Keysh

The face-on galaxy image is credited to Stefan Payne-Wardenaar (https://stefanpw.myportfolio.com/home), whose Twitter and Bluesky bios say, "I make astronomy visualizations in Blender."

is there some sort of gravitational body in the middle that makes everything orbit in galaxies?

No. The Sun's orbit is determined by the total mass of stars, gas, and dark matter interior to the orbit. This is mostly due to the stars (we're not far enough out from the center for dark matter to be the dominant component) and is on the order of several tens of billions of solar masses.

(There is a supermassive black hole at the center of our galaxy, but its mass is only about 4 million solar masses, so it's negligibly small compared to the mass of all the stars.)

I think there is some, but it’s pretty limited. E.g., “He was an old man [who fished alone in a skiff in the Gulf Stream] and …”

"Keplerian" is a term of art based on the rotation curve in Newtonian gravity when all the mass of a system is concentrated at its center. For the Solar System, the rotation curve outside the radius of the Sun is pretty much pure Keplerian: the velocity decreases proportional to the square root of the radius.

For galaxies, which are extended objects, the rotation curve is not Keplerian when you are well inside the galaxy: it first rises to larger radii, then levels off. But since the baryonic matter (stars, gas, dust) in galaxy is rather centrally concentrated, the rotation curve should start looking more and more Keplerian as you get further and further into the outskirts and outside the galaxy.

But that is not what we see. Instead, we see the rotation curves staying roughly constant with radius ("flat"); we call this "non-Keplerian". This is true for almost all galaxies, including ellipticals and lenticulars (this is a recent study of three lenticular galaxies: https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-.... Note the rotation curves in the bottom panels of Figure 3: they do not at any point start decaying, let alone decaying as fast as R^(-1/2).)

Figure 5 of that paper (https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-...) shows the observed rotation curves; it also shows the predicted curves if just the stars and standard Newtonian gravity were operating, with the dotted red lines. Note how these lines first rise to a local peak at small radii, and then decline to larger radii: this is a (quasi-)Keplerian decline. It fails to match the actual rotation curve at large radii.

The conventional response is to postulate some additional form of matter, distributed in a much more extended fashion than the baryonic matter (this produces the dashed black lines in Figure 5 of that paper). The MOND response is to modify gravity (or: to modify the acceleration due to gravity) such that it doesn't show anything like a Keplerian falloff at large radii, even at radii where the gravitating matter (assumed to be just the visible baryonic matter) is well inside.

In the case of the Solar System, the Keplerian decline starts right outside the Sun, where the acceleration is strong enough to be above the MOND threshold. But if you went far enough out and could measure the circular orbital speed, MOND would start to deviate from Keplerian. In the case of galaxies, the outer radii where the rotation curves appear "flat" are where the acceleration due to gravity is low enough for MOND to matter, and so the predicted MOND curves will not be Keplerian.

(I should perhaps point out that I'm a professional astronomer whose been studying galaxies, including lenticulars and ellipticals, for almost 30 years, so attempts to mansplain my field to me won't really impress me.)

It's kind of amusing that you present "keplerian rotation curves" as a "prediction" of MOND, given that the whole point of MOND is a kludge to produce non-Keplerian rotation curves. That is, by definition MOND cannot produce Keplerian rotation curves. This is why the (small) number of dwarf galaxies (not "elliptical and lenticular galaxies"!) which apparently lack dark matter -- and which do not follow the Tully-Fisher relation-- are serious problems for MOND.

The "batting average" is a bit higher than that. For example, measurements of the proper motion (motion across the sky) of Sirius led to the prediction in 1844 that it was in an orbit with an (observed) faint or dark companion; the latter (the white dwarf Sirius B) was not directly observed until 1862, when better telescopes were available.

One could also argue that detections of planets from spectroscopic observations of stars is another example. The first observations of transiting exoplanets -- where the planet blocks some of the light of the star -- were actually cases where the existence of the planet had been previously inferred from Doppler shifting of the parent star (e.g., https://en.wikipedia.org/wiki/HD_209458_b).

As another example, the first evidence for dark matter came from observations in the 1930s of the Doppler shifts of galaxies in galaxy clusters, which suggested much more mass in the clusters than could be explained by the masses of the individual galaxies. Some of this "missing mass" was actually observed in the 1960s and 1970s, when orbiting X-ray telescopes showed X-ray emission from very hot, dilute gas within the clusters (unobservable from the ground because the Earth's atmosphere blocks X-rays). It turns out that the hot, X-ray-emitting gas has about five times the mass of the (stars in) the individual galaxies. So some of the missing mass has been found -- though you still need significant, as-yet-undetected extra mass in clusters to explain why they haven't flown apart long ago.

You don't need a simulation; you just need an understanding of Newtonian gravity, basic algebra and a bit of calculus, and some knowledge of stellar masses, velocities, and space densities. This is a standard part of the grad school curriculum (even the advanced undergrad level) in astronomy; here's an example with the math in some lecture notes from an undergrad course at Caltech (by George Djorgovski): https://sites.astro.caltech.edu/~george/ay20/Ay20-Lec15x.pdf

The mean time for the orbit of a star to be significantly randomized by weak, intermediate-distance interactions (e.g., the kind the Sun is experiencing now from neighboring stars) is the relaxation time, and for a star like the Sun it's of order several trillion years.

The mean time between strong gravitational interactions, where the gravity of a single nearby star significantly changes the orbit of a star (perhaps more like what you were imagining), is of order one quadrillion (10^15) years.

(Note that the numbers are for the density of the stars at the Sun's orbit; further out, where you start to get to the point where dark-matter effects really show up, the density is lower, and so these times would be even longer.)

Those are examples of "extremely rare" even on timescales of the age of the universe.

The problem isn't so much the flatness of the rotation curve, but its continued high value: as you go farther and farther out in distance, it should drop rapidly because most of the visible matter is concentrated toward the center of the galaxy, but it doesn't. This implies that there is more matter, less centrally concentrated than the visible matter.

Note that most "rotation curves" are actually measured from gas, not stars, and also that strong gravitational interactions between individual stars are extremely rare except in very dense star clusters and galactic nuclei, due to the increasingly large distances between stars as you go out from galactic centers. The time required for individual stellar interactions in the main or outer parts of galaxies to significantly affect their motions is much larger than the age of the universe (see, e.g., https://en.wikipedia.org/wiki/Stellar_dynamics).

Finally, this wouldn't address other evidence for dark matter, like the halos of hot (millions or tens of millions of K) intergalactic gas in galaxy clusters. The pressure of the gas should have driven the gas to expand way billions of years ago, if you assume that only the gravity of the individual galaxies and the gas itself is restraining it.

I think "if the training pool is large enough" is a real issue here. We're not talking about living languages with large, properly attested and annotated corpuses.

Indeed, one of the thing you'd probably like the translators to do is identify rare or unique words that can be added to our existing knowledge of these languages.

It sounds like they were excavated in 1881-1882 (e.g., https://en.wikipedia.org/wiki/Sippar#Archaeology) by Hormuzd Rassam ("widely believed to be the first-known Middle Eastern and Assyrian archaeologist from the Ottoman empire."; https://en.wikipedia.org/wiki/Hormuzd_Rassam) working in conjunction with the British Museum.

"From 1877 to 1882, while undertaking four expeditions on behalf of the British Museum, Rassam made some important discoveries. Numerous finds of significance were transported to the museum, thanks to an agreement made with the Ottoman Sultan by Rassam's old colleague Austen Henry Layard, now Ambassador at Constantinople, allowing Rassam to return and continue their earlier excavations and to 'pack and dispatch to England any antiquities [he] found ... provided, however, there were no duplicates.' A representative of the Sultan was instructed to be present at the dig to examine the objects as they were uncovered."

So, not a euphemism for "stolen".

there could be empty regions of space in which billions of years more have elapsed than in e.g. a galaxy.

A problem with that idea would be that the ages of galaxies in low-density regions (including voids) tend to be younger than galaxies in denser regions, suggesting that galaxy evolution proceeds more slowly in voids.

https://www.iaa.csic.es/en/news/galaxies-great-cosmic-voids-...

There have been a longstanding problem that the history of the universe and cosmological distance scale haven't made sense. https://medium.com/starts-with-a-bang/the-hubble-tension-sti...

When I was getting my PhD in condensed matter physics I was going to the department colloquium all the time and seeing astrophysics talks about how some people thought the hubble constant was 40 km/s/Mpc and others thought it was 80 km/s/Mpc. With timescape cosmology maybe they were both right.

You're (mis)remembering a different (old) problem and confusing it with a new one. The problem in the 1970s and 1980s was: what is the local expansion rate of the universe? Where "local" mean "within a few hundred megaparsecs". There were two main groups working on the problem: one group tended to find values of around 50 km/s/Mpc and other values of around 100. Gradually they began to converge (in the early 1990s, the low-H0 group getting values of around 60, the high-H0 group values of around 80), until a consensus emerged that it was in the low 70s, which is where we are now.

The "Hubble tension" is a disagreement between what we measure locally (i.e., a value in the low 70s) and what theory (e.g., LCDM) says we should measure locally, if you extrapolate the best-fitting cosmological models -- based on cosmological observations of the CMB, etc. -- down to now (a value in the upper 60s). This has only become a problem very recently, because the error bars on the local measurement and the cosmological predictions are now small enough to suggest (maybe/probably) meaningful disagreement.

Another longstanding problem in astronomy is that since the 1970s it's been clear we have no idea of how supermassive black holes could have formed in the time we think the universe has existed. With the JWST there are a flood of results that show the first 500 million years of the universe probably lasted a lot more than 500 million years https://iopscience.iop.org/article/10.3847/2041-8213/ac9b22

That's not a "longstanding" problem, it's a problem from the last 25 years or so. In order for there to be a problem, you have to have what you think are reliable estimates for the age of the universe and evidence for large supermassive black holes very early in the universe. This is something that has emerged only relatively recently.

(Your link, by the way, is to a paper that has nothing to do with black holes.)

I like how the arXiv sub-category this paper is in is "Soft Condensed Matter".

Because of course it is.

(Also, the Acknowledgments ends with "We further thank [list of names] for their support and for eating up the sample leftovers.")

I'll admit I've never seen "track" used to mean "write" before.

* I'd like to move the cursor backwards and forwards in long commands easier, maybe even with the mouse (!). Use case is, say, a long curl command and I press up-arrow to repeat it, but want to tweak something. So I press and hold left arrow and wait.

Ctrl-A moves you to the beginning of the line (and Ctrl-E to the end of the line); Option-left-arrow moves you left by word, option-right-arrow right by word.

From Chapter VII of Thomas Kuhn, The Structure of Scientific Revolutions (https://www.lri.fr/~mbl/Stanford/CS477/papers/Kuhn-SSR-2ndEd...):

"If awareness of anomaly plays a role in the emergence of new sorts of phenomena, it should surprise no one that a similar but more profound awareness is prerequisite to all acceptable changes of theory. On this point historical evidence is, I think, entirely unequivocal. The state of Ptolemaic astronomy was a scandal before Copernicus’ announcement. Galileo’s contributions to the study of motion depended closely upon difficulties discovered in Aristotle’s theory by scholastic critics. Newton’s new theory of light and color originated in the discovery that none of the existing pre-paradigm theories would account for the length of the spectrum, and the wave theory that replaced Newton’s was announced in the midst of growing concern about anomalies in the relation of diffraction and polarization effects to Newton’s theory. Thermodynamics was born from the collision of two existing nineteenth-century physical theories, and quantum mechanics from a variety of difficulties surrounding black-body radiation, specific heats, and the photoelectric effect.4 Furthermore, in all these cases except that of Newton the awareness of anomaly had lasted so long and penetrated so deep that one can appropriately describe the fields affected by it as in a state of growing crisis."

Later in the same chapter, he gives three examples of crises that led to paradigmatic revolutions: "a particularly famous case of paradigm change, the emergence of Copernican astronomy."; "the crisis that preceded the emergence of Lavoisier’s oxygen theory of combustion"; and "the late nineteenth century crisis in physics that prepared the way for the emergence of relativity theory."

Kuhn absolutely considered relativity and quantum mechanics to be examples of paradigmatic revolutions, just like Newtonian mechanics in the 17th Century and the earlier Copernican revolution.

If you want to argue that Kuhn was wrong about history, then you can do that (and I would at least partly agree); but if you want to claim Kuhn didn't say what he actually said, that's a different matter.

Physics has obviously had Kuhnian revolutions since Newton, the emergence of relativity and quantum mechanics being two obvious examples.

The Wikipedia entry includes notes on the hypothesized history of modifications to the disk. For example, the non-solar arcs were probably added later (different gold). So the original version had just the lunar crescent.

Not only that, Saxony-Anhalt filed a recent DMCA takedown notice against Wikimedia (https://en.wikipedia.org/wiki/Nebra_sky_disc):

In 2023, the state of Saxony-Anhalt filed a DMCA take down notice requesting removal of nine images of the Nebra sky disc from Wikimedia Commons, asserting that they were the "owner of the exclusive copyright in the Sky Disk of Nebra". Wikimedia Deutschland, a chapter of the Wikimedia Foundation, subsequently filed a DMCA counter-notice stating that since the implementation of Article 14 of the Directive 2019/790 of the European Parliament, there can be no such copyrights on reproductions of visual works that are in the public domain.

Tardigrades are not "extremophiles", which refers to organisms that live (grow, reproduce) in "extreme" environments ("phile" = "like, love"). Tardigrades can temporarily survive some rather extreme conditions, but they generally require fairly ordinary environments to actually live. (As suggested by common names like "water bear" and "moss piglet".)

Those would be "captive" ferrets, not "domesticated" ones. (They want to introduce the offspring from the breeding program into the wild, and have in fact done this for about 10,000 offspring.)

This article in Science about the first cloned black-footed ferret (https://www.science.org/content/article/conservation-first-c...) specifically says, "Cloning endangered species faces unique ethical questions, as well. One is whether the clone, which can hold trace DNA from its surrogate mother, is actually the same as the species that researchers are trying to save. For example, black-footed ferret clones are created using eggs from domestic ferrets, meaning they carry that species’ mitochondrial DNA, which is left in the egg after its nucleus is extracted."

(Later in the article: "... apart from her mitochondrial DNA, most of which comes from her domestic mother, genetic analysis shows she is 100% a blackfooted ferret".)

"Domestic ferret" refers to the domesticated European polecat (e.g., https://www.inaturalist.org/taxa/399272-Mustela-furo).

From the linked Washington Post story: "The cloned animals were made by injecting one of Willa’s cells into an egg from a domesticated ferret."

Which is kind of interesting, because domesticated ferrets are a different species. (I wonder if this means the clones have mitochondrial DNA from the domesticated ferret egg.)

The article says that it was no longer functional (in its communications abilities, at least) by that point

Launched in 1969 ... the spacecraft ceased working a few years later ...

Also, what makes you think the US didn't already have their own satellites for that purpose?

The Tines in Vernor Vinge's A Fire Upon the Deep are pretty good example. (The aliens in A Deepness in the Sky are physically alien, but psychologically pretty close to humans.)

Amusingly, I have made three tries to read that book, and kept giving up (getting a little further each time). Partly it was the peculiar sense of aesthetics (e.g., the idea that of course audience would reject color movies) and partly it was the Looney Tunes solar system, which turned out to be something I could accept in a Daffy Duck cartoon but not in a novel.

(But then I'm an astronomer, so I'm primed to get annoyed by random mistakes and carelessness that other people wouldn't notice or care about. I mean, I could kind of momentarily accept the silliness of a habitable solid surface for Uranus, but when you describe the Sun as fainter than Jupiter seen from that location... and then we're told that wheat can't tolerate cold climates the way rice can, and I realize the botany is just as nonsensical as the astronomy...)

I've read most of one of her short-story collections and really liked it, so it's not a generic problem with her writing.

Do you recall the name of the good translator?

C.J. Cherryh's 1981 novel Wave Without a Shore has something vaguely similar, where the human inhabitants of a planet refuse to notice the non-human natives sharing their cities.

I remember reading this, and looking out my office window where the branches of neighboring tree (denuded of leaves because it was winter) made three or four near-perfect right angles in projection (along with intersections at other angles).

Ah, OK, thanks for the link.

They arrayed three antennas together.

No, they didn’t. The three antennas are in California, Spain, an Australia; they can’t all point at the same point in the sky at once, and even if two could do so, they’re not designed to work as an interferometric array.