Up to now, various observations have strongly constrained most of the mass range for PBHs, leaving only small windows where PBHs could make up a substantial fraction of the dark matter. Here we revisit the PBH constraints for the asteroid-mass window, i.e., the mass range 3.5×1017M<mPBH<4×1012M. We revisit 3 categories of constraints. (1) For optical microlensing, we analyze the finite source size and diffractive effects and discuss the scaling relations between the event rate, mPBH and the event duration. We argue that it will be difficult to push the existing optical microlensing constraints to much lower mPBH. (2) For dynamical capture of PBHs in stars, we derive a general result on the capture rate based on phase space arguments. We argue that survival of stars does not constrain PBHs, but that disruption of stars by captured PBHs should occur and that the asteroid-mass PBH hypothesis could be constrained if we can work out the observational signature of this process. (3) For destruction of white dwarfs by PBHs that pass through the white dwarf without getting gravitationally captured, but which produce a shock that ignites carbon fusion, we perform a 1+1D hydrodynamic simulation to explore the post-shock temperature and relevant timescales, and again we find this constraint to be ineffective. In summary, we find that the asteroid-mass window remains open for PBHs to account for all the dark matter.
Comments:Comments welcome! 43 pages and 8 figures
Subjects:Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as:arXiv:1906.05950 [astro-ph.CO]
 (or arXiv:1906.05950v1 [astro-ph.CO] for this version)

Submission history

From: Paulo Montero-Camacho [view email]
[v1] Thu, 13 Jun 2019 22:20:18 UTC (1,058 KB)

What Causes Fast Radio Bursts?

Fast radio bursts have been observed many times since they were first observed in 2007, but there is still not a good, widely accepted understanding of what causes them. Wikipedia, quoted below with citations omitted, explains what they are, and then, a new review article, that follows this quotation, sums up the current situation. 

The prospects for finding answers to the FRB mystery, as in many lines of research for which astronomy data is relevant, is good, because we are in a golden age of astronomy in which immense waterfalls of data from multiple sources are gushing in and ready to provide answers once they are properly analyzed.
In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to a few milliseconds, caused by some high-energy astrophysical process not yet identified. While extremely energetic at their source, the strength of the signal reaching Earth has been described as 1,000 times less than from a mobile phone on the Moon. The first FRB was discovered by Duncan Lorimer and his student David Narkevic in 2007 when they were looking through archival pulsar survey data, and it is therefore commonly referred to as the Lorimer Burst. Several FRBs have since been found, including two repeating FRBs. Although the exact origin and cause is uncertain, they are almost definitely extragalactic. When the FRBs are polarized, it indicates that they are emitted from a source contained within an extremely powerful magnetic field. The origin of the FRBs has yet to be identified; proposals for their origin range from a rapidly rotating neutron star and a black hole, to extraterrestrial intelligence
The localization and characterization of the first detected repeating source, FRB 121102, has revolutionized the understanding of the source class. FRB 121102 is identified with a galaxy at a distance of approximately 3 billion light years, well outside the Milky Way, and embedded in an extreme environment.

Fast Radio Bursts: An Extragalactic Enigma

We summarize our understanding of millisecond radio bursts from an extragalactic population of sources. FRBs occur at an extraordinary rate, thousands per day over the entire sky with radiation energy densities at the source about ten billion times larger than those from Galactic pulsars. We survey FRB phenomenology, source models and host galaxies, coherent radiation models, and the role of plasma propagation effects in burst detection. The FRB field is guaranteed to be exciting: new telescopes will expand the sample from the current 80 unique burst sources (and a few secure localizations and redshifts) to thousands, with burst localizations that enable host-galaxy redshifts emerging directly from interferometric surveys.  
* FRBs are now established as an extragalactic phenomenon.
* Only a few sources are known to repeat. Despite the failure to redetect other FRBs, they are not inconsistent with all being repeaters.  
* FRB sources may be new, exotic kinds of objects or known types in extreme circumstances. Many inventive models exist, ranging from alien spacecraft to cosmic strings but those concerning compact objects and supermassive black holes have gained the most attention. A rapidly rotating magnetar is a promising explanation for FRB 121102 along with the persistent source associated with it, but alternative source models are not ruled out for it or other FRBs.  
* FRBs are powerful tracers of circumsource environments, `missing baryons' in the IGM, and dark matter.  
* The relative contributions of host galaxies and the IGM to propagation effects have yet to be disentangled, so dispersion measure distances have large uncertainties.
Comments:To appear in Annual Review of Astronomy and Astrophysics. Authors' preprint, 51 pages, 18 figures. A version with higher quality figures is available at: this http URL
Subjects:High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as:arXiv:1906.05878 [astro-ph.HE]
 (or arXiv:1906.05878v1 [astro-ph.HE] for this version)

More Sophisticated Models Of The Bronze Age

Eurogenes calls attention to a notable new ancient DNA paper in a post entitled "Not Bell Beaker, not Corded Ware, but . . . the SGBR Complex."

Since aDNA research suggested a marked gene influx from Eastern into Central Europe in the 3rd millennium bc, outdated, simplistic narratives of massive migrations of closed populations have re-appeared in archaeological discussions. A more sophisticated model of migration from the steppes was proposed recently by Kristiansen et al. As a reaction to that proposal, this paper aims to contribute to this ongoing debate by refining the latter model, better integrating archaeological data and anthropological knowledge. It is argued that a polythetic classification of the archaeological material in Central Europe in the 3rd millennium reveals the presence of a new complex of single grave burial rituals which transcends the traditional culture labels. Genetic steppe ancestry is mainly connected to this new kind of burials, rather than to Corded Ware or Bell Beaker materials. Here it is argued that a polythetic view on the archaeological record suggests more complicated histories of migration, population mixtures and interaction than assumed by earlier models, and ways to better integrate detailed studies of archaeological materials with a deeper exploration of anthropological models of mobility and social group composition and the molecular biological data are explored.
Furholt, Martin, Re-integrating Archaeology: A Contribution to aDNA Studies and the Migration Discourse on the 3rd Millennium BC in Europe, Proceedings of the Prehistoric Society, Published online: 10 June 2019, DOI:

The concept of a population genetic and burial practice movement that corresponds only partially with distributions of Bell Beaker and Corded Ware relics is attractive in the wake of new ancient DNDA data showing that Iberian Bell Beakers have less steppe ancestry and more indigenous ancestry than other Continental and British Isle Bell Beaker individuals.

The abstract is referring to Kristian Kristiansen, who studies the Bronze Age at the University of Gothenburg in Sweden, who is referred to in the journal Nature as on of the field's biggest cheerleaders for Ancient DNA technology, who observes that: “Suddenly there was a lot of free intellectual time to start thinking about prehistoric societies and how they are organized.” (The linked March 2018 review article wonderfully contextualizes the latest movements in the field.) Kristiansen is the lead archaeology in the Copenhagen group, and is associated with a Bronze Age migration model summarized in the following map from a 2018 presentation entitled "The Indo-Europeanization of EuropĂ©."

At least one important conclusion in this presentation, which was plausible for a long time, now seems implausible in light of genetic evidence that Davidski at Eurogenes, in particular, has given a great deal of attention.
The Maikop culture prospered from this Mesopotamian venture for metal, and soon expanded into the steppe, where is became the Kurgan or Yamna culture. . . . More and more prehistoric mines, copper and gold, are being recorded and excavated in the Caucasus to support its bridging role between Mesopotamia/Anatolia and the steppe during this period. . . . A recently published Maikop tumulus used stelae and decorated stone slabs for the construction of the chamber, a tradition (stelae) to be found later also in the steppe. These stelae had apparently been reused from older burials, as a demonstra7on of power
The archaeological and ancient DNA evidence increasingly points to the Balkans (with a nexus localized roughly to contemporary Moldova) as the source of that technology in the Yamna culture, although the Mesopotamian-Anatolian-Steppe archaeological links aren't, as observed, entirely absent.

One possible alternative explanation for the archaeological links between the steppe cultures and the Maikop culture, which would be more consistent with the genetic evidence, is that the direction of archaeological influence ran in the other direction, from steppe to Maikop, rather than the other way around as suggested by Kristiansen.

On the other hand, I do credit Kristiansen for two very important thing: (1) an exceptional level of interdisciplinary analysis that is sadly lacking in a lot of ancient DNA work, and (2) a willingness to connect the dots to create plausible narratives even if there is some possibility that they could be incorrect in some respects. And, some parts of the analysis are, in my humble opinion, in light over the evidence correct even though many scholars aren't willing to stick their necks out to say so in such a clear way:

Bell Beaker groups migrated along the Atlantic seaboard, but also into Central northwestern Europe, where they met Corded Ware groups that stopped their expansion and took over the Bell Beaker package before migrating to England.
I also appreciate the skepticism about linguistic assumptions evidenced in statements like this one:

The western expansion of supposed PIE speaking Yamna groups into the Carpathians and their influence areas, versus supposed Bell Beaker groups of supposed proto-Celtic speaking/Latin speaking populations. Corresponds with gene flow of 1rb male lines from the steppe to England 
The following map of Celtic toponyms is particularly interesting:
The summer 2018 presentation sums up with these rather cryptic statements:
The three models: one for each millennium BCE that contributed to formation and distribution Celtic languages 
• 3rd millennium Beaker migrations to UK and north Iberia spread proto italo-celtic 
• 2nd millennium Bronze Age Atlantic trade systems spread languages of proto-Celtic south but interacted with proto-Germanic speaking population to the north 
• 1st millennium: La Tene migrations from Gaul/Belgium to UK spread a Gaulish version of Celtic to Ireland/UK 
• Thus, this later spread came to dominate. It explains why insular Celtic has virtually no connections to the maritime world. 
The new paper from June 2019 does not appear to be open access, but the bibliography, reproduced below the fold without reformatting, is a nice overview of the major recent work in the field.

Wednesday, June 12, 2019

How To Write An Essay About Hints Of New Physics

A question at the Physics Forums asked for suggestions about how to research and write an article about experimental hints of possible beyond the Standard Model physics, in this case, involving lepton family number violations in B meson decays. Here are some suggestions that I provided. They have broad applicability, so I am reprinting the answer here.

Possible Research Sources

For background information, Wikipedia is always a good starting point, although it should never be your ending point and should only rarely be the reference that you end up actually citing in your article. For example, the articles on Lepton Number, Meson and B Meson in Wikipedia are probably the best starting points in your search because they define key terms and summarize the state of current knowledge at a big picture level.

A comprehensive listing of experimental data on lepton family violating phenomena is found at the Particle Data Group (on the linked page, with different kinds of decays each having a hyperlink to more citations to the relevant literature and materials). There is also a related review article at PDG related to that page. There are also a number of review articles at PDG that discuss the decay of mesons in general and the decay of B mesons in particular.

The most useful resources for reviewing the literature of a search of the arXiv papers for HEP-EX, Google, and Google scholar.

If you haven't found at least three or four fairly recent published articles or pre-prints from respectable authors you haven't researched the literature thoroughly enough and you need to keep looking and you need to try different key words.

A search of posts at Physics Stack Exchange (near the top left of the page, the field kind of blends into the background visually) is also a good way to review available information in a relatively up to date way from people who are on average very knowledgable.
How would you go about writing this paper?
Start With The Basics Of What Is Known Before Discussing New Or Speculative Ideas

In my experience, there is a strong temptation to start out by discussing the mystery and its solutions, and in general, what is unknown. But, this is usually not the best way to organize an essay on new BSM physics discoveries or evidence.

The Basic Outline Of An Essay Discussing Hints Of New Physics

Start With What is Known

Instead, it is usually better to develop as much of a foundation of what is known, what has been measured, what there is consensus upon, and what exactly the Standard Model predicts and allows regarding the matter being discussed.

Many writers omit this assuming that the audience knows it, but this introduction section is easy to write, it grounds all readers in the common set of shared assumptions are taken for granted and the context you are coming at the issue from, its familiarity helps you build solidarity with the reader who shares this knowledge with you making you seem more credible because you and the reader agree on the basics, and it helps a reader build up a little momentum with familiar material before getting into the more challenging new information.

Provide A Brief History Of What Led Up To The Current Experimental Results

Also, in an essay type format discussing new developments, after you lay out the basics or in connection with doing so, it is also often helpful to outline a brief history of the experiments in this area both in terms of who, when and where they too place and how they helped the field make progress and other more tangentially related discoveries. This does several things. One is that it makes it easier for a reader to evaluate references seen in your paper and elsewhere in light of how far developed this line of inquiry was when a particular paper was written. Passing mention of the key researchers in the field will also protect you from accusations that you have failed to attribute work of a researcher whose work you implicitly are relying upon. And, this format also makes it possible to highlight the "where was this point published first by whom" issue, which makes you come across as more authoritative and knowledgable.

This doesn't have to be in narrative form. A timeline or set of bullet points is also acceptable.

For example, unless you are really pressed for words, it would be helpful to discuss in your chronology in LFV in B meson decays, who discovered neutrino oscillation when and where that happened. This is because lepton flavor number is violated pervasively in neutrino oscillation. Until then, it wasn't entirely clear from the experimental data that LFV ever really took place at all, while after this discovery it was manifestly clear that LFN was not a perfectly conserved quantum number in nature.

Lay Out The Essentials Of The Methodology Used And Key Definitions Clearly

Once you get into the new material, in which you will be discussing several related experiments and measurements, it is a good time to discuss as concretely as possible, the experimental setup that the experimenters used in their more recent experiments, simplified so that only the conceptually fundamental key elements of the set up are explained. In this way, you can be clear about where the date come from and what the numbers in your tables and charts are defined to mean. 

Always be meticulous about defining your terms and the units involved, even though more advanced researchers often assume a great deal when they write, and leave a lot to the reader to figure out. If you don't have a PhD or Nobel Prize, you have to prove that you know these things, not force the reader to figure them out.

With This Foundation In Place, Communicate the Results Of The New Experiments Neutrally

As you get into the results, it is key to balance whatever hints have been discovered with a discussion of the experimental uncertainties involved and any other theoretical or circumstantial reasons for skepticism (or lack of skepticism) about the results. 

Do cut and paste the best available screen shots of charts and tables and graphs from your sources, but only if a casual reader will be able to make sense of them without reading the body text carefully first. Everyone looks at the pictures first, before reading the words.

Discuss Possible New Physics Explanations And Do So Very Carefully

Only after you have established all of these basic foundations in as much of a model independent manner as possible should you go on to discuss hypothetical explanations of the data with new physics. 

As you do this be very careful to: (1) identify the source of each proposal typically by leading authors and paper or pre-print, (2) explain the proposal as clearly and simply as possible - take time to really understand each one rather than just trying to cut and paste from another source, (3) write in a manner that clearly distinguishes between different approaches so that a reader doesn't accidentally mash some of them together incorrectly, (4) make clear in the context that they are hypotheses or conjectures rather than proven physical laws, and (5) discuss what motivates a particular BSM approach (i.e. why it makes sense to approach the issue from a particular point of view). Avoid non-standard jargon particular to only one author.

If not one approach is a clear favorite, make a chart or table showing the pros and cons of each possible BSM solution. This makes it easy for the reader to evaluate them and absorb your conclusions. 

Don't pull punches or dance around problems with any particular proposal even if you basically end up discrediting every single idea that has been proposed so far. 

Also, in that chart, always include bad data or other sources of measurement or theoretical error (i.e. that the data is just a fluke or arises as from systemic error) as one of the possibilities. The more strongly the data deviates from the SM expectation, the more you should emphasize this possibility as a likely one.

Write A Conclusion

Finally, (1) sum up the main findings in a conclusion, (2) identify what new experimental data would be most helpful in determining what the correct explanation is, and (3) finally, if you are able to do so, identify when and where new experiments that would provide helpful data are expected to be done in the future.

Write The Abstract Last

Don't write the abstract until you have written everything else and crystalized what you have learned as much as possible in your mind. And, when you do, put as much meat of your conclusion there as you can. Don't just fill the abstract with phases that talk about what your study is doing, but don't convey anything about the results to the reader.

If Space Is Limited, Focus, Don't Survey

Of course, sometimes you won't have the time or space to cover all of this in a single essay. But, if that is the case, first trim material with lots of words that doesn't really communicate much information. Then, err in the direction over covering fewer topics in greater depth, rather than surveying as many subtopics as possible.

Form and Style Issues

First, read your professor's syllabus, website and paradigms, if any, and review any examples of good work from students that he has shared. A professor's preferences always override anything that a random guy on the Internet like me says.

Use headings liberally, even if this means that your essay is broken up into many chunks. Prefer multiple short paragraphs under a heading to one long paragraph unless the entire paragraph really just has one cohesive thought. If you feel a temptation to write a very long or complex sentence, break it up into separate sentences and turn that sentence into a paragraph instead.

It is frequently good form to introduce a subsection with a heading, then to provide the one word or sentence answer, and then to further justify your answer to that question or subquestion. People naturally parse question and answer formats more easily than monologs.

If you use a phrase often, consider defining a term that sums up that phrase. But, resist the urge to create acronyms for phrases or concepts or names that are used only a few times unless the abbreviations are ubiquitous in your field and will instantly be understood by every reader, and even then define them the first time that they are used.

Paper and pixels are cheap and you can cut and paste if you need to in order to save time writing your essay. Time spent puzzling over what you were referring to with an abbreviation or acronym is expensive.

If something is hard to explain in words, supplement it with your own picture. Even a rough hand drawn picture will usually be greatly appreciated and praised. In an article like this one, a Feynman diagrams are often are particularly useful in helping a reader to understand what you are discussing.

Proofread, Review And Having Someone Else Read It If Possible

Always triple proofread your title, the abstract, your first paragraph and your last paragraph for formatting, spelling, grammar, readability, and accuracy, and have another set of eyes look at it too, because errors in these parts of the essay are particularly embarrassing.

Then, review the overall order to see that it flows smoothly, and double check every formula or number that is a result for mistakes.

Keep reviewing and revising your paper in as many drafts as necessary until you get it right. The papers of good students don't look all that great the first time. They look great because they are polished and refined and revised until no further improvements can be made. It should feel normal to have five to twenty-five drafts of a decent sized paper on a complex topic, if not more.

It is especially important to have someone who is native speaker of English read you essay (even if they are not very familiar with your field) if English is not your native language. Otherwise your essay will have awkward ways of saying things that aren't technically incorrect but aren't natural, idiomatic ways of saying something. Having a native speaker read your essay also allows you to avoid using words that have double meanings that you aren't familiar with that are often not documented in dictionaries, which can be embarrassing and distract from your scientific content.

If for some reason it is impossible to have someone else read it at least once, at a minimum, put the finished final draft aside, clear your head by doing something else (ideally sleep on it overnight) , print out the nearly final draft, and then come back to it and review it with a pen in hand, ideally away from your computer.

If you don't take this break or try to edit it entirely on a screen, your mind will often "autocorrect" errors in your head that would be obvious had you been reading your essay freshly for the first time.