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Dark matter

  1. Feb 8, 2014 #1
    What proof do we have that dark matter exists.
  2. jcsd
  3. Feb 8, 2014 #2


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    Why don't you just Google dark matter, or look it up in Wikipedia.
  4. Feb 8, 2014 #3
    Oh well, thx for that.
  5. Feb 8, 2014 #4


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    The moral of the story here is that, there is no reason to ask a question cold in this day and age. You need to first put in some effort to learn things on your own, and then seek help when you encounter a problem. Question such as this, when it can easily be researched online, just shows lack of effort.

  6. Feb 22, 2014 #5
  7. Feb 22, 2014 #6


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    He didn't say that!
  8. Feb 22, 2014 #7
    Lol. No he didn't say that, but that could be a possibility to why he asked this question. The answers that he were given sounded a little mean. That's all.
  9. Feb 22, 2014 #8


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    It's not unreasonable to expect a question that reflects a high school level of knowledge on PF.
  10. Feb 23, 2014 #9
    Thank you for the link.
  11. Feb 23, 2014 #10
    No problem
  12. Feb 23, 2014 #11


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  13. Feb 23, 2014 #12


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    Hi Manraj, welcome to PF! I think you said in another thread that your basis for gaining understanding of interesting questions like this is your 10th grade physics textbook. This is a really good question to be asking at this stage!

    The challenge for us other PF members is basically to go through a summary like the one Drakkith linked to, and interpret in simple everyday language--explain the basic physics content.
    Here is what he linked
    Wikipedia's Dark Matter article section on Observational evidence:

    3 Observational evidence
    3.1 Galaxy rotation curves
    3.2 Velocity dispersions of galaxies
    3.3 Galaxy clusters and gravitational lensing
    3.4 Cosmic microwave background
    3.5 Sky surveys and baryon acoustic oscillations
    3.6 Type Ia supernovae distance measurements
    3.7 Lyman-alpha forest
    3.8 Structure formation

    When I looked at some other questions you asked (as say 16 year old interested in physics) in other posts you struck me as probably pretty quick-witted (excuse the personal comment, hope not impolite) so I am wondering WHICH OF THOSE POINTS do you already understand and which would you like to have discussed?

    Personally I find points 3.3 and 3.8 the most fascinating. 3.3 is about GRAVITATIONAL LENSING. If there is a massive object like a cluster of galaxies in the foreground then light rays coming from sample objects in the background are bent and we can estimate the foreground object MASS from how the background is distorted.
    In particular 3.3 is about WEAK lensing which actually allows to make contour maps of varying density the DM clouds in the foreground. Weak lensing is fascinating type of shape distortion. round circular outlines tend to be flattened into ovals where the flattening is in the direction of increased density. So that STATISTICALLY you get more oval galaxies than you expect with their short width aligned towards the center of the invisible cloud and their longways axis more aligned tangentially along the "circumference" of the extra dense region.

    The computer analysis that goes into the construction of these weak lensing density maps may use the silhouette outline images of on the order of a hundred background galaxies. It is a great achievement. A famous case was published in 2006 and analyzed the DM density map resulting from a collision of two CLUSTERS of galaxies---the so-called "Bullet Cluster".

    I guess you need to tell us what of these 8 points of evidence you want to have some discussion of. I personally feel I have some intuition about some: like 3.1, 3.2, 3.3 and 3.8, and NOT on some of the others.
  14. Feb 23, 2014 #13


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    btw Manraj, I checked back on the question threads you started, since you joined PF two weeks ago:
    https://www.physicsforums.com/search.php?searchid=4037179 [Broken]

    Have to say congratulations, seriously. It is a pretty good list of questions, and you got answers to several of them. That's good for PF, I think, and good for the people who took the trouble to think thru how to explain the answer to an interested 10-grader. So thanks for keeping folks on their toes :biggrin:

    I especially liked your question about GRAVITATIONAL REDSHIFT, or gravitational time dilation. You were asking why a clock that it is deeper down in a gravitational potential well run slower.
    Why does a clock that is higher up or essentially out of the well run faster (other things being equal)?

    There is a pretty good Wikipedia discussion of that in the article on "gravitational redshift"


    Why don't you have a look and see if you have further questions? It should begin to clarify why something down near the event horizon of a black hole seems to a distant observer to be moving very slowly (basically it is the difference between the two clock rates: the high versus the low).
    Last edited by a moderator: May 6, 2017
  15. Feb 24, 2014 #14
    Thanks a lot, Marcus!
  16. Feb 24, 2014 #15


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    My pleasure, Manraj!
    BTW I have no reliable idea about what you can or can't understand. You said you are using a 10th grade physics textbook. You might be a 16 year-old learning basic physics at 10th grade level or you might be a 40-year old sea captain who just happened to pick up a high school textbook to read through as a review.

    I'm a little worried I might be talking "over your head" or giving you links to stuff that is too dense and technical to be helpful. You had better say if something is too much to read or over your head.
    And let people know if some explanation is at the right level and IS actually helpful.

    BTW have you heard of something called the "equivalence principle" in GR? You imagine an physicist in a box doing experiments (say with pendulums or springs or gyroscopes, whatever).
    The idea is he can't tell whether the box is in a uniform gravitational field (approximatey as if resting on the surface of a large planet, but even more uniform) or whether the box is way out in intergalactic space being steadily ACCELERATED by some means.

    The equivalence principle could be related to what you were asking about in the other thread (the upstairs clock runs faster than the downstairs clock: gravitational redshift or "time dilation").

    Or, if the box is out in "zero gravity" space being accelerated, the clock in the front end of the box runs faster than the clock in the rear of the box.
  17. Feb 24, 2014 #16
    Btw, since you ask, i am a fourteen year old who just happens to be really interested in physics.

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  18. Feb 24, 2014 #17


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    OK. Well please let me know if you have met the "equivalence principle" in any of your reading about relativity. I think it comes up in a lot of popular non-technical accounts. People draw cartoons to illustrate it etc etc.

    It's one of these "thought experiment" things. The guy can't tell the difference between the box being uniformly steadily accelerated and the box sitting in a uniform gravitational field.

    So suppose the box is being constantly accelerated and there are two guys, one in the front end and one in the back. They have lasers and they send each other light. Can you see why the guy in the back sees the front-guy's light slightly blue-shifted?

    Because the light takes a moment to travel and during that time the back-guy is moving faster and "coming to meet the light". And the front guy sees the back guy's light slightly red-shifted because due to the slight acceleration of the box (during the time the light was traveling) he is "backing away from the light".

    If the box were moving at constant speed then there wouldn't be any effect. It is the acceleration (while the light is traveling between them) that does it.

    Does that make sense?

    So then stop the box and put it on a large planet so there's a uniform gravitational field.
    the front guy becomes the "upstairs" guy. He send some light downstairs and when it gets there it is BLUE SHIFTED. (IT HAS TO BE BECAUSE OF THE EQUIVALENCE PRINCIPLE because it has to be just the same as when they were out in space with the box being accelerated)

    And when the downstairs guy sends some light upstairs the guy up there will detect that it has beens slightly redshifted. By climbing up the gravitational field, or up out of the potential well, or however you think of it.

    I'm curious to know if that makes sense to you. If you assume the equivalence principle, then does it make the gravitational redshift (which has been checked experimentally) acceptable?
    And might you have already met with this in some of your earlier reading?
  19. Feb 24, 2014 #18
    You know that actually made a lot of sense to me. I've learned a lot in the past 2 hours. Thanks for all the information. And since you were asking, i hadn't read about the equivalence principle before. But i had read something similar in Stephen Hawking's 'Theory of everything.'

    Sent from my iPad using Physics Forums
  20. Feb 24, 2014 #19


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    Good. The wikipedia article I think you said was helpful was


    You could also look at the wiki on "Gravitational time dilation". I think of G-redshift and G-timeslowing as the same phenomenon. So the two articles are both about the same thing! But I only linked to the G-redshift article because I thought it was better written, more complete and understandable.

    The downstairs light looks redder to the upstairs guy, and it actually is redder, to the receptors in his eyes and the atoms in his instruments, because it was made by downstairs atoms which were vibrating slower. The same atoms would vibrate faster upstairs with him. Natural processes actually happen at different rates depending on how deep you are in a gravitational potential well. This is kind of amazing!

    BTW I've been trying to understand a new model of black holes called "planck star" which depends crucially on this timeslowing or "time dilation". A new model is something to be approached at your own risk, and with caution. But I think it is interesting. The collapse of a star actually turns into a BOUNCE but we don't see the explosion because it is way in the distant future.

    From the star's point of view the collapse and rebound is very quick, measured in seconds or fractions of a second. But because the collapse creates extreme density in its own self-made deep deep gravitational potential well, there is extreme time dilation. So what happens in one second (for an imaginary clock if one could ride through the collapse and bounce) would take billions of years if timed by distant clock. Collapse, seen by distant observer and timed by his clock, can actually be a "self-slowing" physical process because it creates such extreme density.

    That's enough about that!!! It's a new idea, just being worked out. Hence risky.

    BTW you can get from the idea of gravitational timeslowing to the idea of gravitational light bending! You have some idea how a convex lens focuses light? thicker in the middle, and light goes slower in glass, so wavefronts advance in smaller steps in the middle. So light rays are bent towards the middle and come together in a focus?

    So when light passes close by a massive object like the sun it travels for a short time in a region where time is dilated, where physical processes happen a bit slower. Lightrays are bent in, as if by a lens. Well, the outer ring of a lens, the star itself blocks the central portion of the gravitational "lens".

    That is the connection with DARK MATTER. We see clouds of dark matter by their gravitational lensing effect on items in the background. And the lensing or light bending can be explained by time going slower in regions deeper down in a gravitational potential well (near a massive body or a comparatively dense cloud of DM)
    Last edited: Feb 24, 2014
  21. Feb 25, 2014 #20
    Thanks, Mr genius. In fact i just have a test coming up on refraction of light through lenses. I guess we have had enough of time dilation for now!
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