Keysh

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.)

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.

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.

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.

"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".

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

> 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.)