8 ms·
The Arxiv[0] paper abstract seems much clearer and more informative than the linked article. In particular, it points to WHY the mass has been revised down so
by ccooffee 3y ago
The Arxiv[0] paper abstract seems much clearer and more informative than the linked article.
In particular, it points to WHY the mass has been revised down so far -- there's seemingly a lot less mass at 19+ kiloparsecs (about 62,000 lightyears) from galactic center than we expected the galaxy to have. I'm not an expert so it isn't clear to me if the discussions in the paper about dark-vs-baryonic matter are a theoretical explanation of the gap.
Here's a 2019 article[1][1b] describing the mass of the galaxy at 1.5 trillion solar masses. This current research lowers that number to just 200 billion solar masses with a 3 sigma confidence.
[0] https://arxiv.org/abs/2309.00048 https://arxiv.org/abs/2309.00048
[1] https://www.sciencealert.com/the-most-accurate-measurement-yet-of-the-milky-way-s-mass-puts-us-ahead-of-andromeda https://www.sciencealert.com/the-most-accurate-measurement-y...
[1b] https://arxiv.org/abs/1804.11348 https://arxiv.org/abs/1804.11348
- slashdev 3y agoDoes this have any implications on dark matter? I think most of the evidence for dark matter is from outside the galaxy, so not really. But might we not have made similar over estimations of baryonic matter at the edges of other galaxies? If they were true, does it imply there must be even more dark matter than we thought?
- layer8 3y agoThe implication is that there may be significantly less dark matter in the Milky Way than previously estimated. From [0]: “Using this newly derived rotation curve, a new mass was deduced for our Milky Way. And surprise... it is much lighter. It is now estimated to be only two hundred billion times that of the Sun (~2.06 x 10^11), so about four to five times lower than the previous estimates. The amount of ordinary matter did not change, thus, there must be a lot less dark matter in the Milky Way than previously thought. The new expectation is that the ordinary matter like stars and gas in the Milky Way now makes up about 1/3 of the Milky Way mass and the other 2/3 are accounted to dark matter.” [0] https://www.cosmos.esa.int/web/gaia/iow_20230927 https://www.cosmos.esa.int/web/gaia/iow_20230927
- benreesman 3y agoI’m not a physicist or cosmologist, just a fan. Is it possible that another turn of the crank or two takes dark matter to zero? Dark matter has always had that string theory vibe: yeah, maybe it sounds ridiculous but it’s the only game in town man.
- 8bitsrule 3y agoApart from the Vera Rubin game, of course.
- eru 3y ago> Is it possible that another turn of the crank or two takes dark matter to zero? Not sure about the Milky Way. But we already found galaxies out there that don't seem to have any dark matter; ie where their rotation curves match what you would predict from the ordinary matter alone. Interestingly enough that is evidence in favour of dark matter: it's easy to conceive of mechanisms that can separate dark matter from visible matter with some low probability in a galaxy merger or near-merger. (Similarly for galaxies having more than the average amount of dark matter.) But alternatives like Modified Newtonian Dynamics (MOND) would have a much harder time accommodating those differences.
- username332211 3y agoWithout knowing anything about MOND, this never made sense to me. If MOND can explain away all cases where m > 0[*], it'd be a bit strange it can't explain m = 0. If MOND can only accommodate a certain range, wouldn't that also present a problem for dark matter? If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any? [*] Where I guess m would be the ratio of dark to ordinary matter.
- eru 3y agoMOND can most easily explain a certain fixed value of what you call m. It has a much harder time explaining lots of different m at the same time. > If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any? They don't all have the same amount of dark matter. There's a distribution. You were right, that a weird distribution would invite investigation.
- kuchenbecker 3y agoWith the original in french, and not a physicist, the three pieces of information i wanted: - What did they revise it from/to? - What is the confidence? - What is the suspected reason? Thanks for answering / alluding to all three :)
- garba_dlm 3y ago> What is the suspected reason? fraud I'm still looking for ways to answer my own followup question how does it make sense that this is the suspected reason? (who is defrauding who? why and how?)
- causality0 3y ago1.5 trillion solar masses seems like an insane number for a galaxy that's over the hill on star formation, has about 100 billion stars, over half of which are red dwarfs.
- db48x 3y agoThe vast majority of the mass in most galaxies is dark matter anyway, so the number of stars is not so important. Remember, normal matter is only about 5% of all the stuff in the universe.
- tuatoru 3y agoWow, that's confidence!
- db48x 3y agoWhat do you mean by that?
- AnimalMuppet 3y agoGiven that they just revised the mass of the Milky Way by a factor of five, we should not regard all the other numbers as being absolutely certain. They may be the best numbers we have now, but they are subject to revision, sometimes drastically.
- eru 3y agoThere's already a lot of variation in the proportion of dark matter to visible matter in different galaxies. Revising one galaxy doesn't change the statistics that much. (However you are right that from a Bayesian point of view, the Milky Way is the galaxy we have the most data on. So should count for a lot more than a random galaxy.)
- AnimalMuppet 3y ago