32 ms·

Why modifying gravity doesn’t add up

96 points by raattgift 4y ago

wwarner 4y ago
Another great article from Ethan Siegel. I think he makes a pretty strong case for cold dark matter, but it relies pretty heavily on theoretical cosmological arguments. For the pro-mond argument, there’s a recent interview with Milgrom on Dr Brian Keating’s physics podcast Into the Impossible [0]. There is also a talk about all the ways DM does not fit observations from Pavel Kroupa (University of Bonn) on youtube [1]. In particular, Kroupa argues that when galaxies pass through one another, DM should cause them to move slower after the collision, because DM would be drawn behind each system in a turbulent wake; but no such slowdown is observed. MOND plus visible matter predicts the motion of these systems well. [0] https://youtu.be/oQ0Cr0hxN40 https://youtu.be/oQ0Cr0hxN40 [1] https://youtu.be/PVgwLWVETIM https://youtu.be/PVgwLWVETIM
- andrewflnr 4y ago
  Turbulence entirely due to gravitational interaction? Does that work? I guess I should watch it, but I can't take it seriously with the title "On the non-existence of dark matter" when there's so much evidence for dark matter. That's clearly unwarranted confidence in search of attention. That said, if true, I think it more likely indicates that both MOND and LCDM have significant truth. The bar for a theory is "as simple as possible, but no simpler." If the evidence truly demands both MOND and DM, we just have to suck it up.
  - wwarner 4y ago
    The DM is moved around by the gravity of the moving galaxy. The DM theory is that each galaxy is surrounded by a huge spherical cloud of DM that (by definition) only experiences gravitation and is invisible and insubstantial in every other way. When Voyager gained speed from passing near Jupiter, it caused Jupiter to slow down by a tiny amount. Similarly, when galaxies pass through each other, the DM should take a measurable amount of momentum from the galaxies. And, yeah, it's a provocative title, but he believes that his measurements falsify DM theory. It's partly a tussle between experimentalists and theoreticians. Kroupa is arguing that the claims of the theory can be tested, and that it doesn't match observations. The theoreticians have a nice story for the big bang, inflation, baryonic nucleosynthesis and the variations seen in the CMB, and the amount of DM it predicts happens to match precisely the amount of DM that we observe.
throwawaymaths 4y ago
Suppose we made an observation that mercury's orbit precesses by a fraction of a degree every orbit. Heretofore every observation of perturbances has correctly presaged the presence of a large mass (a planet) that was quickly found, but this time we have looked and looked but not found anything. Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? > You can postulate an additional separate modification of gravity that comes into play on cluster scales. Or it might be something else altogether. Anyways, MOND has made predictions that people set out to disprove MOND went looking for negative evidence and had to turn around (e.g. external field effect), as well as straightforward predictions JWT may have confirmed (early galaxies) and explains many things that dark matter cannot, like how satellite galaxies don't get ripped apart by the dark matter halo.
- tshaddox 4y ago
  > Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Are these substantially different? Of course using “must” isn’t appropriate, but spouting off the best theory you have at the time seems okay to me. Hopefully no one is saying that we cannot possibly ever find another better theory the same way Einstein’s was better than Newton’s or that we shouldn’t ever look for one.
  - 2muchcoffeeman 4y ago
    I guess the other side effects might be different right? E.g. we talk about relativistic mass. But when they did experiments it was found that no other attributes that come with increased mass was found. So the answer is right but there is something off with our understanding.
  - throwawaymaths 4y ago
    > Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.
hinkley 4y ago
My top, crackpot theory is we live in a black hole and dark matter is the accretion disk. My second crackpot theory is we are four dimensional beings living in a five dimensional universe. Though I also like Cixin Liu’s idea that we are living in a universe that used to be five dimensions and has (mostly) decayed to four, with bits of five dimensional debris holding on.
- pryelluw 4y ago
  You mean there could be black holes inside other black holes?
  - Filligree 4y ago
    Mathematically, the universe looks a little bit like a black hole running backwards in time. But only a little bit.
    - pryelluw 4y ago
      More please.
      - tanqueraythc 4y ago
        Ehh, read Steven Hawking. Black holes (singularities) are strikingly similar to what we know of the big bang. If we are in a black hole, it could explain the accelerating expansion of the universe.
        somat 4y ago
        my pet science fiction theory is that passing the event horizon removes half a dimension. so outside the universe you can move freely back and forth in time, but here in the universe it is constrained you can only go forward. when you pass another event horizon one of what we call a physical dimension gets constrained. you can't move toward the wall of the event horizon, that dimension is gone.
ars 4y ago
Something I've always wondered: Right after the Big Bang there was a massive annihilation of matter and anti-matter, with just a bit of extra matter. But the energy resulting from that annihilation did not go away. So where is it? No matter what form it takes, energy gravitates, so it should be able to be observed.
- frankchn 4y ago
  My impression (I might well be wrong about this) is that the energy from that forms part of the cosmic microwave background radiation.
- exmadscientist 4y ago
  A lot of it just went back into producing particle-antiparticle pairs again. Some of it is the CMB, yes, but not all. Why we didn't just end up with even numbers of particles and antiparticles repeating this dance endlessly is the single biggest open question in particle physics.
- beeforpork 4y ago
  Photons. Each annihilation event emitted a photon, and they are still around. IIRC, about 5e9 times as many photons are observed as there are baryons, and that seems to match up well with the calculated imbalance of the baryosynthesis.
debacle 4y ago
We are in the "epicycles" phase of physics discovery, at both the micro and macro level. I may not live to see it, but I'm excited for when everything just "clicks" and the model becomes more simple and elegant, as it was before the 1920s.
- MichaelZuo 4y ago
  That's an intuitive way to put it, ironic considering epicycles themselves are viewed as the opposite.
  - wumpus 4y ago
    When you run into physics/astronomy people mentioning epicycles, it's not ironic, it's saying that we're missing out on a simpler rule.
    - MichaelZuo 4y ago
      Clearly I'm referring to how the idea of epicycles themselves are viewed by the general population.
colechristensen 4y ago
My fun, science fiction theory to solve the gravity problem is that the universe is actually full of intelligent life mostly billions of years old civilizations and all of the extra mass we can’t see is actually the galactic version of cities built from materials that don’t interact with light much. Likewise we haven’t met any other intelligent species because the growth period before reaching this common technology is short from thousands to millions of years so there just aren’t collisions in space or time where two intelligent species would interact.
- russdill 4y ago
  Except an important property of dark matter is they it's interacts weakly with itself. But still good for sci-fi
  - Beldin 4y ago
    If that is true, where are the dark matter black holes? And why hasn't the majority of DM collapsed into black holes?
    - andrewflnr 4y ago
      The same reason the rest of the matter didn't collapse into black holes, not quite dense enough, or blowing up too fast. And dark matter doesn't have it's own kind of black holes. Related is the idea of "primordial black holes".
      - gpderetta 4y ago
        In fact I think it is harder for dark matter to collapse into black holes because it can't easily shed momentum (angular of otherwise) and 'spiral down' .
        Beldin 4y ago
        That's what i used to think, but GP alluded to weak interaction between DM particles. I thinkt interaction consumes energy. If so, DM particles would be able to shed some momentum... but then they (eventually) could clump into black holes.
rapjr9 4y ago
Maybe dark matter does have electromagnetic interactions with normal matter, but they are at an exceedingly high (or low) frequency. Science has limits in what frequencies it can probe, matter may not have the same limitations. A century long EM wave would look like background noise on a human scale. Petahertz (and beyond) vibrations could have extremely short range interactions that mostly occur in the near field.
Beldin 4y ago
I've got two problems with this. First: if theory and observation don't match up, that can be due to lacking theory or lacking observations. Both should be investigated. Second: all candidates for dark matter i know posit an enormous number of places where this dark matter is. It is not that "dark matter solves this problem". It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass. So not only is this theory supposing that most material in the universe is invisible to us, but its properties other than mass are unknown to us except it allows us to fill in the blanks exactly as needed. E.g, we're not expecting a lot of dark matter black holes, as the hypothesised stuff doesn't even interact with itself and so cannot slow down. Basically, "dark matter" is fairy dust that physicists sprinkle over everything till it works. And you know what: maybe that is the way to go. Probably it plays at least some part in answering these questions. But until we have a clearer picture, we may as well call it "magical fairy dust". That at least reminds us how much we're fudging.
- pfortuny 4y ago
  Until it is experimentally tested, DM is essentially equivalent to the ether. A useful unexplained tool which to date seems to work. Until another Michelson-Morley experiment comes (and MOND might have something to say, or at least they seem to be testing DM experimentally), yes: as untestable as string theory is.
- DiogenesKynikos 4y ago
  > It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass It's actually an extremely simple initial distribution of dark matter in the early universe. You can specify the entire distribution with one number: a spectral index. The thing is, we already know that the universe had that distribution of energy density anyways, because of the Cosmic Microwave Background. If you take that very simple distribution and let the laws of physics run for 13 billion years, you get the required present-day distribution of dark matter. That distribution can be verified in several ways, including gravitational lensing, the rotation curves of galaxies, the motions of galaxies inside clusters, and the temperatures of hot gas inside galaxy clusters. The fact that such a simple set of initial conditions can explain pretty much everything we can see on scales ranging from stars to the entire visible universe is why cold dark matter is overwhelmingly the favored paradigm.
bowsamic 4y ago
Yes but dark matter is explanatory while MOND is predictive. We see a set of problems, we fix them by choosing our unobservable dark matter halo in order to fix those problems. That should automatically be extremely suspicious for any physicist. On the other hand, MOND predicts the correct galaxy rotation curves of galaxies we have not looked at yet. Sure DM explains more than MOND, but it is also basically not a scientific theory, since it has far too many degrees of freedom that can explain away any problem and is extremely difficult to falsify.
- rini17 4y ago
  If you have to repeatedly modify "predictive" theory to match observation, then it was not predictive in first place. The situation with MOND appears much worse than for example Planck's theory for blackbody radiation. He revised it once and that was it. Generally I don't get this splitting hairs. Things that we can only observe but can't reproduce in lab are not a science then?
- DiogenesKynikos 4y ago
  Lambda CDM makes very few assumptions. It only has 6 free parameters.[0] If you start with those 6 parameters, and then let the laws of physics work, you get the universe we see today. There are a huge number of different types of phenomena that the theory has to correctly predict (e.g., the abundance of different elements, the numbers of galaxies with different masses, the rotation curves of galaxies), and it does so extremely well. 0. https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters
  - naasking 4y ago
    > Lambda CDM makes very few assumptions. It only has 6 free parameters.[0] If you start with those 6 parameters, and then let the laws of physics work, you get the universe we see today. Not including the DM distribution needed to match each galaxy's rotation curve, or external field effects, and dozens more observations: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm Contrast that with MOND's 1 parameter, visible mass.
LatteLazy 4y ago
I thought the problem with modifying gravity was that there was no one modification you could make to explain observations. It's plausible that 1000s of different galaxies spin differently because they each have different amount of mass of dark matter in them. But it's not really possible to explain 1000s of different data points without 1000s of different gravity equations. If there were some rule (eg on large scales gravity is actually an exponential) then someone would have found it and we would have a candidate for a new law. But they haven't... Is that wrong?
- naasking 4y ago
  > It's plausible that 1000s of different galaxies spin differently because they each have different amount of mass of dark matter in them. You also need a plausible explanation why all of these galaxies have such different distributions even though they have similar visible mass. If DM is only supposed to interact gravitationally then there should be a tight correspondence with little deviation, but there are galaxies entirely free of DM and distributions that differ across visible mass. There are also observations that don't even match DM (external field effects, tidal effects, and more). There's clearly a lot more to the story.
Tepix 4y ago
Looking back at this episode - after we figure this out - will be similar to looking back at the period before the discovery of general relativity. Right now, we're really quite clueless.
twawaaay 4y ago
It is epicycles all over again. I think modifying gravity might be useful. Probably people who were slapping more terms to epicycles knew what they were doing is wrong but did it anyway because it allowed them to predict what they needed.
- fallingknife 4y ago
  Isn't dark matter an epicycle?
  - twawaaay 4y ago
    I am not a physicist, but I think it is not. It seems they can account for mass in various places in universe, they can model how things should be moving (being affected by gravity), they can measure how things are actually moving and from this they can infer where this missing mass actually is placed. This is currently all pretty early stage but the resolution is expected to increase with time. So no, I don't think dark matter is an epicycle. It is more or less like the case of gravitational disturbance affecting planets in solar system and somebody theorising a planet placed somewhere would explain the effect, we just can't yet measure things accurately enough to tell where the planet is exactly.

96 points by raattgift 4y ago

Another great article from Ethan Siegel. I think he makes a pretty strong case for cold dark matter, but it relies pretty heavily on theoretical cosmological arguments. For the pro-mond argument, there’s a recent interview with Milgrom on Dr Brian Keating’s physics podcast Into the Impossible [0]. There is also a talk about all the ways DM does not fit observations from Pavel Kroupa (University of Bonn) on youtube [1]. In particular, Kroupa argues that when galaxies pass through one another, DM should cause them to move slower after the collision, because DM would be drawn behind each system in a turbulent wake; but no such slowdown is observed. MOND plus visible matter predicts the motion of these systems well. [0] https://youtu.be/oQ0Cr0hxN40 https://youtu.be/oQ0Cr0hxN40 [1] https://youtu.be/PVgwLWVETIM https://youtu.be/PVgwLWVETIM

andrewflnr 4y ago
Turbulence entirely due to gravitational interaction? Does that work? I guess I should watch it, but I can't take it seriously with the title "On the non-existence of dark matter" when there's so much evidence for dark matter. That's clearly unwarranted confidence in search of attention. That said, if true, I think it more likely indicates that both MOND and LCDM have significant truth. The bar for a theory is "as simple as possible, but no simpler." If the evidence truly demands both MOND and DM, we just have to suck it up.
- wwarner 4y ago
  The DM is moved around by the gravity of the moving galaxy. The DM theory is that each galaxy is surrounded by a huge spherical cloud of DM that (by definition) only experiences gravitation and is invisible and insubstantial in every other way. When Voyager gained speed from passing near Jupiter, it caused Jupiter to slow down by a tiny amount. Similarly, when galaxies pass through each other, the DM should take a measurable amount of momentum from the galaxies. And, yeah, it's a provocative title, but he believes that his measurements falsify DM theory. It's partly a tussle between experimentalists and theoreticians. Kroupa is arguing that the claims of the theory can be tested, and that it doesn't match observations. The theoreticians have a nice story for the big bang, inflation, baryonic nucleosynthesis and the variations seen in the CMB, and the amount of DM it predicts happens to match precisely the amount of DM that we observe.

throwawaymaths 4y ago

Suppose we made an observation that mercury's orbit precesses by a fraction of a degree every orbit. Heretofore every observation of perturbances has correctly presaged the presence of a large mass (a planet) that was quickly found, but this time we have looked and looked but not found anything. Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? > You can postulate an additional separate modification of gravity that comes into play on cluster scales. Or it might be something else altogether. Anyways, MOND has made predictions that people set out to disprove MOND went looking for negative evidence and had to turn around (e.g. external field effect), as well as straightforward predictions JWT may have confirmed (early galaxies) and explains many things that dark matter cannot, like how satellite galaxies don't get ripped apart by the dark matter halo.

tshaddox 4y ago
> Should we presume that it must be an invisible ball of matter that gravitates but we have no other way of detecting except through the same effect we use to infer its existence, or should we tweak Newton's laws of physics? Are these substantially different? Of course using “must” isn’t appropriate, but spouting off the best theory you have at the time seems okay to me. Hopefully no one is saying that we cannot possibly ever find another better theory the same way Einstein’s was better than Newton’s or that we shouldn’t ever look for one.
- 2muchcoffeeman 4y ago
  I guess the other side effects might be different right? E.g. we talk about relativistic mass. But when they did experiments it was found that no other attributes that come with increased mass was found. So the answer is right but there is something off with our understanding.
- throwawaymaths 4y ago
  > Hopefully no one is saying that we cannot possibly ever The OP article might as well. The thrust of the article's argument is, "heaven forbid we add third parameter to MOND" meanwhile, draco, fornax, and Andromeda have wildly different dark matter densities so that's three parameters right there.

hinkley 4y ago

My top, crackpot theory is we live in a black hole and dark matter is the accretion disk. My second crackpot theory is we are four dimensional beings living in a five dimensional universe. Though I also like Cixin Liu’s idea that we are living in a universe that used to be five dimensions and has (mostly) decayed to four, with bits of five dimensional debris holding on.

pryelluw 4y ago
You mean there could be black holes inside other black holes?
- Filligree 4y ago
  Mathematically, the universe looks a little bit like a black hole running backwards in time. But only a little bit.
  - pryelluw 4y ago
    More please.
    - tanqueraythc 4y ago
      Ehh, read Steven Hawking. Black holes (singularities) are strikingly similar to what we know of the big bang. If we are in a black hole, it could explain the accelerating expansion of the universe.
      - somat 4y ago
        my pet science fiction theory is that passing the event horizon removes half a dimension. so outside the universe you can move freely back and forth in time, but here in the universe it is constrained you can only go forward. when you pass another event horizon one of what we call a physical dimension gets constrained. you can't move toward the wall of the event horizon, that dimension is gone.

ars 4y ago

Something I've always wondered: Right after the Big Bang there was a massive annihilation of matter and anti-matter, with just a bit of extra matter. But the energy resulting from that annihilation did not go away. So where is it? No matter what form it takes, energy gravitates, so it should be able to be observed.

frankchn 4y ago
My impression (I might well be wrong about this) is that the energy from that forms part of the cosmic microwave background radiation.
exmadscientist 4y ago
A lot of it just went back into producing particle-antiparticle pairs again. Some of it is the CMB, yes, but not all. Why we didn't just end up with even numbers of particles and antiparticles repeating this dance endlessly is the single biggest open question in particle physics.
beeforpork 4y ago
Photons. Each annihilation event emitted a photon, and they are still around. IIRC, about 5e9 times as many photons are observed as there are baryons, and that seems to match up well with the calculated imbalance of the baryosynthesis.

debacle 4y ago

We are in the "epicycles" phase of physics discovery, at both the micro and macro level. I may not live to see it, but I'm excited for when everything just "clicks" and the model becomes more simple and elegant, as it was before the 1920s.

MichaelZuo 4y ago
That's an intuitive way to put it, ironic considering epicycles themselves are viewed as the opposite.
- wumpus 4y ago
  When you run into physics/astronomy people mentioning epicycles, it's not ironic, it's saying that we're missing out on a simpler rule.
  - MichaelZuo 4y ago
    Clearly I'm referring to how the idea of epicycles themselves are viewed by the general population.

colechristensen 4y ago

My fun, science fiction theory to solve the gravity problem is that the universe is actually full of intelligent life mostly billions of years old civilizations and all of the extra mass we can’t see is actually the galactic version of cities built from materials that don’t interact with light much. Likewise we haven’t met any other intelligent species because the growth period before reaching this common technology is short from thousands to millions of years so there just aren’t collisions in space or time where two intelligent species would interact.

russdill 4y ago
Except an important property of dark matter is they it's interacts weakly with itself. But still good for sci-fi
- Beldin 4y ago
  If that is true, where are the dark matter black holes? And why hasn't the majority of DM collapsed into black holes?
  - andrewflnr 4y ago
    The same reason the rest of the matter didn't collapse into black holes, not quite dense enough, or blowing up too fast. And dark matter doesn't have it's own kind of black holes. Related is the idea of "primordial black holes".
    - gpderetta 4y ago
      In fact I think it is harder for dark matter to collapse into black holes because it can't easily shed momentum (angular of otherwise) and 'spiral down' .
      - Beldin 4y ago
        That's what i used to think, but GP alluded to weak interaction between DM particles. I thinkt interaction consumes energy. If so, DM particles would be able to shed some momentum... but then they (eventually) could clump into black holes.

rapjr9 4y ago

Maybe dark matter does have electromagnetic interactions with normal matter, but they are at an exceedingly high (or low) frequency. Science has limits in what frequencies it can probe, matter may not have the same limitations. A century long EM wave would look like background noise on a human scale. Petahertz (and beyond) vibrations could have extremely short range interactions that mostly occur in the near field.

Beldin 4y ago

I've got two problems with this. First: if theory and observation don't match up, that can be due to lacking theory or lacking observations. Both should be investigated. Second: all candidates for dark matter i know posit an enormous number of places where this dark matter is. It is not that "dark matter solves this problem". It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass. So not only is this theory supposing that most material in the universe is invisible to us, but its properties other than mass are unknown to us except it allows us to fill in the blanks exactly as needed. E.g, we're not expecting a lot of dark matter black holes, as the hypothesised stuff doesn't even interact with itself and so cannot slow down. Basically, "dark matter" is fairy dust that physicists sprinkle over everything till it works. And you know what: maybe that is the way to go. Probably it plays at least some part in answering these questions. But until we have a clearer picture, we may as well call it "magical fairy dust". That at least reminds us how much we're fudging.

pfortuny 4y ago
Until it is experimentally tested, DM is essentially equivalent to the ether. A useful unexplained tool which to date seems to work. Until another Michelson-Morley experiment comes (and MOND might have something to say, or at least they seem to be testing DM experimentally), yes: as untestable as string theory is.
DiogenesKynikos 4y ago
> It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass It's actually an extremely simple initial distribution of dark matter in the early universe. You can specify the entire distribution with one number: a spectral index. The thing is, we already know that the universe had that distribution of energy density anyways, because of the Cosmic Microwave Background. If you take that very simple distribution and let the laws of physics run for 13 billion years, you get the required present-day distribution of dark matter. That distribution can be verified in several ways, including gravitational lensing, the rotation curves of galaxies, the motions of galaxies inside clusters, and the temperatures of hot gas inside galaxy clusters. The fact that such a simple set of initial conditions can explain pretty much everything we can see on scales ranging from stars to the entire visible universe is why cold dark matter is overwhelmingly the favored paradigm.

bowsamic 4y ago

Yes but dark matter is explanatory while MOND is predictive. We see a set of problems, we fix them by choosing our unobservable dark matter halo in order to fix those problems. That should automatically be extremely suspicious for any physicist. On the other hand, MOND predicts the correct galaxy rotation curves of galaxies we have not looked at yet. Sure DM explains more than MOND, but it is also basically not a scientific theory, since it has far too many degrees of freedom that can explain away any problem and is extremely difficult to falsify.

rini17 4y ago
If you have to repeatedly modify "predictive" theory to match observation, then it was not predictive in first place. The situation with MOND appears much worse than for example Planck's theory for blackbody radiation. He revised it once and that was it. Generally I don't get this splitting hairs. Things that we can only observe but can't reproduce in lab are not a science then?
DiogenesKynikos 4y ago
Lambda CDM makes very few assumptions. It only has 6 free parameters.[0] If you start with those 6 parameters, and then let the laws of physics work, you get the universe we see today. There are a huge number of different types of phenomena that the theory has to correctly predict (e.g., the abundance of different elements, the numbers of galaxies with different masses, the rotation curves of galaxies), and it does so extremely well. 0. https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters https://en.wikipedia.org/wiki/Lambda-CDM_model#Parameters
- naasking 4y ago
  > Lambda CDM makes very few assumptions. It only has 6 free parameters.[0] If you start with those 6 parameters, and then let the laws of physics work, you get the universe we see today. Not including the DM distribution needed to match each galaxy's rotation curve, or external field effects, and dozens more observations: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm Contrast that with MOND's 1 parameter, visible mass.

LatteLazy 4y ago

I thought the problem with modifying gravity was that there was no one modification you could make to explain observations. It's plausible that 1000s of different galaxies spin differently because they each have different amount of mass of dark matter in them. But it's not really possible to explain 1000s of different data points without 1000s of different gravity equations. If there were some rule (eg on large scales gravity is actually an exponential) then someone would have found it and we would have a candidate for a new law. But they haven't... Is that wrong?

naasking 4y ago
> It's plausible that 1000s of different galaxies spin differently because they each have different amount of mass of dark matter in them. You also need a plausible explanation why all of these galaxies have such different distributions even though they have similar visible mass. If DM is only supposed to interact gravitationally then there should be a tight correspondence with little deviation, but there are galaxies entirely free of DM and distributions that differ across visible mass. There are also observations that don't even match DM (external field effects, tidal effects, and more). There's clearly a lot more to the story.

Tepix 4y ago

Looking back at this episode - after we figure this out - will be similar to looking back at the period before the discovery of general relativity. Right now, we're really quite clueless.

twawaaay 4y ago

It is epicycles all over again. I think modifying gravity might be useful. Probably people who were slapping more terms to epicycles knew what they were doing is wrong but did it anyway because it allowed them to predict what they needed.

fallingknife 4y ago
Isn't dark matter an epicycle?
- twawaaay 4y ago
  I am not a physicist, but I think it is not. It seems they can account for mass in various places in universe, they can model how things should be moving (being affected by gravity), they can measure how things are actually moving and from this they can infer where this missing mass actually is placed. This is currently all pretty early stage but the resolution is expected to increase with time. So no, I don't think dark matter is an epicycle. It is more or less like the case of gravitational disturbance affecting planets in solar system and somebody theorising a planet placed somewhere would explain the effect, we just can't yet measure things accurately enough to tell where the planet is exactly.