4 ms·

It is of course entirely plausible that the validity of general relativity breaks down much closer to our own home than at the edge of the hypothetical end of t

It is of course entirely plausible that the validity of general relativity breaks down much closer to our own home than at the edge of the hypothetical end of the universe. Is it really? Have people formulated specific break downs, then gone to look for evidence?

fhars 7y ago
A more damning quote seems to be Another recent probe found galaxies inconsistent with the theory of dark matter, which posits this hypothetical substance to be everywhere. There is no reference given, but I have the suspicion that it refers to the discovery of galaxies with almost no dark matter (cf. https://news.ycombinator.com/item?id=16701248 https://news.ycombinator.com/item?id=16701248). That does indeed refute the assumpion that dark matter is everywhere, but that is not an assumption anyone makes and is not part of any relevant theory of dark matter. On the contrary, the existence of galaxies like that is generally seen as one of the strongest arguments for the existence of dark matter as an independent, if elusive physical entity.
- nabnob 7y ago
  Couldn't it just as easily mean that our predictions based on general relativity are wrong? Either 1) the prediction for a particular galaxy is right, and there's no "dark energy" or "dark matter" or 2) the prediction is wrong, and you add a bunch of dark energy or dark matter to make the equations work out. It's not testable at all - no matter whether predictions for a particular galaxy are right or wrong, you're still allowing yourself to think that the theory is correct. How is this scientific?
  - darkmighty 7y ago
    It's not that simple. It's not like a) Some galaxies fit predictions, b) Some do not, and we add unknown dark matter. It's that when they do not fit predictions, there is a single parameter (the mass of dark matter) that does explain its behavior, whose model is that of a weakly interacting gas halo. So dark matter tries to fix things in a very precise way, it's not an arbitrary curve fitting exercise. Now whether there is really weakly interacting matter out there or some other explanation is valid is still an open question afaik; but the evidence is quite strong so far.
    - astrobe_ 7y ago
      > Now whether there is really weakly interacting matter out there Not "out there". It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day.
      - pdonis 7y ago
        > It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day. It's >80% of the average density of the matter in the universe. But that's an average over all of the universe. On the scale of our planet, or of the solar system, that density is negligible--as in "30 orders of magnitude smaller than the density of the stuff you eat" negligible.
  - fhars 7y ago
    No, the only consistent explanation for both the overwhelming majority of galaxies as well as the few outliers is general relativity and independent dark mater. All other theories that make testable predictions (modified newtonian dynamics or MOND seems to be the only class that is not completely add odds with almost all observations) cannot explain the behaviour of those galacies that are as big as other galaxies but rotate wih a different speed. But the difference is easily explained with different amounts of dark matter in the galaxies, and the bullet cluster cited elsewhere in this thread give a plausible explanation of how these galaxies might form.
  - pdonis 7y ago
    > Couldn't it just as easily mean that our predictions based on general relativity are wrong? No, because general relativity with dark matter (and dark energy--the best current cosmological model we have) predicts a lot more than just galaxy rotation curves. And the whole point of the model is to explain all the things it makes predictions about, not just one.
- czbond 7y ago
  Not necessarily. This great video from PBS Spacetime is a fairly detailed, but good for a 'nearly' layman on the findings in depth. https://www.youtube.com/watch?v=5t0jaE--l0Y https://www.youtube.com/watch?v=5t0jaE--l0Y A discussion on how a galaxy could be missing dark matter - or how it could be separated from a galaxy
AnimalMuppet 7y ago
Well... assuming that the local rules are the same as the rules billions of light-years away is an assumption, based on a philosophical presupposition that the rules are uniform. What probability should we assign to that assumption? 100% in the absence of evidence to the contrary? Or should we reserve some skepticism, remembering that it is an assumption? Your question is reasonable: Have people formulated specific breakdowns, and gone looking? But it's also unreasonable. An assumption shouldn't necessarily be treated beyond plausible doubt until we can prove it wrong.
- jerf 7y ago
  "Have people formulated specific breakdowns, and goon looking?" They have. Considerable effort has been expended trying to detect changes in various constants related to light, including speed but also some other things. It is expected that if constants were changing, spectra would change as well. So far as I know, all those results have been negative, but it's not for lack of trying. Similar arguments can be deployed for the nuclear forces as well w.r.t. spectrum, rates of fusion, various things you'd be able to observe in certain types of supernovae, etc. One of the things I don't like about this genre of articles is that it's typically quite mistaken, if not outright disingenuous, about the current state of cosmology. From what I could see, the idea that cosmologists are all stodgy old fogies brought up in a paradigm of the Big Bang and who won't entertain any other thoughts is diametrically opposed to the truth. To my eyes, the field is a bubbling cauldron of various exotic ideas and attempts to resolve all these issues. There's no lack of theories in cosmology and attempts to resolve these issues, including many quite radical ones (the holographic principle, changing constants over time, heck even "Dark Energy" is quite radical in many ways even if it is also the accepted theory at the moment). What we lack are theories that explain all the observations. Dark matter is favored not because it's the only idea that anyone has come up with, but because it's still the one that best fits the facts. (As PBS Space Time pointed out last week, ironically, the galaxies recently discovered to not contain dark matter are themselves quite strong evidence dark matter must exist: https://www.youtube.com/watch?v=5t0jaE--l0Y https://www.youtube.com/watch?v=5t0jaE--l0Y , because almost all other theories to explain galactic rotations are based on characteristics that can not vary by galaxy like matter distributions can. "MOND", for instance, is not "MOND, except in these two galaxies".) If you can solve all these problems, as near as I can tell, you'll be celebrated, not shut down by the old fogerati. But be prepared for your theory to need to cover a lot of existing observations; we're centuries past "Well, what if gravity just worked differently?" or "What if the constants aren't constant?" You're going to need a great deal more detail than that.

fhars 7y ago

A more damning quote seems to be Another recent probe found galaxies inconsistent with the theory of dark matter, which posits this hypothetical substance to be everywhere. There is no reference given, but I have the suspicion that it refers to the discovery of galaxies with almost no dark matter (cf. https://news.ycombinator.com/item?id=16701248 https://news.ycombinator.com/item?id=16701248). That does indeed refute the assumpion that dark matter is everywhere, but that is not an assumption anyone makes and is not part of any relevant theory of dark matter. On the contrary, the existence of galaxies like that is generally seen as one of the strongest arguments for the existence of dark matter as an independent, if elusive physical entity.

nabnob 7y ago
Couldn't it just as easily mean that our predictions based on general relativity are wrong? Either 1) the prediction for a particular galaxy is right, and there's no "dark energy" or "dark matter" or 2) the prediction is wrong, and you add a bunch of dark energy or dark matter to make the equations work out. It's not testable at all - no matter whether predictions for a particular galaxy are right or wrong, you're still allowing yourself to think that the theory is correct. How is this scientific?
- darkmighty 7y ago
  It's not that simple. It's not like a) Some galaxies fit predictions, b) Some do not, and we add unknown dark matter. It's that when they do not fit predictions, there is a single parameter (the mass of dark matter) that does explain its behavior, whose model is that of a weakly interacting gas halo. So dark matter tries to fix things in a very precise way, it's not an arbitrary curve fitting exercise. Now whether there is really weakly interacting matter out there or some other explanation is valid is still an open question afaik; but the evidence is quite strong so far.
  - astrobe_ 7y ago
    > Now whether there is really weakly interacting matter out there Not "out there". It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day.
    - pdonis 7y ago
      > It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day. It's >80% of the average density of the matter in the universe. But that's an average over all of the universe. On the scale of our planet, or of the solar system, that density is negligible--as in "30 orders of magnitude smaller than the density of the stuff you eat" negligible.
- fhars 7y ago
  No, the only consistent explanation for both the overwhelming majority of galaxies as well as the few outliers is general relativity and independent dark mater. All other theories that make testable predictions (modified newtonian dynamics or MOND seems to be the only class that is not completely add odds with almost all observations) cannot explain the behaviour of those galacies that are as big as other galaxies but rotate wih a different speed. But the difference is easily explained with different amounts of dark matter in the galaxies, and the bullet cluster cited elsewhere in this thread give a plausible explanation of how these galaxies might form.
- pdonis 7y ago
  > Couldn't it just as easily mean that our predictions based on general relativity are wrong? No, because general relativity with dark matter (and dark energy--the best current cosmological model we have) predicts a lot more than just galaxy rotation curves. And the whole point of the model is to explain all the things it makes predictions about, not just one.
czbond 7y ago
Not necessarily. This great video from PBS Spacetime is a fairly detailed, but good for a 'nearly' layman on the findings in depth. https://www.youtube.com/watch?v=5t0jaE--l0Y https://www.youtube.com/watch?v=5t0jaE--l0Y A discussion on how a galaxy could be missing dark matter - or how it could be separated from a galaxy

AnimalMuppet 7y ago

Well... assuming that the local rules are the same as the rules billions of light-years away is an assumption, based on a philosophical presupposition that the rules are uniform. What probability should we assign to that assumption? 100% in the absence of evidence to the contrary? Or should we reserve some skepticism, remembering that it is an assumption? Your question is reasonable: Have people formulated specific breakdowns, and gone looking? But it's also unreasonable. An assumption shouldn't necessarily be treated beyond plausible doubt until we can prove it wrong.

jerf 7y ago
"Have people formulated specific breakdowns, and goon looking?" They have. Considerable effort has been expended trying to detect changes in various constants related to light, including speed but also some other things. It is expected that if constants were changing, spectra would change as well. So far as I know, all those results have been negative, but it's not for lack of trying. Similar arguments can be deployed for the nuclear forces as well w.r.t. spectrum, rates of fusion, various things you'd be able to observe in certain types of supernovae, etc. One of the things I don't like about this genre of articles is that it's typically quite mistaken, if not outright disingenuous, about the current state of cosmology. From what I could see, the idea that cosmologists are all stodgy old fogies brought up in a paradigm of the Big Bang and who won't entertain any other thoughts is diametrically opposed to the truth. To my eyes, the field is a bubbling cauldron of various exotic ideas and attempts to resolve all these issues. There's no lack of theories in cosmology and attempts to resolve these issues, including many quite radical ones (the holographic principle, changing constants over time, heck even "Dark Energy" is quite radical in many ways even if it is also the accepted theory at the moment). What we lack are theories that explain all the observations. Dark matter is favored not because it's the only idea that anyone has come up with, but because it's still the one that best fits the facts. (As PBS Space Time pointed out last week, ironically, the galaxies recently discovered to not contain dark matter are themselves quite strong evidence dark matter must exist: https://www.youtube.com/watch?v=5t0jaE--l0Y https://www.youtube.com/watch?v=5t0jaE--l0Y , because almost all other theories to explain galactic rotations are based on characteristics that can not vary by galaxy like matter distributions can. "MOND", for instance, is not "MOND, except in these two galaxies".) If you can solve all these problems, as near as I can tell, you'll be celebrated, not shut down by the old fogerati. But be prepared for your theory to need to cover a lot of existing observations; we're centuries past "Well, what if gravity just worked differently?" or "What if the constants aren't constant?" You're going to need a great deal more detail than that.