Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Measuring dark matter's repulsive force

  1. Jan 6, 2010 #1


    User Avatar
    Gold Member

    I'm trying to find the amount of repulsive force dark matter has on matter.

    To my understanding, the presence of dark matter is deduced from the fact that galaxies are being pushed away from one another instead of toward each other. The absence of matter between the galaxies suggests an unknown form of matter is causing this force.

    I'm trying to find or calculate the force that dark matter presents between two galaxies based upon the mass of each galaxy, distance apart and 'amount' of dark matter between the galaxies.

    Can anyone point me in the right direction?


  2. jcsd
  3. Jan 6, 2010 #2


    User Avatar
    Science Advisor

    You have the force backwards. Dark matter is considered to have exactly the same attractive force (gravity) as matter. Dark matter is inferred from the need to hold things together; not drive them apart. It works both to hold galaxies together more strongly together than would apply with only the matter we can see; and also to pull galaxies of the whole universe towards one another ... that is, dark matter slows down expansion of the universe in the same way as ordinary matter.

    Cheers -- sylas
  4. Jan 6, 2010 #3
  5. Jan 6, 2010 #4


    User Avatar
    Gold Member

    Thank you for the correction Sylas
  6. Jan 7, 2010 #5


    User Avatar
    Science Advisor

    Dark Energy is twice as repulsive as normal matter is attractive. Is that enough for you?
  7. Jan 7, 2010 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    As was said earlier, dark matter is attractive to itself and to ordinary matter. It collects in clumps and helps form galaxies and clusters of galaxies. It helps keep galaxies together in clusters. The effect of dark matter is to make each galaxy more massive (because the DM is clumped around the galaxy) and therefore more attractive to its neighbors in the usual Newtonian way.

    You were really talking about dark energy instead of DM, so would it be useful to try editing your original question and study a modified version?
    If that is what you want to ask about, then you could start by noting these basics. In the standard cosmology picture, dark energy has a constant uniform density of about 0.6 nanojoules per cubic meter---constant over all space and for all time. That is about 0.6 joules per cubic kilometer.

    Dark energy does not clump. It stays uniformly distributed throughout all space. (Even though we cannot see all space, the standard model assumes for simplicity that it all resembles what we can see---it all has the same amounts of dark energy and matter that is in the part we can see.)

    Dark energy does not thin out as distances expand. Matter and ordinary energy thins out, but not DE. It stays 0.6 nanojoules per cubic meter.

    If you look at any one object, like a galaxy, there is as much DE on one side as there is on the other. It would be impossible for DE to be "pushing" the galaxy in any one particular direction.

    These are just the basic premises of DE. You may want to think about these basics for a while. For most people they take some getting used to, since they are not especially intuitive. You are encouraged to ask more questions!
    Last edited: Jan 7, 2010
  8. Jan 7, 2010 #7


    User Avatar
    Science Advisor

    Well, and in our model uiniverse, there is as much matter on one side than on the other. So it can't pull a galaxy in a perticular direction, but it is still pulling all galaxies together, thus slowing the expansion.
    GR is not different from Newtonian gravitation in this respect, with one notable exception: in GR, you can prove that in a homogeneous universe, the dynamics of a spherical region is not affected by the rest of the universe. That renders the old, Newtonian argument for a static universe -...as there are masses everywhere, their net effect cancels out...- invalid.
    So there is one source for cosmological acceleration (or deceleration), and that's the trace of the Stress-Energy-Tensor.
    As long as there is no shear and no high velocities, this trace is at the base of all "normal" gravitational effects too, see e.g. http://math.ucr.edu/home/baez/einstein/einstein.html" [Broken]. There is simply no difference between cosmological deceleration and "normal" gravitational attraction - or, for that matter, between cosmological acceleration and not-so-normal gravitational repulsion.
    For "fluids", the source of gravitation is [itex]\rho + 3 p[/itex], which is [itex]\rho[/itex] for dust and [itex]-2\rho[/itex] for Dark Energy.
    Last edited by a moderator: May 4, 2017
  9. Jan 7, 2010 #8
    I would just make one point, and that is while DE may well be responsible for the ACCELERATION in the recession speed between galaxies, that shouldn't be confused with UNIVERSAL expansion. It's probably also worth noting that DE is a solution to a mathematical problem which is an excellent description of the forces at work, but really not much in the way of the SOURCE of that force. Theories centered on vacuum expectation energy fall far short of the needed energy, and other contenders depend on theories that lie outside of The Standard Model and GR.

    I guess the idea of a cosmological constant (although sudden acceleration 3 billion years ago isn't exactly CONSTANT) wasn't as silly as Einstein eventually came to believe. Granted this isn't quite the same thing, but damned if it isn't a bit creepy nonetheless.
  10. Jan 7, 2010 #9


    User Avatar
    Science Advisor

    It should be identified with universal expansion. The recession between widely separated galaxies is a universal phenomenon, and DE applies universally.

    There is no proposal of a "sudden acceleration". What is proposed is indeed a cosmological constant term. This refers to a constant energy density in the vacuum, as opposed to the energy density of matter, for example, which reduces as matter disperses. As matter disperses, the mean density of energy throughout the universe approaches in the limit the cosmological constant value. There's a smooth transition from deceleration to acceleration, in the flat ΛCDM model, as the density of matter drops and the dark energy density remains constant.

    Cheers -- sylas
  11. Jan 7, 2010 #10
    I stand corrected on two points, however, the expected vacuum energy is insufficient to explain the apparant effect of dark energy. I was however, completely wrong about the "sudden" terminology, and I was sorely wrong about the universal nature of the expansion. I stand corrected, and thank you Sylas.
  12. Jan 7, 2010 #11


    User Avatar
    Science Advisor

    No problem. You are delving into really interesting areas, and everyone needs correction as they start to explore the subject. Don't let that stop you! One my own favourite sayings:
    They say we learn from our mistakes. That must make me the smartest person in the world.

    With that in mind... here's another. The theoretically expected dark energy from quantum field theory is actually much much larger than what is needed to explain apparent effects in the present. The "expected vacuum energy" according to existing quantum field theory is enormous; about 120 orders of magnitude more than what is used in modern cosmology. This is a major unsolved problem in modern physics.

    And if I have that wrong, someone can correct me as well, please!

    Cheers -- sylas
  13. Jan 7, 2010 #12
    I thought it was roughly 100 order of magnitude less... Call me crazy but I'm going to go with your version!
  14. Jan 15, 2010 #13
    Re: Dark Matter Distribution

    I have always had a problem with the Dark Matter theory. In order to explain the unexpected higher velocity of the outer stars in a galaxy, it is theorized that there must be some unknown matter at the edge of those galaxies that causes the outer stars to orbit the center of their galaxy at a speed higher than what would be expected based on gravitational law. That doesn't pass the common sense test. Why would the dark matter always congregate at the outer edge of the galaxies? And why would it congregate in just the right amounts from galaxy to galaxy in order to maintain those higher outer star velocites? Well, the answer to the last question is it doesn't. I think dark matter may be just a property of space.

    Recent work by Univ of Maryland astronomer Stacy McGaugh indicates the ratio of normal matter to dark matter decreases as the size of a galaxy decreases and the relationship is systematic with scale. So large galaxies appear to have less dark matter for their size than the small galaxies. How does McGaugh come up with the information to measure dark matter to normal matter ratios between the galaxies? He compares the expected outer star velocities to that based solely on graviational law and the discrepancy is then assumed to be dark matter and from there the amount of dark matter is calculated so as to explain the outer star velocities. McGaugh's results showed a linear variation from 17% normal matter for large galaxies to only .2% normal matter for small galaxies. If one were to believe that dark matter is related to normal matter and is part of the development of a galaxy then those numbers should be fairly constant. But if "dark matter" is nothing more than a property of the continuum of space or a change in the property of gravity with distance then these numbers make more sense. It's almost like an effect that is relational to the amount of space occupied. Almost as if the universe has a built in gravitational constant or acceleration constant across all space. (Maybe those 'zero-point energy' guys are headed in the right direction.)
    Last edited: Jan 15, 2010
  15. Jan 15, 2010 #14

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    Re: Dark Matter Distribution

    No, it doesn't say that this matter is concentrated at the edge.

    That makes most of the rest of your post rather moot.
  16. Jan 15, 2010 #15
    I want to know where on earth you HEARD or read that dark matter is concentrated at the edges of anything. There is a well known map that purports to show the distribution of dark matter, and it's nothing like you describe. Beyond that... "Dark matter related to normal matter..." wel... in theory everything we percieve came from the same source, so yeah... Otherwise no, dark matter would be very unlike normal matter. Not to mention that dark matter (WIMPs perhaps) are the VAST majority, not "normal" matter... whatever that is.

    You know what, Vanadium 50 was right to ignore the rest of your post.
  17. Jan 16, 2010 #16
    What about conservation of energy (or energy-momentum four vector)?

    This sounds like spontaneous creation of matter (or energy-momentum four vector). Next you will be telling me that maggots form spontaneously inside rotting meat. :)
  18. Jan 16, 2010 #17
    energy is not conserved in cosmology
  19. Jan 16, 2010 #18
    But may be you're right, it is not conserved even locally (in a closed region)
  20. Jan 17, 2010 #19
    No, we are talking about DE
  21. Jan 17, 2010 #20
    Frame Dragger, check page 1.
    The title of the thread is incorrect, the poster actually meant DE, not DM
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook