A Blog About the Science and Sociology of Cosmology and Dark Matter

Case Western Reserve University

Stacy McGaugh

Cosmology is challenged at present by two apparently unrelated problems: the apparent formation of large galaxies at unexpectedly high redshift observed by JWST, and the tension between the value of the Hubble constant obtained by traditional methods and that found in multi-parameter fits to the acoustic power spectrum of the cosmic microwave background (CMB).   Maybe they’re not unrelated?

Early Galaxy Formation and the Hubble Constant Tension

A surprising and ultimately career-altering result that I encountered while in my first postdoc was that low surface brightness galaxies fell precisely on the Tully-Fisher relation. This surprising result led me to test the limits of the relation in every conceivable way. Are there galaxies that fall off it? How far is it applicable?

Super spirals on the Tully-Fisher relation

The Milky Way Galaxy in which we live seems to be a normal spiral galaxy. But it can be hard to tell. Our perspective from within it precludes a “face-on” view like the picture above, which combines some real data with a lot of artistic liberty. Some local details we can measure in extraordinary detail, but the big picture is hard. Just how big is the Milky Way?

A Precise Milky Way

Vera Rubin passed away a few weeks ago. This was not surprising: she had lived a long, positive, and fruitful life, but had faced the usual health problems of those of us who make it to the upper 80s. Though news of her death was not

 surprising

, it was deeply saddening. It affected me more than I had anticipated, even armed with the intellectual awareness that the inevitable must be approaching.

Ode to Vera

It has been proposal season for the Hubble Space Telescope, so many astronomers have been busy with that. I am no exception. Talking to others, it is clear that there remain many more excellent Hubble projects than available observing time. So I haven’t written here for a bit, and I have other tasks to get on with. I did get requests for a report on the last conference I went to, Beyond WIMPs: from Theory to Detection.

Critical Examination of the Impossible

To continue… we had been discussing the baryon content of the universe, and the missing baryon problem. The problem exists because of a mismatch between the census of baryons locally and the density of baryons estimated from Big Bang Nucleosynthesis (BBN). How well do we know the latter? Either extremely well, or perhaps not so well, depending on which data we query.  At the outset let me say I do not doubt the basic BBN picture.

What is the Baryon Density, Anyway?

We have a new paper that introduces SPARC: Spitzer Photometry &amp; Accurate Rotation Curves. SPARC is a database of 175 galaxies with measured HI rotation curves and homogeneous near-infrared [3.6 micron] surface photometry obtained with the Spitzer Space Telescope. It provides the largest cohesive dataset currently available of disk galaxy mass models. SPARC represents all known types of rotating galaxies.

SPARC

Galaxies are gravitationally bound condensations of stars and gas in a mostly empty, expanding universe. The tens of billions of solar masses of baryonic material that comprise the stars and gas of the Milky Way now reside mostly within a radius of 20 kpc. At the average density of the universe, the equivalent mass fills a spherical volume with a comoving radius a bit in excess of 1 Mpc.

Galaxy Formation – a few basics

When we look up at the sky, we see stars. Stars are the building blocks of galaxies; we can see the stellar disk of the galaxy in which we live as the vault of the Milky Way arching across the sky. When we look beyond the Milky Way, we see galaxies. Just as stars are the building blocks of galaxies, galaxies are the building blocks of the universe.

Primer on Galaxy Properties

The last post was basically an introduction to this one, which is about the recent work of Pengfei Li. In order to test a theory, we need to establish its prior. What do we expect? The prior for fully formed galaxies after 13 billion years of accretion and evolution is not an easy problem. The dark matter halos need to form first, with the baryonic component assembling afterwards.

Galaxy models in compressed halos

I’ve been busy. There is a lot I’d like to say here, but I’ve been writing the actual science papers. Can’t keep up with myself, let alone everything else. I am prompted to write here now because of a small rant by Maury Goodman in the neutrino newsletter he occasionally sends out. It resonated with me.   First, some context. Neutrinos are particles of the Standard Model of particle physics.

The neutrino mass hierarchy and cosmological limits on their mass

A unique prediction of MOND One curious aspect of MOND as a theory is the External Field Effect (EFE). The modified force law depends on an absolute acceleration scale, with motion being amplified over the Newtonian expectation when the force per unit mass falls below the critical acceleration scale a

 0

= 1.2 x 10

 -10

m/s/s. Usually we consider a galaxy to be an island universe: it is a system so isolated that we need

Statistical detection of the external field effect from large scale structure

A Philosophical Approach to MOND

is a new book by David Merritt. This is a major development in the both the science of cosmology and astrophysics, on the one hand, and the philosophy and history of science on the other. It should be required reading for anyone interested in any of these topics.

A Philosophical Approach to MOND

Vincent: Want to talk about MOND? Jules: No man, I don’t consider MOND. Vincent: Are you biased? Jules: Nah, I ain’t biased, I just don’t dig MOND, that’s all. Vincent: Why not? Jules: MOND is an ugly theory. I don’t consider ugly theories. Vincent: MOND makes predictions that come true. Fits galaxy data gooood.

Vincent

: Want to talk about MOND?

 Jules

: No man, I don’t consider MOND.

 Vincent

: Are you biased?

 Jules

: Nah, I ain’t biased, I just don’t dig MOND, that’s all.

 Vincent

: Why not?

 Jules

: MOND is an ugly theory. I don’t consider ugly theories.

 Vincent

: MOND makes predictions that come true. Fits galaxy data gooood.

Pulp Science

After writing the commentary on the latest fin du MOND, it occurred to me that there are many issues that I consider to be obvious. But I’ve been thinking about them for a quarter century, so perhaps they may need to be clearly elucidated for those who don’t share that background. I am thinking, in particular, of galaxy formation modelers and theorists.

The Grand Observational Challenge for Galaxy Formation Simulations

People often ask for a straight up comparison between ΛCDM and MOND. This is rarely possible because the two theories are largely incommensurable. When one is eloquent the other is mute, and vice-versa. It is possible to attempt a comparison about how bad the missing baryon problem is in each.

Missing baryons in LCDM and MOND

OK, I’m not even going to try to answer that one. But I am going to do some comparison exploration. A complaint often leveled against MOND is that it is not a theory. Or not a complete theory. Or somehow not a proper one. Sometimes people confuse MOND with the empirical observations that display MONDian phenomenology. I would say that MOND is a hypothesis, as is dark matter.

What is theory?

I should perhaps explain a little about the title of the last post. It is perfectly obvious to me. But probably not to anyone else. Our brains work in subtly different ways. One thing that mine does, whether I like it or not, is memorize lines and make obscure links between them.

Plate of Shrimp

Cosmology used to be called the hunt for two numbers. It was simple and elegant. Nowadays we need at least six. It is neither simple nor elegant. So how did we get here? The two Big Numbers are, or at least up till the early-90s were, the Hubble constant H

 0

and the density parameter Ω. These told us Everything. Or so we thought. The Hubble constant is the expansion rate of the universe.

Two Numbers

Why does MOND get any predictions right?

That’s the question of the year, and perhaps of the century. I’ve been asking it since before this century began, and I have yet to hear a satisfactory answer.

Question of the Year (and a challenge)

I’ve reached the point in the semester teaching cosmology where we I’ve gone through the details of what we call the three empirical pillars of the hot big bang: Hubble Expansion Primordial [Big Bang] Nucleosynthesis (BBN) Relic Radiation (aka the Cosmic Microwave Background;

Tooth Fairies &amp; Auxiliary Hypotheses

I noted last time that in the rush to analyze the first of the JWST data, that “some of these candidate high redshift galaxies will fall by the wayside.” As Maurice Aabe notes in the comments there, this has already happened. I was concerned because of previous work with Jay Franck in which we found that photometric redshifts were simply not adequately precise to identify the clusters and protoclusters we were looking for.

By the wayside

One does not simply form stars from 100% of the available baryons.

There has been a veritable feeding frenzy going on with the first JWST data. This is to be expected. Also to be expected is that some of these early results will ultimately prove to have been premature. So –

 caveat emptor!

That said, I want to highlight one important aspect of these early results, there being too many to do all them all justice.

An early result from JWST

I want to take another step back in perspective from the last post to say a few words about what the radial acceleration relation (RAR) means and what it doesn’t mean. Here it is again:  This information was not available when the dark matter paradigm was developed. We observed excess motion, like flat rotation curves, and inferred the existence of extra mass. That was perfectly reasonable given the information available at the time.

Why’d it have to be MOND?

Galaxies are big. Our own Milky Way contains about fifty billion solar masses of stars, and another ten billion of interstellar gas, roughly speaking. The average star is maybe half a solar mass, so crudely speaking, that’s one hundred billion stars. Give or take. For comparison, the population of the Earth has not quite reached eight billion humans.

Tracing the baryons in star forming galaxies

Yes, some. That much is a step forward from a decade ago, when a common assumption was that the Milky Way’s rotation curve remained flat at the speed at which the sun orbited. This was a good guess based on empirical experience with other galaxies, but not all galaxies have rotation curves that are completely flat, nor can we be sure the sun is located where that is the case.

Is the Milky Way’s rotation curve declining?

I think the time has come for another update on wide binaries. These were intensely debated at the conference in St. Andrews, with opposing camps saying they did or did not show MONDian behavior. Two papers by independent authors have recently been refereed and published: Chae (2023) in the

 Astrophysical Journal

and Hernandez (2023) in

 Monthly Notices

. These papers both find evidence for MONDian behavior in wide binaries.

Wide Binary Results Favoring MOND

This clickbait title is inspired by the clickbait title of a recent story about high redshift galaxies observed by JWST. To speak in the same vernacular: LOL! What they mean, as I’ve discussed many times here, is that it is difficult to explain these observations in LCDM. LCDM does not encompass all of science.

 Science

* predicted exactly this.

Can’t be explained by science!

Reality check   Before we can agree on the interpretation of a set of facts, we have to agree on what those facts are. Even if we agree on the facts, we can differ about their interpretation. It is OK to disagree, and anyone who practices astrophysics is going to be wrong from time to time. It is the inevitable risk we take in trying to understand a universe that is vast beyond human comprehension.

Divergence

There are two basic approaches to cosmology: start at redshift zero and work outwards in space, or start at the beginning of time and work forward. The latter approach is generally favored by theorists, as much of the physics of the early universe follows a “clean” thermal progression, cooling adiabatically as it expands.

The next cosmic frontier: 21cm absorption at high redshift

I have been musing for a while on the idea of writing about Naturalness in science, particularly as it applies to the radial acceleration relation. As a scientist, the concept of Naturalness is very important to me, especially when it comes to the interpretation of data. When I sat down to write, I made the mistake of first Googling the term. The top Google hits bear little resemblance to what I mean by Naturalness.

What is Natural?

or why Vera Rubin and Albert Bosma deserve a Nobel Prize

 Natural Law: a concise statement describing some aspect of Nature.

In the sciences, we teach about Natural Law all the time. We take them for granted. But we rarely stop and think what we mean by the term. Usually Natural Laws are items of textbook knowledge.

Natural Law

I’m just back from the World Science Festival where I participated in the discussion shaking up the dark universe. It was an interesting experience, and mostly positive despite some offstage diva behavior. One thing it drove home to me was how hard it is to communicate across a gulf of different training and experience. Not just from scientists to the public, but between scientists.

New York’s all right if you like saxophones

People have been asking me about comments in a recent video by Sabine Hossenfelder. I have not watched it, but the quote I’m asked about is “the higher the uncertainty of the data, the better MOND seems to work” with the implication that this might mean that MOND is a systematic artifact of data interpretation.

Full speed in reverse!

I would like to write something positive to close out the year. Apparently, it is not in my nature, as I am finding it difficult to do so. I try not to say anything if I can’t say anything nice, and as a consequence I have said little here for weeks at a time. Still, there are good things that happened this year. JWST launched a year ago. The predictions I made for it at that time have since been realized.

Remain Skeptical

I want to start by thanking those of you who have contributed to maintaining this site. This is not a money making venture, but it does help offset the cost of operations. The title is not related to this, but rather to a flood of papers addressing the questions posed in recent posts. I was asking last time “take it where?” because it is hard to know what cosmology under UT will look like. In particular, how does structure formation work?

Ask and receive

In science, all new and startling facts must encounter in sequence the responses 1. It is not true! 2. It is contrary to orthodoxy. 3. We knew it all along. Louis Agassiz (circa 1861) This expression exactly depicts the progression of the radial acceleration relation.

This expression exactly depicts the progression of the radial acceleration relation. Some people were ahead of this curve, others are still behind it, but it quite accurately depicts the mass sociology. This is how we react to startling new facts. For quotation purists, I’m not sure exactly what the original phrasing was.

A brief history of the Radial Acceleration Relation

People often ask me of how “perfect” MOND has to be. The short answer is that it agrees with galaxy data as “perfectly” as we can perceive – i.e., the scatter in the credible data is accounted for entirely by known errors and the expected scatter in stellar mass-to-light ratios. Sometimes it nevertheless looks to go badly wrong. That’s often because we need to know both the mass distribution and the kinematics perfectly.

How things go mostly right or badly wrong

One of the most interesting and contentious results concerning MOND this year has been the dynamics of wide binaries. When last I wrote on this topic, way back at the end of August, Chae (2023) and Hernandez (2023) both had new papers finding evidence for MONDian behavior in wide binaries. Since that time, they each have written additional papers on the subject.

Wide binary debate heats up again

Predictive power in science isn’t everything. It’s the only thing.

This is a wise truth that has often been poorly interpreted. I despise some of the results that this sports quote has had in American culture. It has fostered a culture of bad sportsmanship in some places: an acceptance, even a dictum, that the ends justify the means – up to and including cheating, provided you can get away with it.  Winning every time is an impossible standard.

Predictive Power in Science

The results from the high redshift universe keep pouring in from JWST. It is a full time job, and then some, just to keep track. One intriguing aspect is the luminosity density of the universe at z &gt; 10. I had not thought this to be problematic for LCDM, as it only depends on the overall number density of stars, not whether they’re in big or small galaxies. I checked this a couple of years ago, and it was fine.

The evolution of the luminosity density

We have a new paper on the arXiv. This is a straightforward empiricist’s paper that provides a reality check on the calibration of the Baryonic Tully-Fisher relation (BTFR) and the distance scale using well-known Local Group galaxies. It also connects observable velocity measures in rotating and pressure supported dwarf galaxies: the flat rotation speed of disks is basically twice the line-of-sight velocity dispersion of dwarf spheroidals.

Leveling the Playing Field of Dwarf Galaxy Kinematics

Last time, I expressed despondency about the lack of progress due to attitudes that in many ways remain firmly entrenched in the 1980s. Recently a nice result has appeared, so maybe there is some hope.

The RAR extended by weak lensing

I was contacted today by a colleague at NASA’s Goddard Space Flight Center who was seeking to return some photographic plates of Halley’s comet that had been obtained with the Burrell Schmidt telescope. I at first misread the email – I get so many requests for data, I initially assumed that he was

 looking

for said plates.

Fuzzy Thing!

I haven’t written here since late January, which not coincidentally was early in the Spring semester. Let’s just say it was… eventful. Mostly in an administrative way, which is neither a good way, nor an interesting way. Not that plenty interesting hasn’t happened. I had a great visit to Aachen for the conference Dark Matter &amp; Modified Gravity. Lots of emphasis on the philosophy of science, as well as history and sociology.

Shameless commercialism

In the last post, I noted some of the sociological overtones underpinning attitudes about dark matter and modified gravity theories. I didn’t get as far as the more scientifically  interesting part, which  illustrates a common form of reasoning in physics. About modified gravity theories, Bertone &amp; Tait state  Leaving aside just which observations need to be mimicked so precisely (I expect they mean power spectrum;

It Must Be So. But which Must?

A common objection to MOND is that it does not entirely reconcile the mass discrepancy in clusters of galaxies. This can be seen as an offset in the acceleration scale between individual galaxies and clusters.

The Fat One – a test of structure formation with the most massive cluster of galaxies

I happened to visit this blog as a visitor from a computer not mine. Seeing it that way made me realize how obnoxious the ads had become. So WordPress’s extortion worked; I’ve agreed to send them a few $ every month to get rid of the ads. With it comes a new domain name: tritonstation.com. Bookmarks to the previous website (tritonstation.wordpress.com) should redirect here. Let me know if a problem arises, or the barrage of ads fails to let up.

New web domain

My last post about the Milky Way was intended to be a brief introduction to our home galaxy in order to motivate the topic of binary stars. There’s too much interesting to say about the Milky Way as a galaxy, so I never got past that. Even now I feel the urge to say more, like with this extended rotation curve that I included in my contribution to the proceedings of IAU 379.

Wide Binary Weirdness

Last time, I commented on the developing situation with binary stars as a test of MOND. I neglected to enable comments for that post, so have done so now. Indranil Banik has shared his perspective on wide binaries in a talk on the subject that is available on Youtube, included below. Indranil and his collaborators are not seeing a MOND effect in wide binaries. Others have, as I discussed in the previous post.

Commentary on Wide Binaries

The missing mass problem has been with us many decades now. Going on a century if you start counting from the work of Oort and Zwicky in the 1930s. Not quite a half a century if we date it from the 1970s when most of the relevant scientific community started to take it seriously. Either way, that’s a very long time for a major problem to go unsolved in physics.

A Significant Theoretical Advance

This Thanksgiving, I’d highlight something positive. Recently, Bob Sanders wrote a paper pointing out that gas rich galaxies are strong tests of MOND. The usual fit parameter, the stellar mass-to-light ratio, is effectively negligible when gas dominates. The MOND prediction follows straight from the gas distribution, for which there is no equivalent freedom.

Hypothesis testing with gas rich galaxies

Britain and America are two nations divided by a common language. attributed to George Bernard Shaw Physics and Astronomy are two fields divided by a common interest in how the universe works. There is a considerable amount of overlap between some sub-fields of these subjects, and practically none at all in others.

attributed to George Bernard Shaw  Physics and Astronomy are two fields divided by a common interest in how the universe works. There is a considerable amount of overlap between some sub-fields of these subjects, and practically none at all in others. The aims and goals are often in common, but the methods, assumptions, history, and culture are quite distinct.

Two fields divided by a common interest

I have become despondent for the progress of science. Despite enormous progress both observational and computational, we have made little progress in solving the missing mass problem. The issue is not one of technical progress. It is psychological. Words matter. We are hung up on missing mass as literal dark matter.

Despondency

There is a tendency when teaching science to oversimplify its history for the sake of getting on with the science.

A personal recollection of how we learned to stop worrying and love the Lambda

Science progresses through hypothesis testing. The primary mechanism for distinguishing between hypotheses is predictive power. The hypothesis that can predict new phenomena is “better.” This is especially true for surprising, a priori predictions: it matters more when the new phenomena was not expected in the context of an existing paradigm. I’ve seen this happen many times now. MOND has had many predictive successes.

A script for every observational test

I am a white American male. As such, I realize that there is no way for me to grasp and viscerally appreciate all the ways in which racism afflicts black Americans. Or, for that matter, all the ways in which sexism afflicts women. But I can acknowledge that these things exist. I can recognize when it happens. I’ve seen it happen to others, both friends and strangers. I can try not to be part of the problem.

The Other

People seem to like to do retrospectives at year’s end. I take a longer view, but the end of 2020 seems like a fitting time to do that. Below is the text of a paper I wrote in 1995 with collaborators at the Kapteyn Institute of the University of Groningen. The last edit date is from December of that year, so this text (in plain TeX, not LaTeX!) is now a quarter century old. I am just going to cut &amp; paste it as-was;

25 years a heretic

In the previous post, I related some of the history of the Radial Acceleration Relation (henceforth RAR). Here I’ll discuss some of my efforts to understand it. I’ve spent more time trying to do this in terms of dark matter than pretty much anything else, but I have not published most of those efforts. As I related briefly in this review, that’s because most of the models I’ve considered are obviously wrong.

What should we expect for the radial acceleration relation?

In the previous post, I wrote about a candidate parent relativistic theory for MOND that could fit the acoustic power spectrum of the cosmic microwave background (CMB). That has been a long time coming, and probably is not the end of the road. There is a long and largely neglected history behind this, so let’s rewind a bit. I became concerned about the viability of the dark matter paradigm in the mid-1990s.

A pre-history of the prediction of the amplitude of the second peak of the cosmic microwave background

Kuhn noted that as paradigms reach their breaking point, there is a divergence of opinions between scientists about what the important evidence is, or what even counts as evidence. This has come to pass in the debate over whether dark matter or modified gravity is a better interpretation of the acceleration discrepancy problem. It sometimes feels like we’re speaking about different topics in a different language.

What we have here is a failure to communicate

Recent results from the third data release (DR3) from Gaia has led to a flurry of papers. Some are good, some are great, some are neither of those. It is apparent from the comments last time that while I’ve kept my pledge to never dumb it down, I have perhaps been assuming more background knowledge on the part of readers than is adequate.

Recent Developments Concerning the Gravitational Potential of the Milky Way. I.

Avi Loeb has a nice recent post

 Recalculating Academia

, in which he discusses some of the issues confronting modern academia. One of the reasons I haven’t written here for a couple of months is despondency over the same problems. If you’re here reading this, you’ll likely be interested in what he has to say. I am not eager to write at length today, but I do want to amplify some of the examples he gives with my own experience.

Some Outsider Perspective from Insiders

It often happens that data are ambiguous and open to multiple interpretations. The evidence for dark matter is an obvious example. I frequently hear permutations on the statement  This is said in all earnestness by serious scientists who clearly believe what they say.

Bias all the way down

There is a rule of thumb in scientific publication that if a title is posed a question, the answer is no.  It sucks being so far ahead of the field that I get to watch people repeat the mistakes I made (or almost made) and warned against long ago. There have been persistent claims of deviations of one sort or another from the Baryonic Tully-Fisher relation (BTFR). So far, these have all been obviously wrong, for reasons we’ve discussed before.

Are there credible deviations from the baryonic Tully-Fisher relation?

It is common to come across statements like “There is overwhelming astrophysical and cosmological evidence that most of the matter in our Universe is dark matter.” This is a gross oversimplification. The astronomical data that indicates the existence of acceleration discrepancies also test the ideas we come up with to explain them.

Checking in on Troubles with Dark Matter

Dark matter remains undetected in the laboratory. This has been true for forever, so I don’t know what drives the timing of the recent spate of articles encouraging us to keep the faith, that dark matter is still a better idea than anything else. This depends on how we define “better.”  There is a long-standing debate in the philosophy of science about the relative merits of accommodation and prediction.

Define “better”

In previous posts, I briefly described some of the results that provoked a crisis of faith in the mid-1990s. Up until that point, I was an ardent believer in the cold dark matter paradigm. But it no longer made sense as an explanation for galaxy dynamics. It didn’t just not make sense, it seemed strewn with self-contradictions, all of which persist to this day.

When you have eliminated the impossible…

Screw the Earth and its smoking habit. The end of 2023 approaches, so let’s talk about the whole universe, which is its own special kind of mess. As I’ve related before, our current cosmology, LCDM, was established over the course of the 1990s through a steady drip, drip, drip of results in observational cosmology – what Peebles calls the classic cosmological tests. There were many

 contributory

results;

Holiday Concordance

Continuing from last time, let’s compare recent rotation curve determinations from Gaia DR3:  These are different analyses of the same dataset. The Gaia data release is immense, with billions of stars. There are gazillions of ways to parse these data. So it is reasonable to have multiple realizations, and we shouldn’t expect them to necessarily agree perfectly: do we look exclusively at K giants? A stars?

Recent Developments Concerning the Gravitational Potential of the Milky Way.&nbsp;II. A Closer Look at the Data

To start the new year, I provide a link to a discussion I had with Simon White on Phil Halper’s YouTube channel:  In this post I’ll say little that we don’t talk about, but will add some background and mildly amusing anecdotes. I’ll also try addressing the one point of factual disagreement. For the most part, Simon &amp; I entirely agree about the relevant facts; what we’re discussing is the interpretation of those facts.

Discussion of Dark Matter and Modified Gravity

At the dawn of the 21st century, we were pretty sure we had solved cosmology. The Lambda Cold Dark Matter (LCDM) model made strong predictions for the power spectrum of the Cosmic Microwave Background (CMB). One was that the flat Robertson-Walker geometry that we were assuming for LCDM predicted the location of the first peak should be at ℓ = 220.

Second peak bang on

I just got back from a visit to the Carnegie Institution of Washington where I gave a talk and saw some old friends. I was a postdoc at the Department of Terrestrial Magnetism (DTM) in the ’90s. DTM is so-named because in their early days they literally traveled the world mapping the magnetic field. When I was there, DTM

 +

had a small extragalactic astronomy group including Vera Rubin*, Francois Schweizer, and John Graham.

It is not linear

I’m back from the meeting in St. Andrews, and am mostly recovered from the jet lag and the hiking (it was hot and sunny, we did not pack for that!) and the driving on single-track roads like Mr. Toad. The A835 north from Ullapool provides some spectacular mountain views, but the A837 through Rosehall is more perilous carnival attraction than well-planned means of conveyance. As expected, the most contentious issue was that of wide binaries.

The MOND at 40 conference

I read somewhere – I don’t think it was Kuhn himself, but someone analyzing Kuhn – that there came a point in the history of science where there was a divergence between scientists, with different scientists disagreeing about what counts as a theory, what counts as a test of a theory, what even counts as evidence. We have reached that point with the mass discrepancy problem.

I started this blog as a place to discuss science, and have refrained from discussing overtly political matters. This is no longer possible. Today is June 10, 2020 – the date set to strike for black lives. I want to contribute in a tiny way by writing here. If that seems inappropriate to you or otherwise makes you uncomfortable, then that probably means that you need to read it and reflect on the reasons for your discomfort.

Black Lives Matter

I recently traveled to my first international meeting since the Covid pandemic began. It was good to be out in the world again. It also served as an excellent reminder of the importance of in-person interactions. On-line interactions are not an adequate substitute. I’d like to be able to recount all that I learned there, but it is too much. This post will touch on one of the much-discussed topics, our own Milky Way Galaxy.

A few words about the Milky Way

A common refrain I hear is that MOND works well in galaxies, but not in clusters of galaxies. The oft-unspoken but absolutely intended implication is that we can therefore dismiss MOND and never speak of it again. That’s silly. Even if MOND is wrong, that it works as well as it does is surely telling us something.

Clusters of galaxies ruin everything

The following is a guest post by Indranil Banik, Moritz Haslbauer, and Pavel Kroupa (bios at end) based on their new paper

 
 Modifying gravity to save cosmology
 

Cosmology is currently in a major crisis because of many severe tensions, the most serious and well-known being that local observations of how quickly the Universe is expanding (the so-called ‘Hubble constant’) exceed the prediction of the standard cosmological

Big Trouble in a Deep Void

It’s early in the new year, so what better time to violate my own resolutions? I prefer to be forward-looking and not argue over petty details, or chase wayward butterflies. But sometimes the devil is in the details, and the occasional butterfly can be entertaining if distracting. Today’s butterfly is the galaxy AGC 114905, which has recently been in the news.

The curious case of AGC 114905: an isolated galaxy devoid of dark matter?

I had written most of the post below the line before an exchange with a senior colleague who accused me of asking us to abandon General Relativity (GR). Anyone who read the last post knows that this is the opposite of true. So how does this happen? Much of the field is mired in bad ideas that seemed like good ideas in the 1980s.

Take it where?

I am primarily an extragalactic astronomer – someone who studies galaxies outside our own. Our home Galaxy is a subject in its own right. Naturally, I became curious how the Milky Way appeared in the light of the systematic behaviors we have learned from external galaxies. I first wrote a paper about it in 2008;

Recent Developments Concerning the Gravitational Potential of the Milky Way. III. A Closer Look at the RAR Model

Imagine if you are able that General Relativity (GR) is correct yet incomplete. Just as GR contains Newtonian gravity in the appropriate limit, imagine that GR itself is a limit of some still more general theory that we don’t yet know about. Let’s call it Underlying Theory (UT) for short.

Imagine if you can

I have been wanting to write about dwarf satellites for a while, but there is so much to tell that I didn’t think it would fit in one post. I was correct. Indeed, it was worse than I thought, because my own experience with low surface brightness (LSB) galaxies in the field is a necessary part of the context for my perspective on the dwarf satellites of the Local Group.

Dwarf Satellite Galaxies. II. Non-equilibrium effects in ultrafaint dwarfs

Like the Milky Way, our nearest giant neighbor, Andromeda (aka M31), has several dozen dwarf satellite galaxies. A few of these were known and had measured velocity dispersions at the time of my work with Joe Wolf, as discussed previously. Also like the Milky Way, the number of known objects has grown rapidly in recent years – thanks in this case largely to the PAndAS survey.

Dwarf Satellite Galaxies. III. The dwarfs of Andromeda

I promised more results from SPARC. Here is one. The dynamical mass surface density of a disk galaxy scales with its central surface brightness. This may sound trivial: surface density correlates with surface brightness. The denser the stars, the denser the mass. Makes sense, yes? Turns out, this situation is neither simple nor obvious when dark matter is involved.

The Central Density Relation

In 1984, I heard Hans Bethe give a talk in which he suggested the dark matter might be neutrinos. This sounded outlandish – from what I had just been taught about the Standard Model, neutrinos were massless. Worse, I had been given the clear impression that it would screw everything up if they did have mass. This was the pervasive attitude, even though the solar neutrino problem was known at the time. This did not compute!

Neutrinos got mass!

I travel to Cambridge, MA tomorrow to participate in the workshop Rethinking the Dark Matter Paradigm (I had nothing to do with the choice of title). I went to college at MIT in the ’80s, so is a bit back to the future for me in space as well as time. There is a lot to rethink, or nothing at all, depending on who you ask.

Rethinking the Dark Matter Paradigm

The Star Forming Main Sequence relates star formation rate to stellar mass. By exploring a large dynamic range in mass with dwarf galaxies, we require the mathematically expected slope that has eluded most work on the subject.

A subject of long-standing interest in extragalactic astronomy is how stars form in galaxies. Some galaxies are “red and dead” – most of their stars formed long ago, and have evolved as stars will: the massive stars live bright but short lives, leaving the less massive ones to linger longer, producing relatively little light until they swell up to become red giants as they too near the end of their lives.

The Star Forming Main Sequence – Dwarf Style

There has been some hand-wringing of late about the tension between the value of the expansion rate of the universe – the famous Hubble constant, H

 0

, measured directly from observed redshifts and distances, and that obtained by multi-parameter fits to the cosmic microwave background.

Tension in the Hubble constant

One experience I’ve frequently had in Astronomy is that there is no result so obvious that someone won’t claim the exact opposite. Indeed, the more obvious the result, the louder the claim to contradict it. This happened today with a new article in Nature Astronomy by Rodrigues, Marra, del Popolo, &amp; Davari titled

 Absence of a fundamental acceleration scale in galaxies

. This title is the opposite of true.

The Acceleration Scale in the Data

So the radial acceleration relation is a new law of nature. What does it mean? One reason we have posed it as a law of nature is that it is interpretation-free. It is a description of how nature works – in this case, a rule for how galaxies rotate.

 Why

nature behaves thus is another matter.

Four Strikes

The radial acceleration relation is consistent with a single effective force law with a universal critical acceleration scale.

The radial acceleration relation connects what we see in visible mass with what we get in galaxy dynamics. This is true in a statistical sense, with remarkably little scatter. The SPARC data are consistent with a single, universal force law in galaxies. One that appears to be sourced by the baryons alone. This was not expected with dark matter. Indeed, it would be hard to imagine a less natural result.

RAR fits to individual galaxies

A quick note to put the acceleration discrepancy in perspective. The acceleration discrepancy, as Bekenstein called it, more commonly called the missing mass or dark matter problem, is the deviation of dynamics from those of Newton and Einstein. The quantity D is the amplitude of the discrepancy, basically the ratio of total mass to that which is visible.

Astronomical Acceleration Scales

Note: this is a guest post by David Merritt, following on from his paper on the philosophy of science as applied to aspects of modern cosmology.

Stacy kindly invited me to write a guest post, expanding on some of the arguments in

 my paper

. I’ll start out by saying that I certainly don’t think of my paper as a final word on anything.

Cosmology and Convention (continued)

There has been another attempt to explain away the radial acceleration relation as being fine in ΛCDM. That’s good; I’m glad people are finally starting to address this issue. But lets be clear: this is a beginning, not a solution. Indeed, it seems more like a rush to create truth by assertion than an honest scientific investigation.

Triton Station

What we have here is a failure to communicate