5 ms·

There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lens

There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg). MOND only explains 1. Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.

deleted 5mo ago
[deleted]
dnautics 5mo ago
MOND explains 1. 2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). 3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g. MOND explains several things LCDM cannot: - why most elliptical galaxies seem to "not have dark matter" (effectively a prediction) - external field effect (predicted and confirmed) - renzo's rule - DM halos that are way too big - early galaxies (this was a prediction) HM. people have been downvoting. Anyone care to post a substantive rebuttal?
- lukan 5mo ago
  "HM. people have been downvoting. Anyone care to post a substantive rebuttal?" Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
  - dnautics 5mo ago
    I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.
    - lukan 5mo ago
      Well, I rather meant there are guidelines to keep this space meta debate free and I do think they make sense. (And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)
- Tuna-Fish 5mo ago
  2 needs a more substantive rebuttal. LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be. There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)
  - dnautics 5mo ago
    > LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be. It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation. you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.
- pdonis 5mo ago
  > nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter! In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.
- firebot 5mo ago
  2. It's called relativistic MOND. That exists as a category. RelMOND(for short) includes TeVeS, Einstein–Aether variants, BIMOND, and others. 3. I would wager that what we call the CMB is more of a local effect than a big bang remnant. I lack the karma to downvote, don't blame me. :)
  - dnautics 5mo ago
    2. not a physicist but iiuc the relativistic MONDs have been disappointing? but not enough to rule out
    - firebot 5mo ago
      They have varying degrees of 'inelegance' or 'complexity.' But generally they closely conform to Einstein gravity/standard model, as they incorporate relativity. But that's still the thing. No theory can explain gravity across the spectrum of variables (from micro to macro scales, apparent lack of dark matter in some galaxies, black holes, why galaxies don't fly apart, etc.) We have a dark gravity problem regardless of which theory you like best.
throwaway27448 5mo ago
> Only catch is that it hasnt been directly observed. Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
- wvenable 5mo ago
  > Ok, so... why do people take it seriously as a concept? Historically, science has predicted lots of true things about the universe that could not be observed at the time.
  - sigbottle 5mo ago
    I don't know what or why our science education is like this, but it seems like everybody's understanding of science bottomed out at a straw man version of popper & positivism. And to be clear, falsification and being empirical & skeptical about theoretical claims is great. What I see all too often on the Internet is just pattern matching to the words "observable" and "falsification" without a second thought, without actually looking into how science develops, and any and all narratives are historically rewritten to fit only those two categories. Which is why it's even more impressive to be a real scientist, to actually be able to navigate the muddy waters properly where it's not just some simple adjective checklists to run through. (As a non scientist)
    - wwarner 5mo ago
      I feel the same, but then I remind myself of how I used to feel about dark matter (I really disliked it). Having an arena where no scientific question is out of bounds is great.
  - magicalhippo 5mo ago
    > Historically, science has predicted lots of true things about the universe that could not be observed at the time. Just some examples I could think of: The neutrino[1], predicted in 1930 due to missing mass in beta decays, discovered in 1956. Gravitational waves[2], predicted by Einstein in 1916, indirectly detected via pulsars in 1974 and directly by LIGO in 2015. Higgs boson[3], predicted in 1964 to solve the problem of the Standard Model at the time requiring particles to be massless, which we knew they weren't. Hints from LEP at the turn of the millennium, detection by the LHC in 2012. Cosmic microwave background[4], predicted in 1948 as a consequence of the expanding universe on the early radiation. Detected by accident in 1964. Gravitational lensing[5] was first proposed in 1801 as a consequence of Newtonian corpuscular theory of light. Einstein realized in 1915 that GR predicted a larger value. Weak lensing by the sun was observed in 1919. Strong lensing by a distant galaxy of an object behind the galaxy was was predicted in 1937, and first observed in 1979. [1]: https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal [2]: https://en.wikipedia.org/wiki/Gravitational_wave#History https://en.wikipedia.org/wiki/Gravitational_wave#History [3]: https://en.wikipedia.org/wiki/Higgs_boson#History https://en.wikipedia.org/wiki/Higgs_boson#History [4]: https://en.wikipedia.org/wiki/Cosmic_microwave_background#History https://en.wikipedia.org/wiki/Cosmic_microwave_background#Hi... [5]: https://en.wikipedia.org/wiki/Gravitational_lens#History https://en.wikipedia.org/wiki/Gravitational_lens#History
joquarky 5mo ago
It can't be observed, but it affects all these things. This reminds me of arguments on the alt.athiesm newsgroup back in the day.
Sniffnoy 5mo ago
I mean there are modified gravity candidates other than MOND. I think those are worth more consideration. MOND seems pretty well disproven at this point, yet somehow it's still what people focus on in terms of modified gravity! Where are the tests of other modified gravity theories? Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.
zamalek 5mo ago
Associating dark matter with epicycles is unfair, but it's still a risk (CDM, WDM, SIDM, and probably more by next year). In that light, of course it has more observations (by virtue of creating branches to explain incompatible observations) - but counterpoint is that it has made some predictions. My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?

deleted 5mo ago

[deleted]

dnautics 5mo ago

MOND explains 1. 2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). 3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g. MOND explains several things LCDM cannot: - why most elliptical galaxies seem to "not have dark matter" (effectively a prediction) - external field effect (predicted and confirmed) - renzo's rule - DM halos that are way too big - early galaxies (this was a prediction) HM. people have been downvoting. Anyone care to post a substantive rebuttal?

lukan 5mo ago
"HM. people have been downvoting. Anyone care to post a substantive rebuttal?" Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
- dnautics 5mo ago
  I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.
  - lukan 5mo ago
    Well, I rather meant there are guidelines to keep this space meta debate free and I do think they make sense. (And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)
Tuna-Fish 5mo ago
2 needs a more substantive rebuttal. LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be. There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)
- dnautics 5mo ago
  > LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be. It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation. you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.
pdonis 5mo ago
> nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter! In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.
firebot 5mo ago
2. It's called relativistic MOND. That exists as a category. RelMOND(for short) includes TeVeS, Einstein–Aether variants, BIMOND, and others. 3. I would wager that what we call the CMB is more of a local effect than a big bang remnant. I lack the karma to downvote, don't blame me. :)
- dnautics 5mo ago
  2. not a physicist but iiuc the relativistic MONDs have been disappointing? but not enough to rule out
  - firebot 5mo ago
    They have varying degrees of 'inelegance' or 'complexity.' But generally they closely conform to Einstein gravity/standard model, as they incorporate relativity. But that's still the thing. No theory can explain gravity across the spectrum of variables (from micro to macro scales, apparent lack of dark matter in some galaxies, black holes, why galaxies don't fly apart, etc.) We have a dark gravity problem regardless of which theory you like best.

throwaway27448 5mo ago

> Only catch is that it hasnt been directly observed. Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.

wvenable 5mo ago
> Ok, so... why do people take it seriously as a concept? Historically, science has predicted lots of true things about the universe that could not be observed at the time.
- sigbottle 5mo ago
  I don't know what or why our science education is like this, but it seems like everybody's understanding of science bottomed out at a straw man version of popper & positivism. And to be clear, falsification and being empirical & skeptical about theoretical claims is great. What I see all too often on the Internet is just pattern matching to the words "observable" and "falsification" without a second thought, without actually looking into how science develops, and any and all narratives are historically rewritten to fit only those two categories. Which is why it's even more impressive to be a real scientist, to actually be able to navigate the muddy waters properly where it's not just some simple adjective checklists to run through. (As a non scientist)
  - wwarner 5mo ago
    I feel the same, but then I remind myself of how I used to feel about dark matter (I really disliked it). Having an arena where no scientific question is out of bounds is great.
- magicalhippo 5mo ago
  > Historically, science has predicted lots of true things about the universe that could not be observed at the time. Just some examples I could think of: The neutrino[1], predicted in 1930 due to missing mass in beta decays, discovered in 1956. Gravitational waves[2], predicted by Einstein in 1916, indirectly detected via pulsars in 1974 and directly by LIGO in 2015. Higgs boson[3], predicted in 1964 to solve the problem of the Standard Model at the time requiring particles to be massless, which we knew they weren't. Hints from LEP at the turn of the millennium, detection by the LHC in 2012. Cosmic microwave background[4], predicted in 1948 as a consequence of the expanding universe on the early radiation. Detected by accident in 1964. Gravitational lensing[5] was first proposed in 1801 as a consequence of Newtonian corpuscular theory of light. Einstein realized in 1915 that GR predicted a larger value. Weak lensing by the sun was observed in 1919. Strong lensing by a distant galaxy of an object behind the galaxy was was predicted in 1937, and first observed in 1979. [1]: https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal [2]: https://en.wikipedia.org/wiki/Gravitational_wave#History https://en.wikipedia.org/wiki/Gravitational_wave#History [3]: https://en.wikipedia.org/wiki/Higgs_boson#History https://en.wikipedia.org/wiki/Higgs_boson#History [4]: https://en.wikipedia.org/wiki/Cosmic_microwave_background#History https://en.wikipedia.org/wiki/Cosmic_microwave_background#Hi... [5]: https://en.wikipedia.org/wiki/Gravitational_lens#History https://en.wikipedia.org/wiki/Gravitational_lens#History

joquarky 5mo ago

It can't be observed, but it affects all these things. This reminds me of arguments on the alt.athiesm newsgroup back in the day.

Sniffnoy 5mo ago

I mean there are modified gravity candidates other than MOND. I think those are worth more consideration. MOND seems pretty well disproven at this point, yet somehow it's still what people focus on in terms of modified gravity! Where are the tests of other modified gravity theories? Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.

zamalek 5mo ago

Associating dark matter with epicycles is unfair, but it's still a risk (CDM, WDM, SIDM, and probably more by next year). In that light, of course it has more observations (by virtue of creating branches to explain incompatible observations) - but counterpoint is that it has made some predictions. My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?