Is Dark Matter Equivocal

[cosmocrazy]

This is my basic understanding of what dark matter is, and why it is implied to exist;

"Dark Matter" is inferred from the observational movements of galaxies based on our current accepted understanding of gravity from general relativity. It is deemed - dark matter - because, if it exists in reality, it doesn't appear to interact, or possibly at best extremely weakly, with normal matter or light . It does however interact gravitationally, which therefore very nicely explains the anomaly of the rotation of galaxies. We are aware that our complete understanding of gravity is flawed and/or incomplete. This is because we have yet to find a theory which unites quantum gravity with general relativity. (If I have this basically wrong please correct me in your reply to my question)

So my question is, if we know our understanding of gravity is incomplete, why would we infer dark matter as a solution to the observed gravitational anomaly of the rotation of galaxies, which are much different from how they should rotate based on what our models describe using general relativity?

[Van Rijn]

So my question is, if we know our understanding of gravity is incomplete, why would we infer dark matter as a solution to the observed gravitational anomaly of the rotation of galaxies, which are much different from how they should rotate based on what our models describe using general relativity?

The basic issue is that scientists *have* tried to find a modification of gravity that fits all the data but haven’t found one that fits. On the other hand, adding dark matter does fit. It also isn’t just about galaxy rotation.

[grant hutchison]

And the scale on which we know general relativity is necessarily incomplete is very different from the scale on which we invoke dark matter. Quantum gravity needs to kick in at minute distances and huge densities, such as those encountered in the Big Bang and inside black holes. We don't have any theoretical reason to believe GR would fail at large distances and low densities.

Grant Hutchison

[Noclevername]

Hasn't DM also been detected by its lensing?

[Shaula]

Also the scales on which our models for gravity are known to break down are not the ones we are looking at. For it to be a quantum gravity effect we'd expect to see problems in the strong field domain. What we are seeing are weak field issues.

And as Van Rijn say there are a number of other issues dark matter fixes that modifying gravity doesn't seem to handle well. Early universe structure formation, cluster dynamics and nucleogenesis are among the other things that are fixed by dark matter but all seem to require a different and incompatible fix for gravity.

Edit: Clearly I am a slow typer...

[profloater]

Dark matter is inferred by these strange orbital rotation speeds of stars in observed galaxies. There was a recent paper by Chae , Kyu-Hyun et al. Published in the astrophysical journal, where they were testing the strong equivalence principle. That is the assumption that gravity mass is the same thing as inertial mass. It certainly is in our earthbound experiments. They looked at the external field effect which is The idea that nearby galaxies would affect by means of gravity, the rotation of stars in a Galaxy. To their embarrassment, and they really checked their data carefully, they found there was a statistically significant effect. That single experiment , Which will no doubt be repeated, brings back the idea of modified Newtonian gravity , And raises a question About dark matter. The effect seen would be too small to find on Earth but becomes significant at cosmological scales.

[cosmocrazy]

The basic issue is that scientists *have* tried to find a modification of gravity that fits all the data but haven’t found one that fits. On the other hand, adding dark matter does fit. It also isn’t just about galaxy rotation.

In addition to galaxy rotation, what other effects have been observed that could be the result of dark matter?

I'm interested to learn more on the subject.

Thanks

[cosmocrazy]

And the scale on which we know general relativity is necessarily incomplete is very different from the scale on which we invoke dark matter. Quantum gravity needs to kick in at minute distances and huge densities, such as those encountered in the Big Bang and inside black holes. We don't have any theoretical reason to believe GR would fail at large distances and low densities.

Grant Hutchison

I'd heard different, but to be honest it was on a science vlog on you tube so not a very reliable source! It raised my interest though.

[cosmocrazy]

Also the scales on which our models for gravity are known to break down are not the ones we are looking at. For it to be a quantum gravity effect we'd expect to see problems in the strong field domain. What we are seeing are weak field issues.

And as Van Rijn say there are a number of other issues dark matter fixes that modifying gravity doesn't seem to handle well. Early universe structure formation, cluster dynamics and nucleogenesis are among the other things that are fixed by dark matter but all seem to require a different and incompatible fix for gravity.

Edit: Clearly I am a slow typer...

Ok, so this is why dark matter seems to be the best fit, since GR is a theory that is very well established and tested to be correct many times it makes sense that other theories would need to cover everything that GR does, plus find a solution that omits dark matter.

Thanks

[cosmocrazy]

Dark matter is inferred by these strange orbital rotation speeds of stars in observed galaxies. There was a recent paper by Chae , Kyu-Hyun et al. Published in the astrophysical journal, where they were testing the strong equivalence principle. That is the assumption that gravity mass is the same thing as inertial mass. It certainly is in our earthbound experiments. They looked at the external field effect which is The idea that nearby galaxies would affect by means of gravity, the rotation of stars in a Galaxy. To their embarrassment, and they really checked their data carefully, they found there was a statistically significant effect. That single experiment , Which will no doubt be repeated, brings back the idea of modified Newtonian gravity , And raises a question About dark matter. The effect seen would be too small to find on Earth but becomes significant at cosmological scales.

(my Bold) is that the MOND theory I have heard about?

[Shaula]

In addition to galaxy rotation, what other effects have been observed that could be the result of dark matter?

Simulations of early structure development end up looking much more like what we currently see if we model the universe as two fluids - one that interacts gravitationally only and one that interacts via gravity and EM forces. If you just have the second fluid then you get a very different looking universe.

Galactic cluster dynamics don't look particularly great unless you include dark matter. The velocity distribution across the cluster members doesn't match what we would expect when we look at the amount of matter present inferred from things like the X-ray shocks and other emission sources.

Nucleosysnthesis has some issues but they get worse without dark matter. Dark matter acts as a dilutant and reduces the baryon density without messing with the photon density. Without it you end up with too much of some elements and not enough of elements that are easily destroyed in high energy environments.

Acoustic oscillations in the early universe (which leave a signature in the cosmic microwave background) are very senstive to the balance between baryon density and everything else. Again, dark matter acts as a dilutant and leaves measurable signatures in the power spectrum of the oscillations that we can observe today.

So you can see how the different lines of evidence stack up (and are pretty independent). Without dark matter we have some serious gaps in our understanding of nuclear processes (which would be a surprise as we measure these in the labs - we'd need to add in a magicium solution to fix the early universe), of fluid dynamics (which we can measure in the lab as well), of plasma dynamics (ditto) and of gravity at the galactic and cluster scales (which would be different, no MOND solution exists that fits all of the different observations).

Not saying dark matter is perfect - and it would be very nice to actually have a beaker full of it to prod. But it gets us a long way to understanding a wide range of phenomena without requiring that physics somehow works very differently once we get beyond our planet. Which makes me laugh because some people keep saying "Until I get a lab based detection of dark matter I won't accept it!" but at the same time are perfectly happy to propose that as soon as you get outside the lab gravity and atomic physics suddenly behave totally differently.

I'll be honest and say I've never been a fan of the dark matter and especially the dark energy solution - but that is part of being a scientist. If the evidence fits a solution better than all of the others you have to accept it. You can work at finding alternatives all you like but until they do a better job you have to accept what the data and models are telling you. Which is how I keep ending up defending things I'm not a great fan of on science boards...

[Shaula]

(my Bold) is that the MOND theory I have heard about?

Yes - MOND is MOdified Newtonian Dynamics

Edit: It is called tihs because at the scales and energies we see a deviation from what we expect GR is not that relevant. Newton works fine as an approximate solution.

[cosmocrazy]

Simulations of early structure development end up looking much more like what we currently see if we model the universe as two fluids - one that interacts gravitationally only and one that interacts via gravity and EM forces. If you just have the second fluid then you get a very different looking universe.

Galactic cluster dynamics don't look particularly great unless you include dark matter. The velocity distribution across the cluster members doesn't match what we would expect when we look at the amount of matter present inferred from things like the X-ray shocks and other emission sources.

Nucleosysnthesis has some issues but they get worse without dark matter. Dark matter acts as a dilutant and reduces the baryon density without messing with the photon density. Without it you end up with too much of some elements and not enough of elements that are easily destroyed in high energy environments.

Acoustic oscillations in the early universe (which leave a signature in the cosmic microwave background) are very senstive to the balance between baryon density and everything else. Again, dark matter acts as a dilutant and leaves measurable signatures in the power spectrum of the oscillations that we can observe today.

So you can see how the different lines of evidence stack up (and are pretty independent). Without dark matter we have some serious gaps in our understanding of nuclear processes (which would be a surprise as we measure these in the labs - we'd need to add in a magicium solution to fix the early universe), of fluid dynamics (which we can measure in the lab as well), of plasma dynamics (ditto) and of gravity at the galactic and cluster scales (which would be different, no MOND solution exists that fits all of the different observations).

Not saying dark matter is perfect - and it would be very nice to actually have a beaker full of it to prod. But it gets us a long way to understanding a wide range of phenomena without requiring that physics somehow works very differently once we get beyond our planet. Which makes me laugh because some people keep saying "Until I get a lab based detection of dark matter I won't accept it!" but at the same time are perfectly happy to propose that as soon as you get outside the lab gravity and atomic physics suddenly behave totally differently.

I'll be honest and say I've never been a fan of the dark matter and especially the dark energy solution - but that is part of being a scientist. If the evidence fits a solution better than all of the others you have to accept it. You can work at finding alternatives all you like but until they do a better job you have to accept what the data and models are telling you. Which is how I keep ending up defending things I'm not a great fan of on science boards...

Thank you for this response, its very informative! It appears then that, although we both share the same view (my bold), dark matter is a good candidate for what we observe and fits into our current models that are based on tested and accepted mainstream theories.

Wouldn't it be great if we could get sample "to prod" as you stated.

[grant hutchison]

And the scale on which we know general relativity is necessarily incomplete is very different from the scale on which we invoke dark matter. Quantum gravity needs to kick in at minute distances and huge densities, such as those encountered in the Big Bang and inside black holes. We don't have any theoretical reason to believe GR would fail at large distances and low densities.

Grant Hutchison

I'd heard different, but to be honest it was on a science vlog on you tube so not a very reliable source! It raised my interest though.

Really? That's interesting. What theoretical reason did they advance?
We know the theory of GR must come into conflict with quantum mechanics at small scales and high densities. But there doesn't seem to be any existing theory for GR to conflict with at large scales and low densities.

Grant Hutchison

[cosmocrazy]

Really? That's interesting. What theoretical reason did they advance?
We know the theory of GR must come into conflict with quantum mechanics at small scales and high densities. But there doesn't seem to be any existing theory for GR to conflict with at large scales and low densities.

Grant Hutchison

I would have to try and re-view it on youtube, but IIRC it wasn't so much a theory, but more of a statement about the limitations of observation over larger scales. I interpreted (correctly or not) that to mean that the further away we observer the less accurate our data/observations are, so it becomes more difficult to accurately test current theories.
I don't believe this to be a substantial reason to imply that GR could be flawed at the extreme larger scales, but it raised my interest to ask the question.

I have very limited understanding of GR so please bear with me.
Thanks

[profloater]

Putting my neck out, this discrepancy In the paper I mentioned above, seems to be consistent with GR in that we interpret mass accelerations as distortion of spacetime. Gravity wells. That is why it looks like there has to be more mass in the outer areas of galaxies than we can account for, because in GR mass curves spacetime. So dark matter curves spacetime but we cannot see it. Alternatively MOND is called up, modifying Newton’s gravity just a tiny bit, but Newton and Einstein both offer models which neatly explain observations. The fun is that some observations don't fit either model although many do. One trouble with MOND, apparently, is that it predicts gravity waves do not travel at c, and recent observations demonstrate that they do, a tick for GR. so orbit speeds of stars in distant galaxies is a hot topic. We infer those speeds from redshifts, mostly. We infer star masses from our models of stars.

[Shaula]

Putting my neck out, this discrepancy In the paper I mentioned above, seems to be consistent with GR in that we interpret mass accelerations as distortion of spacetime.

My understanding is that GR satisfies the strong equivalence principle. If it breaks then GR needs extra fields to be added or to be replaced. So I don't think it is consistent at all.

[cosmocrazy]

There is an hypothesis stating that gravity itself could have mass. Where the elusive graviton might be a massive particle unlike the other force carriers?

[profloater]

My understanding is that GR satisfies the strong equivalence principle. If it breaks then GR needs extra fields to be added or to be replaced. So I don't think it is consistent at all.

I did not explain very well that if one can alter force=mass x acceleration by a fraction which only has significance at huge distances, for MOND, ( without a cause,) then one can similarly alter GR by the same factor to change the curvature of spacetime. The interpretation of why inertial mass should vary from gravity mass does suggest either a new field or the standard fields have that factor which becomes significant at large distances. We cannot expect to find that within the solar system, or maybe even within our galaxy, when the scale effect is so many orders of magnitude for separated galaxies.

[Shaula]

I did not explain very well that if one can alter force=mass x acceleration by a fraction which only has significance at huge distances, for MOND, ( without a cause,) then one can similarly alter GR by the same factor to change the curvature of spacetime. The interpretation of why inertial mass should vary from gravity mass does suggest either a new field or the standard fields have that factor which becomes significant at large distances. We cannot expect to find that within the solar system, or maybe even within our galaxy, when the scale effect is so many orders of magnitude for separated galaxies.

Pretty sure the background dependence that the paper was looking at would not be a simple change to GR at all. I'd need to work through it but at first glance it seems that it breaks a number of the symmetries and principles used to make solving the field equations tractable.

I mean, conceptually it is easy. Just like conceptually modifying a dog to carry a letter across the Atlantic is easy.

[profloater]

I should add that the adjustment to Newton is in the acceleration, introducing a fundamental acceleration a0 and an empirical “interpolating function” , suggested by Milcrom last century. If the strong equivalence of gravity mass and inertial mass is broken, there could be a weak equivalence that introduces quantum effects. This has been tested to less than one part in 10^15 while that acceleration new constant was suggested to be “only” 10^-10.

But just galaxy observations does seem to support MOND using that empirical correction, while the standard model supports dark matter, to explain why outer stars centripetal acceleration is not dependent on their distance from a galactic centre. (Rotation curves are “flat”)

[profloater]

Pretty sure the background dependence that the paper was looking at would not be a simple change to GR at all. I'd need to work through it but at first glance it seems that it breaks a number of the symmetries and principles used to make solving the field equations tractable.

I mean, conceptually it is easy. Just like conceptually modifying a dog to carry a letter across the Atlantic is easy.

Fair enough, if I understand correctly, which is as unlikely as a modified dog, that would force gravity to be different from the other fundamental forces and thus unlikely to get unified. Unification is attractive as a beautiful solution, but then a little thing like star rotation could force theory to accept that major modification is required. Which is what makes physics so interesting.

[Noclevername]

There is an hypothesis stating that gravity itself could have mass. Where the elusive graviton might be a massive particle unlike the other force carriers?

I have never heard of such an idea. Where does it come from?

[Jens]

I have never heard of such an idea. Where does it come from?

There’s a long article in Wikipedia about it. I’m not really qualified to judge it though. There are lots of sigmas and stuff.

https://en.m.wikipedia.org/wiki/Massive_gravity

[profloater]

There is an hypothesis stating that gravity itself could have mass. Where the elusive graviton might be a massive particle unlike the other force carriers?

The graviton is the hypothetical equivalent to the photon , the particle of the gravity field, and if it had mass would have to be sub luminal. It seems gravity waves arrive at the same time as EM waves from cosmic clashes, so at the moment , quantum gravity gravitons are massless. But not yet found.

[kzb]

I don't like either dark matter or modified gravity. They are both ad hoc inventions with the only evidence to support their existence being the phenomena they were invented to explain.

I don't know why people are not intensively investigating extreme scattering events, which seem to show there is a huge mass of small gas clouds out there, currently not accounted in galaxy mass budgets. Why is this almost a fringe subject, when it could be very important?

Also we hear very little about the KATRIN experiment, which may find neutrinos have mass. If so, this could be the "cluster mass".

[Shaula]

I don't know why people are not intensively investigating extreme scattering events, which seem to show there is a huge mass of small gas clouds out there, currently not accounted in galaxy mass budgets. Why is this almost a fringe subject, when it could be very important?

Because baryonic matter won't fix the other lines of evidence I mentioned, because enough mass to fix things would be detectable via things like cluster level ram shocks, because there are not enough of them, because to be undetectable you need to fine tune their properties to a point that makes dark matter distributions look natural. And given that there is a steady stream of papers and the subject gets time on big telescopes it isn't fringe, it's just not seen as the best bet given the current evidence.

Also we hear very little about the KATRIN experiment, which may find neutrinos have mass. If so, this could be the "cluster mass".

Neutrinos do have mass. That is pretty much a given based on the fact that they oscillate. You can't have that oscillation with a flavour/mass eigenstate mismatch and mass differences to drive it. But neutrinos are not cold and warm dark matter is almost as bad a fit to observations as baryonic matter.

Both of these and lots of other alternatives have been considered, rejected, reviewed and are regularly revisited in light of observations. They just are not a good fit to the data.

[profloater]

I noticed this “in the week magazine from Karl Popper:
Whenever a theory appears to you as the only possible one, take this as a sign that you neither understand the theory nor the problem which it was intended to solve.

[Shaula]

I noticed this “in the week magazine from Karl Popper:
Whenever a theory appears to you as the only possible one, take this as a sign that you neither understand the theory nor the problem which it was intended to solve.

Not relevant or applicable in this case. As has been obvious from the commentary in this thread there were lots of possible theories. Off the top of my head MOND, TeVeS, Brans Dicke theory, WDM (including neutrinos, sterile neutrinos, axions and a host of other alternatives), CDM, HDM (including light neutrinos and other more exotic solutions), Mixed Dark Matter theories, baryonic matter (dust clouds, compact bodies, gas halos, gas clouds, black holes), extra dimensions (string theory bulk/brane, fifth force theories, shadow dimension theories), vacuum modification theories (including superfluid vacuum, versions of twistor theory), modified GR... Then we can add in the dark energy ideas including quintessence, L(t), more extra dimensions, more modified gravity theories, and the cosmological constant approach.

So that, without having to do more than think back to papers I have seen in the last two decades, is at least one hundred distinct dark energy and dark matter combinations. Some are closer to the data that others. LCDM is currently the best fit. If you are saying that whenever one theory fits the data best then we clearly don't understand the theory or the problem then I question how you believe actually doing anything resembling science is possible. Because it would imply that whenever a theory works it is a sign we should discard it immediately.

[profloater]

Not relevant or applicable in this case. As has been obvious from the commentary in this thread there were lots of possible theories. Off the top of my head MOND, TeVeS, Brans Dicke theory, WDM (including neutrinos, sterile neutrinos, axions and a host of other alternatives), CDM, HDM (including light neutrinos and other more exotic solutions), Mixed Dark Matter theories, baryonic matter (dust clouds, compact bodies, gas halos, gas clouds, black holes), extra dimensions (string theory bulk/brane, fifth force theories, shadow dimension theories), vacuum modification theories (including superfluid vacuum, versions of twistor theory), modified GR... Then we can add in the dark energy ideas including quintessence, L(t), more extra dimensions, more modified gravity theories, and the cosmological constant approach.

So that, without having to do more than think back to papers I have seen in the last two decades, is at least one hundred distinct dark energy and dark matter combinations. Some are closer to the data that others. LCDM is currently the best fit. If you are saying that whenever one theory fits the data best then we clearly don't understand the theory or the problem then I question how you believe actually doing anything resembling science is possible. Because it would imply that whenever a theory works it is a sign we should discard it immediately.

I did not say that, I just noticed a quote which does seem to argue against finding one theory the only one. Popper of course was clear about ever testing to try to falsify theories, and theories work until they don’t. The line between hypothesis and theory is largely how testable it is. Tests seem to challenge both MOND and DM at the moment. But I think your post comes closest to answering the OP and it helps me too.