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Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence com

Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence commonly cited, including but not limited to galaxy rotation curves. Honestly skepticism of dark matter is understandable, but what the heck else are you expecting to pop up to explain redshift, a phenomenon that happens always and only when an object is moving away from you? What's the mystery there? You might be confusing universe expansion with dark energy (which IMO might well disappear, being vulnerable to improvements in the cosmic distance ladder), or cosmic inflation (apparently well substantiated but quite ad hoc, but if so you should really learn your terminology before you try to make predictions about future discoveries.

grishka 4y ago
> but what the heck else are you expecting to pop up to explain redshift For example, some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform and we're in an area that has properties slightly different from the surrounding space. What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?
- ben_w 4y ago
  > some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform I think people have explored the photons thing with regard to “solving” dark matter, and found the evidence lacking. As for spacetime… trivially yes, but I don’t think this is what you meant? I mean, spacetime being non-uniformly curved just means there’s stuff in it, and that is literally one of the things we can directly observe, and we see that in e.g. the Bullet Cluster the dark matter kept moving when the visible matter collided and slowed down. > What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang? If, after careful work to exclude all other possibilities, then a party and a Nobel prize.
- andrewflnr 4y ago
  > some kind of phenomenon that happens to photons but only when they travel great distances Tried, found wanting. Repeatedly. https://en.wikipedia.org/wiki/Tired_light https://en.wikipedia.org/wiki/Tired_light Anisotropy of space is (a) again adding complications and (b) unlikely to produce such a spherically symmetric redshift. The CMB is infamous for how smooth it is. The different region of space would have to be perfectly spherical with us in the dead center. May as well revive geocentrism. Look, if we did find evidence like that from JWST or whatever, we'd re-evaluate what best meets Occam's razor for the totality of the evidence. But that doesn't change the fact that with our current evidence, that explanation is expansion of space. Other stuff is sci-fi at best. Fun to think about, but no reason to actually believe it. The story for dark matter is more complicated, but similar in broad outline. The more the alternative theories try to fit all the evidence, the closer they get to reinventing dark matter. You have something that walks like matter and quacks like matter, so the smart money says it's matter.
scatters 4y ago
Redshift also occurs to light emitted from within a gravity well. Obviously that isn't what's happening at a cosmic scale (our light isn't blueshifted to other observers) but it's at least conceivable that some third phenomenon could be the cause.

grishka 4y ago

> but what the heck else are you expecting to pop up to explain redshift For example, some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform and we're in an area that has properties slightly different from the surrounding space. What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?

ben_w 4y ago
> some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform I think people have explored the photons thing with regard to “solving” dark matter, and found the evidence lacking. As for spacetime… trivially yes, but I don’t think this is what you meant? I mean, spacetime being non-uniformly curved just means there’s stuff in it, and that is literally one of the things we can directly observe, and we see that in e.g. the Bullet Cluster the dark matter kept moving when the visible matter collided and slowed down. > What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang? If, after careful work to exclude all other possibilities, then a party and a Nobel prize.
andrewflnr 4y ago
> some kind of phenomenon that happens to photons but only when they travel great distances Tried, found wanting. Repeatedly. https://en.wikipedia.org/wiki/Tired_light https://en.wikipedia.org/wiki/Tired_light Anisotropy of space is (a) again adding complications and (b) unlikely to produce such a spherically symmetric redshift. The CMB is infamous for how smooth it is. The different region of space would have to be perfectly spherical with us in the dead center. May as well revive geocentrism. Look, if we did find evidence like that from JWST or whatever, we'd re-evaluate what best meets Occam's razor for the totality of the evidence. But that doesn't change the fact that with our current evidence, that explanation is expansion of space. Other stuff is sci-fi at best. Fun to think about, but no reason to actually believe it. The story for dark matter is more complicated, but similar in broad outline. The more the alternative theories try to fit all the evidence, the closer they get to reinventing dark matter. You have something that walks like matter and quacks like matter, so the smart money says it's matter.

scatters 4y ago

Redshift also occurs to light emitted from within a gravity well. Obviously that isn't what's happening at a cosmic scale (our light isn't blueshifted to other observers) but it's at least conceivable that some third phenomenon could be the cause.