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"Annila shows that, when gravitational lensing is analyzed with this concept, it does not require dark matter to explain the results." Both dark energy and dar

by J3L2404 15y ago

"Annila shows that, when gravitational lensing is analyzed with this concept, it does not require dark matter to explain the results." Both dark energy and dark matter seem contrived and virtually anything would be less hand-wavy. Just for aesthetic reasons I hope these are found to be the modern equivalents of "ether" as a medium for EM waves.

jonnathanson 15y ago
Something's always struck me as very "ethereal" about the concepts of dark matter and dark energy, and I'd be inclined to agree with you. Sooner or later, a robust theoretical model will come along that suffices without either of these constructs, which will be the kiss of death for them.
- badger7 15y ago
  They're ethereal by design - they're more like placeholders for gaps in our understanding than they are concepts in their own right, they're there to be removed or explained.
bluedanieru 15y ago
Dark energy especially so, I think. I do agree with you somewhat, but dark matter at least is not so preposterous, simply being matter that only interacts with other matter via the gravitational force.
- J3L2404 15y ago
  Yes, I think you have a point. Galaxy rotation is going to be hard explain without some missing mass.
  - dgottlieb 15y ago
    Now I'm merely trying to raise awareness, not offer an argument, but there are alternate gravitational theories for galaxy rotation (that I believe are more elegant, but at the expense of breaking more things than it fixes) such as MOND. http://en.wikipedia.org/wiki/Modified_Newtonian_Dynamics http://en.wikipedia.org/wiki/Modified_Newtonian_Dynamics Dark matter being less elegant in the galaxy rotation problem being a) it hasn't been detected and b) if it does exist, why does it fall into the same halo distribution pattern in different galaxies to account for the same rotational observation of these aforementioned spiral galaxies. But as I said before, MOND seems to break more physics than it solves, but much smarter people that actually work in physics are trying to put those pieces together which makes it legitimate enough for me to take seriously. Edit: Actually reading who's responding to what, I learned you're more than well aware of this :) I like to believe that galaxy rotation is solved with a single theory and not a significant combination of explanations (Occam's Razor). In any event I'll leave this comment here as an awareness booster for younger dabbling cosmologists.
splat 15y ago
I have to disagree with you about dark matter and dark energy seeming contrived. First it's hard to call dark energy "contrived" when it hasn't really been contrived, conceived, or created at all. That is, no one has really made any strong, convincing claims as to what it actually is. The term is really just a placeholder for the fact that something is providing a long-scale repulsive force (and so probably has negative energy density). Although it can be "explained" by a cosmological constant, that just punts the question to what the cosmological constant physically is. As for dark matter, it is the simplest, most elegant way to explain the variety of observations that point to its existence. These observations include small-scale dynamics of dwarf spheroidal galaxies, the rotation curves of galaxies, gravitational lensing due to galaxy clusters, and more. All of them can be explained if there exists some unknown particle which interacts only weakly with other particles. While it's true that the standard model predicts no such particle, it's also true that it's known that the standard model is an incomplete theory. You might argue that it could just as well be that our theory of gravity is wrong. (Certainly at the quantum level we know for sure that that is the case.) But inaccuracies in our theory of gravity at large scales have difficulty explaining the dark matter problem in an elegant matter. Consider dwarf spheroidals. These are the smallest of galaxies; in terms of luminous mass, they are only slightly more massive than globular clusters. Yet globular clusters are 1000s of times smaller than dwarf spheroidals. How can they possibly remain gravitationally bound on the scale of 1000s of light-years when globular clusters are only bound on scales of a few light-years? Dark matter can explain this by simply saying that dwarf spheroidals have relatively massive dark matter halos, whereas globular clusters have almost no dark matter. (Which one would expect in standard galaxy formation scenarios.) But modified gravity theories have difficulty explaining why two systems with similar masses interact so differently. We can similarly see that galaxies of all sizes exhibit a range of dark matter fractions. If the problem was merely with our theory of gravity one would expect all galaxies of a given luminous mass to have the same dark matter fraction. Any modified theory of gravity which explained these observations would not be aesthetic. The aesthetic explanation would be some hitherto unknown weakly interacting massive particle.
- J3L2404 15y ago
  >The term is really just a placeholder for the fact that something is providing a long-scale repulsive force How is that not contrived? >You might argue that it could just as well be that our theory of gravity is wrong. As an armchair physicist, I was rather hoping the Pioneer anomaly would lead us down that road, oh well. >Any modified theory of gravity which explained these observations would not be aesthetic. Well aesthetics do invoke subjective qualities. Relativity, modifying Newtonian physics, could have be seen as ugly by some. E = mc^2 is so elegant as to seem improbable.
lutorm 15y ago
"Dark energy" isn't that contrived. When Einstein formulated GR, he showed that when you set up the theory, there is an additional free parameter. People assumed that it was zero, but there was really no empirical reason for this. But it's just as much part of the equations of GR as matter-energy is. If a dark energy had been not been allowed by GR and still been observed, then the hand-waving would have started.

by J3L2404 15y ago

jonnathanson 15y ago

Something's always struck me as very "ethereal" about the concepts of dark matter and dark energy, and I'd be inclined to agree with you. Sooner or later, a robust theoretical model will come along that suffices without either of these constructs, which will be the kiss of death for them.

badger7 15y ago
They're ethereal by design - they're more like placeholders for gaps in our understanding than they are concepts in their own right, they're there to be removed or explained.

bluedanieru 15y ago

Dark energy especially so, I think. I do agree with you somewhat, but dark matter at least is not so preposterous, simply being matter that only interacts with other matter via the gravitational force.

J3L2404 15y ago
Yes, I think you have a point. Galaxy rotation is going to be hard explain without some missing mass.
- dgottlieb 15y ago
  Now I'm merely trying to raise awareness, not offer an argument, but there are alternate gravitational theories for galaxy rotation (that I believe are more elegant, but at the expense of breaking more things than it fixes) such as MOND. http://en.wikipedia.org/wiki/Modified_Newtonian_Dynamics http://en.wikipedia.org/wiki/Modified_Newtonian_Dynamics Dark matter being less elegant in the galaxy rotation problem being a) it hasn't been detected and b) if it does exist, why does it fall into the same halo distribution pattern in different galaxies to account for the same rotational observation of these aforementioned spiral galaxies. But as I said before, MOND seems to break more physics than it solves, but much smarter people that actually work in physics are trying to put those pieces together which makes it legitimate enough for me to take seriously. Edit: Actually reading who's responding to what, I learned you're more than well aware of this :) I like to believe that galaxy rotation is solved with a single theory and not a significant combination of explanations (Occam's Razor). In any event I'll leave this comment here as an awareness booster for younger dabbling cosmologists.

splat 15y ago

I have to disagree with you about dark matter and dark energy seeming contrived. First it's hard to call dark energy "contrived" when it hasn't really been contrived, conceived, or created at all. That is, no one has really made any strong, convincing claims as to what it actually is. The term is really just a placeholder for the fact that something is providing a long-scale repulsive force (and so probably has negative energy density). Although it can be "explained" by a cosmological constant, that just punts the question to what the cosmological constant physically is. As for dark matter, it is the simplest, most elegant way to explain the variety of observations that point to its existence. These observations include small-scale dynamics of dwarf spheroidal galaxies, the rotation curves of galaxies, gravitational lensing due to galaxy clusters, and more. All of them can be explained if there exists some unknown particle which interacts only weakly with other particles. While it's true that the standard model predicts no such particle, it's also true that it's known that the standard model is an incomplete theory. You might argue that it could just as well be that our theory of gravity is wrong. (Certainly at the quantum level we know for sure that that is the case.) But inaccuracies in our theory of gravity at large scales have difficulty explaining the dark matter problem in an elegant matter. Consider dwarf spheroidals. These are the smallest of galaxies; in terms of luminous mass, they are only slightly more massive than globular clusters. Yet globular clusters are 1000s of times smaller than dwarf spheroidals. How can they possibly remain gravitationally bound on the scale of 1000s of light-years when globular clusters are only bound on scales of a few light-years? Dark matter can explain this by simply saying that dwarf spheroidals have relatively massive dark matter halos, whereas globular clusters have almost no dark matter. (Which one would expect in standard galaxy formation scenarios.) But modified gravity theories have difficulty explaining why two systems with similar masses interact so differently. We can similarly see that galaxies of all sizes exhibit a range of dark matter fractions. If the problem was merely with our theory of gravity one would expect all galaxies of a given luminous mass to have the same dark matter fraction. Any modified theory of gravity which explained these observations would not be aesthetic. The aesthetic explanation would be some hitherto unknown weakly interacting massive particle.

J3L2404 15y ago
>The term is really just a placeholder for the fact that something is providing a long-scale repulsive force How is that not contrived? >You might argue that it could just as well be that our theory of gravity is wrong. As an armchair physicist, I was rather hoping the Pioneer anomaly would lead us down that road, oh well. >Any modified theory of gravity which explained these observations would not be aesthetic. Well aesthetics do invoke subjective qualities. Relativity, modifying Newtonian physics, could have be seen as ugly by some. E = mc^2 is so elegant as to seem improbable.

lutorm 15y ago

"Dark energy" isn't that contrived. When Einstein formulated GR, he showed that when you set up the theory, there is an additional free parameter. People assumed that it was zero, but there was really no empirical reason for this. But it's just as much part of the equations of GR as matter-energy is. If a dark energy had been not been allowed by GR and still been observed, then the hand-waving would have started.