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

Dark matter and dark energy

  1. Nov 30, 2011 #1


    User Avatar

    If it existed a particle that had such properties that it both pushed away other particles of the same kind and it pushed away ordinary matter ,could this then explain dark matter and dark energy ? If the big voids of the universe was filled with such a particle, the net force of the particles in the universe would be outwards ,so the expansion would accelerate.

    If particles of this type surrounded a galaxy,they would push the edges of that galaxy inwards, so that the stars far from the center of that galaxy would increase their speed ,and it has been observed that the stars in a galaxy have a greater velocity than expected.

    In a cluster of galaxy this type of particle could push the galaxies together, allowing them to have a greater velocity than expected without escaping from the cluster,and this greater than expected speed has been observed.

    This particle could also explain the big voids in the universe,it would be a natural consequence of the particles properties.
  2. jcsd
  3. Nov 30, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Dark matter is not repulsive, it is attractive through gravity both with itself and to normal matter. The speed of stars orbiting a galaxy is only one phenomenon believed to be caused by dark matter. There is also things like Gravitational Lensing that must be explained.

    Also I don't believe this would fit the models for the expansion of the universe. For example, in your case the repulsive force would decrease over time at the same rate as gravitation does due to the average density of the universe decreasing. Instead we see that as the density decreases and gravity becomes weaker, the repulsive force does not. Hence we see increasing acceleration.
  4. Dec 1, 2011 #3


    User Avatar
    Science Advisor

    Unfortunately it isn't that easy. First, as Drakkith noted, dark matter produces pretty large gravitational potential wells: it attracts matter quite strongly. You also won't get dark matter that clumps if it doesn't attract itself, and the dark matter we observe is pretty clumpy.

    As for dark energy, the problem there is that matter that pushes itself away has the wrong sign to pressure. Matter that pushes itself away would experience positive pressure. Dark energy has negative pressure.
  5. Dec 4, 2011 #4
    Does not exists any particle that can explain the properties of dark matter or dark energy alone, less still both at once.
  6. Dec 4, 2011 #5


    User Avatar
    Science Advisor

    Uh, what? Dark matter is easily explained by a WIMP, of which many theoretical ideas have been proposed. Dark energy is most likely to be the cosmological constant, which is not a particle. But there are many scalar fields that could also do the same thing.
  7. Dec 6, 2011 #6
    I did mean a real particle. Of course, you can imagine hypothetical particles with all properties that you want.
  8. Dec 6, 2011 #7


    User Avatar
    Science Advisor

    (Leaving dark energy out of this, for the moment...)
    1. No particle in the standard model explains dark matter. So if it's a particle, it has to be a new particle we haven't seen in the lab before. The most reasonable numbers for a particle that would explain our cosmological observations predict that it would be extremely hard to detect anyway, so this isn't a surprise.
    2. Non-particle solutions to the dark matter problem are far more exotic than particle solutions.

    So the most reasonable conclusion is that it's probably a WIMP.
  9. Dec 7, 2011 #8
    That was my point.

    A lot of modern theoretical 'physics' is so safe because focuses on the experimental range that goes from the «extremely hard to detect» up to the limit of the undetectable :rolleyes:

    A more reasonable conclusion is that dark matter is a fictitious distribution of mass and thus WIMPs do not exist, what means that people's money would not be wasted in their search.
  10. Dec 7, 2011 #9


    User Avatar
    Gold Member

    So which of the following is it that you believe, or is it both
    (1) physicists are so stupid that they are incorrect in their conclusions regarding the existence of dark matter AND the likelihood that it is WIMPS
    (2) physicists are so corrupt that they pretend dark matter exists so they can get research money to pretend to study it.

    Oh ... there is a third possibility:

    (3) you don't know what you are talking about.
  11. Dec 7, 2011 #10


    User Avatar
    Science Advisor

    To that I say look at the evidence. It is quite conclusive.
  12. Dec 7, 2011 #11


    User Avatar
    Gold Member

    Chanlnoth, you are SO much more polite than I am. I should follow your example, and I DO try most of the time, but sometimes I find that, in the words of my favorite poet, e e cummings, "there is some s***, up with which I will not put".
  13. Dec 7, 2011 #12


    User Avatar
    Science Advisor

    Don't let this one example fool you ;)
  14. Dec 8, 2011 #13
    Effectively the several premature claims of detection have gone (although are recorded by historians of science :uhh: ) and stuff as WIMPs continue without being real particles...
  15. Dec 8, 2011 #14


    User Avatar
    Science Advisor

    What's your point? The astrophysical and cosmological observations are firm and verified.
  16. Dec 9, 2011 #15
    And another premature claim of observation of dark matter has gone when latest Fermi studies have found no trace of dark matter (once again)


    But not true believer in dark matter would worry because all the tests of direct observation have failed... because the 'observations' are so «firm and verified» as the 'observations' of caloric were :rofl:
  17. Dec 9, 2011 #16


    User Avatar
    Science Advisor

    I see you are completely ignoring the cosmological evidence such as WMAP or gravitational lensing studies.
  18. Dec 9, 2011 #17
    Evidently neither WMAP nor gravitational lensing are direct observations of dark matter {*}. That is the reason which experiments as that reported by Physics World (link given before) have been performed and all they have failed. Not a surprise that each experiment looking for dark matter, WIMPS, and all that stuff has failed, since dark matter is so fictitious as caloric was centuries ago. And recall that many clever guys said to the ignorants that caloric had been observed in hundred of observations and experiments :rofl:

    {*} In fact, discrepancies between observation and theory are, a posteriori, interpreted as resulting from some hypothetical invisible matter.

    To say more, since you seem ignorant of the history of the scientific topic that your pretend to know. The first prediction of what WMAP would detect was done by a non-DM theory. Whereas the predictions done by DM-theory (lambda 1999 model) miserably failed and only after the observations were obtained that the DM model was modified (pure curve fitting) to adapt it to the WMAP observations a posteriori.

    However, the non-DM model predicted exactly the first and second peaks, i.e., before the observations.
    Last edited: Dec 9, 2011
  19. Dec 9, 2011 #18


    User Avatar
    Science Advisor

    Then what, pray tell, is the explanation?

    This is pure, unadulterated fantasy. The dark matter model used in nearly all WMAP data analysis is the simplest possible: zero-temperature dark matter. This model fits the data exceedingly well. There is no curve fitting done with respect to dark matter in analyzing the WMAP data, except in terms of estimating the values of the various parameters. The simplest model for WMAP uses the following parameters:

    1. Dark matter density.
    2. Normal matter density.
    3. Hubble expansion rate.
    4. Scalar spectral index (an inflation parameter).
    5. Optical depth to the CMB (basically, how transparent the universe is between us and the CMB).
    6. Amplitude of fluctuations.

    Each one of these parameters is physically-motivated. There is no "curve fitting" going on. Each one of the parameters which can be estimated independently with other experiments shows the same result for those parameters. The various extensions of this minimalist model, such as allowing spatial curvature to vary, or allowing the scalar spectral index to change with scale, so far show no evidence of deviation from this simplest description.
  20. Dec 10, 2011 #19
    What you call «pure, unadulterated fantasy» is just the orange line in the next figure.


    The orange line is the prediction done by the dark-matter model before WMAP data (and Boomerang data) was known. The dark matter model was clearly falsified... unless you are blind.

    The prediction done by the non-dark matter model for both first and second peaks was remarkably verified... unless again you are blind. The third peak could not be explained because the model used then was non-relativistic, but some recent relativistic extensions seem to fit the third as well.

    Once the WMAP data was known, DM cosmologists did an exercise in curve fitting (all of dark matter is pure curve fitting) and changed the parameters of the dark matter model until fitting the data.

    Moreover, when forcing the dark matter model to fit the available data, DM cosmologists chose values for its free parameters that contradict other independent tests. For example, the baryon density assumed in the current dark matter cosmological model is much larger (about 3x) that the baryon density measured from other independent tests.

    However, if you do not assume this high value of the density then you cannot fit the WMAP data using the dark matter cosmological model, generating a contradiction.

    Therefore we have a contradictory dark matter model, which fails to explain many data (TFL, fine tunning of galactic rotation curves...) and whose hypothetical new kind of matter has been systematically not found in a large list of experiments performed since the 80s. I cited the recent results of Fermi not finding any evidence of the hypothetical dark matter, but before Fermi was Xenon100 the experiment that found nothing and before was...
    Last edited: Dec 10, 2011
  21. Dec 10, 2011 #20


    User Avatar
    Science Advisor

    Two points on this:
    1. Did you even look at the parameters for the no dark matter case? Generally the parameters for these types of "alternative" theories are completely and utterly falsified by other observations.
    2. WMAP has since measured the third peak pretty accurately, completely breaking the degeneracy between dark matter and the other parameters. The dark matter model fits, models without dark matter do not.

    Edit: Oh, and let me just point out that the baryon density discrepancy is trivially explained by just noting that most of the normal matter has not fallen into gravitational potential wells and is thus invisible.
  22. Dec 11, 2011 #21
    This remind me of a typical anti-evolutionist argument that says that fossils were put by God for testing the faith of the Christians.

    It is not fascinating that whereas the DM prediction (red orange) failed miserably, the non-DM theorists were so lucky that used false values of parameters just to predict the result of the WMAP before was known. Those non-DM guys would play bonoloto all the weeks

    As you said the evidence for dark matter is firm. It is so firm that numerous experiments designed to find it have all failed to find it merely for improving the true faith of DM-believers.

    The measurement of the third peak is not so accurate as you believe and new experiments are being prepared to measure it with more accuracy. Of course, the third peak in no way proves that the dark matter is correct.

    Edit: Oh and I believed that the baryon density discrepancy still remains unexplained in the literature, but you can give a reference on the contrary please.
  23. Dec 11, 2011 #22


    User Avatar

    Staff: Mentor

    juanrga, please provide citations to demonstrate that your view has at least some currency in professional circles. If this is just your view, then publish it somewhere and make it part of professional discussion, and then we can consider it here.

    If legitimate controversy already exists in professional circles, then it is fair game for discussion here; but this is not the place to start such controversy.
  24. Dec 11, 2011 #23


    User Avatar
    Science Advisor

    You are truly demonstrating a shocking ignorance of the science you are so keen to criticize.

    There is no unique power spectrum predicted for the CMB given the existence of dark matter. The specific power spectrum you get depends upon the amount of dark matter (and other things). However, dark matter produces an extremely distinct signature on the CMB power spectrum: it suppresses the even-numbered peaks with respect to the odd-numbered peaks.

    This is why I went on about the third peak and why its measurement is important. It is possible to fudge the first and second peaks merely by adjusting the parameters which control how rapidly the overall power spectrum drops at smaller angular scales. But once you've measured the third peak, this degeneracy is broken, as now you can directly measure the overall decay of the power spectrum (by comparing the first and third peaks), and then if the second peak is comparatively low against this overall decay, then that is a definitive and conclusive signature of dark matter.

    And that is precisely what we see in the data.
  25. Dec 11, 2011 #24
    What you call «my view» is a well-known astrophysics/cosmological topic that has received large coverage even in science news services.

    For instance, you can find some coverage about the most recent evidence against dark matter

    http://news.sciencemag.org/sciencenow/2011/02/more-evidence-against-dark-matte.html?ref=ra [Broken]


    Regarding the figure in my post #19 and my claim that the dark matter model predictions were falsified (orange line in that figure), take a look to http://xxx.lanl.gov/abs/astro-ph/0008188 (Astrophys.J. 541, 2000, L33-L36), you would find the first figure in page 10, which gives the correct prediction using a model without dark matter (No-CDM), whereas the second figure gives the well-known fiasco of the prediction done using a dark matter model (Lambda-CDM).

    A review of a cosmology without dark matter is given in Class. Quant. Grav. 26: 143001, 2009 (http://arxiv.org/abs/0903.3602) although I do not like TeVeS.

    If you search in this thread you would find my posts alluding to the null results by Xenon100 and by Fermi experiments in their search for the [STRIKE]aether[/STRIKE] dark matter. If you do not know what Fermi is or what measures, please search my posts here and follow the link to Physics World.

    Now, it would be fine :rolleyes: if you also ask to Chalnoth to provide references supporting his many bold claims. He has just replied now to me but once again he refuses to provide references albeit I am asking to him for at least one...
    Last edited by a moderator: May 5, 2017
  26. Dec 11, 2011 #25


    User Avatar
    Science Advisor

    What a bunch of steaming crap. Not one shred of evidence is presented here.

    Except it doesn't fit. Not when you include the third peak. It is simply not possible to fit the first three peaks without dark matter.

    I'd also point out that he simply has the wrong parameter values. That can be forgiven the paper's author because these parameters were not known in detail when this paper was written. The parameters are, however, known in detail today.

    What, you want a textbook in basic cosmology? Here is how the various parameters impact the power spectrum of the CMB:
    Last edited by a moderator: May 5, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook