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Is there any evidence of "dark matter" or "dark energy" actually existing? I find it more likely that we just don't understand the real mechanisms behind galax

by umvi 5y ago

Is there any evidence of "dark matter" or "dark energy" actually existing? I find it more likely that we just don't understand the real mechanisms behind galaxy rotation and universe expansion and so we just made up unfalsifiable stopgaps to make the numbers work out.

JudasGoat 5y ago
I always find it curious that both dark matter and dark energy aren't available locally to study.
- AnIdiotOnTheNet 5y ago
  It likely is, we just can't detect it on such small scales. It would be a form of matter that interacts pretty much only via gravity, and since gravity is already 40 orders of magnitude weaker than the electromagnetic force we usually use to detect things, it's understandable that it is kinda hard to see.
- AnimalMuppet 5y ago
  They might be, but only in very small quantities. The amount of dark matter that we would need to "fix" the galaxy rotation problem... would it affect the orbits of things in the solar system at all? (OK, yes, it would, but observably?) I haven't done the math, but I strongly suspect that the answer is "no".
dahfizz 5y ago
Dark matter is especially dubious to me. Is it possible its literally matter which is dark? i.e. there are more planets and asteroids and mass than we can see because it is not illuminated? How can we be confident in our prediction of the mass of distant galaxies? Maybe that is what is meant when a physicist says dark matter, and its the media that mysticizes it.
- jasonwatkinspdx 5y ago
  So dark matter doesn't just mean "not illuminated" it means it doesn't interact with the EM field at all. However it does interact gravitationally, and this has a measurable impact on matter we can see with our telescopes, the cosmic microwave background, etc. Cosmology today is, in general, very sophisticated. I mean no offense but armchair contrarian takes here are very much just coming from a place of ignorance. Any criticism we can imagine as lay people has already been plumbed to a depth and detail we will never be capable of in our lives. One can point out that there's always some degree of uncertainty in absolute terms, but I think the appropriate understanding is Bayesian, and I'd sooner bet on getting hit in the head by an astroid walking through my neighborhood than that cosmologists have this all fundamentally wrong. The wikipedia page does a reasonably good job of going through the multiple independent lines of evidence we have for dark matter. Sean Carrol has some great posts / videos in this topic area as well. The reason we're so confident of the current state of knowledge is not somehow that physicists and cosmologists suffer from hubris that blinds them to sophomoric criticisms, though sadly there's a number of people who continue to grind that grift.
AnIdiotOnTheNet 5y ago
As mentioned elsewhere, I'm an idiot not a physicist, but as I understand it dark matter is favored over modified gravity theories because all such theories are considered to be lacking in parsimony. Essentially, instead of showing how the behavior we observe arises from a fundamental simplicity, they are adding and tuning parameters to fit. With enough parameters one can fit any arbitrary shape, but this doesn't give the model any predictive power. I also get the impression that while dark matter is popularly accepted as "actually existing", that is much less the case for dark energy.
ajkjk 5y ago
There is vast amounts of evidence for dark matter existing. Dark energy is an unrelated concept with a similar name, and there is not really evidence for it; it is speculated because the equations of general relativity imply the need for an additional term to justify the inflation of the universe, but it's not at all clear what this term corresponds to in terms of matter.
- nimish 5y ago
  No there isn't. There's a bunch of evidence that the current model used for cosmological dynamics (Friedmann's equations) don't work unless there's a lot of undetected stuff called dark matter. Until someone rules out stuff like using the full nonlinear Einstein equations vs Friedmann equations, or that the presumption of homogeneity/isotropy is wrong, (there's more stuff here that hasn't quite been excluded by observation) then the best we can say is that "We need some extra matter called dark matter for the standard cosmological model to work"

by umvi 5y ago

JudasGoat 5y ago

I always find it curious that both dark matter and dark energy aren't available locally to study.

AnIdiotOnTheNet 5y ago
It likely is, we just can't detect it on such small scales. It would be a form of matter that interacts pretty much only via gravity, and since gravity is already 40 orders of magnitude weaker than the electromagnetic force we usually use to detect things, it's understandable that it is kinda hard to see.
AnimalMuppet 5y ago
They might be, but only in very small quantities. The amount of dark matter that we would need to "fix" the galaxy rotation problem... would it affect the orbits of things in the solar system at all? (OK, yes, it would, but observably?) I haven't done the math, but I strongly suspect that the answer is "no".

dahfizz 5y ago

Dark matter is especially dubious to me. Is it possible its literally matter which is dark? i.e. there are more planets and asteroids and mass than we can see because it is not illuminated? How can we be confident in our prediction of the mass of distant galaxies? Maybe that is what is meant when a physicist says dark matter, and its the media that mysticizes it.

jasonwatkinspdx 5y ago
So dark matter doesn't just mean "not illuminated" it means it doesn't interact with the EM field at all. However it does interact gravitationally, and this has a measurable impact on matter we can see with our telescopes, the cosmic microwave background, etc. Cosmology today is, in general, very sophisticated. I mean no offense but armchair contrarian takes here are very much just coming from a place of ignorance. Any criticism we can imagine as lay people has already been plumbed to a depth and detail we will never be capable of in our lives. One can point out that there's always some degree of uncertainty in absolute terms, but I think the appropriate understanding is Bayesian, and I'd sooner bet on getting hit in the head by an astroid walking through my neighborhood than that cosmologists have this all fundamentally wrong. The wikipedia page does a reasonably good job of going through the multiple independent lines of evidence we have for dark matter. Sean Carrol has some great posts / videos in this topic area as well. The reason we're so confident of the current state of knowledge is not somehow that physicists and cosmologists suffer from hubris that blinds them to sophomoric criticisms, though sadly there's a number of people who continue to grind that grift.

AnIdiotOnTheNet 5y ago

As mentioned elsewhere, I'm an idiot not a physicist, but as I understand it dark matter is favored over modified gravity theories because all such theories are considered to be lacking in parsimony. Essentially, instead of showing how the behavior we observe arises from a fundamental simplicity, they are adding and tuning parameters to fit. With enough parameters one can fit any arbitrary shape, but this doesn't give the model any predictive power. I also get the impression that while dark matter is popularly accepted as "actually existing", that is much less the case for dark energy.

ajkjk 5y ago

There is vast amounts of evidence for dark matter existing. Dark energy is an unrelated concept with a similar name, and there is not really evidence for it; it is speculated because the equations of general relativity imply the need for an additional term to justify the inflation of the universe, but it's not at all clear what this term corresponds to in terms of matter.

nimish 5y ago
No there isn't. There's a bunch of evidence that the current model used for cosmological dynamics (Friedmann's equations) don't work unless there's a lot of undetected stuff called dark matter. Until someone rules out stuff like using the full nonlinear Einstein equations vs Friedmann equations, or that the presumption of homogeneity/isotropy is wrong, (there's more stuff here that hasn't quite been excluded by observation) then the best we can say is that "We need some extra matter called dark matter for the standard cosmological model to work"