4 ms·

Note that this isn’t a “get out of dark matter free” card, as there are clear signatures of dark matter in the cosmic microwave background. https://physicsworld

Note that this isn’t a “get out of dark matter free” card, as there are clear signatures of dark matter in the cosmic microwave background. https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic-microwave-background/ https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic...

willis936 3y ago
Only if we've constrained ourselves to take inflation as true.
- Vecr 3y ago
  Inflation in what era? There has to have been some inflation at some point, right, otherwise how could the very hot period that's considered to be right after the start of the universe be low entropy? Sure, maybe you could say there's no big bang and time runs the other way on the other side of that low entropy state, but there has to be some sort of low entropy state.
  - tsimionescu 3y ago
    It could be that it is actually not low entropy, and there are some other processes that explain a recution in entropy at some scale. That is, maybe the second law of thermodynamics is not that universal.
    - Vecr 3y ago
      That would be really weird. I'd be more willing to buy galaxies just coming into existence somehow in the gaps as the universe expands, as that at least drives the arrow of time consistently in the right direction. The arrow of time can survive local entropy reduction, but probably not what you're talking about.
  - willis936 3y ago
    >Inflation in what era? Why the inflationary epoch of course. https://en.wikipedia.org/wiki/Inflationary_epoch https://en.wikipedia.org/wiki/Inflationary_epoch I'm saying the very notion of the big bang warrants more scrutiny. All singularities do.
chongli 3y ago
Yes and also in gravitational lensing. The most famous example of this is the Bullet Cluster [1], which displays a pattern of lensing that doesn’t line up with the distribution of light (visible matter) but does line up with where we might expect dark matter to end up after a collision. [1] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster
a_gnostic 3y ago
Could we infer these effects to be from an average distribution of matter across multiversal spacetime, and how would we test for that?
raattgift 3y ago
From the preprint link at the top's page 6, after Eqn (23): "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies. These now form an important set of tools with which to test [our] theory". Other lines of evidence like the one at your link just increase the ability to test their theory (which is meant to solve some quantum gravity problems at very small length scales) at very large length scales. They are not motivated by the desire to prove MOND is correct or that cold dark matter doesn't exist. Rather they can show that in certain restricted circumstances their theory allows for nearly-MOND-like orbits. So their theory survives a small but important hurdle imposed by our observations of nature (we observe MOND-like orbits). Elsewhere in the comments someone asked the good question of (highly paraphrasing) whether this success is wiped out if a distribution of dark matter is added to their SdS-like universe as the generator of some of the observations normally taken as proof of \Lambda-CDM. That question is closely related to the final sentence in the quote above, and answers are hard to guess at.

willis936 3y ago

Only if we've constrained ourselves to take inflation as true.

Vecr 3y ago
Inflation in what era? There has to have been some inflation at some point, right, otherwise how could the very hot period that's considered to be right after the start of the universe be low entropy? Sure, maybe you could say there's no big bang and time runs the other way on the other side of that low entropy state, but there has to be some sort of low entropy state.
- tsimionescu 3y ago
  It could be that it is actually not low entropy, and there are some other processes that explain a recution in entropy at some scale. That is, maybe the second law of thermodynamics is not that universal.
  - Vecr 3y ago
    That would be really weird. I'd be more willing to buy galaxies just coming into existence somehow in the gaps as the universe expands, as that at least drives the arrow of time consistently in the right direction. The arrow of time can survive local entropy reduction, but probably not what you're talking about.
- willis936 3y ago
  >Inflation in what era? Why the inflationary epoch of course. https://en.wikipedia.org/wiki/Inflationary_epoch https://en.wikipedia.org/wiki/Inflationary_epoch I'm saying the very notion of the big bang warrants more scrutiny. All singularities do.

chongli 3y ago

Yes and also in gravitational lensing. The most famous example of this is the Bullet Cluster [1], which displays a pattern of lensing that doesn’t line up with the distribution of light (visible matter) but does line up with where we might expect dark matter to end up after a collision. [1] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster

a_gnostic 3y ago

Could we infer these effects to be from an average distribution of matter across multiversal spacetime, and how would we test for that?

raattgift 3y ago

From the preprint link at the top's page 6, after Eqn (23): "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies. These now form an important set of tools with which to test [our] theory". Other lines of evidence like the one at your link just increase the ability to test their theory (which is meant to solve some quantum gravity problems at very small length scales) at very large length scales. They are not motivated by the desire to prove MOND is correct or that cold dark matter doesn't exist. Rather they can show that in certain restricted circumstances their theory allows for nearly-MOND-like orbits. So their theory survives a small but important hurdle imposed by our observations of nature (we observe MOND-like orbits). Elsewhere in the comments someone asked the good question of (highly paraphrasing) whether this success is wiped out if a distribution of dark matter is added to their SdS-like universe as the generator of some of the observations normally taken as proof of \Lambda-CDM. That question is closely related to the final sentence in the quote above, and answers are hard to guess at.