5 ms·

Does Dark Matter Affect the Motion of the Solar System?

40 points by GrantS 12y ago

karmakaze 12y ago
I have a question--please let me know if I'm way off base here. Gravity distorts space and we postulate the existence of dark matter from the distortion of space without corresponding mass/matter. Why must space without matter/mass be non-distorted to begin with?
- sarciszewski 12y ago
  Empirical observations support distortion of spacetime. See also, the gravitational lensing effect observed during a solar eclipse.
  - sarciszewski 12y ago
    Okay, why the hell was THAT one downvoted? https://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun https://en.wikipedia.org/wiki/Tests_of_general_relativity#De...
    - DanBC 12y ago
      Because the question is "Why must space without matter/mass be non-distorted to begin with?" Which your answer doesn't seem to answer. (I didn't downvote you)
      - sarciszewski 12y ago
        Oh you're right! I seem to have misread the question. Apologies.
- tjradcliffe 12y ago
  > Why must space without matter/mass be non-distorted to begin with? It doesn't have to be. De Sitter came up with solutions to the equations of GR that were free of ordinary matter but included a cosmological constant that is the effect of an "inherent" curvature of space-time: https://en.wikipedia.org/wiki/De_Sitter_universe https://en.wikipedia.org/wiki/De_Sitter_universe However, the kind of curvature you get from a cosmological constant is not sufficient to explain everything that is observed, so even though modern cosmologies typically have a "dark energy" term (which is another name for a cosmological constant) they require an extra matter term (dark matter) to describe the universe we see.
  - karmakaze 12y ago
    Great responses--different observations and types of curvature to explain. I realize now that my question is inherently flawed--where space 'is' distorted and we don't find ordinary matter we still need some explanation matterlike or otherwise. I also found out about MOND and TeVeS "which try to account for the anomalous observations without requiring additional matter" although minor, does probe in a different direction altogether.
- cozzyd 12y ago
  Evidence for dark matter is not just from gravitational lensing. As for your question, I'm no cosmologist, but my understanding is that we have not observed any curvature unassociated with mass ( we believe the universe to be flat on large scales).
cozzyd 12y ago
On the other hand, the motion of the solar system is essentially why we may be able to measure dark matter at Earth. If dark matter is made up of WIMPs, then our ability to observe its direct (non-gravitational) effects depends heavily on the velocity distribution function of dark matter at Earth. Because the non-dark matter component of the galaxy formed a disk while the dark matter halo will likely have remained roughly spherical, we expect that Earth is moving through the dark matter halo at O(230 km/s). This has a few important impliciations. First, it increases the expected kinetic energy of WIMPs in our frame, thus resulting in more energetic WIMP-nucleon interactions, making direct detection (which essentially consists of having a bunch of material and waiting for an unexplained interaction within it) significantly more feasible. Secondly, it leads to an annual modulation effect (as reportedly, controversially. observed by DAMA or COGENT) related to the Earth's motion around the sun. Thirdly, it means that recoils from interactions with WIMPs will have a preferred direction away from the direction of the Earth's propagation through the galaxy. Because no potential terrestrial background can have this signature, a number of groups, including the one I work with, are trying to develop detectors sensitive to the direction of low-energy recoils in order to potentially unambiguously detect WIMPs.
omgitstom 12y ago
Interesting article, I was hoping that dark matter would prove to be observable in our solar system. I'm more curious about dark energy though. Hopefully in my lifetime, we can figure out what dark energy is. I'm curious what dark energy does to time since it accelerates the expansion of space. The next golden age of science resides in our understanding of the unknown matter and energy that we are currently observing.
- InclinedPlane 12y ago
  It's highly likely that within the next decade or two we will have a very good idea about what dark matter is actually made of and have direct observational evidence of it. The most likely candidate for dark matter is a hypothesized supersymmetric particle that happens to be weakly interacting, there are several good candidates for such. But these particles should be detectable in future particle colliders and should give rise to characteristic signals in x-ray/gamma ray emissions from galaxies as well as perhaps being directly detectable in "dark matter wind" type experiments (which leverage the significant difference in speed relative to the galactic disk the Earth experiences at different times of the year). Dark energy is still a rather large mystery, but it's possible someone will make a big breakthrough at some point.
  - lurcio 12y ago
    Dark Matter, or dark energy: i can't but think its this era's ether. That is, an impasse waiting for a sizeable conceptual clarification/shake up.
    - InclinedPlane 12y ago
      That's a common sentiment among people who aren't terribly familiar with the scientific background for dark matter. I would urge you to go take a look at the wikipedia entry on dark matter and read through it, it's a good overview of a lot of the lines of evidence. The fact is that scientists have always been reluctant to believe in the existence of some exotic, new form of matter that hasn't been seen or directly detected, but in trying to disprove its existence or to prove that it's really just something else they've consistently, time after time, been stuck with that as the only option that explains the evidence. And there is quite a lot of evidence, from cosmological simulations to gravitational lensing studies to galactic rotational curves and so much more. If dark matter is what we think it probably is then it would actually make for a fairly elegant explanation. The overwhelmingly likely story is that dark matter is a kind of "supersymmetric" particle which requires very high energy conditions to create, conditions which do not generally exist much anymore, and which interacts very weakly or not at all with electromagnetic and strong nuclear forces. Highly energetic conditions were quite common in the early period of the Universe just after the big bang though, and if it were at all possible for such weakly interacting particles to exist then they would have been created in abundance, sapping a lot of the total mass/energy away from the Universe as they "fell out" of the energetic soup of the Universe. Eventually the Universe would have cooled enough to cause creation of such particles to halt, after which it would have gone on to create the more ordinary matter we know today (photons, electrons, protons, etc.) Fortunately we're likely to get a lot more data on the subject in the next few decades. As I mentioned our capabilities of directly testing the existence of dark matter particles are increasing to the point where within one to two decades we will likely have direct confirmation of their existence. I put the chances of dark matter being something other than a weakly interacting massive particle to be fairly low, though not impossible of course. As for dark energy, the evidence for the accelerating expansion of the Universe is much newer and the hypotheses attempting to explain it much more circumspect, I wouldn't be surprised if it turned out to be something other than the crude guesses we've made at present.

40 points by GrantS 12y ago

I have a question--please let me know if I'm way off base here. Gravity distorts space and we postulate the existence of dark matter from the distortion of space without corresponding mass/matter. Why must space without matter/mass be non-distorted to begin with?

sarciszewski 12y ago
Empirical observations support distortion of spacetime. See also, the gravitational lensing effect observed during a solar eclipse.
- sarciszewski 12y ago
  Okay, why the hell was THAT one downvoted? https://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun https://en.wikipedia.org/wiki/Tests_of_general_relativity#De...
  - DanBC 12y ago
    Because the question is "Why must space without matter/mass be non-distorted to begin with?" Which your answer doesn't seem to answer. (I didn't downvote you)
    - sarciszewski 12y ago
      Oh you're right! I seem to have misread the question. Apologies.
tjradcliffe 12y ago
> Why must space without matter/mass be non-distorted to begin with? It doesn't have to be. De Sitter came up with solutions to the equations of GR that were free of ordinary matter but included a cosmological constant that is the effect of an "inherent" curvature of space-time: https://en.wikipedia.org/wiki/De_Sitter_universe https://en.wikipedia.org/wiki/De_Sitter_universe However, the kind of curvature you get from a cosmological constant is not sufficient to explain everything that is observed, so even though modern cosmologies typically have a "dark energy" term (which is another name for a cosmological constant) they require an extra matter term (dark matter) to describe the universe we see.
- karmakaze 12y ago
  Great responses--different observations and types of curvature to explain. I realize now that my question is inherently flawed--where space 'is' distorted and we don't find ordinary matter we still need some explanation matterlike or otherwise. I also found out about MOND and TeVeS "which try to account for the anomalous observations without requiring additional matter" although minor, does probe in a different direction altogether.
cozzyd 12y ago
Evidence for dark matter is not just from gravitational lensing. As for your question, I'm no cosmologist, but my understanding is that we have not observed any curvature unassociated with mass ( we believe the universe to be flat on large scales).

cozzyd 12y ago

On the other hand, the motion of the solar system is essentially why we may be able to measure dark matter at Earth. If dark matter is made up of WIMPs, then our ability to observe its direct (non-gravitational) effects depends heavily on the velocity distribution function of dark matter at Earth. Because the non-dark matter component of the galaxy formed a disk while the dark matter halo will likely have remained roughly spherical, we expect that Earth is moving through the dark matter halo at O(230 km/s). This has a few important impliciations. First, it increases the expected kinetic energy of WIMPs in our frame, thus resulting in more energetic WIMP-nucleon interactions, making direct detection (which essentially consists of having a bunch of material and waiting for an unexplained interaction within it) significantly more feasible. Secondly, it leads to an annual modulation effect (as reportedly, controversially. observed by DAMA or COGENT) related to the Earth's motion around the sun. Thirdly, it means that recoils from interactions with WIMPs will have a preferred direction away from the direction of the Earth's propagation through the galaxy. Because no potential terrestrial background can have this signature, a number of groups, including the one I work with, are trying to develop detectors sensitive to the direction of low-energy recoils in order to potentially unambiguously detect WIMPs.

omgitstom 12y ago

Interesting article, I was hoping that dark matter would prove to be observable in our solar system. I'm more curious about dark energy though. Hopefully in my lifetime, we can figure out what dark energy is. I'm curious what dark energy does to time since it accelerates the expansion of space. The next golden age of science resides in our understanding of the unknown matter and energy that we are currently observing.

InclinedPlane 12y ago
It's highly likely that within the next decade or two we will have a very good idea about what dark matter is actually made of and have direct observational evidence of it. The most likely candidate for dark matter is a hypothesized supersymmetric particle that happens to be weakly interacting, there are several good candidates for such. But these particles should be detectable in future particle colliders and should give rise to characteristic signals in x-ray/gamma ray emissions from galaxies as well as perhaps being directly detectable in "dark matter wind" type experiments (which leverage the significant difference in speed relative to the galactic disk the Earth experiences at different times of the year). Dark energy is still a rather large mystery, but it's possible someone will make a big breakthrough at some point.
- lurcio 12y ago
  Dark Matter, or dark energy: i can't but think its this era's ether. That is, an impasse waiting for a sizeable conceptual clarification/shake up.
  - InclinedPlane 12y ago
    That's a common sentiment among people who aren't terribly familiar with the scientific background for dark matter. I would urge you to go take a look at the wikipedia entry on dark matter and read through it, it's a good overview of a lot of the lines of evidence. The fact is that scientists have always been reluctant to believe in the existence of some exotic, new form of matter that hasn't been seen or directly detected, but in trying to disprove its existence or to prove that it's really just something else they've consistently, time after time, been stuck with that as the only option that explains the evidence. And there is quite a lot of evidence, from cosmological simulations to gravitational lensing studies to galactic rotational curves and so much more. If dark matter is what we think it probably is then it would actually make for a fairly elegant explanation. The overwhelmingly likely story is that dark matter is a kind of "supersymmetric" particle which requires very high energy conditions to create, conditions which do not generally exist much anymore, and which interacts very weakly or not at all with electromagnetic and strong nuclear forces. Highly energetic conditions were quite common in the early period of the Universe just after the big bang though, and if it were at all possible for such weakly interacting particles to exist then they would have been created in abundance, sapping a lot of the total mass/energy away from the Universe as they "fell out" of the energetic soup of the Universe. Eventually the Universe would have cooled enough to cause creation of such particles to halt, after which it would have gone on to create the more ordinary matter we know today (photons, electrons, protons, etc.) Fortunately we're likely to get a lot more data on the subject in the next few decades. As I mentioned our capabilities of directly testing the existence of dark matter particles are increasing to the point where within one to two decades we will likely have direct confirmation of their existence. I put the chances of dark matter being something other than a weakly interacting massive particle to be fairly low, though not impossible of course. As for dark energy, the evidence for the accelerating expansion of the Universe is much newer and the hypotheses attempting to explain it much more circumspect, I wouldn't be surprised if it turned out to be something other than the crude guesses we've made at present.