6 ms·

> Therefore dark matter cannot exist. How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. This is a grand claim

by rob_c 4y ago

> Therefore dark matter cannot exist. How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. This is a grand claim from one piece of work and feels more like a straw man grab at further research funding rather than finding the heffalon at the LHC https://arxiv.org/abs/1303.7367 https://arxiv.org/abs/1303.7367 Or in other words Occam's razor says you have a stripey horse in northern Europe rather than a zebra.

pdonis 4y ago
> How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. As I understand it, the "behavior" of dark matter in Kroupa's analysis is the behavior that is claimed for it by dark matter proponents. So if dark matter doesn't behave the way dark matter proponents say it does, the burden would be on the dark matter proponents to modify their models to say how it does behave.
- rob_c 4y ago
  Trust me this is a straw man picking at a hole of a yet incomplete theory to claim the whole deck of cards is invalid. AFAIK none of these alternatives offer a serious explanation of gravitational lensing for instance (yet, I'm happy for someone to make a theoretical breakthrough that explains away the need for an unknown new particle, but that just opens more questions around CP violation and so on)
  - pdonis 4y ago
    > Trust me Why? Who made you an infallible authority? > none of these alternatives The basic point I described is not proposing an alternative to dark matter, and I'm not asking for a refutation of Kroupa's well-known preference for alternative gravity theories. I'm just pointing out that Kroupa has described a prediction of the dark matter hypothesis that does not appear to match observations. I'm wondering if any proponent of the dark matter hypothesis has a counter argument to this: either an explanation of why the dark matter hypothesis doesn't make the prediction Kroupa is describing, or an explanation of why that prediction does match observations after all. This seems to me to be a perfectly reasonable question that does not require any discussion of alternative theories at all.
    - rob_c 4y ago
      > Who made you an infallible authority? I'm not infallible but I do have a PhD in the study of CPV in particle physics so I understand this at a level enough to pass a doctoral viva. Alternate gravity theories struggle with gravitational lensing. This is a proven observable optical phenomena in the study of the universe. Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. This is colloquially referred to as Dark Matter. A gravitational (matter like) effect, which doesn't interact optically via standard EM as we can see the galaxies and stars. Hence the name Dark Matter.
      - pdonis 4y ago
        > Alternate gravity theories Are not what I'm asking about here. I know Kroupa pushes them, but that's not the part of Kroupa's work that I'm asking about. > Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. Not in the specific item I'm asking about. In that specific item, he's not talking about any alternative theories of gravity at all. He's pointing out what he claims is a conflict between the predictions of the dark matter hypothesis and actual observations. I've already explained what a counter argument to that would look like. Do you have one?
        rob_c 4y ago
        Unfortunately you are getting a little crossed from your perspective. You're asking a question that to experts is structured oddly because you can't disentangle things you're trying to separate. I'll try to explain. Again dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models. There "classical dark matter hypothesises" is just that. That these observations can be explained by a quantizable matter candidate of some form with some yet to be determined properties. The only non particle descriptions of "dark matter phenomena" is a modified gravity framework. These attempt to address the "classical dark matter phenomena" whilst disentangling it from the "CPV phenomena". This is fine but breaks the big bang model at the stage of baryogenesis (matter formation from pure energy). There is the possibility that you can explain "classical dark matter phenomena" without demanding it also be responsible for CPV. However, if you do that the CPV step of the big bang model demands additional particle candidates which then raises the problem of what do they look like and where are they and do they interact with the Higgs field and therefore have mass. There are no counter arguments to "classical dark matter phenomena" because they are just that. Observed natural phenomena which do not fit into our current understanding of the universe. A "complete dark matter theory" tends to explain CPV+DM. These again don't make many testable predictions other than a (potentially TeV scale) weekly interacting particle candidate. So the testable prediction is that a dark matter candidate exists and we can observe it. > I've already explained what a counter argument to that would look like. Do you have one? I'll choose to read that in a relaxed manner, albeit it's a bit direct. There is no counter argument to natural phenomena. The main 2 theories are that dark matter particles exist and we can find them or we can't. MOND et al make few predictions and struggle to observe defined "gravitational phenomena", hence why there is little interest in them. There are differences between DM particle models but almost all agree that the major premise is that we might/should be able to see certain particle types given how they interact.
nico 4y ago
Interestingly, a similar argument (ie. “if it exists, it should behave like this, and it doesn’t”) was used to disprove the existence of the aether[1]. 1: Michelson-Morley experiment: https://physics.stackexchange.com/a/472611 https://physics.stackexchange.com/a/472611
- HelloNurse 4y ago
  We consider particles with oscillatory behaviour sufficiently different from aether to abandon the name and the concept as useless. But contrary to aether, dark matter is still a new, vague and complicated concept: there is ample room to evolve dark matter theories without giving up on the idea of dark matter. In any case competing theories will need to converge towards new experimental evidence and therefore towards each other, and then names and starting principles will be unimportant.
- rob_c 4y ago
  Yes, and it is a very nice experiment. However, "dark matter" is an admittedly lose framework it could be due to 1 new type of particles, 10, or higher dimensional effects and new physics we don't understand yet. Saying "therefore it doesn't exist" is ignoring the mountain of decades old solid data showing gaps in our knowledge exist. And as for dark energy... "The universal expansion is accelerating". Well done you now know as much as 99% of the physicists in the world on this strange unexplained problem.
- stefantalpalaru 4y ago
somenameforme 4y ago
Your statement implies dark matter cannot be falsified by a contradicting observation, because it (dark matter) hasn't yet been observed. But if it doesn't exist, then it will never be observed. So it, therefore, can never be falsified? If I'm straw manning you, it is unintentional. How would you propose dark matter be falsified?
- alrlroipsp 4y ago
  The idea is to propose a model that does not need dark matter to describe observations.
  - rob_c 4y ago
    Unfortunately a lot of the models try to remove DM completely from either a gravity or CP perspective and both have direct consequences as to what sort of stable structures in the universe that allow to form. And the models often conflict with other observations showing theyre no better. It's asif Newton observed an extra solar comet and therefore decided the Principia must otherwise all be wrong. It's a big step to say shinny thing in sky makes the Apple not fall as we observe it.
- rob_c 4y ago
  Dark matter as a particle has to have certain properties in order to satisfy what we know about a) it's EW interactions and b) the fact we haven't seen it yet. Over the next 20 years or so we will likely cover a huge amount of the phase space of direct observation of dark matter like candidates assuming they weakly interact with the standard model. If we observe it, great we now know a huge chunk more than ever before of the universe. If we fail to observe it then we have to go away and explain the Dark Matter problem another way. Axions are my personal favourite here because I claim nature for her beauty is sometimes horrible. But this could (in the most scary scenario) potentially be non quantisable interactions between unknown aspects of the universe. At that point we need to further our understanding of potential graviton candidates to evolve our theories of spacetime. DM isn't a solid prediction like an aether it's a statement of a big gap in our knowledge that has implications to asto and particle physics models. The big problem is then CP violation (why more matter than not, real why are we here at all stuff). This can't be explained by the standard model and stuff we can see and touch as far as we know. So we assume that this huge amount of it that we can't explain must come from interactions with new particles somewhere in some way at some energy scale. Physicists try to narrow down that phase space of where and how but a lot of this stuff makes the search for the higgs, look like cheating and watching where your parents hid the Easter eggs. There is scary little to directly go on so theoreticians have to get clever. Edit: There is also little to say that extra-SM CP violation _must_ be from the same source as Dark Matter astro candidates. The combination of the 2 fields is much more a car of. "Hmm we both have a missing piece in our jigsaw that has 3 of the same sides and a 4th we've never seen. That's odd let's look together to try and find it." Modeling of the early universe to today relies on some unknown amount of CP violation occurring and we know we see it in particle physics which describes everything we see eat and touch. So why the amounts are so different (orders of magnitude different) keeps some scientists up at night.

by rob_c 4y ago

pdonis 4y ago

> How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. As I understand it, the "behavior" of dark matter in Kroupa's analysis is the behavior that is claimed for it by dark matter proponents. So if dark matter doesn't behave the way dark matter proponents say it does, the burden would be on the dark matter proponents to modify their models to say how it does behave.

rob_c 4y ago
Trust me this is a straw man picking at a hole of a yet incomplete theory to claim the whole deck of cards is invalid. AFAIK none of these alternatives offer a serious explanation of gravitational lensing for instance (yet, I'm happy for someone to make a theoretical breakthrough that explains away the need for an unknown new particle, but that just opens more questions around CP violation and so on)
- pdonis 4y ago
  > Trust me Why? Who made you an infallible authority? > none of these alternatives The basic point I described is not proposing an alternative to dark matter, and I'm not asking for a refutation of Kroupa's well-known preference for alternative gravity theories. I'm just pointing out that Kroupa has described a prediction of the dark matter hypothesis that does not appear to match observations. I'm wondering if any proponent of the dark matter hypothesis has a counter argument to this: either an explanation of why the dark matter hypothesis doesn't make the prediction Kroupa is describing, or an explanation of why that prediction does match observations after all. This seems to me to be a perfectly reasonable question that does not require any discussion of alternative theories at all.
  - rob_c 4y ago
    > Who made you an infallible authority? I'm not infallible but I do have a PhD in the study of CPV in particle physics so I understand this at a level enough to pass a doctoral viva. Alternate gravity theories struggle with gravitational lensing. This is a proven observable optical phenomena in the study of the universe. Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. This is colloquially referred to as Dark Matter. A gravitational (matter like) effect, which doesn't interact optically via standard EM as we can see the galaxies and stars. Hence the name Dark Matter.
    - pdonis 4y ago
      > Alternate gravity theories Are not what I'm asking about here. I know Kroupa pushes them, but that's not the part of Kroupa's work that I'm asking about. > Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. Not in the specific item I'm asking about. In that specific item, he's not talking about any alternative theories of gravity at all. He's pointing out what he claims is a conflict between the predictions of the dark matter hypothesis and actual observations. I've already explained what a counter argument to that would look like. Do you have one?
      - rob_c 4y ago
        Unfortunately you are getting a little crossed from your perspective. You're asking a question that to experts is structured oddly because you can't disentangle things you're trying to separate. I'll try to explain. Again dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models. There "classical dark matter hypothesises" is just that. That these observations can be explained by a quantizable matter candidate of some form with some yet to be determined properties. The only non particle descriptions of "dark matter phenomena" is a modified gravity framework. These attempt to address the "classical dark matter phenomena" whilst disentangling it from the "CPV phenomena". This is fine but breaks the big bang model at the stage of baryogenesis (matter formation from pure energy). There is the possibility that you can explain "classical dark matter phenomena" without demanding it also be responsible for CPV. However, if you do that the CPV step of the big bang model demands additional particle candidates which then raises the problem of what do they look like and where are they and do they interact with the Higgs field and therefore have mass. There are no counter arguments to "classical dark matter phenomena" because they are just that. Observed natural phenomena which do not fit into our current understanding of the universe. A "complete dark matter theory" tends to explain CPV+DM. These again don't make many testable predictions other than a (potentially TeV scale) weekly interacting particle candidate. So the testable prediction is that a dark matter candidate exists and we can observe it. > I've already explained what a counter argument to that would look like. Do you have one? I'll choose to read that in a relaxed manner, albeit it's a bit direct. There is no counter argument to natural phenomena. The main 2 theories are that dark matter particles exist and we can find them or we can't. MOND et al make few predictions and struggle to observe defined "gravitational phenomena", hence why there is little interest in them. There are differences between DM particle models but almost all agree that the major premise is that we might/should be able to see certain particle types given how they interact.

nico 4y ago

Interestingly, a similar argument (ie. “if it exists, it should behave like this, and it doesn’t”) was used to disprove the existence of the aether[1]. 1: Michelson-Morley experiment: https://physics.stackexchange.com/a/472611 https://physics.stackexchange.com/a/472611

HelloNurse 4y ago
We consider particles with oscillatory behaviour sufficiently different from aether to abandon the name and the concept as useless. But contrary to aether, dark matter is still a new, vague and complicated concept: there is ample room to evolve dark matter theories without giving up on the idea of dark matter. In any case competing theories will need to converge towards new experimental evidence and therefore towards each other, and then names and starting principles will be unimportant.
rob_c 4y ago
Yes, and it is a very nice experiment. However, "dark matter" is an admittedly lose framework it could be due to 1 new type of particles, 10, or higher dimensional effects and new physics we don't understand yet. Saying "therefore it doesn't exist" is ignoring the mountain of decades old solid data showing gaps in our knowledge exist. And as for dark energy... "The universal expansion is accelerating". Well done you now know as much as 99% of the physicists in the world on this strange unexplained problem.
stefantalpalaru 4y ago

somenameforme 4y ago

Your statement implies dark matter cannot be falsified by a contradicting observation, because it (dark matter) hasn't yet been observed. But if it doesn't exist, then it will never be observed. So it, therefore, can never be falsified? If I'm straw manning you, it is unintentional. How would you propose dark matter be falsified?

alrlroipsp 4y ago
The idea is to propose a model that does not need dark matter to describe observations.
- rob_c 4y ago
  Unfortunately a lot of the models try to remove DM completely from either a gravity or CP perspective and both have direct consequences as to what sort of stable structures in the universe that allow to form. And the models often conflict with other observations showing theyre no better. It's asif Newton observed an extra solar comet and therefore decided the Principia must otherwise all be wrong. It's a big step to say shinny thing in sky makes the Apple not fall as we observe it.
rob_c 4y ago
Dark matter as a particle has to have certain properties in order to satisfy what we know about a) it's EW interactions and b) the fact we haven't seen it yet. Over the next 20 years or so we will likely cover a huge amount of the phase space of direct observation of dark matter like candidates assuming they weakly interact with the standard model. If we observe it, great we now know a huge chunk more than ever before of the universe. If we fail to observe it then we have to go away and explain the Dark Matter problem another way. Axions are my personal favourite here because I claim nature for her beauty is sometimes horrible. But this could (in the most scary scenario) potentially be non quantisable interactions between unknown aspects of the universe. At that point we need to further our understanding of potential graviton candidates to evolve our theories of spacetime. DM isn't a solid prediction like an aether it's a statement of a big gap in our knowledge that has implications to asto and particle physics models. The big problem is then CP violation (why more matter than not, real why are we here at all stuff). This can't be explained by the standard model and stuff we can see and touch as far as we know. So we assume that this huge amount of it that we can't explain must come from interactions with new particles somewhere in some way at some energy scale. Physicists try to narrow down that phase space of where and how but a lot of this stuff makes the search for the higgs, look like cheating and watching where your parents hid the Easter eggs. There is scary little to directly go on so theoreticians have to get clever. Edit: There is also little to say that extra-SM CP violation _must_ be from the same source as Dark Matter astro candidates. The combination of the 2 fields is much more a car of. "Hmm we both have a missing piece in our jigsaw that has 3 of the same sides and a 4th we've never seen. That's odd let's look together to try and find it." Modeling of the early universe to today relies on some unknown amount of CP violation occurring and we know we see it in particle physics which describes everything we see eat and touch. So why the amounts are so different (orders of magnitude different) keeps some scientists up at night.