Dark Matter Theory Question

In summary, the dark matter theory was proposed due to inconsistencies with observed mass vs the calculated required mass for a galaxy to exist. Just wondering how we know its not just stray exoplanets or possibly even smaller collapsed stars, really any kind of mass that wouldn't emit light. Seeing how we can only observe stars that emit light.
  • #1
TL;DR Summary
Dark Matter Theory Question
I was thinking about how the dark matter theory was proposed due to inconsistencies with observed mass vs the calculated required mass for a galaxy to exist. Just wondering how we know its not just stray exoplanets or possibly even smaller collapsed stars, really any kind of mass that wouldn't emit light. Seeing how we can only observe stars that emit light.
 
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Dark matter does not interact with light at all the way regular matter does. It does not emit, absorb, or reflect light. It is theorized to be six times more plentiful than regular matter. See https://home.cern/science/physics/dark-matter
 
  • #3
I understand the idea of it but isn't it an assumption to assume it doesn't interact with light? I mean we can observe stars because they emit light and we can observe planets around stars from the dip in light that comes from a planet passing in between the star and our field of view. but the vast majority of matter doesn't emit light unless its extremely hot like a star. So isn't it possible the extra matter could be matter that hasn't combined with stars or matter that is not extremely hot, and if thats not the case, how do we know its not?
 
  • #4
One piece of evidence for non-baryonic dark matter is the abundances of the light elements. Big Bang Nucleosynthesis provides quite excellent agreement with the observed primordial abundances of 1H, 2H, and He. Those predictions depend on the amount of baryonic matter. Were dark matter baryonic, the predicted primordial abundances would not be in agreement with observations.

The relative amplitudes of the peaks in the cosmic microwave power spectrum (temperature anisotropies) also depend on the densities of baryonic matter and non-baryonic dark matter.

Both sets of observations are consistent with baryonic matter making up about 5% of the critical density of the universe and non-baryonic dark matter making up about 25% of the critical density.
 
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  • #5
The peaks in the CMB power spectrum are my favourite, because they come from a separate topic unrelated to galaxy dynamics and are unexplained without non-baryonic matter.

Another point, though, is that you need an awful lot of planets to add up to the amount of dark matter you need. They can't be much bigger than Jupiter or they'd be small stars, but you need more than a hundred Jupiters to make up the mass of the Sun. And typically you have more smaller things than larger ones, so you would expect there to be a lot of Earth-sized planets floating around out there. But we don't see them occluding stars (the ones we do see do it regularly, so are in orbit). Basically, there's a lot of absence of evidence that's quite tricky to explain if there really are swarms of planets floating around out there.
 
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  • #6
GreenLemon said:
I understand the idea of it but isn't it an assumption to assume it doesn't interact with light? I mean we can observe stars because they emit light and we can observe planets around stars from the dip in light that comes from a planet passing in between the star and our field of view. but the vast majority of matter doesn't emit light unless its extremely hot like a star. So isn't it possible the extra matter could be matter that hasn't combined with stars or matter that is not extremely hot, and if thats not the case, how do we know its not?
One possibility for dark matter is lots of neutrinos. Another is a large population of primordial black holes (i.e. black holes that formed early in the universe, rather than from stellar collapse).
 
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  • #7
GreenLemon said:
I understand the idea of it but isn't it an assumption to assume it doesn't interact with light?
Assumption, guess, educated guess, prediction, theoretical condition... you could call it many different things.
GreenLemon said:
So isn't it possible the extra matter could be matter that hasn't combined with stars or matter that is not extremely hot, and if thats not the case, how do we know its not?
Possible, perhaps. But we're talking about a LOT of missing mass since something like 85% of the mass of the universe is dark matter. It can't all be cold hydrogen-helium gas otherwise we would see it in the rates of stellar formation, absorption rates of various EM frequencies, and other things. It can't all be dust or we would see it obscuring background targets, measure different stellar metallicity, and in other ways. Having it made up of sub-stellar matter (planets, asteroids, comets, etc) doesn't fit with either observations or theoretical models of how star systems form, where the vast majority of the mass is in the stars and only a small fraction is contained in the sub-stellar objects surrounding the stars. We've catalogued a lot of star systems and none of them have hugely abnormal amounts sub-stellar matter.

These problems continue when you consider all other known objects or types of matter. There just isn't a way that we've found to reconcile the behaviors of galaxies and galaxy clusters with missing 'normal' matter. The observations just don't support it.

That's not to say that dark matter is a perfect candidate. It's not. It's just the best model we have right now.

Also, remember that we already know of matter that interacts via four, three, and two of the fundamental forces (quarks, electrons, and neutrinos respectively, as an example of each). Is it THAT much of a stretch to imagine a type of matter that only interacts via gravity?
 
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Drakkith said:
Assumption, guess, educated guess, prediction, theoretical condition... you could call it many different things.

Possible, perhaps. But we're talking about a LOT of missing mass since something like 85% of the mass of the universe is dark matter. It can't all be cold hydrogen-helium gas otherwise we would see it in the rates of stellar formation, absorption rates of various EM frequencies, and other things. It can't all be dust or we would see it obscuring background targets, measure different stellar metallicity, and in other ways. Having it made up of sub-stellar matter (planets, asteroids, comets, etc) doesn't fit with either observations or theoretical models of how star systems form, where the vast majority of the mass is in the stars and only a small fraction is contained in the sub-stellar objects surrounding the stars. We've catalogued a lot of star systems and none of them have hugely abnormal amounts sub-stellar matter.

These problems continue when you consider all other known objects or types of matter. There just isn't a way that we've found to reconcile the behaviors of galaxies and galaxy clusters with missing 'normal' matter. The observations just don't support it.

That's not to say that dark matter is a perfect candidate. It's not. It's just the best model we have right now.

Also, remember that we already know of matter that interacts via four, three, and two of the fundamental forces (quarks, electrons, and neutrinos respectively, as an example of each). Is it THAT much of a stretch to imagine a type of matter that only interacts via gravity?
I guess its not that much of a stretch but its just hard to directly find evidence of it. I imagine the ideal way to experiment to find it would be find localized regions outside our solar system where a lot of dark matter is predicted to be and measure distortions in gravity outside what is normal. Humans are a long way off from ever doing something like that if it even is possible.
 
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  • #9
GreenLemon said:
I imagine the ideal way to experiment to find it would be find localized regions outside our solar system where a lot of dark matter is predicted to be and measure distortions in gravity outside what is normal. Humans are a long way off from ever doing something like that if it even is possible.
On the contrary, it has been done repeatedly and those observations are what led to the concept of dark matter in the first place. The first observations were considered anomalous because what was "normal" was not what was being observed.
 
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  • #10
GreenLemon said:
I guess its not that much of a stretch but its just hard to directly find evidence of it. I imagine the ideal way to experiment to find it would be find localized regions outside our solar system where a lot of dark matter is predicted to be and measure distortions in gravity outside what is normal. Humans are a long way off from ever doing something like that if it even is possible.
There is the galaxy rotation data. We're not short of data on the gravitational profile. The mystery is what sort of "dark" matter is out there. If it really is cold, dark matter that does not interact other than gravitationally, then that is the problem, in terms of identifying what it is more specifically.

That said, if it were primordial black holes, then they would be more detectable.
 
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  • #11
GreenLemon said:
I understand the idea of it but isn't it an assumption to assume it doesn't interact with light? I mean we can observe stars because they emit light and we can observe planets around stars from the dip in light that comes from a planet passing in between the star and our field of view. but the vast majority of matter doesn't emit light unless its extremely hot like a star. So isn't it possible the extra matter could be matter that hasn't combined with stars or matter that is not extremely hot, and if thats not the case, how do we know its not?
Well, for one thing, the amount of such "cold" matter would have to out mass the "hot" matter by a considerable amount. This would change the overall elemental composition of the Universe. And we would expect to see this reflected in the spectral lines of the "Hot" objects.
This is similar to the reason as to why MACHOs (MAssive Compact Halo Objects) like Black holes and neutron stars can't be a major component of DM. That part of the universe we can see would not look like it does to us if the conditions existed for that many MACHOs to form.
The other is that not "interacting with light" is only a part of the picture. It doesn't interact electromagnetically in any way. This mean the types of collisions and clumping we see with "normal" matter doesn't occur, because these are all due to electromagnetic interactions.
These interactions are why, in a spiral galaxy, the visible matter tends to form a disk. If DM were composed of this type of matter (even if cold) it would do the same. But for DM to have the observed effect it does on galactic rotation curves, it has to be distributed as a spherical cloud engulfing the disk of the galaxy, meaning it isn't subject to the same influences that formed the disk of the galaxy.
 
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  • #12
Exoplanets?

Do you know how many Jupier-sized objects per solar system are needed to match observation? Like 10,000.
We have one.
 
  • #13
GreenLemon said:
TL;DR Summary: Dark Matter Theory Question

I was thinking about how the dark matter theory was proposed due to inconsistencies with observed mass vs the calculated required mass for a galaxy to exist. Just wondering how we know its not just stray exoplanets or possibly even smaller collapsed stars, really any kind of mass that wouldn't emit light. Seeing how we can only observe stars that emit light.
This possibility, called the MACHO hypothesis (for "massive compact halo object) was ruled out decades ago. The main points are that:

* The amount of ordinary matter in the universe inferred from Big Bang Nucleosynthesis isn't great enough.
* In the local part of our universe that are close enough to compare the ratio of matter in stars to matter in dim objects other than stars (in part, directly, and in part, indirectly through micro-lensing), the proportion of MACHO candidates isn't remotely close enough to explain the dynamics of the Milky Way galaxy or other places where dark matter phenomena are observed. The proportion of ordinary matter that is either in stars, or in diffuse interstellar gases made up of hydrogen with trace amounts of other light elements, is very high. Planets and other dim objects that are MACHO candidates make up a very small percentage of the total, but they need to make up the lion's share of all matter in the galaxy to fit the MACHO dark matter candidate hypothesis.
* The dynamics of how MACHOs would be distributed and influence the movement of visible matter is inconsistent with what is observed. The dynamics that are observed in visible matter demand a "nearly collisionless" dark matter candidate, but MACHOs obviously, don't fit that description.
 
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GreenLemon said:
I guess its not that much of a stretch but its just hard to directly find evidence of it.
The following references might be of interest. Both are short, freely available, and written by experts.

Nata A. Bahcall, Dark matter universe, Proceedings of the National Academy of Sciences (PNAS), 112 (40) 12243-12245 (2015).

Joseph Silk, Dark Matter from Martin White's website at University of California, Berkeley.

There is of course a great deal of more technical literature available.
 
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Vanadium 50 said:
Exoplanets?

Do you know how many Jupiter-sized objects per solar system are needed to match observation? Like 10,000.
We have one.
I know 10000 jupiter like planets is a stretch, but I was more thinking the possibility of there existing more mass in deep space than anything, burnt out or collapsed dwarf stars and their exo planets, asteroids etc. Just a summation of masses that don't give off light and cant be detected that could maybe add up to the difference or at least some of it. I'm not as familiar with the research on dark matter and what led to the current theories so I was curious as to why its been ruled out as a possibility but I got my answer now.
 
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