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The reason work on alternative theories of gravity is important: We have one reality R, for which we have various possibly imperfect theories T(R) and a large

by Beldin 6y ago

The reason work on alternative theories of gravity is important: We have one reality R, for which we have various possibly imperfect theories T(R) and a large set of possibly imperfect observations O(R). For gravity, we know that general relativity doesn't perfectly match current observations -- GR(R) != O(R). Since R is the same in both sides, this is not supposed to happen. There are three ways to resolve this: - GR() is incomplete, - O() is incomplete, - both are incomplete. Dark matter focuses on the second, MOND on the first. We shouldn't give up on any option until there's either a solution or a counterexample.

Beldin 6y ago
With respect to the Bullet Cluster (which is often mentioned as a counterexample to MOND): discussion of why that does not rule out MOND: https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html https://web.archive.org/web/20160721044735/http://www.astro....
- pdonis 6y ago
  His argument is basically "well, we still need some dark matter with MOND, just not as much". That seems like a weak argument.
  - dogma1138 6y ago
    Not exactly if you need a little as in just enough to be explained by “missing” baryonic matter and other DM candidate that fit the standard model it’s not a weak argument on its own.
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  - narag 6y ago
    Wasn't it estimated to be 90%?
  - btilly 6y ago
    Not as weak as you think. When we go looking for sources of dark matter, we have lots of options of things that aren't lit up. For example looking at the distribution of the sizes of the stars, we expect there to be a lot of giant gas balls that weren't quite big enough to start fusion. We don't see them, but we've got good reasons to believe that they are there and can even make a good guess as to how much of them there are. However when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies. This is when we are getting into the territory of imagining new kinds of particles that physics hasn't yet discovered. Therefore the debate isn't about whether dark matter exists - it obviously does. It is about whether exotic forms of dark matter need to exist.
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    - pdonis 6y ago
      > when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies The dynamics of galaxies are not the only place where non-baryonic dark matter is needed. Our understanding of baryogenesis in the early universe places limits on how much total baryonic matter there can be in the universe. But the expansion history of the universe requires much more matter than that (about 5 times as much IIRC).
beervirus 6y ago
What observations differ from general relativity?
- pdonis 6y ago
  Observed galaxy rotation curves (the speed of rotation of stars around the galaxy's center as a function of distance from the center) cannot be accounted for using GR and the observed distribution of visible matter in the galaxies. You need to postulate some extra matter that is not visible ("dark matter") to account for the observed rotation curves using GR. That is the discrepancy that originally prompted physicists to postulate dark matter. Since then the same postulate has also been used to improve the match between theory and observation in other areas, notably our model of how the early universe evolved.
- Beldin 6y ago
  For a start: the speed at which stars rotate around the center of a galaxy. This should be due to gravity, yet the speed of stars further from the center isn't slower (as it should be). This holds true for the vast majority of galaxies. A specific amount of extra (not yet observed) mass distributed in the right way throughout the galaxy could explain this.
blamestross 6y ago
> We have one reality R For the record, physics takes this on desperate faith. It's an assumption in the face of a perceived to be hopeless alternative. Special Relativity itself, Quantum Physics and this new modified MOND that is based on gravity not being universally constant are all pairing down the whole "there is one consensus reality" notion. This isn't to say that there isn't consensus reality, only that the consensus bit keeps getting smaller and smaller as we learn more.
bsdetector 6y ago
> alternative theories of gravity To me it all just sounds like quantum vs continuous. Like at our scales we normally see only the continuous averages and we'd expect that to be even more so at galactic scales. But what if at the fringes of a galaxy gravity is so 'small' that it's really the quantum that ends up determining the macro results? Say some near object should experience 5 quadrillion plus 0.1 gravity from continuous equations, whatever quantum happens with the 0.1 doesn't affect the results and maybe we can't even measure it. But way out in deep space where it would be 5.1 gravity that 0.1 matters a lot, for instance if it's 'rounded up' in some way (maybe it represents a graviton being just close enough or whatever). Obviously this is just nonsense spitballing, but I wonder what you guys think about the idea of galactic outcomes reflecting the quantum rather than the continuous.

by Beldin 6y ago

Beldin 6y ago

With respect to the Bullet Cluster (which is often mentioned as a counterexample to MOND): discussion of why that does not rule out MOND: https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html https://web.archive.org/web/20160721044735/http://www.astro....

pdonis 6y ago
His argument is basically "well, we still need some dark matter with MOND, just not as much". That seems like a weak argument.
- dogma1138 6y ago
  Not exactly if you need a little as in just enough to be explained by “missing” baryonic matter and other DM candidate that fit the standard model it’s not a weak argument on its own.
  - deleted 6y ago
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  - deleted 6y ago
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- narag 6y ago
  Wasn't it estimated to be 90%?
- btilly 6y ago
  Not as weak as you think. When we go looking for sources of dark matter, we have lots of options of things that aren't lit up. For example looking at the distribution of the sizes of the stars, we expect there to be a lot of giant gas balls that weren't quite big enough to start fusion. We don't see them, but we've got good reasons to believe that they are there and can even make a good guess as to how much of them there are. However when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies. This is when we are getting into the territory of imagining new kinds of particles that physics hasn't yet discovered. Therefore the debate isn't about whether dark matter exists - it obviously does. It is about whether exotic forms of dark matter need to exist.
  - deleted 6y ago
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  - pdonis 6y ago
    > when we try to make estimates for each kind of dark matter that we think of, we come up a lot short of the amount of dark matter which is required to explain the gravitational dynamics of galaxies The dynamics of galaxies are not the only place where non-baryonic dark matter is needed. Our understanding of baryogenesis in the early universe places limits on how much total baryonic matter there can be in the universe. But the expansion history of the universe requires much more matter than that (about 5 times as much IIRC).

beervirus 6y ago

What observations differ from general relativity?

pdonis 6y ago
Observed galaxy rotation curves (the speed of rotation of stars around the galaxy's center as a function of distance from the center) cannot be accounted for using GR and the observed distribution of visible matter in the galaxies. You need to postulate some extra matter that is not visible ("dark matter") to account for the observed rotation curves using GR. That is the discrepancy that originally prompted physicists to postulate dark matter. Since then the same postulate has also been used to improve the match between theory and observation in other areas, notably our model of how the early universe evolved.
Beldin 6y ago
For a start: the speed at which stars rotate around the center of a galaxy. This should be due to gravity, yet the speed of stars further from the center isn't slower (as it should be). This holds true for the vast majority of galaxies. A specific amount of extra (not yet observed) mass distributed in the right way throughout the galaxy could explain this.

blamestross 6y ago

> We have one reality R For the record, physics takes this on desperate faith. It's an assumption in the face of a perceived to be hopeless alternative. Special Relativity itself, Quantum Physics and this new modified MOND that is based on gravity not being universally constant are all pairing down the whole "there is one consensus reality" notion. This isn't to say that there isn't consensus reality, only that the consensus bit keeps getting smaller and smaller as we learn more.

bsdetector 6y ago

> alternative theories of gravity To me it all just sounds like quantum vs continuous. Like at our scales we normally see only the continuous averages and we'd expect that to be even more so at galactic scales. But what if at the fringes of a galaxy gravity is so 'small' that it's really the quantum that ends up determining the macro results? Say some near object should experience 5 quadrillion plus 0.1 gravity from continuous equations, whatever quantum happens with the 0.1 doesn't affect the results and maybe we can't even measure it. But way out in deep space where it would be 5.1 gravity that 0.1 matters a lot, for instance if it's 'rounded up' in some way (maybe it represents a graviton being just close enough or whatever). Obviously this is just nonsense spitballing, but I wonder what you guys think about the idea of galactic outcomes reflecting the quantum rather than the continuous.