Notices
Results 1 to 53 of 53
Like Tree4Likes
  • 1 Post By Strange
  • 1 Post By Strange
  • 1 Post By Strange
  • 1 Post By Strange

Thread: Dark Matter

  1. #1 Dark Matter 
    Forum Radioactive Isotope
    Join Date
    Feb 2009
    Posts
    3,790
    I am embarassed (as usual ) to ask but could it be possible that the areas in the sky that show up as dark matter (which I have seen on some recent charts) are in fact receeding at speeds greater than c , the speed of light?

    I understand that the universe is supposed to be expanding and that some galaxies are far enough distant that their part in the expansion means that they are "moving" relative to one another at speeds greater than the speed of light.

    (I appreciate that my "theory" has probably been entertained elsewhere and dismissed but I have not heard about it....)


    Last edited by geordief; November 25th, 2013 at 07:00 AM.
    Reply With Quote  
     

  2.  
     

  3. #2  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    The trouble is that the effects of dark matter are seen within our own galaxy and nearby galaxies, and nearby galaxy clusters. So they can't be receding at any significant speed.


    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  4. #3  
    Forum Radioactive Isotope
    Join Date
    Feb 2009
    Posts
    3,790
    thanks. There is no answer to that!
    Reply With Quote  
     

  5. #4  
    Suspended
    Join Date
    Nov 2013
    Posts
    238
    Is there any dark matter in our solar system?
    Reply With Quote  
     

  6. #5  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Ninja Pancakes View Post
    Is there any dark matter in our solar system?
    Almost certainly (but we haven't detected any yet).
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  7. #6  
    Suspended
    Join Date
    Nov 2013
    Posts
    238
    Why almost certainly?
    Reply With Quote  
     

  8. #7  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Ninja Pancakes View Post
    Why almost certainly?
    Because it is distributed throughout the galaxy and it would be a little odd if the was a "hole" where we are.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  9. #8  
    Suspended
    Join Date
    Nov 2013
    Posts
    238
    So the issue is that the effect is so small, we can't distinguish it at the scale of the solar system, but in a galactic scale, there's something happening that we can't explain without it?
    Reply With Quote  
     

  10. #9  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Ninja Pancakes View Post
    So the issue is that the effect is so small, we can't distinguish it at the scale of the solar system, but in a galactic scale, there's something happening that we can't explain without it?
    Exactly. There is enough dark matter within the galaxy to affect the rotational velocities of stars. But, because the galaxy is so huge, the overall density is very low and so has no noticeable effect within the solar system. And attempts to directly detect dark matter particles have, so far, been unsuccessful.
    sir ir r aj likes this.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  11. #10  
    Suspended
    Join Date
    Nov 2013
    Posts
    238
    Is there some kind of map of the distribution of dark matter in a galaxy?

    What is the difference between the distribution of dark matter in a spiral galaxy and the distribution of dark matter in a globular cluster?
    Reply With Quote  
     

  12. #11  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Ninja Pancakes View Post
    Is there some kind of map of the distribution of dark matter in a galaxy?
    https://en.wikipedia.org/wiki/Galaxy...nsity_profiles

    Annoyingly, I have never found a good graphical representation.

    What is the difference between the distribution of dark matter in a spiral galaxy and the distribution of dark matter in a globular cluster?
    Good question.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  13. #12  
    Forum Freshman Cudamerica's Avatar
    Join Date
    Nov 2013
    Location
    U.S
    Posts
    21
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Ninja Pancakes View Post
    Why almost certainly?
    Because it is distributed throughout the galaxy and it would be a little odd if the was a "hole" where we are.

    Well isn't it something like everything we see (stars, planets, everything on earth, pretty much everything observable thus far) only composes 4% of the Universe? The rest is either dark energy or dark matter. Given random distribution under that percentage I'm assuming it would be nearly impossible to not have at least SOME dark matter or energy in our solar system.
    Reply With Quote  
     

  14. #13  
    Forum Freshman
    Join Date
    Dec 2013
    Posts
    15
    my speculation is it possible,that dark energy is Only the lack of any matter in the vast area outside our universe i.e. the space past the galaxies ,our "viewable universe"is nothing more than an empty vacuum that is pulling normal matter outward causing us to see a view from here as "expansion" like water spreads out when poured on a flat surface, and at all times with no resistance moving ever faster ,when all our visible universe is just rushing outward to fill the infinite void of space? may there also be various sized black holes and undiscovered things like black holes throughout space 'invisible' to us? causing the dark matter effect of holding galaxies and solar systems etc together ? while at all times matter is just trying to fill the emptiness of space?
    Last edited by kreep; December 8th, 2013 at 02:18 PM.
    Reply With Quote  
     

  15. #14  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    I applaud your description of the above as a speculation (rather than a theory). How would you go about testing this idea? I don't mean personally, but supposing you had access to any of the currently available observing methods.
    Reply With Quote  
     

  16. #15  
    Forum Radioactive Isotope
    Join Date
    Feb 2009
    Posts
    3,790
    .
    Last edited by geordief; December 9th, 2013 at 08:51 AM.
    Reply With Quote  
     

  17. #16  
    Forum Freshman Laurieag's Avatar
    Join Date
    Apr 2013
    Posts
    93
    Quote Originally Posted by Strange View Post
    Annoyingly, I have never found a good graphical representation.
    This is one possible graphical representation of how you could increase the circular velocity of a source at large heights above the mid plane (by using R2 or R3 instead of R).



    Maybe some of the more informed forum members could put forward their own graphical interpretations of what actually is the difference between the dark matter paper and the no dark matter paper as per the abstract below.

    [1205.4033] On the local dark matter density


    An analysis of the kinematics of 412 stars at 1-4 kpc from the Galactic mid-plane by Moni Bidin et al. (2012) has claimed to derive a local density of dark matter that is an order of magnitude below standard expectations. We show that this result is incorrect and that it arises from the assumption that the mean azimuthal velocity of the stellar tracers is independent of Galactocentric radius at all heights. We substitute the assumption, supported by data, that the circular speed is independent of radius in the mid-plane. We demonstrate that the assumption of constant mean azimuthal velocity is implausible by showing that it requires the circular velocity to drop more steeply than allowed by any plausible mass model, with or without dark matter, at large heights above the mid-plane. Using the approximation that the circular velocity curve is flat in the mid-plane, we find that the data imply a local dark-matter density of 0.008 +/- 0.003 Msun/pc^3 = 0.3 +/- 0.1 GeV/cm3, fully consistent with standard estimates of this quantity. This is the most robust direct measurement of the local dark-matter density to date
    Reply With Quote  
     

  18. #17  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Laurieag View Post
    Quote Originally Posted by Strange View Post
    Annoyingly, I have never found a good graphical representation.
    This is one possible graphical representation of how you could increase the circular velocity of a source at large heights above the mid plane (by using R2 or R3 instead of R).
    Thanks. But it was a simple graphical representation of the density of dark matter I was looking for (I have only seen various graphs and diagrams showing orbital velocity of gas and stars). I originally started looking to counter the belief some people have that, when astronomers talk about a "halo" of dark matter they mean a ring of dark matter (like modern representations of angels) rather than a sphere of dark matter (as in the original representations of halos).
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  19. #18  
    Forum Freshman Laurieag's Avatar
    Join Date
    Apr 2013
    Posts
    93
    Quote Originally Posted by Strange View Post
    Thanks. But it was a simple graphical representation of the density of dark matter I was looking for (I have only seen various graphs and diagrams showing orbital velocity of gas and stars). I originally started looking to counter the belief some people have that, when astronomers talk about a "halo" of dark matter they mean a ring of dark matter (like modern representations of angels) rather than a sphere of dark matter (as in the original representations of halos).

    I hope nobody mistakes the dark area in my graphic for dark matter as the distinction being made was with regards to the differences in velocities obtained by using a different radius of rotation, the back calculation of the mass from different orbital velocities based on the different radius used, and the measurement of dark matter densities or not.


    That's what was so unusual about the wording of the abstract, it makes a distinction about assumptions of mean azimuthal velocity, circular velocity and dependence on galactocentric radius, explicitly related to large heights above the mid plane and then uses an approximation based on flat velocity curves in the mid plane to obtain 'the most robust direct measurement of the local dark-matter density to date'.


    Incidentally, I was looking at [1210.1294v1] The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole and noted that, while the period of the star was 11.5 years, no velocity or radius was given for various reasons. Using a rough circular approximation (not a Keplerian orbit) that takes into consideration the ratio of the velocity to c i.e. distance travelled in one rotation period = 11.5 * C = (2 * Pi * R) * c/v, where C is the distance traveled by light in a year and v is the orbital velocity. R = (11.5 * C * v)/(2 * Pi * c), and plotting the radius of rotation R vs a range of velocities v (as a fraction of c), reveals a linear relationship.
    Reply With Quote  
     

  20. #19  
    Suspended
    Join Date
    Oct 2013
    Posts
    1,839
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Ninja Pancakes View Post
    Is there any dark matter in our solar system?
    Almost certainly (but we haven't detected any yet).
    I do not understand what you mean here, how can you be almost certain when you have not detected any yet?
    Reply With Quote  
     

  21. #20  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Stargate View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Ninja Pancakes View Post
    Is there any dark matter in our solar system?
    Almost certainly (but we haven't detected any yet).
    I do not understand what you mean here, how can you be almost certain when you have not detected any yet?
    It appears to be evenly distributed throughout and around the galaxy (with an increasing density towards the centre). There may be local variations in this density but it would be oddly implausible if we happened to be in a area where there was a complete absence of it.

    Gravitational effects (which is how dark matter is normally identified) would be far too small to measure within the solar system. We have not yet detected dark matter directly.
    sir ir r aj likes this.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  22. #21  
    Forum Freshman Laurieag's Avatar
    Join Date
    Apr 2013
    Posts
    93
    Nobody has mentioned Galaxy rotation curve - Wikipedia, the free encyclopedia yet.

    The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the Newtonian-Keplerian prediction, assuming a centrally-dominated mass associated with the observed luminous material. When masses of galaxies are calculated solely from the luminositiesand mass-to-light ratios in the disk, and if core portions of spiral galaxies are assumed to approximate to those of stars, the masses derived from the kinematics of the observed rotation and the law of gravity do not match. This discrepancy can be accounted for by a large amount of dark matter that permeates the galaxy and extends into the galaxy's halo.
    Reply With Quote  
     

  23. #22  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Laurieag View Post
    Maybe not explicitly, but isn't that assumed as soon as one says "dark matter"?
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  24. #23  
    Suspended
    Join Date
    Oct 2013
    Posts
    1,839
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Stargate View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Ninja Pancakes View Post
    Is there any dark matter in our solar system?
    Almost certainly (but we haven't detected any yet).
    I do not understand what you mean here, how can you be almost certain when you have not detected any yet?
    It appears to be evenly distributed throughout and around the galaxy (with an increasing density towards the centre). There may be local variations in this density but it would be oddly implausible if we happened to be in a area where there was a complete absence of it.

    Gravitational effects (which is how dark matter is normally identified) would be far too small to measure within the solar system. We have not yet detected dark matter directly.
    I take it that this is purely speculative, right? to be very honest with you I am not sure if I understood any of what you said here.
    Reply With Quote  
     

  25. #24  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by Stargate View Post
    I take it that this is purely speculative, right? to be very honest with you I am not sure if I understood any of what you said here.
    No, it is all based on evidence.

    There are two main ways of observing dark matter; both based on its gravitational effects.

    Firstly it has an effect on the speed at which stuff (stars, dust, gas) orbits in galaxies and the speed at which galxies orbit one another in clusters.

    Secondly, the presence of mass causes "gravitational lensing" (light is bent by the effects of gravity).

    Both of these can be used to measure the density and distribution of dark matter.

    They both show that dark matter is everywhere in the universe. There is more where there are concentrations of matter (e.g. in and around galaxies).

    As we are in a galaxy, we are almost certainly surrounded by dark matter.

    But we can only detect the effects of dark matter on large scales (galaxies and galaxy clusters). The effect of dark matter on the orbits of the planets in the solar system is too small to measure.

    Is that clearer?
    sir ir r aj likes this.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  26. #25  
    Suspended
    Join Date
    Oct 2013
    Posts
    1,839
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Stargate View Post
    I take it that this is purely speculative, right? to be very honest with you I am not sure if I understood any of what you said here.
    No, it is all based on evidence.

    There are two main ways of observing dark matter; both based on its gravitational effects.


    Firstly it has an effect on the speed at which stuff (stars, dust, gas) orbits in galaxies and the speed at which galxies orbit one another in clusters.

    Secondly, the presence of mass causes "gravitational lensing" (light is bent by the effects of gravity).

    Both of these can be used to measure the density and distribution of dark matter.

    They both show that dark matter is everywhere in the universe. There is more where there are concentrations of matter (e.g. in and around galaxies).

    As we are in a galaxy, we are almost certainly surrounded by dark matter.

    But we can only detect the effects of dark matter on large scales (galaxies and galaxy clusters). The effect of dark matter on the orbits of the planets in the solar system is too small to measure.

    Is that clearer?
    Yes, absolutely.
    Reply With Quote  
     

  27. #26  
    Forum Freshman Laurieag's Avatar
    Join Date
    Apr 2013
    Posts
    93
    Quote Originally Posted by Strange View Post
    Maybe not explicitly, but isn't that assumed as soon as one says "dark matter"?
    I don't doubt that you know that Strange and, considering the wide variety of assumptions people make on this site, I didn't want anybody thinking otherwise.


    That's why I referred to the 11.5 year period star close to our own galactic center in my other post in this thread. As the galactic rotation curve drops off substantially towards the center, and the radius of rotation of the short period star shrinks according to a linear relationship with its circular velocity, there are just as many issues with calculating the velocity of stars close to our galactic center (should be faster or expanded) as there are with stars that are distant from our galactic center (should be slower or contracted).
    Reply With Quote  
     

  28. #27  
    Forum Freshman
    Join Date
    Dec 2013
    Posts
    7
    One thing I have always been curious about with dark matter. When we look around our own galaxy we can only observe matter that is compressed and clumpy like gas or dust clouds with the addition of stars that are nothing more than super compressed hydrogen and other gasses super heated and put off light. What we detect is the photons in the way of light or heat or xrays or any other number of ways we can detect matter at a distance. Doesn't all this require heat or reactions from compression? What is stopping dark matter from actually being normal matter that is just too spread out and too cold to be detected. For instance if all the space in between stars was full of hydrogen but at such a low density and temperature its virtually undetectable? Haha sorry if this is a stupid question but I really am curious.
    Reply With Quote  
     

  29. #28  
    Forum Senior
    Join Date
    Sep 2012
    Location
    Western Australia
    Posts
    319
    Doesn't all this require heat or reactions from compression? What is stopping dark matter from actually being normal matter that is just too spread out and too cold to be detected.
    i think if that were the case then we would see absorption lines as more distant radiation passed through that region on its way to our detectors. also we have a pretty good handle on the amount of baryonic matter in the universe and this rules out DM being that. there is just too much.
    Sometimes it is better not knowing than having an answer that may be wrong.
    Reply With Quote  
     

  30. #29  
    KJW
    KJW is online now
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,431
    Quote Originally Posted by Chrispen Evan View Post
    Doesn't all this require heat or reactions from compression? What is stopping dark matter from actually being normal matter that is just too spread out and too cold to be detected.
    i think if that were the case then we would see absorption lines as more distant radiation passed through that region on its way to our detectors. also we have a pretty good handle on the amount of baryonic matter in the universe and this rules out DM being that. there is just too much.
    One thing that tells us that dark matter doesn't interact electromagnetically is that there is no scattering of background light sources.
    There are no paradoxes in relativity, just people's misunderstandings of it.
    Reply With Quote  
     

  31. #30  
    Forum Freshman
    Join Date
    Oct 2013
    Location
    Poconos, PA
    Posts
    15
    The concept that the Universe is expanding at a faster rate due to "Undetectable Dark Mater" sounds too much like the Theory of Ether. It is based on the idea that there is not enough detectable mass within galaxies for gravity to hold them together. Why is is so hard to imagine that over 14 billion years the majority of large suns have run out of fuel to burn and are now dead masses of carbon that emit no light and no radiation for us to detect. Furthermore, there are probably 10 times as many dead planets and an uncountable number of asteroids out there that are not detectable.


    Does lack of Mass Create Time ????
    Reply With Quote  
     

  32. #31  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by tsafa View Post
    The concept that the Universe is expanding at a faster rate due to "Undetectable Dark Mater" sounds too much like the Theory of Ether.
    It sounds like something you have made up. The universe is not expanding because of dark matter. And dark matter has been detected.

    Does lack of Mass Create Time ????
    No!!!!
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  33. #32  
    Genius Duck Moderator Dywyddyr's Avatar
    Join Date
    Jan 2013
    Location
    Scunthorpe, UK
    Posts
    11,701
    Quote Originally Posted by tsafa View Post
    It is based on the idea that there is not enough detectable mass within galaxies for gravity to hold them together.
    No it's not.
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
    Reply With Quote  
     

  34. #33  
    Forum Freshman
    Join Date
    Oct 2013
    Location
    Poconos, PA
    Posts
    15
    I heard that there is a popular theory that say that Dark Mater is pushing the Universe apart. I heard the theory a Second Time on a Science Show "How the Universe Works". They said that the way Dark Mater has been detected is by watching light bend through space. Similar to the way light has been observed to bend around the sun during an eclipse.

    Why jump to the conclusion that there is some mystical dark matter bending the light and not just an ordinary dead sun or small black hole?
    Reply With Quote  
     

  35. #34  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by tsafa View Post
    I heard that there is a popular theory that say that Dark Mater is pushing the Universe apart.
    Apparently, you misheard. That would be dark energy.

    Why jump to the conclusion that there is some mystical dark matter bending the light and not just an ordinary dead sun or small black hole?
    Because if there was that much normal matter / black holes then it would be readily detectable. We are not talking about an extra 10% or so, which could be missed. We are talking about many times more dark matter than visible matter.

    We can see light being bent but we can't see the thing bending the light. What does that tell you?

    And there is nothing "mystical" about dark matter. We just don't know what it is yet. That is normal for science.

    In 1781 it was noticed that there were irregularities in the orbit of Uranus. It was proposed there might be another "invisible" planet or planets that caused the effect. 65 years later, Neptune ("the original dark matter") was discovered and explained the perturbations.

    In 1930 Pauli hypothesized that there might be an undetected particle emitted in beta radiation to explain conservation of energy, spin and momentum. 26 years later, the "invisible" neutrino was detected.

    Why are you so sceptical about the normal progress of science?
    KJW likes this.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  36. #35  
    Forum Sophomore
    Join Date
    Dec 2013
    Location
    2720'06.53"N 8232'48.35"W
    Posts
    176
    Is dark matter concidered mystery matter? Or is it more likely regular matter distributed in interstellar space?
    Reply With Quote  
     

  37. #36  
    Forum Sophomore
    Join Date
    Dec 2010
    Posts
    117
    Quote Originally Posted by geordief View Post
    I am embarassed (as usual ) to ask but could it be possible that the areas in the sky that show up as dark matter (which I have seen on some recent charts) are in fact receeding at speeds greater than c , the speed of light?

    I understand that the universe is supposed to be expanding and that some galaxies are far enough distant that their part in the expansion means that they are "moving" relative to one another at speeds greater than the speed of light.

    (I appreciate that my "theory" has probably been entertained elsewhere and dismissed but I have not heard about it....)
    Dude - you need to read about comoving distance
    Reply With Quote  
     

  38. #37  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    I think that dark matter doesn't exist, and it was just made up ad hoc to fudge the standard theory, but somehow occam's razor was not applied, im not sure why.
    Reply With Quote  
     

  39. #38  
    Bullshit Intolerant PhDemon's Avatar
    Join Date
    Feb 2013
    Location
    Newcastle-upon-Tyne, UK
    Posts
    5,234
    So how do you account for the observed gravitational effects? Or are you just another loon who disagrees with current theory because they don't understand it?
    Reply With Quote  
     

  40. #39  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by 514void View Post
    I think that dark matter doesn't exist, and it was just made up ad hoc to fudge the standard theory
    Unlike, say, MOND which is just an ad-hoc fudge to the standard theory. And doesn't explain all the evidence.

    What would you prefer scientists do? Ignore new data? Pretend there is nothing wrong with current models?

    Science doesn't progress by sticking its fingers in its ears and shouting "can't hear you" at the universe.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  41. #40  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    Quote Originally Posted by PhDemon View Post
    So how do you account for the observed gravitational effects? Or are you just another loon who disagrees with current theory because they don't understand it?
    I don't understand it completely, how is there some stuff that makes up 95% of the universe but it is undetectable? i am not brimming with confidence about this one.
    Reply With Quote  
     

  42. #41  
    Bullshit Intolerant PhDemon's Avatar
    Join Date
    Feb 2013
    Location
    Newcastle-upon-Tyne, UK
    Posts
    5,234
    Well unless you can come up with an alternative that explains:

    Dark matter - Wikipedia, the free encyclopedia

    at least as well as current theory I couldn't give a damn about your confidence levels.
    Reply With Quote  
     

  43. #42  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by 514void View Post
    I think that dark matter doesn't exist, and it was just made up ad hoc to fudge the standard theory
    Unlike, say, MOND which is just an ad-hoc fudge to the standard theory. And doesn't explain all the evidence.

    What would you prefer scientists do? Ignore new data? Pretend there is nothing wrong with current models?

    Science doesn't progress by sticking its fingers in its ears and shouting "can't hear you" at the universe.
    I suppose that alternative theories should be looked at.
    It seems that every time the data doesn't agree with the hypothesis, the hypothesis is propped up with new variables.
    Reply With Quote  
     

  44. #43  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    Quote Originally Posted by 514void View Post
    I don't understand it completely, how is there some stuff that makes up 95% of the universe but it is undetectable? i am not brimming with confidence about this one.
    We do not know how it exists, or even if, for certain it does exist. What we do know is that it currently the best explanation for the anomalous rotation rates of stars within galaxies. These rotation rates can be explained if there is a large amount of matter not present in the form of stars, planets, assorted debris, gas clouds, dust clouds, black holes, and the like. Why is it undetectable by other than its gravitational effects? We don't know yet. However, our ignorance is never a good reason for rejecting an idea that fits the facts.

    It seems that every time the data doesn't agree with the hypothesis, the hypothesis is propped up with new variables.
    Apart from the perjorative use of "propped up", this exactly right. This is what science is. This is how science works. This is why science is successful. (And it is amazingly successful.) If you did not prveiously understand this, then you did not previously understand science.

    Welcome to the real world. (And to the forum.)
    Reply With Quote  
     

  45. #44  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    Welcome to the real world. (And to the forum.)[/QUOTE]

    cheers.
    Reply With Quote  
     

  46. #45  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by 514void View Post
    I suppose that alternative theories should be looked at.
    I'm sorry, but that makes no sense. Instead of looking at alternative theories, they should look for alternative theories?

    Dark matter was initially a name for an unexpected set of observations. A large number of "alternative theories" (actually, hypotheses, but we'll let that pass) were proposed. These can probably be divided into three broad classes:

    1. Modifications to the way gravity works: MOND is the best known, but there are also relativistic versions. Unfortunately none of these really fit the data. You can kludge them to work for some data, but then they fail for others. And the observation of gravitational lensing by dark matter pretty much kills this one.

    2. Some form of "stuff" that we can't see. There have been at least a dozen different ideas in various classes such as hot, warm and cold dark matter. The evidence currently favours some form of cold dark matter. There are probably half a dozen or more suggestions of what these weakly-interacting massive particles could be: super-symmetric particles, sterile neutrinos, mirror matter, etc. Tests have been proposed to narrow this down. Some have already been ruled out. Experiments are being performed.

    Note that dark matter as "stuff" also solves a number of other problems in cosmology so it is definitely the favourite right now.

    3. Other. There are various more "out there" ideas such as general relativistic fluid that would unify darkl matter and dark energy; a novel quantum field; blah blah blah ...

    So, with all these alternative theories out there, what you want is an "alternative theory"? Got it.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  47. #46  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    Quote Originally Posted by Strange View Post
    So, with all these alternative theories out there, what you want is an "alternative theory"? Got it.
    yes, one that predicts the "dark matter" inherently within the system, without the need for extra ad hoc bits.
    Reply With Quote  
     

  48. #47  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by 514void View Post
    yes, one that predicts the "dark matter" inherently within the system, without the need for extra ad hoc bits.
    I'm not sure what you think is ad hoc. If it turns out that a new, modified, theory of gravity is required to explain it then we will have a new theory that includes the effects currently labelled "dark matter".

    If (when) we detect the particles which make up dark matter as stuff, then we will have a new theory that includes dark matter. This might be one of the currently proposed extensions to the standard model (e.g. super symmetry) or something completely new (that, therefore, we can't yet predict).

    As John says, that is how science works. Once upon a time, some unexpected behaviour was noticed in the motion of Uranus. It was hypothesized (in an ad hoc manner, according to you) that there might be another planet. A few decades later, Neptune ("the original dark matter") was detected.

    Then, it was noticed that the behaviour of Mercury wasn't quite as expected. All sorts of "ad hoc" theories were proposed (undiscovered planets, new forces, etc). In the end a new theory of gravity (GR) provided the answer.

    Then, it was noticed that there was a problem with conservation of energy in certain types of nuclear decay. All sorts of "ad hoc" theories were proposed (including a suggestion that perhaps conservation of energy didn't always apply - so much for those blinkered, closed minded scientists). One of these ridiculous "ad hoc" ideas was a new particle. Experiments were suggested and, several decades later, the neutrino was discovered.

    I really don't know what you want from science. This is how it works: look at the data, propose some hypotheses, test them, eventually you have a new theory. You might wish for a shortcut but I'm afraid there aren't any.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  49. #48  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    What I meant is that newton had a good theory that worked for alot of things, but then when it started to not predict things as good, a new theory was invented that explained things better.
    Reply With Quote  
     

  50. #49  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Quote Originally Posted by 514void View Post
    What I meant is that newton had a good theory that worked for alot of things, but then when it started to not predict things as good, a new theory was invented that explained things better.
    That was one of the examples I used above. There were several competing new theories. They were tested against the data and, in the end, all but one rejected.

    And that is the process we are going through now. So I still don't see what is wrong. It is science doing its thing.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  51. #50  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    Cool, but the process seem unscientific.
    Reply With Quote  
     

  52. #51  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    To me it looks exactly the same as the process whenever new, unexpected observations are made.

    BTW, thanks for engaging in this discussion. I have heard a few people make similar objections but have never been able to get them to explain anything beyond "we need something new".

    Well of course we do! And that is what people are trying to come up with. It might turn out to be a new theory of gravity (looking unlikely now), it may be some new subatomic particle(s), it might be something we just haven't thought of yet. But it will be new!
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  53. #52  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    162
    cool, I wanna be around for the next epic theory that changes everything.
    Reply With Quote  
     

  54. #53  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,825
    Me too. But you are more likely to be around!

    I shouldn't be greedy. There have been at least three major paradigm shifts in different areas of science in my lifetime.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

Similar Threads

  1. Black holes, dark matter & dark energy
    By Cuete in forum Astronomy & Cosmology
    Replies: 12
    Last Post: May 3rd, 2013, 05:33 PM
  2. Dark Matter, Dark Energy and the Hyperbolic Gravitational Field
    By Gary Anthony Kent in forum Astronomy & Cosmology
    Replies: 58
    Last Post: April 28th, 2013, 03:17 PM
  3. Replies: 5
    Last Post: March 20th, 2013, 12:12 AM
  4. Dark energy, Dark matter, Fine tuning problem,Negative mass!
    By icarus2 in forum Personal Theories & Alternative Ideas
    Replies: 18
    Last Post: July 31st, 2011, 12:12 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •