3 ms·

I feel like I should say this in every post about this kind of topic. "Dark matter" was a term used by astronomers to refer to anything that was not shining in

I feel like I should say this in every post about this kind of topic. "Dark matter" was a term used by astronomers to refer to anything that was not shining in a star. Astronomers work with photons in all manner of ways. If it emits photons in a star, then you might call it "light matter." In the original definition, if it was a bit of dust between stars or even galaxies? It's dark matter. Asteroid ejected from a solar system? Dark matter. Itty-bitty planet you cannot resolve with a telescope because it is so close to the parent star and so tiny? It's dark matter. Pinpoint black hole? Dark matter. An electron anti-neutrino sliding unnoticed through galaxies and the hearts of stars for the last few billion years? Dark matter. If it isn't emitting photons due to recent, local fusion and you have had to infer its existence somehow? It's dark matter. The term itself has evolved and now often has an implicit "non-baryonic" in the mix, but don't always count on that.

raattgift 6y ago
> I feel like I should say this in every post about this kind of topic. Please don't. Just link to the actual paper which makes it clear. For example: Preprint version : https://arxiv.org/abs/2005.13161 https://arxiv.org/abs/2005.13161 (n.b. "Comments: Published online in Nature 27 May, 2020) "census of baryons" (title) and "More than three quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to observe, with only a small fraction directly observed in galaxies and galaxy clusters." (abstract's first sentence) gets the point across. I don't know where "dark matter" got to you in the context of the linked article, but it certainly wasn't from the paper or its preprint. It wasn't even from the article linked at the top, which carefully and correctly says: 'Astronomy is full of missing stuff. Most of the universe is understood to be "dark matter" and "dark energy", which nobody has ever directly seen. But even more of a mystery for astronomers was that they couldn't find about half the ordinary matter in the universe.' Further down it says '"dark matter" [...] is undetectable using ordinary methods', which is a bit less careful, but reasonably correct. > In the original definition I doubt there is anyone who can actually submit to arXiv:astro-ph who would fail to distinguish baryonic matter from dark matter in the way you do. > don't always count on it Ok, there may be some on vixra. Lastly: > electron anti-neutrino We should distinguish here by saying that electron anti-neutrinos are not cold dark matter, in the \Lambda-CDM sense, but otherwise are a very-low-mass form of weakly-interacting massive particle (WIMP). Electron neutrinos and their antis are so light that they are almost always ultra-relativistic (viz. https://physics.stackexchange.com/questions/71874/ultrarelativitistic-particle-what-kind-of-a-particle-is-this https://physics.stackexchange.com/questions/71874/ultrarelat... and https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_Special_Relativity_(Crowell)/04%3A_Dynamics/4.01%3A_Ultrarelativistic_particles https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_S... both of which will take you to deeper treatments), importantly including in the cosmological frame. Cold dark matter is non-relativistic in the cosmological frame. Hot dark matter like neutrinos (described 1934, detected 1956) was easily excluded as a significant part of the explanation for the observations detailed at Rubin, Ford & Thonnard (1980) https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstract https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstra... : neutrinos shine out of galaxies and don't linger at the edges of them, or deep in galaxy clusters, very much like electromagnetic radiation shines away rather than lingering.
- at_a_remove 6y ago
  I am speaking not of the paper or of any paper in specific but of people's usual reactions to the term.

raattgift 6y ago

> I feel like I should say this in every post about this kind of topic. Please don't. Just link to the actual paper which makes it clear. For example: Preprint version : https://arxiv.org/abs/2005.13161 https://arxiv.org/abs/2005.13161 (n.b. "Comments: Published online in Nature 27 May, 2020) "census of baryons" (title) and "More than three quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to observe, with only a small fraction directly observed in galaxies and galaxy clusters." (abstract's first sentence) gets the point across. I don't know where "dark matter" got to you in the context of the linked article, but it certainly wasn't from the paper or its preprint. It wasn't even from the article linked at the top, which carefully and correctly says: 'Astronomy is full of missing stuff. Most of the universe is understood to be "dark matter" and "dark energy", which nobody has ever directly seen. But even more of a mystery for astronomers was that they couldn't find about half the ordinary matter in the universe.' Further down it says '"dark matter" [...] is undetectable using ordinary methods', which is a bit less careful, but reasonably correct. > In the original definition I doubt there is anyone who can actually submit to arXiv:astro-ph who would fail to distinguish baryonic matter from dark matter in the way you do. > don't always count on it Ok, there may be some on vixra. Lastly: > electron anti-neutrino We should distinguish here by saying that electron anti-neutrinos are not cold dark matter, in the \Lambda-CDM sense, but otherwise are a very-low-mass form of weakly-interacting massive particle (WIMP). Electron neutrinos and their antis are so light that they are almost always ultra-relativistic (viz. https://physics.stackexchange.com/questions/71874/ultrarelativitistic-particle-what-kind-of-a-particle-is-this https://physics.stackexchange.com/questions/71874/ultrarelat... and https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_Special_Relativity_(Crowell)/04%3A_Dynamics/4.01%3A_Ultrarelativistic_particles https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_S... both of which will take you to deeper treatments), importantly including in the cosmological frame. Cold dark matter is non-relativistic in the cosmological frame. Hot dark matter like neutrinos (described 1934, detected 1956) was easily excluded as a significant part of the explanation for the observations detailed at Rubin, Ford & Thonnard (1980) https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstract https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstra... : neutrinos shine out of galaxies and don't linger at the edges of them, or deep in galaxy clusters, very much like electromagnetic radiation shines away rather than lingering.

at_a_remove 6y ago
I am speaking not of the paper or of any paper in specific but of people's usual reactions to the term.