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Is dark matter’s “nightmare scenario” true?
- tablespoon 4y agoHonestly, I hope so. The universe isn't totally amenable to our inspection, and it'd be nice to have an example that just stared us in the face all the time.
- aszantu 4y agoI've been wondering for a while if light could be the thing that pushes away the boundaries of the universe and that's the reason for the expansion, if u can push a solar sail... anyways, I don't understand enough about this.
- SomewhatLikely 4y agoThere are no known boundaries of the universe. The universe expands everywhere continuously. I think of it as though a little bit of extra volume is injected in between everything on a regular basis. Also when we look out as far back in time as we can, everything looks very similar, and there's no "boundary". Each year we can see a little bit farther out into the universe because those photons from billions of years ago finally got to us and so far things keep looking the same the farther out we can see. So as far as we know the universe is far larger than what we can see (what has had enough time to send us photons). I also don't understand enough about this. https://en.wikipedia.org/wiki/Expansion_of_the_universe https://en.wikipedia.org/wiki/Expansion_of_the_universe https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe
- wkearney99 4y agoBut at the same time we're also racing away. There is light from objects that will never get to us because we're moving away from it, due to expansion, faster than it'd ever get to us. Eventually we'll have moved so far away from everything that we'll start seeing less and less (granted, "we" likely won't be around, our planet having long since been incinerated when Sol dies).
- jay_kyburz 4y agoI think its likely we will be around in some form, though perhaps very different to how we see ourselves today.
- negativelambda 4y agoWe can calculate the force exerted on an object by a photon of light of a given frequency. I'm guessing it is not sufficient to produce the effect you envisage, although I have not done the calculation. Interestingly this photon pressure does put an upper limit on the mass of a star; the Eddington Limit [1]. If a star is too large (and therefore luminous) its photon pressure pushes away other material and stops is from growing larger! [1] https://en.wikipedia.org/wiki/Eddington_luminosity https://en.wikipedia.org/wiki/Eddington_luminosity
- stillworks 4y agoWould recommend reading Chapter 3 "From Eternity to Here" by Sean Carroll (or maybe read the entire book) for a "light weight" introduction. If you don't have the time then TL;DR : universe is expanding means literally that, i.e it is almost as if new space is getting created rather than "cosmological scale objects" hurtling away from each other because they "gained momentum (if one can even call it that)" because of that big bang.
- woleium 4y agoIf the title is a question, the answer is no.
- someotherpersom 4y agoThat’s likely not the case here, but nice try with the trendy Reddit generalization
- miika 4y agoNightmare for who? How hard it is to admit that their equations are incorrect? Even amateur like myself can see that the dark matter is adhoc value that was calculated in order to patch the standard model. There are several better alternatives but do we need to wait until these stubborn believers have passed away until the world can move forward?
- negativelambda 4y agoAt the moment there are no better alternatives that satisfy all the observations. If you try to replace General Relativity with something like Modified Newtonian Dynamics (MOND) [1], then you can get good fits to galaxy rotation curves. However, there are many sources of astronomical/cosmological data that will not bend to alternative gravity models, e.g the precise fluctuations of the cosmic microwave background put stringent bound on the matter components of the Universe. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Outstanding_problems_for_MOND https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ou...
- rocqua 4y agoI vaguely recall hearing that there are galaxies "without any dark-matter". Or, more accurately, galaxies whose rotational speed is consistent with general relativity without needing to presume dark matter exists in those galaxies. If that is correct, does it not immediately invalidate MOND?
- dotancohen 4y agoOne funeral at a time, as the saying goes.
- exmadscientist 4y agoThere are many good reasons to think that dark matter is a good solution to certain cosmological problems. There are also a few good reasons to think that dark matter is total BS. It is a genuine open question. No theory in this area is a good fit for all available evidence. This is not a case of "progress by funeral" like, say, the Alzheimer's situation also on today's front page. The linked article is a good summary.
- account-5 4y agoAm I right in thinking no one has been able to detect dark matter despite decades of attempts and research? My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them?? If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real? How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over? Like planets, rocks, dust, etc. Couldn't that stuff, which to my mind there would be vastly more of than stars, make up the extra gravity? I'm aware I might be talking complete shit because I'm completely ignorant of the detail here. I would love an understandable explanation as to why that's wrong.
- negativelambda 4y ago> Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research? Yup [1] > My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them?? Yup [2] > If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real? It produces certain effects in various different enviromennts or domains, which lends more support to that hypothesis. By certain different domains, I mean like Galaxy and cluster dynamics, gravitational lensing around galaxies and clusters and the precise fluctuations seen in the cosmic microwave background, these are all very different regimes but all seem to support the hypothesis of extra mass which is not interacting electromagnetically. > How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over? There are research efforts to look for exactly this kind of stuff; Massive compact halo object (MACHOS). However they don't see enough of it to account for the observations we see in galaxies and galaxy clusters. [3] [1] https://lss.fnal.gov/archive/2022/conf/fermilab-conf-22-180-v.pdf https://lss.fnal.gov/archive/2022/conf/fermilab-conf-22-180-... [2] https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... [3] https://en.wikipedia.org/wiki/Massive_compact_halo_object#Detection https://en.wikipedia.org/wiki/Massive_compact_halo_object#De...
- exmadscientist 4y agoI hate to be that guy, but many of those points are exactly the subject of this article.
- habibur 4y agoDark matter and dark energy are like what if there was no Einstein to discover relativity in early 1900s. We later continue to discover that gravity can't explain planetary motions correctly and therefore search for any explanation. There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.
- superjan 4y agoThere is certainly new physics involved, the issue is more that for dark matter, the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with (standard model). It might be that the correct theory of dark matter is already found and debated, we just have no way to tell it from the false ones, as for all that we know, dark matter might be a shadow world of spaghetti monsters of which we only know its existence via their gravitation. *) Except gravity.
- throwawaymaths 4y ago> the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with I think this is why people get frustrated with the DM story. It's not the evidence that points to that. It's the lack of evidence that points to that. Specifically speaking, we keep hypothesizing limits on interaction in the hopes of finding it below some threshold but keep pushing those limits back. A multimodal observation would be clutch, but we keep confirming that a multimodal observation can't happen.
- fsh 4y agoYou are cherry-picking one particular example. The neutrino and the planet Neptune were "dark matter" for decades until better detectors became available.
- throwawaymaths 4y agoIronically Pluto was found the same way that Neptune was, but it turned out to be a calculation error.
- akomtu 4y agoCall it stellar matter at least, for its influence on stars. Calling it dark only adds confusion.
- febusravenga 4y agoBut the very definition of this phenomenon is something interacting with gravity (like matter) and not with EM... And EM in layman terms equals light, thus dark... It totally makes sense to call it "dark matter"
- 6nf 4y agoThe author is implying that other solutions like Modified Gravity are dead in the water, I don't think they are.
- jjgreen 4y agoThe interested reader might find Roscoe's work on MOND worth a browse: https://www.researchgate.net/profile/David-Roscoe-3 https://www.researchgate.net/profile/David-Roscoe-3
- 6nf 4y agoWhy did my comment get multiple downvotes, I can't work it out?
- jjgreen 4y agoMe neither, MOND seems entirely reasonable intellectually, scientifically; I don't have a strong view as to whether it's right or not, but I know serious people who do. "Meh, HN eh?" probably the best response :-)
- continuational 4y agoThere's a chance dark matter is black holes created in the aftermath of the Big Bang. Some of these then merged into the supermassive black holes we see in the center of galaxies today. https://news.yale.edu/2021/12/16/black-holes-and-dark-matter-are-they-one-and-same https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...
- scotty79 4y agoI'm huge fan of that because in my view it requires least amount of making stuff up. So I probably never be convinced by any other explanation unless this one is solidly falsified for all possible blackhole masses.
- titzer 4y agoYet black holes do interact with ordinary matter--they can consume it--and they emit Hawking radiation. Dark matter does not appear to be doing either of those things.
- choeger 4y agoMy intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated. I fail to see how dark matter could ever be falsified. The best potential replacement would be a theory that's simpler, but even then dark matter would probably explain the same observations. The second question is: Is dark matter even matter if it only interacts via gravity? Wouldn't it be better named a property of space-time? That is, dark matter is a name for a recursive property of gravity?
- namaria 4y agoUnless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.
- AnIdiotOnTheNet 4y agoRight? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.
- npc12345 4y ago
- doliveira 4y agoDark matter is the simplest theory.
- TEP_Kim_Il_Sung 4y ago*hypothesis Theory implies they found out what it is.
- doliveira 4y agoWhy do people in the comments always ignore that dark matter has way more evidence for it than just galaxy rotation curves (something very well explained in the article)? It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.
- scotty79 4y agoDo you know of any good derivation of existence of dark matter a layman could follow through and be convinced?
- ctchocula 4y agoThis is a good write-up that justifies dark matter using 7 independent pieces of evidence: https://medium.com/starts-with-a-bang/7-independent-pieces-of-evidence-for-dark-matter-3692126a2283 https://medium.com/starts-with-a-bang/7-independent-pieces-o...
- pja 4y agoYou could read through the relevant Wikipedia pages for an overview: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
- rocqua 4y agoWikipedia is rarely a good introduction for laymen. It tends to be a great reference for people already in the field.
- ciphol 4y agoThat's true in mathematics, but I find for any other subject it's a great reference for anyone intelligent enough to understand the subject. (One might remark that I'm just not intelligent enough to understand advanced mathematics, and they might be right...)
- kryptiskt 4y ago
- scotty79 4y agoHow small a black hole would have to be to be completely undetectable by us even when it flies through the solar system?
- LatteLazy 4y agoThe minimum mass for a black hole is about 3 times the size of the sun. So we would definitely detect such an object. In fact it would have pretty catastrophic consequences if it entered the solar system...
- mr_mitm 4y agoYou mean stellar black holes, right? Primordial black holes could be much smaller, if they exist. I just looked up what would happen in that case, and it sounds really bizarre: > Nevertheless, a PBH would have a notable local impact, Rahvar notes. “The passage of a black hole could melt a cylinder along the interior of the Earth with a radius of almost 10 cm. After a short time, this tunnel would freeze,” he explains. In theory, this would produce a distinct metamorphic fingerprint of the passage in the rock record. But given that we might expect only eight of these to have been produced in the Earth’s lifetime (one on each side of the planet per collision episode), “I would expect that such a geological trace is difficult to detect,” Rahvar concedes. https://physicsworld.com/a/concerning-primordial-black-holes/ https://physicsworld.com/a/concerning-primordial-black-holes...
- LatteLazy 4y agoThe issue with that is you're really just talking about another form of dark matter: something else theoretically not impossible but also never observed despite decades of looking...
- scotty79 4y agoAt least it's never observed in a sense that neutrino was never observed until it was. We just lack powerful enough sensors. The other forms of dark matter were never observed in a sense tachyons were never observed. As in we don't just lack sensors, we lack exact understanding of what we are trying to observe.
- deleted 4y ago[deleted]
- Haga 4y ago
- atemerev 4y ago“Gravitationally only interacting dark matter” is sort of synonymous with “non-homogeneous spacetime”, or am I missing something? Ah, yes, we also need to have some sort of bound energy, to balance Einstein’s equations. So, geons! https://en.wikipedia.org/wiki/Geon_(physics) https://en.wikipedia.org/wiki/Geon_(physics)
- Doorstep2077 4y agoAlways been interested in physics since I was a kid, so seeing this kind of stuff with dark matter and being able to understand how it'd be used in practical applications is fascinating to me.
- eisvogel 4y agoThe only real nightmare is that a couple of Jesuits, the same guys who imprisoned Galileo for daring to suggest that the Earth revolves around the sun, came up with the Big Bang theory to explain cosmological redshift, thereby carving out another century of confusion and creationist psychosis under the psychological rule of their lawful-evil overlords in the vatican.
- hilbert42 4y ago"There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy." Hamlet William Shakespeare
- tlogan 4y agoIf mass of dark matter is created by Higg effect, does it mean dark matter particle needs to interact via weak force?
- AnIdiotOnTheNet 4y agoI find it very interesting that every time Dark Matter, a theory that actual physicists largely find very convincing, comes up a bunch of non-physicist HNers come out to explain why it doesn't make sense.
- plasticchris 4y agoIt feels like this is implying that questioning is unscientific? I mean, I get being annoyed with laypeople but if you didn’t question the scientific consensus the historic record and the philosophy of science are against you.
- AnIdiotOnTheNet 4y agoIt's more the level of confidence they seem to have in their take, given their relative lack of understanding of the subject.
- exmadscientist 4y agoFor me it's the fact that so many of these commenters seem to think they've come up with something special, found some secret idea. That's completely false. Most physicists aren't very happy with dark matter as a theory (especially since the failure of the big direct particle searches). So the commenters are trying to convince the professionals of something most of them already believe. It's silly. (My credentials: my fingerprints are all over one of those (pilot) experiments.)
- TillE 4y agoHN is big into contrarianism across a wide variety of topics.
- sylware 4y agoIsn't there a modified MOND which fits all the observation data?
- pfdietz 4y agoI don't believe so. If it did exist, it would be more complex than LCDM.
- puffoflogic 4y ago> This isn't evidence against dark matter's existence, If the repeated failure to find tangible evidence of something in the physical universe isn't evidence against its existence, then you're no longer doing science, but rather playing some religious game
- graderjs 4y agoUgh..this writing style, so unreadable. I think the first paragraph could be rewritten better as: "Dark matter puzzles us. Einstein's theory of gravity, plus known matter and radiation in the Universe, including particles and antiparticles described by the Standard Model, cannot explain cosmic observations. We need an additional gravity source - dark matter."
- api 4y agoI’m sure physicists have already thought about this but… Has anyone considered modeling the effect currently explained via dark matter as a new kind of force or field? Like a distinct force generated by mass and its relationship to space time? This wouldn’t be modified gravity but a proposal for an entirely new force whose effect is what we currently explain with the “fudge factor” of dark matter. From the rough picture I do have of dark matter this force would be one whose relationship to distance is different from the usual inverse square law that governs forces. That would be weird but maybe no weirder than particles that can never be detected or MOND.
- mr_mitm 4y agoSomeone has: https://en.wikipedia.org/wiki/Self-interacting_dark_matter https://en.wikipedia.org/wiki/Self-interacting_dark_matter They even think about dark chemistry: https://iopscience.iop.org/article/10.3847/1538-4357/ac75ef https://iopscience.iop.org/article/10.3847/1538-4357/ac75ef Didn't have much support last time I checked.
- igiveup 4y agoIf the unmodified gravity and this new force always oocur together, why not just call them one force? What is it that makes them "distinct"?
- api 4y agoDo we know they always occur together?
- SAI_Peregrinus 4y ago"Dark matter" implies a new field. Or rather, a new field or force that affects gravity implies some sort of interaction exactly like a massive particle that interacts only gravitationally. Particles are just localized disturbances in fields (in quantum field theory), and new fields can have such disturbances and thus have particles. In this (trivial) sense, MOND is a type of "dark matter" theory that posits a particle with only gravitational interactions. Of course we don't have a quantum field theory for gravity, so the above is hand-waving away a HUGE issue of what that new field would be like, how its particles would act, etc, since we don't even know what the quantum gravitational field is like and what its particles are like (assuming such a thing even makes sense).
- thewatcher23 4y agothere's no such thing as dark matter
- rapjr9 4y agoReading this I realized that dark matter is starting to sound somewhat like the idea of the "ether". It's everywhere, it affects everything, but we can't see it or interact with it easily. It's somewhat different than the idea of the ether in that it is not thought to be a homogenous, pervasive field; instead it clumps around gravity. But it has some very ether-like aspects, maybe some we have not discovered yet. I always thought we abandoned the idea of an ether too soon. Why does it have to be evenly spread? Why does it have to respond to things like direction or EM waves? Maybe there are some simple experiments that could explore the idea of an ether/dark matter further. Pure speculation but that is my intuition.