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I Dark matter/energy, why must it be there?

  1. Aug 29, 2017 #1
    Hey everyone. This question is coming from someone who is just a science enthusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

    To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

    My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason that I do not know?

    Best regards,

    Pedro
     
    Last edited: Aug 29, 2017
  2. jcsd
  3. Aug 29, 2017 #2

    thierrykauf

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    Incorporating dark matter and dark energy, is a way of saying our models are (were) wrong. However, save for the common adjective, dark energy and dark matter have opposite roles. Dark energy is as dark as black body is black: not at all. Dark energy is a vacuum residual energy density. This energy density appears in Einstein's equations with the same sign as a cosmological constant. That is, it contributes to the expansion of the universe (hence the name repulsive energy). It is necessary to provoke inflation, this hyper rapid expansion of the universe that preceded the big bang. Last, dark matter, for one, is really dark. The reason is that visible matter only accounts for 10% of the density of the universe. To wrap this up: physicists will never call an equation wrong until they can replace it with a better one.
     
  4. Aug 29, 2017 #3

    Janus

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    No. Dark energy is not required to explain why the galaxy is expanding. Science was quite content with an expanding universe long before the concept of Dark Energy was raised. The idea of dark energy didn't come into things until we carefully measured that expansion and noted that its rate had increased over time (We expected it to decrease over time. For the time being, "dark energy" more or less is a placeholder term for what causes this.
    For dark energy, a new model might prove to be the answer. For dark matter this is increasingly less and less likely to be the case. A new model of gravity would have to explain all the observations we have made. The most damning observation against a new gravity model is that of the Bullet cluster.

    The bullet cluster shows the result of a collision between two galaxy clusters. Now while we can't directly see dark matter, we can detect it by how its gravity bends light, which produces a gravitational lens effect. What we see with the Bullet cluster is a region of space with no visible matter, but does produce a gravitational lens. This is exactly what you would expect to see if the dark matter had been separated away from the visible matter in the collision. This is not an observation that can be made to fit into a different model of gravity, unless that model also includes dark matter.

    The case for particular dark matter has become increasingly stronger over time and the case for a modified model of gravity has become weaker.
     
  5. Aug 29, 2017 #4
    So, dark matter must be something that has mass but doesn't interact with light which makes it invisible.

    Thanks Janus, that made a lot of sense.
     
  6. Aug 29, 2017 #5

    CWatters

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    Dark matter does interfere with light (it bends it) but it doesn't form stars that produce light.
     
  7. Aug 29, 2017 #6
    And by that you mean scattered dust, as in halos surrounding many galaxies?
     
  8. Aug 30, 2017 #7

    thierrykauf

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    I'm not doing research in this area and I thought I could answer your question without checking with recent papers. I was wrong. So rather than sharing outdated information on the subject, let me direct you to papers that will help you better. For the status on the search (detection) of dark matter, I recommend1" whose abstract I quote verbatim here "Much like ordinary matter, dark matter might consist of elementary particles, and weakly interacting massive particles are one of the prime suspects. During the past decade, the sensitivity of experiments trying to directly detect them has improved by three to four orders of magnitude, but solid evidence for their existence is yet to come. We overview the recent progress in direct dark matter detection experiments and discuss future directions." As for theory, there is an over-abundance of proposals, which I will try to summarize for you in my next post.
    1 http://www.nature.com/nphys/journal/v13/n3/abs/nphys4039.html
     
  9. Aug 30, 2017 #8

    benorin

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    I'm yet another physics enthusiast (my training was in math) with a follow up question: we talk about detecting gravitational lensing of light from galaxies, the light is bent by more gravity than the visible matter in the area produces; my question is detection on a different scale. Can we detect the gravity on a smaller scale of particles? Could we look at a small region and calculate the gravity do to the regular matter in it and infer the presence of dark matter particles, like a detector? (I saw on Discovery Science channel program that they are using a purified xeon detector hoping for an interaction with dark matter shielded inside a mountain)

    -Ben Orin
     
    Last edited: Aug 30, 2017
  10. Aug 31, 2017 #9

    thierrykauf

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    The principle of equivalence means that gravity only acts at scales that are not infinitesimal, unlike any purely local interaction. That is why detection efforts focus on large scale events. Otherwise, your idea was correct. As for the Xeon detector, results from the Lux Dark Matter experiment which consisted of 332 live days were largely negative, with only three events identified as possible candidates for dark matter.
     
  11. Sep 1, 2017 #10
    Just so you have some numbers in head even if they're impossible to grasp
    Gravity is 10^38 times weaker than the strong force wich holds the neutrons and protons to form the nuclei
    So demonstrating thus "seeing" gravity at an atomic level is impossible with our curent technology
     
  12. Sep 1, 2017 #11

    Drakkith

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    Dark matter doesn't interact through any of the known fundamental forces other than gravity, so it cannot absorb, emit, or scatter EM radiation, no. Note that this kind of behavior isn't so unprecedented. Of all the fundamental particles, most do not interact through all 4 fundamental forces. Electrons do not interact via the strong force. Neutrinos do not interact via the strong for nor the electromagnetic force. It seems almost natural to find a particle which only interacts through gravity in my opinion.

    I don't think we could detect it by its gravitational interactions at that scale. Perhaps they were looking to see if there were any odd interactions that couldn't be explained by the standard model?
     
  13. Sep 6, 2017 #12
    I've read the book "Electric Universe". The book is for consideration only for those who are electrotechnically competent and at the same time have at least basic knowledge of astrophysics, plasma physics and nuclear physics. Unlike the author, I am a supporter of Big Bang, but I also have my own views that do not cover the current hypotheses of dark matter and dark energy.
    The dark mass of the universe are forming the proton clouds, which "failed" to connect with the electrons to the hydrogen, and the electron clouds and neutron clouds. This mass is therefore dark because it does not appear electromagnetically (we can not measure). Expanding the universe causes dark energy, the repelling electrical charges of the same sign.
    In the future, proton and electron clouds will overlap again, part of dark matter, protons and electrons will change to hydrogen and other molecules, and new stars formation will occur. After several oscillations of the electron cloud around the proton cloud, space expansion will stop, gravity and later collapse of the universe will take place in a small space.
     
  14. Sep 6, 2017 #13

    Drakkith

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    Unfortunately this is quite impossible. Non-neutral electromagnetically charged particles (in this case, protons and electrons not combined into neutral atoms) react strongly with electromagnetic radiation and we would see mountains of evidence for clouds of charged particles floating out in space. Nor would such clouds have anything to do with dark energy and the expansion of the universe. Beyond that, there is no known way for clouds of like-particles to form in the first place. All particle creation events obey charge conservation, so you would need to break a fundamental conservation law on a massive scale to get these clouds from local annihilation/creation events.

    Please note that we don't generally allow serious discussion of personal theories, ideas, or hypotheses. See PF Terms and Rules for more information.
     
  15. Sep 6, 2017 #14
    Very few cosmologists say it, but dark matter may not be matter at all. Evidence for "it" may be a clue that we do not understand gravity as well as we think we do. General relativity is really great but not complete. Dark matter may be the 21st century luminiferous aether--look for it as we may, it just isn't there.
     
  16. Sep 6, 2017 #15
    Your question is answered in Wiki:

    https://en.wikipedia.org/wiki/Dark_matter
     
  17. Sep 6, 2017 #16

    kimbyd

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    The short answer is that when the observations didn't line up, theorists worked in many different directions to try to explain them. As more and more observations have been collected, most of those alternatives have been proven to just not fit observation at all, or to require additional assumptions that make them unreasonably complicated. For now, the only viable models really seem to be dark matter and dark energy. There are still some theorists who are working on other alternatives, such as TeVeS for explaining dark matter as a feature of gravity. But so far they haven't been able to present a convincing case.
     
  18. Sep 7, 2017 #17
    You're right if you mean cloud of plasma. Thank you for your response.
     
  19. Sep 8, 2017 #18
    One aspect of dark matter has so far been absent from the discussion. I understand that the following is a bit of oversimplification.

    The ratio of the density of dark matter to ordinary matter (having quarks as constituents) is sufficiently large that it is known (with a high degree of certainty) that dark matter is not made of quarks. The reason for this conclusion is related to the density of deuterium measured by astronomical observation. It is known that some deuterium is consumed as stars produce energy by nucleosynthesis. All the deuterium that still exists now was created during the primordial nucleosynthesis period (PNP) which took place during the first few minutes following the big bang. If dark matter did consist of quarks, almost all of the deuterium that exists now would have been consumed in the PNP by the synthesis of helium.

    Here is one refinement detail. One current idea about what dark matter might be made of is black holes that were formed before the PNP. These primordial black holes would have been formed from quarks, but these quarks would not be able to interact with the ordinary matter quarks during PNP because they were inside the event horizon of these black holes.
     
  20. Sep 9, 2017 #19
    The galaxy is not expanding .The main premise that supports dark matter is that the galaxy should expand because of the amount of observable matter in it and the gravity that matter exerts plus the speed of it's rotation is not enough to stop it flying apart.
    It is the universe that is expanding carrying the galaxy and others along with it.
     
  21. Sep 9, 2017 #20

    ISamson

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    I think our observations are wrong, because the galaxies are so far away, that our telescopes might not be able to see all the particularities.
     
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