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Why do we believe dark matter exists?

  1. Jul 19, 2015 #1
    <<Mentor note: Thread split from Is Dark matter homogeneous in Universe?>>

    Not to be rude..... but how do you know it even exists? Dark matter is a place holder used to make a hypothesis work mathematically with actual observations. Dark energy.... Ditto. And the original hypothesis itself is based on the assumption that gravity is the dominant force in observed cosmological phenomenon.

    It sounds like a contrivance based on an assumption. If we have to create some exotic "thing" like dark matter, and dark energy, to make the math support our hypothesis - we should seriously consider that our hypothesis might be wrong.

    If I can make an analogy.
    Theists have their "god of the gaps" and cosmologists have their "cosmology of the gaps"

    Sorry, Dark matter / dark energy; it sounds like voodoo.
     
    Last edited by a moderator: Jul 21, 2015
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  3. Jul 19, 2015 #2

    Chronos

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    I would argue DM a case compelled by observational evidence. No viable alternative yet exists. Otherwise, you are forced to concede scientists are modern day temple priests bound by the tenants of theory. I would argue there is no temple of science - modern science is strictly motivated by observational evidence and math. I agree the 'dark' sector appears to be an invocation attempting to justify dogma to those unfamiliar with the highly rigorous methodology employed by modern science.
     
  4. Jul 19, 2015 #3
    Are there really no alternative hypothesis worth investigating?
    Does the scientific community support the research efforts of those seeking an alternative? Really, do they?

    Let me make another analogy.
    The geocentric model of our solar system predicted that certain planets would be at certain places, at certain times. When observations proved this wrong - the scientists of the day contrived new epicycles (orbits). That fixed the problem, until the next failed prediction then they added more epicycles.... etc...etc...

    Cosmologists believe in inflationary cosmology. When the observable mass of the universe does not fit the inflationary model, they invented dark matter. That solves the problem, until it was observed that the universe is accelerating - so they invented dark energy. Problem solved.
     
  5. Jul 19, 2015 #4
    I am not an scientifitic person.But the thing is I guess the human kind always want to keep things simple.Simplicity is the solution of complexity.When we think the universe, theres lots of mystery to solve.We are calling names some of them which we cant solve.Dark matter dark energy and other theories.

    They dont need to be true.They are just "problem names" wanted to solve

    We have some assumptions but thats the all thing we got.
     
  6. Jul 19, 2015 #5
    I agree. We have to ask questions, make some assumptions and investigate. But don't get stuck on a hypothesis if observations say otherwise.

    What I'm saying is that if we have a hypothesis, and then observations don't match - it's not good science to arbitrarily invent a substance (or orbit) that makes the hypothesis work again. If observations don't match our hypothesis, we change the hypothesis to match the observations. Observations are real - hypothesis are not.

    Yet cosmologists, astrophysicists and others, talk about dark matter as if they know it exists, how it's distributed etc... as if they forget it was contrived to make the math work, to support a hypothesis (that they talk about like a theory).
     
  7. Jul 19, 2015 #6

    Chronos

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    It is our nature to 'invent' explanations fot observations that do not fit our expectations. The 'invention' of DM was not arbitrary, it was a consequence of exploring and rejecting all other reasonable explanations - it was a classic example of scientific inquiry. All deductions resulted from this same process of scientific inquiry - hypothesize, observe, analyze and reformulate the hypothesis as appropriate. While the outcome remains open to inquiry, the process is beyond reproach. Cosmologists still cling to DM because it is the only conclusion still viable in the face of a mountain of evidence.
     
  8. Jul 19, 2015 #7
    Badseed,I understand your idea.Why are we stuck the idea of dark matter or dark energy ? The main reason which Chronos said theres no other better theory to explain this situation
    If we find a better theory we will left the idea of dark matter and dark energy.

    Think aether.The physicist made experiments about it.Experiments show that aether is wrong but they still stuck that idea.Until Einstein came.

    Michelson-Morley Experiment made in 1890 I guess which Einstein write article about SR in 1905(I dont know other theories which says aether is wrong between 1890 and 1905)

    So this how science works I guess
     
  9. Jul 19, 2015 #8

    Chronos

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    Yes, this is how science works. We rely on effective theories, like Newtonian gravity, which produce good predictions, until a better [and adequately confirmed] theory comes along that is even more predictive. The test of a new theory is it must reproduce predictions of the old theory along with observations not otherwise accounted for under the old theory.
     
  10. Jul 19, 2015 #9

    Orodruin

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    Yes, there are some alternatives. Yes, they are being investigated. No, they do not seem to work as well as the dark matter solution. What you seem to not grasp is that there is an overwhelming plethora of observational evidence in different epochs of the universe that indicate that dark matter, or something very similar to it, exists.

    You are looking at it in a distorted fashion. Dark matter was introduced because a theory which is very well tested indicates that the matter distribution looks in a certain way, but there happend to be not enough luminous matter there. Since then, there is a large amount of evidence pointing in the same direction to the point that it is very unlikely the universe could exist and look the way it does without the existence of dark matter.

    You also seem to think "just making the math come out" is a bad thing, making the math come out right is the only thing quantitative science is about. If you have two theories with predictions that work equally well, you may start to apply Occam's razor.


    This analogy is bad. In the case of epicycles, it was necessary to include more and more of them to explain new observations. In the case of dark matter, it has already been successful in explaining several observations which were not the same as that in which it was first discovered.

    Let me offer a better analogy: Planetary motion and. Newtonian gravity has already been used to infer mass distributions, which were later discovered. In the 19th century, the orbit of Uranus was not in exact agreement with predictions. This could be explained by the existence of a new planet, which nobody had seen, and eventually led to the discovery of Neptune and later on Pluto.
     
  11. Jul 20, 2015 #10
    Understand, if there is some knowledge to "grasp", I really do want to grasp it.

    What "theory which is very well established" are we talking about? the big bang, inflationary cosmology? what makes them well established - what observations that is?

    "there happens to be not enough luminous matter" that's sounds like a nice way to put it;
    or you could say "the well established theory predicted a certain amount of matter and observations did not match that prediction, in response we created DM as a place holder for the missing matter; that worked for a while. Then we observed the universe expanding, contradicting the well established theories predictions again, so we created DE as place holder for a mysterious unknown force no one can see or measure" - It just seams fundamentally flawed to me

    Like what? I would like to know more.


    Not at all. The math has to work or your wrong, right?; but I don't think it's a good idea to create "magic" matter and "magic" energy to make the math work. Sorry for the bluntness, that's just what it sounds like to me. If it was put forward as a thought experiment, academic conjecture or a hypothesis to be tested I could understand; but DM/DE are discussed as if someone has it in a jar in their lab.

    Occam's razor - Is there really no hypothesis with fewer assumptions? Quantitatively, it appears to me that DM/DE are assumptions that make up about 95% of the universe (but no ne can see it or measure it)- we can't find a hypothesis with less substantial assumptions?

    Maybe DM and DE are the answer, I don't know - but I find it intellectually "unsatisfying". It just reminds me of a voodoo Dr. trying to convince me of the magic forces I can't see or measure; and the unseen worlds that exist all around me. Yes, I know I'm going to get flamed for that comment . It's the truth - let the flaming begin!

    Didn't they have to add DE on top of DM to keep the inflationary model going? That's what egocentrics did with epicycles.

    When was DM "discovered"? That's kind of what I'm talking about, when I say people talk about DM like they have some in a jar. Have you seen some DM, has anyone? can I way it in a lab? has a satellite scooped some up?

    In the case of Neptune, Newtonian gravity predicted an orbit for Uranus that was different from the observed orbit. It was hypothesized that a near planet (Neptune) could explain the observed orbit - They looked for the planet where the hypothesis predicted, they found it- we have pictures of it, we have spectral analysis of it etc... do you have a picture of dark matter? do you know what it's made of? do you have some in a lab where you can demonstrate it's gravitational effect?
     
  12. Jul 20, 2015 #11

    Chronos

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    You could make the same argument for rejecting the existence of subatomic particles. Much of modern science is based on inferences deduced from unseen entities. Are lasers, GPS and i phones voodoo, or representative of our ability to understand and apply knowledge of unseen entitities?
     
  13. Jul 20, 2015 #12

    Orodruin

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    General relativity.

    It is not a prediction, it is an observation. Dark matter is not based upon the big bang or cosmology, it is based on gravitational effects. There are then two options, GR is wrong or there is dark matter.

    No, this is just wrong, you need to get your facts checked. The universe was observed to be expanding long before dark energyg became an issue and it would be even if there was no dark energy. It is how the expansion proceeds that is the issue. It was Einstein who tried to save the stationary state model by introducing what was effectively dark energy, he could have foreseen the expansion of the universe but did not simply because he was too certain the universe should be static. Later Hubble discovered expansion and dark energy was removed, only to turn up again almost a century later, but in a much much different role. The discovery of dark matter has nothing to do with this.

    Also, do you believe in neutrinos? You cannot see them and they rarely interact with matter at all. (This is an extension of Chalnoth's comment.) If you only believe in things you can see or touch you will not get very far in science. This is what we have instruments for.


    Several have been mentioned in this thread already. Galaxy rotation curves, the virial theorem applied to galaxy clusters, the large scale structure of the universe, CMB observations, gravitational lensing, the bullet cluster, dwarf spheroidal galaxies, to name a few.

    Inventing hypothesis and checking them is the entire basis of science. This particular one has been tested and found to give a good description. That you do not like the conclusions is irrelevant. To some extent, this is no different from the people we get in the relativity and QM forums who do not believe in SR or QM because "it does not make sense" to them. Science does not care, it only cares if you are able to make a good prediction or not.

    But this is how it was originally presented! It was conjectured by Zwicky to explain the movements of the galaxies in the Virgo cluster and has later on been checked by several experriments as already mentioned. Do you want to start every paper based on relativity by discussing the Michelson-Morely experiment?

    The alternative assumption is "GR is wrong" and it is being persued. However, you need to come up with a theory that looks essentially exactly as GR but is not GR on very large scales. Such theories generally give you a few of the observations presented as evidence of dark matter, but not all of them. You are left with one theory which predicted the phenomena (all of them) and one that is merely reproducing a subset of them. Guess which one scientists tend to favour.

    This is exactly the argument made by SR and QM nonbelievers (yet somehow they still use computers and GPS). Science does not care what you find satisfying, it cares only about what you can predict and not.

    No, this is not why dark energy was introduced. It has nothing to do with inflation, nor does dark matter.

    It was discovered in 1933 by Fritz Zwicky. You cannot have dark matter in a jar or see it by definition. What we can see is the effects of dark matter and, as already mentioned, these are plentiful.

    We do have pictures mappping out the gravitational effects of dark matter, so yes. Or are photons the only type of messenger you accept? Why not restrict yourself to photons in the visible spectrum? Or only to photons in the visible spectrum which happen to be absorbed in your eyes?

    I can tell you what dark matter is not made of, it is not made of any standard model particle. This should get you excited, it means hat here are new physics to be discovered, either in the form of a new theory of gravitation or by discovering what DM is made from.

    Do you believe in the Higgs boson?
     
  14. Jul 21, 2015 #13
    Or maybe GR is correct, there is no dark matter and gravity is not solely responsible for the orbital velocities of galaxies etc....
    Just a thought..... but maybe there is invisible stuff, that can't be seen or measured directly. I don't rule it out.

    I was referring to discovery of the acceleration of the expansion.

    Cautiously, I would say they probably do exist - because they can be detected indirectly in a lab.

    And I thought with the exception of CMB they are all based on indirect (not of dark matter) observations that start with the assumption that gravity is solely responsible for everything we see in space - am I wrong on this?

    It's not the conclusion that I have an issue with, it is the method used to get there and the certainty used when discussing it.

    We must have different definitions of "discovered". This is the first sentence of the description in the oxford dictionary for dark matter: "(in some cosmological theories) nonluminous material that is postulated to exist in space..... Does "postulated" mean it has been discovered? How about Wikipedia: "Dark matter is a hypothetical kind of matter that cannot be seen with telescopes..."

    You can see plentiful effects of something. It could turnout to be dark matter (maybe even likely that it is) - or something else.

    You have pictures of the effects of something.

    It is exciting.


    It probably exists. My understanding is that a candidate particle has been detected, measured and analyzed.
     
  15. Jul 21, 2015 #14
    Last edited: Jul 21, 2015
  16. Jul 21, 2015 #15
    Subatomic particles can be detected, measured and used to make things work - show me a GPS, laser or Iphone based on dark matter and I'll believe it exists.

    The voodoo Dr. claims to apply knowledge of unseen entities also; If he can make a spirit give me GPS coordinates - I would believe him. No, I don't really think cosmology is "voodoo", but the certainty in things like dark matter and dark energy perplex me. Especially given the past certainties that proved to be wrong.
     
  17. Jul 21, 2015 #16

    Orodruin

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    You do realise that assuming a new force also is assuming new particles, in this case interacting very weakly with matter at all scales except very large ones (or we would have seen it), and thereby introducing a dark sector? Introducing a new long range force also comes with extensive theoretical problems unless done properly, which would generally imply introducing a dark matter sector as well.

    This is why we believe there is dark energy. It is not how it was invented. In its simplest form, and one that is perfectly consistent with observations, dark energy is a cosmological constant. As mentioned earlier, it was seen as a possibility already by Einstein in his field equations. There is no reason it should not be there in GR, so the thing that puzzles most physicists is not "why is there dark energy" but "why is the cosmological constant so small"?

    Which is also all you have for electrons or neutrinos.
    And you do realise that the Higgs lifetime is far to short for it to reach even the first parts of the detector? All we can see is the decay products and these come with enormous backgrounds. The Higgs was mainly discovered in the gamma-gamma channel, where by plotting your events in a certain way gives a bump. Not very direct, but it is exactly what was predicted by the theory.

    But this is the very essence of science. You may not realise it reading popular science, but the entire point of science is to be proven wrong. The current working assumption is there because it is the best description we have. It is there because it has proven predictive. Saying you do not believe it because it does not "appeal" to you is just silly. Science cares absolutely nothing fir that. Whatever model you come up with is going to have to reproduce observations already made which are well described by the current theory. In the case of dark matter, these are so plentiful that you might just call the new thing dark matter as well.
     
  18. Jul 21, 2015 #17

    Chronos

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    But, you appear to dismiss DM largely on the basis of lack of evidence for electromagnetic interactions. Is evidence for gravitational interactions unpursuasive? Not in the opinion of the vast majority of cosmologists.
     
  19. Jul 21, 2015 #18

    marcus

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    That was an interesting discovery by the PhD student Cameron Yozin and somebody else. The ordinary matter (baryonic matter) in these galaxies is so spread out, so thin, so diffuse, that they are not making stars. But they have a lot of DM in them and that holds them together by its gravity.
    In case anybody wants to look at the technical article here it is:
    http://arxiv.org/abs/1507.05161
     
  20. Jul 21, 2015 #19
    Perhaps it is not a new force, but effects of a fundamental force we already know about in addition to gravity? (or maybe it's just gravity and dark matter?).

    (Why not call it luminiferous aether, lol - I'm just joking)
    Does GR need a cosmological constant - especially if another fundamental force is at work? Certainly Einstein did not think so. The question is "is there a good reason for it to be there" before we are going to take the extraordinary step of declaring the existence of something exotic - should we have extraordinary proof first?



    For neutrinos, this is true. That's why I said "cautiously" and "probably". The fact that we can recreate it's "effects" in an experiment give it some weight in my opinion.
    For electrons I also have my IPhone, GPS and a thousand other applications. Not just equations and postulations.

    That's why I said "probably". The fact that we can recreate it's "effects" in an experiment give it some weight in my opinion.

    I don't think I said "appeal" (to lazy to look) - I said "Intellectually satisfying" as in "not good enough" for me. Am I wrong for this? I think that is how we progress. The Ptolemaic system predicted the motions of the solar system as accurately as the heliocentric model for a 1,000 years, yet it was wrong.
     
  21. Jul 21, 2015 #20
    I don't dismiss it, I'm just not convinced enough to talk as though it is a fact of reality. It might be the answer.
     
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