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You can lead a theorist to data, but you can’t make him think.
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Jul 20 28 tweets 6 min read
For those watching the astro community freak out about bright, high redshift galaxies being detected by JWST, some historical context in an amusing anecdote… The 1998 October conference was titled “After the dark ages, when galaxies were young (the universe at 2 < z < 5).” That right there tells you what we were expecting. Redshift 5 was high - when the universe was a mere billion years old. Before that, not much going on (dark ages).
Jan 14, 2020 8 tweets 2 min read
Oversimplification Today. So, the snarky twitter reply is to wonder Who these “esteemed astronomers” are. The obvious Twitter-stupid answer:
Jan 8, 2020 5 tweets 3 min read
@ewinsberg There are many distinct threads here that need to be deconstructed: the physics of SNIa, their use as standard candles, and the implications for cosmology. I wrote a post that addresses only the latter:… @ewinsberg One reason the SNIa result indicating dark energy was widely and rapidly accepted is that we had already decided it was needed for completely unrelated reasons. It was a confirmation of 1990s cosmology, not a discovery as it is retroactively portrayed.
Oct 8, 2019 6 tweets 1 min read
Jim Peebles is both a great scientist and a great human being. Brilliant and kind can come in the same person. The award of the Nobel prize to Peebles goes some way to correct an historical quirk. Decades ago, the 1978 prize (also split with a completely unrelated discovery in low temperature physics) was awarded to Penzias & Wilson for discovering the cosmic microwave background.
Sep 30, 2019 16 tweets 5 min read
Who wants to hear about our Galaxy, the Milky Way?
I’m slowly recovering from jet lag induced from visiting Bonn for a conference last week. As it happened, a paper was accepted while there. Managed to post it on the arXiv from the hotel, but haven’t had time to talk about it. Our home Galaxy is a normal spiral galaxy. So I wondered what we might learn by putting it in that context with known scaling relations for external spirals like the radial acceleration relation (RAR; see, e.g.,

Quite a lot, as it turns out.
Feb 27, 2019 14 tweets 4 min read
Here follows a thread on the use and misuse of the exponential disk approximation in extragalactic astronomy. Exponential disk is a common and useful way to quantitively approximate the light distribution of rotating galaxies. By azimuthally averaging around ellipses, we can plot the surface brightness as a function of radius. Here is an example for the LSB galaxy UGC 1230.
Dec 19, 2018 22 tweets 6 min read
Dark Matter Hunters Pivot After Years of Failed Searches - WIRED

“Pivot” is an interesting choice of words. Sorta implies that one is pivoting from one thing (WIMPS) to something else. In which case the “something else” needs to be specified.… The whole dark matter search community reminds me of Daffy Duck in the episode quoted from IMDb here. [shockingly, I do not instantly find a gif of Daffy falling off said cliff.]
Nov 9, 2018 12 tweets 3 min read
An example from my own work how difficult the distances to individual objects (galaxies in this case) can be. Blue points are objects I believe to have reasonably well determined distances. X’s are more suspect. Nice relation for the “good” data. Less so for the rest. Note that many of the less trustworthy data follow the same relation as the better data. Most of the time one gets it right, albeit with big uncertainties. But there are also some whopper outliers. Note, for example, the group of points that are “too high” at low rotation speed.
Aug 28, 2018 15 tweets 4 min read
What we have here is a failure to communicate. Over and over I see statements from esteemed colleagues - genuinely brilliant people - that are simply incorrect, for simple lack of fact checking. Like “using only two out of 10 pieces” of evidence. How does one count? Or “MOND doesn’t and dark matter does.” Does what, exactly?
Aug 19, 2018 4 tweets 1 min read
Strange choice of headline for a mission that was planned to run for 3 months but ran successfully for 14 years. This is a skilled and subtle form of propaganda. By putting the price tag up front (how is this relevant to the story?) the immediate and natural reaction is to scoff “What a waste!” when by the standards of space missions this was incredibly successful and long-lived.
Aug 19, 2018 6 tweets 2 min read
Minor astronomical mystery last night. Looked out just before bed & noticed the first quarter moon was... dimmer than usual. This was pronounced enough that I went out to look. Indeed. Even a slight shade of red. Also fewer stars than usual. But no clouds in the sky, nor cirrus. Woke up this morning; still no clouds. The color of the sky seemed off though, and the sun a bit orangish even once well above the horizon. What the heck? Then I remembered all the wildfires in the west, and decided to look up a smoke map at @NOAA
Jul 31, 2018 8 tweets 2 min read
Came across 1954 classic “Them” on TCM. Apparently standard issue for New Mexico State police back then included sub-machine guns. Hollywood hype? Or just ahead of their time? I mean, you meet a freakish giant ant in the desert. Pistol fire doesn’t phase it. So the state trooper runs back to the patrol car for extra firepower. Smart move. I’m expecting a shotgun, maybe a rifle. But dude comes back with a sub-machine gun. WHY is that in the trunk?
Jul 27, 2018 16 tweets 4 min read
Oh, Ethan. There you go again. Ethan Siegel has made it clear that he hated MOND. Over the years he has made it a personal crusade to persecute the heretics who dare speak of such things. Because nothing says scientific rationality like as hominem attacks.

Mostly I ignore him.

Jul 23, 2018 17 tweets 3 min read
I like the starting point: “collapse [at] z<10+” The “+” makes it. Collapse could happen any time. Redshift less than 10. Or redshift greater than 10. Whenever.

I really empathize with Simon here. This is a problem I’ve often faced as an observer. How do I express the predictions of a theory that makes no [agreed] prediction?
Jul 20, 2018 4 tweets 1 min read
Shocked Conservative Learns Obama Doesn’t Hate White People - New York Magazine… Nails it:
Jun 28, 2018 4 tweets 2 min read
I know a number of serious dark matter experimentalist. They’re really talented scientists. It is amazing the progress they’ve made in increasing the sensitivity of DM detection experiments, especially those based on Noble gases like Xenon. fabulous work. But they have no result The common wisdom for why the physics Nobel committee never recognized Vera Rubin for discovering dark matter was that this was such an outlandish proposition that it had to be confirmed by laboratory detection. That may not have been the only reason, but that was part of it.
Jun 19, 2018 5 tweets 2 min read
Here is a histogram of characteristic accelerations from the SPARC data (our data fit by For each galaxy, g+=x*Vf^4/(G*M). The range of the x-axis is set to match their Fig. 1. No fitting of any sort has been done. The scale a0 is in the data. Vf is the flat rotation velocity. M is the baryonic mass - the sum of stars & gas assuming fixed M/L for all galaxies. G is Newton’s constant. X=0.8 accounts for the geometry of disk galaxies (they’re not spherical cows and we can tell. See Binney & Tremaine). There at no knobs.
Jun 18, 2018 6 tweets 2 min read
Here’s the cumulative distribution of chi^2 from Pengfei’s paper. Red line is a fixed RAR, basically equivalent to MOND. The other lines let g+ vary with Gaussian or flat priors. One gains nothing meaningful by letting g+ vary. Note that this distribution does not look right for chi^2. There are too many “bad” fits (high chi^2). That’s true. But it doesn’t help to vary g+ or anything else. Dark halo fits have a CDF with the same shape. One can’t best-fit one’s way into a sensible CDF. So what’s up??
Jun 14, 2018 4 tweets 2 min read
New blog post summarizing recent work by grad student Pengfei Li.… The scientific version of the paper is in press at A&A; you can get the preprint at
Jun 13, 2018 7 tweets 2 min read
I never cease to be amazed at the human capacity for self-deception.… The precursor of the radial acceleration relation, the mass discrepancy-acceleration relation, has been known since the previous century. In 2004, I provided a general version for simulators to shoot at:…. The response was deafening silence.
Apr 15, 2018 15 tweets 4 min read
You must be thinking of the badly outdated “falling” rotation curve of DDO 154. That wasn’t very persuasive at the time, and was shown to be incorrect by THINGS a decade ago. DDO 154 has M*<Mgas and is well for by MOND with essentially zero freedom. This is a microcosm of everything wrong with science today. We don’t like MOND, so dismiss it the instant one scrap of dodgy data looks slightly out of place (eg DF2). But we like CDM, so spot it endless amounts of special pleading. The bullet cluster is a good example of that.