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So, getting back to this assumption in your original question, as previously pointed out (first by Grant), our understanding of gravity at the scale and in the context of dark matter is not known to be a problem. General Relativity has passed every test in that regime. Everything else you listed in your "basic understanding," though, is pretty accurate.Originally Posted by cosmocrazy
Everyone is entitled to his own opinion, but not his own facts.
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I do not understand the wording in the context of “equivocal”, we have no fit with dark matter in the standard model, so the scale issue is part of a problem, which is that the dark matter hypothesis fits some but not all observations but we don’t know what it is. So has GR passed every test if extra fields are necessary in some (recent) observations? Or are you saying dark matter is accepted in GR ?
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
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GR is a theory of gravitation. It doesn't care if matter is dark or not. In fact it doesn't even make that distinction.
At the moment comes down to two scenarios:
- GR breaks down at both high and low energy extremes, somehow acting very differently over a range of large scales than it does in the lab, our solar system or our galactic local area.
- Our understanding of the properties of matter is incomplete and there is a sector of 'dark' particles or similar that we have not seen because they are so hard to detect.
GR is a theory that has been tested to very high precision by a number of techniques and so far the theory has proven sound. Thus to see it suddenly break down at galactic and intergalactic scales (but not all galactic and intergalactic scales - just some) is difficult to reconcile with the wealth of evidence that it is a strong theory. Add to that the fact that no proposed modification to the theory fits more than a small subset of observations (essentially the modifications work so long as you tune them to a particular system but generally fail for other systems) and it is hard to feel comfortable writing GR off just yet.
On the other hand we have been here before with unknown or suspected particles. The neutron. Neutrinos. Truth/Beauty (sue me I like the old names). Half of the eightfold path. Pentaquarks. The Higgs. there are sound theoretical reasons for not proposing more families of particles but there are none for ruling out more particles. And some hints that there should be more out there (supersymmetry, although that is looking less attractive by the year, the chiral zoo etc).
At the end of the day the dark matter hypothesis fits more observations and seems more likely to be correct (for a given value of correct). Modifying GR seems like the worse option because of the issues I've mentioned, especially how you need to retune the model everey few minutes for reasons we don't understand. Another point is testing it. With DM it makes specific, testable predictions we can use to probe our understanding of it (lensing and other effects). You predict a distribution and look for evidence of it. Most of the time you find it. Whereas with MOND it is generally the case that you posit a change to GR, it works for some cases. Because it is an arbitrary change you then retune it for other cases where it doesn't work (for example in TeVeS you just keep layering on extra fields and hoping). You can't test it because you have no idea what is going on. And no, the paper you mentioned didn't test it. It looked for correlations between the size of the observed effect and local gravitational conditions. They conclude there is a correlation, which could be a pointer so something interesting or a coincidence in the data they were using.
So, and I feel like I have said this a few times, the situation is that we have a current favoured theory. We know it is not perfect and we are looking to improve it. We also have a large number of other ideas that did not fit as well. These other ideas are still being looked into and revisited (as you can see from the stream of papers on them). Seems to me like things are working as intended and we are not in a bad place. There is a current front runner and a healthy level of challenge and investment in potential alternatives.
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Hear him! Hear him!
Grant Hutchison
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Thanks, all beautifully put.
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
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Thanks. I should probably add, though, that if we discover some underlying principle that allows us to predict the changes we need to make to GR and then test it against observations suddenly it would become a more attractive proposition. (If it passed the observational tests reasonably well). Especially if knowing how GR has to change opens up new physics. At the moment MOND-like theories are stuck in this awkward spot where they are effectively saying "Hey, if you fit an arbitrary line to a line then you get a line that roughly fits a line".
There is some hope for these theories though - before the observational campaign of the last decade Dark Matter theories were in the same place. They were saying "Hey, if you change the distribution of matter to fit the observed orbits then the observed orbits fit the distribution of matter". At that point there wasn't really much that was testable. Since then observations, simulations and other tools have actually taken us to a much better, more scientific place.
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Is it permitted then to ask a supplementary question, not really about the ambiguity of dark matter but its effect on EM radiation like the gravitational lensing seen around galaxies.? The dark matter does not emit or reflect EM but it does contribute to gravitational lensing?
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
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Yes, that is correct. All mass (strictly all stress-energy-momentum) contributes to the gravitational field (as per the Einstein field equations). It is not the dark matter particles that interact with EM radiation - it is a secondary effect where the dark matter partciles create a gravitational field which interacts with the EM radiation. No electromagnetic interactions take place.
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Thank you Shaula for your contribution to this thread, as profloater said "beautifully put" and also very enlightening.
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I thought this to be the case (my bold), but was intrigued why it would be suggested as such which is why I asked the question.
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Also places like the Bullet Cluster where the center of gravity is not in the same place as the center of visible matter
Established Member
This is an old article (1999) which purported to find hitherto unknown large amounts of molecular hydrogen in galaxies.
Articles like this just seem to be forgotten.
FIRST EXTRAGALACTIC DIRECT DETECTION OF LARGE-SCALE MOLECULAR HYDROGEN
IN THE DISK OF NGC 891
We present direct observations of molecular hydrogen in the disk of the nearby edge-on spiral galaxy
NGC 891. With Infrared Space Observatory’s Short-Wavelength Spectrometer (SWS) [EDIT]
profiles with CO and H i data. The observed line ratios indicate relatively warm ( K) molecular T 5 150–230
clouds scattered throughout the disk in addition to a massive cooler ( K) component which dominates T 5 80–90
the signal in the outer regions.
[EDIT]
This factor matches well the mass required to resolve the problem of the missing matter of spiral galaxies within at least
the optical disk.
https://citeseerx.ist.psu.edu/viewdo...=rep1&type=pdf
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They were not forgotten or ignored. They were an important part of the debate and still are. However they don't fit the evidence - these signals are not widely observed, have a lot of assumptions built into them, don't match structure/galaxy formation models, don't work for galactic clusters, don't match the CMBR observations and don't match the nucleosynthesis data. The CO tracer method has been tried and doesn't match rotaton curve observations for most galaxies. This led to the idea that the baryonic matter had to be clumped, which led to a long observational campaign. They were considered, led to large observation campaigns and from the evidence gathered found to be non-universal and not fit observations as well as non-baryonic dark matter does.
And there is still a steady trickle of papers looking to fix dark matter with baryons. I don't know why you think these papers were forgotten - they were an important part of the lively discussion about what DM was likely to be.
These extended gas halos might have important answers to the 'missing baryon' problem, but so far they have not shown up enough to even work for that.
Established Member
I think the whole point is that the tracer methods do not always work. Molecular hydrogen itself is very difficult to detect, and of course if you don't look, you don't see.
Here is more food for thought:
A Heavy Baryonic Galactic Disc
We investigate the possibility that the observed rotation of galaxies can be accounted for by invoking a massive baryonic disc with no need for non-baryonic dark matter or a massive halo.
...there have been an increasing number of recent observations that imply that X_CO may be ten times higher in the outer Galaxy. This `dark' gas may provide adequate mass to account for galaxy rotation.
https://arxiv.org/abs/1204.4649
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You can mine arxiv for pre-prints of just about any topic you like and find something. It just shows that there are still people looking into it - which is a very good thing as diversity of thought is important. But neither the paper or your comment addresses the simple fact that baryonic dark matter is a worse fit to the full range of observations than non-baryonic matter. And it fails a number of observational tests (mostly to do with galactic interactions where you would see this material being subject to collisional effects).
I really don't know why you are holding these up as food for thought, though. Anyone working in the area is familair with the baryonic halo proposals. It is a bit like holding up a paper on Chomsky's Universal Grammar at a Linguistics conference. Yes, we know. No, it is not convincing with the current state of evidence and theoretical work.
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The abstract of this 2012 paper ends with "We discuss... where sufficient `hidden' baryons might be found." Have they been found yet?
Everyone is entitled to his own opinion, but not his own facts.
Established Member
I believe they might exist in the small gas clouds causing extreme scattering events in the radio. The sky coverage of these events imply a huge mass in such clouds is possible, yet it is almost a fringe subject.
Also, if you read that heavy baryonic disc paper instead of just scanning the abstract, it will certainly make you think.
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I find cosmic units hard to relate to Earthy ones so, if I did the conversions correctly:
Using just one significant figure:
Hydrogen in galaxies might be from 1 to 10 atoms per cubic centimeter,
Hydrogen at one atom per cc. is a density of 2. 10^-21 kg/m^3
Obviously then at 10 atoms per cc 2.10^-20 kg/m^3;
A galaxy density might be 5 . 10^-19kg/m^3
( I used the Andromeda galaxy mass 2.10^42 kg and volume 4.10^60 m^3, converted from 8.10^11suns at 2.10^30 kg and 22000 LY diameter at LY = 10^16m as per WP.)
Just looking for orders of magnitude, and done in haste, so maybe a few orders out.!
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
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Just noticed this, and wanted to say that I'm really pleased to know that I'm not the only one that feels this way.
Conserve energy. Commute with the Hamiltonian.
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It's a shame Keats could never get his head round it, though.
Beauty is truth, truth beauty,—that is all
Ye know on earth, and all ye need to know.
Grant Hutchison
Established Member
A cylinder of diameter 67kpc and h= 2kpc has a volume of 2.0E+68 cm^3 (back of envelope calculation).
At 1 atom of H per cm^3, there are obviously 2.0 E+68 H atoms.
This is 2.0 E+68/6.02E+23 = 3.4 E+44 moles of H = 3.4 E+41 kg of H.
The mass of Andromeda galaxy, let's say 1.5 E+12 Msun = 2.98 E+42kg (there are widely varying estimates of the mass but I've picked a middling value)
So at an average of 1 H atom/cm^3 the gas mass is 11% of the mass. We only need to raise this by a factor of 10 to obtain 110% of the entire mass.
Interesting, but they would say this amount of gas is not detected, and its presence would lead to absorption phenomena which are not seen to the extent necessary.
But the gas cloud theory does not postulate the gas is evenly spread, it says it is present in compact clouds of a few AU radius and of planetary mass. Predominantly cold molecular dihydrogen clouds which are practically indetectable and also highly transparent to light. They are only detected by causing extreme scattering events in the radio, and also by gravitational microlensing of quasars seen through galactic discs.
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They also need to be non-collisional, difficult to ionise, have completely different composition to the rest of the universe and mysteriously stable. We also have no idea how they formed and how they evolve to this state without being detectable. Or how they don't affect galactic evolution. We then also need new physics to fix nucleosynthesis, structure formation, the CMBR spectrum and galactic cluster dynamics.
Established Member
Nevertheless they seem to be there. If you doubt they are there, maybe the astronomy community should invest more effort in researching what else is causing the phenomena?
Many of the problems you note are explained if you look into the topic.
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I don't doubt that there is material there, however the consensus seems to be that there isn't enough based on observational evidence. Yes, you can find papers that claim otherwise. Just like you can find MOND papers and lots of other theories being promoted as the answer to the question of dark matter. Which, as I've said, is good.
And I have read into the topic, mainly I have focused on peer reviewed work on the issues. The questions I've raised are still open. I've seen some poor answers, some speculative answers and some highly contrived answers. But nothing that compelling for the general case.
So as far as I can tell the community are still looking into these events (which is why there are papers and observational campaigns). And the consensus is that it doesn't fit the data as well as LCDM. You keep implying that it is a simple fix for everything that is being ignored - which just isn't true. That's the only reason I'm still replying, to be honest. It isn't simpler and it isn't being ignored. It just isn't as good an answer. Maybe that will change, maybe not.
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In a very naiive way, there might be a mix of simple particles like protons, electrons, neutrons, hydrogen atoms and molecules, baryonic matter in other words, spread out at low density so their spacing is in centimeters or millimetres.
At that spacing they are rarer than hens’ teeth in diameter terms than stars, by hundreds of thousands of times. At any distance bigger than atomic, electrostatic forces are also many orders bigger than gravity, so protons repel each other as do electrons. The attraction of electrons to protons will accelerate them to relativistic speeds because at those separations, most will shoot past the slower moving protons. That is because the mass of the electron is so much less than the proton.
So there would be a sea of expanding protons, akin to pressure, plus high speed electrons buzzing about. Any neutrons, hydrogen atoms or molecules will ignore that and attract by the much weaker gravity force forming clumps locally which would dominate the distant star attraction.
The density is so low in volume terms that interactions are rare. The factor must be 10^-12 or so. So the big question is how to observe them? Photons passing through must get absorbed and dispersed by rare interactions. So the spectrum gets changed because of the quantum nature of such interactions.
The average temperature expected or observed must be complex, because photons are only emitted from those rare interactions.
I am not suggesting any conclusion, just musing more on the hard to grasp density of these baryons, so far away.
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
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But then; we know moving charges create magnetic fields and fields make charged particles accelerate. So any protons or electrons in this interstellar space have complex motions and some will accumulate higher energies than others. Through them come the particles energetic enough to escape their galaxy, cross space and reach us! These add to our observations of EM photons.
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
Established Member
I know we agreed that it doesn't take that much more gas to make up the missing mass, but spread out and ionised as you describe, it would be detected. High temperature gas has been found in galaxy halos, and at the time it was hailed as the "missing baryons". Subsequently that claim was downgraded to a lot of baryons, but not enough to make up the missing baryons, and nowhere near enough to make up the missing mass.
Anyhow, the least detectable form of hydrogen is the plain old dihydrogen molecule.
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Is not H2 found by its n1234 spectral lines absorbed from starlight?
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G
Established Member
I'm no expert in this but I think you are referring to atomic hydrogen emission lines here. Also, lab physics demonstrations of these emissions start with hydrogen gas (i.e diatomic molecules), but as it is heated it dissociates into atomic hydrogen, and it is this that is responsible for the emissions.
The dihydrogen molecule has no dipole.
From what I read it is only detectable from above the atmosphere. Perhaps others can help.
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