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Dark matter is predicted at every level of astrophysics. It's not just "hey, galaxies look weird". The amount of visible matter only accounts for 4% of the co

Dark matter is predicted at every level of astrophysics. It's not just "hey, galaxies look weird". The amount of visible matter only accounts for 4% of the cosmic microwave background. The CMB is extremely smooth, but visible matter is highly clumped up. This can only be explained by a huge amount of matter that doesn't clump up ie non-baryonic. The ratio of deuterium to hydrogen rises based on the density, since there's more pressure from gravity. There's way more deuterium than can be explained by visible matter, so there must be some extra compression happening. Galaxies have more kinetic energy than they can have given their visible matter and how fast it's moving. This implies a huge amount of invisible mass. Gravitational lensing around the edges of galaxy clusters indicates a halo of invisible mass. Colliding galactic clusters show their mass splits into two parts: one that clumps up into a bright, dense blob, and another that has much higher inertia and continues past the bright blob as if it didn't interact with it in any way besides gravity. We can map variations in empty space where there is more mass than is expected, in a way that can't be explained by a self-consistent theory of gravity but can be explained by invisible mass. Dark matter as a theory hasn't changed for 80 years and every single one of these discrepancies is solved exactly by using the measurements of dark matter that already existed. They all agree with each other perfectly. There is no theory that even competes with dark matter. Nobody has come up with another idea that explains all of those or even most of those problems.

effie 9y ago
> The amount of visible matter only accounts for 4% of the cosmic microwave background. Could you explain what do you mean by 'accounts for 4% of the cosmic microwave background'? How is one supposed to account for CMB by matter density, when its spectrum is almost equilibrium one (thus consistent with almost any value of matter density)?
- hwillis 9y ago
  The CMB has a certain spectrum and temperature. The amount of energy in the universe correlates to the amount of mass in the universe. The more energy and mass was present in the big bang, the hotter the CMB will be. The amount of visible matter corresponds to a CMB temperature that is far lower than what we see.
  - effie 9y ago
    > The amount of visible matter corresponds to a CMB temperature that is far lower than what we see. Why would there be a relation between CMB temperature and amount of (visible+invisible) matter? I suppose the CMB temperature decreases in time. Would`s the relation you talk about imply that the mass or density of mass of the Universe then has to decrease as well?
scythe 9y ago
>The CMB is extremely smooth, but visible matter is highly clumped up. This can only be explained by a huge amount of matter that doesn't clump up ie non-baryonic This is not correct. Datk matter's presence is inferred from the CMB anisotropy not because it doesn't "clump up" but because it does not interact with photons (EM field). Inflation is responsible for excess smoothness. In fact dark matter drives regular matter to "clump". See eg: https://physics.stackexchange.com/questions/61239/how-the-cmb-anisotropy-is-linked-to-the-existence-of-cold-dark-matter-and-dark-e https://physics.stackexchange.com/questions/61239/how-the-cm... >every single one of those discrepancies is solved exactly This is quite an oversell as far as I know. The margins of error on these calculations are often large. In particular the version with a slightly modified gravity field and smaller amounts of unobserved regular matter is not too outlandish. What would be needed to exclude this is a proof that the amount of unobserved matter cannot be reduced significantly by modifying G, which has not been written yet.
dnautics 9y ago
> solved exactly by using the measurements of dark matter that already existed... Just be careful. Dark matter distributions are highly paramterized quasi-continuous functions, so getting measurements to match exactly is a function of their definition, not an emergent property. While we do feel confident that there could something reasonable there, a priori exactitude cannot be evidence because that's begging the question and also possibly over fitting parameters.
- hwillis 9y ago
  I have become a little aggravated by people who don't believe in dark matter
  - dnautics 9y ago
    why? Until direct evidence and predictions of properties emerge, from an epistemological point of view it's still a very shaky model. If it all turned out to be epicycles it wouldn't be surprising. Or maybe a better analogy is Vulcan. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) The closest we have come to direct evidence are the independent calculations of the properties if it's a self-annhilating particle that agree with a certain set of observations. Even so that seriously breaks a conservation rule, because if there's no proposed way to create dark matter, the universe makes no sense.

effie 9y ago

> The amount of visible matter only accounts for 4% of the cosmic microwave background. Could you explain what do you mean by 'accounts for 4% of the cosmic microwave background'? How is one supposed to account for CMB by matter density, when its spectrum is almost equilibrium one (thus consistent with almost any value of matter density)?

hwillis 9y ago
The CMB has a certain spectrum and temperature. The amount of energy in the universe correlates to the amount of mass in the universe. The more energy and mass was present in the big bang, the hotter the CMB will be. The amount of visible matter corresponds to a CMB temperature that is far lower than what we see.
- effie 9y ago
  > The amount of visible matter corresponds to a CMB temperature that is far lower than what we see. Why would there be a relation between CMB temperature and amount of (visible+invisible) matter? I suppose the CMB temperature decreases in time. Would`s the relation you talk about imply that the mass or density of mass of the Universe then has to decrease as well?

scythe 9y ago

>The CMB is extremely smooth, but visible matter is highly clumped up. This can only be explained by a huge amount of matter that doesn't clump up ie non-baryonic This is not correct. Datk matter's presence is inferred from the CMB anisotropy not because it doesn't "clump up" but because it does not interact with photons (EM field). Inflation is responsible for excess smoothness. In fact dark matter drives regular matter to "clump". See eg: https://physics.stackexchange.com/questions/61239/how-the-cmb-anisotropy-is-linked-to-the-existence-of-cold-dark-matter-and-dark-e https://physics.stackexchange.com/questions/61239/how-the-cm... >every single one of those discrepancies is solved exactly This is quite an oversell as far as I know. The margins of error on these calculations are often large. In particular the version with a slightly modified gravity field and smaller amounts of unobserved regular matter is not too outlandish. What would be needed to exclude this is a proof that the amount of unobserved matter cannot be reduced significantly by modifying G, which has not been written yet.

dnautics 9y ago

> solved exactly by using the measurements of dark matter that already existed... Just be careful. Dark matter distributions are highly paramterized quasi-continuous functions, so getting measurements to match exactly is a function of their definition, not an emergent property. While we do feel confident that there could something reasonable there, a priori exactitude cannot be evidence because that's begging the question and also possibly over fitting parameters.

hwillis 9y ago
I have become a little aggravated by people who don't believe in dark matter
- dnautics 9y ago
  why? Until direct evidence and predictions of properties emerge, from an epistemological point of view it's still a very shaky model. If it all turned out to be epicycles it wouldn't be surprising. Or maybe a better analogy is Vulcan. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) The closest we have come to direct evidence are the independent calculations of the properties if it's a self-annhilating particle that agree with a certain set of observations. Even so that seriously breaks a conservation rule, because if there's no proposed way to create dark matter, the universe makes no sense.