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I Question About Dark Matter

  1. Dec 12, 2016 #1
    Hello, my name is Pedraam and I am newly getting into Astrophysics and stuff. It is very interesting to me so far!

    My question has to do with dark matter. I'm curious as to why we look at Newton and Einstein as such gods that if they're theory is slightly off, even in the cosmic world, we have to make up matter out of thin air that is absolutely unidentifiable to us?

    I know there are theories out there, where they propose very slight changes to Einstein and Newtons gravitational laws. But I just don't understand what makes dark matter such a widely accepted theory, like 97% of scientists agree upon it.
     
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  3. Dec 12, 2016 #2

    phinds

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    How about the fact that evidence for it is indisputable? We don't know what it IS but there is a plethora of consistent evidence about what it DOES. Google "Bullet Cluster" and/or read some of the thousands of posts on this forum for further discussion. A good place to start is likely to be the links at the bottom of this thread.
     
  4. Dec 12, 2016 #3

    Vanadium 50

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    You might want to lose the 'tude, bro.

    That's not why we think there is Dark Matter. The reason is that simple modifications of gravity don't explain the data at all, and complicated modifications of gravity do no better.
     
  5. Dec 12, 2016 #4

    Drakkith

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    Well, here's a little story for you. Back in the 1930s, an age so long ago that it's shrouded in legend and myth, a man named Wolfgang Pauli had a problem. The problem was that the laws of conservation of energy and momentum appeared to be violated in a certain type of radioactive decay called beta decay. In this process, a neutron emitted an electron and turned into a proton, and precise measurements confirmed that not all of the energy and momentum was being carried away by the electron and the proton.

    This was a problem.

    But, it was a problem that had several possible answers. The first, was that perhaps conservation of energy and momentum were only true in a statistical sense. In other words, perhaps sometimes you lose a bit of energy and sometimes you gain a bit of extra energy, and the two tend to average out over time. Another man, Niels Bohr, a nobel prize winning physicist, supported this idea. However, the idea of modifying these conservation laws was controversial. They had been verified as accurate time and time again, and if they truly only held in a statistical sense then further experiments needed to be done.

    At the same time, Pauli proposed that perhaps a new particle was carrying away that lost energy and momentum, and the conservation laws didn't need to be modified at all. This, of course, was also controversial. If there was a new particle, why couldn't they observe it? The immediate explanation was that the new particle was not electrically charged, and it was given the tentative name of "neutron". But, around the same time, another scientist announced that he'd found another neutral particle, heavier than a proton, and he also named it "neutron".

    Now, this latter particle was much too heavy to be the particle Pauli was looking for. His particle would have to be tiny in order to fit with the data. Utterly tiny. So light that he wasn't even sure it had mass. So it was named "neutrino", short for either "little neutral one" or "diminutive one who doesn't take sides in arguments". We're not sure, as records from this era are scarce.

    So, which side was correct? Was there a new particle, or were the conservation laws inaccurate? Until conclusive evidence for one side was discovered, there would be no consensus.

    A few years went by. New experiments were done. If this statistical conservation law was accurate, then during some of these decays there should be an excess of energy. But that's not what they showed. There was never an excess of energy. There was only a loss.

    Were the conservations laws simply wrong? Were they not even statistically correct? Pauli didn't agree with this. Three new particles had been discovered in the last thirty to forty years: the electron in 1896, the proton in 1917 (or 1919, or 1920, depending on your preference), and the neutron in 1932. Why would another new particle be less preferable to modifying some very important laws of nature? Despite observations showing that Bohr's modified conservations laws weren't accurate, there was no consensus until 1942, when an ingenious experiment showed that something was carrying energy away from nuclear reactions and into nearby protons, turning them into neutrons.

    That was it. There was the missing energy Pauli had been looking for. This new experiment showed that the energy wasn't lost, it was simply transported elsewhere. Since the idea that energy can transport itself to and fro is, well, extremely unpopular with scientists, this was taken as evidence that a new particle existed. It was named the "neutrino".

    So there you have it. We invented a new matter particle, right out of thin air, that was at first completely and utterly undetectable to all experiments at the time. And it worked beautifully. Neutrinos have since been detected in a great many experiments and we've even discovered multiple types of neutrinos, named "flavors", because the scientists were eating popsicles at the time. Or something like that. As I said, this was a time of myth and legend. Of neutrinos and neutrons and statistical conservation laws. Strange, exciting times indeed.
     
    Last edited: Dec 12, 2016
  6. Dec 12, 2016 #5

    Drakkith

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    Put simply, it is the simplest theory that most accurately explains our observations. It's not perfect, but it works much better than modifications to gravity do in almost every area. And what's wrong with proposing new types of matter? How do you think we've come to include 17 fundamental particles in the standard model of particle physics? We had to "invent" them to explain our observations, and they work extraordinarily well. The current idea that dark matter explains why our observations of the universe don't quite match up with our laws of gravity is exactly where we were for most of the 20th century when we were developing quantum theory and most of our current cutting-edge theories. It's just taking a little longer to discover whether our theories are accurate or not, as dark matter has proven very difficult to either confirm or disprove.
     
  7. Dec 12, 2016 #6

    Ken G

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    I think the possible existence of "dark matter" gets significantly demystified when we recognize that there's really only one type of particle, the electron, that is really any good at all at emitting and absorbing light. So although other particles can do it, most of the light we observe is from electrons. So classes of matter, such as the above-mentioned neutrino, that come without elecrons, always make pretty good "dark matter" (though neutrinos are the grand champion). It isn't so much that the matter needs to be "dark" (or more correctly, "invisible") that presents the problem, it is that models of the early universe show it also has to be non-baryonic, unless we have something very wrong. So even if the stuff did emit light, we'd still have a big problem figuring out what it is, since we'd still need something non-baryonic in the models. That it doesn't emit light, or couple to regular matter at all well, presents an even more serious problem for detecting it with our regular-matter instruments. But who ever promised the universe would only involve things that our eyes can see or our instruments can detect? We always had to just hope that we could detect the constituents of the universe, and keep trying as hard as we can-- but it is never a given.
     
  8. Dec 12, 2016 #7

    What Newton did was make it so we can explain these strange physical interactions with math formulas. That followed a logic, we had previously not used nor even realized we could use math to make these explanations. I don't really go goo-goo over Newton, but I do certainly recognize the importance of math and calculus to explain physics.

    What Einstein did, was before E=MC^2, we thought everything was just little pieces of atoms, floating around, but we couldn't explain why gravity was not made of pieces of this stuff. Then Einstein realized that all of space, even the space, between space, filled with nothing, is made of a 'fabric'. This finally showed how gravity could move all this stuff around without actually being made of anything. This space time fabric is very important, as particles are physical pieces of information, the Einstein fabric of space-time is the 'fluid' that all these particles float around in and how they interact. We may very well modify Einsteins 'space-time and fabric, but it is such an important explanation for the 'empty space' that's its near impossible to get rid of it. Then we find out he made a prediction, 100 years ago to be true, with the gravitational wave discovery. 100 years ago... Its the equivalent of someone writing a math problem today, and 100 years from now, in 2116, we find out it to be true. Einstein space-time fabric is very important as it permeates everything in the universe, in all of our experiments, so we have to keep it, or only slightly modify it because it explains 90% of everything.
     
  9. Dec 12, 2016 #8

    phinds

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    Seriously? Have you every heard of, for example, Johannes Kepler? Newton was a giant in physics but let's not go overboard.

    "Fabric" turns out to be a terrible way to describe things in cosmology and is not used by serious physicists, just pop-science presentations that get a lot of other stuff wrong as well.

    Even if there WERE an aether/fabric (which there isn't) it would hardly be reasonable to say that it explains 90% of everything. It would have nothing to do with biology or classical mechanics just as a couple of examples.
     
  10. Dec 12, 2016 #9
    That's why I said space-time fluid. Fabric is a way of pixelating the fluid. There most certainly is a space-time fabric fluid.
    I also was trying to explain it to someone who is a beginner. I understand your considerations.
     
  11. Dec 12, 2016 #10

    phinds

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    Citations? I don't get what you think is the difference between fabric and fluid. This sounds like a personal theory.
     
  12. Dec 12, 2016 #11
    Dark matter and non-newtonian gravity from general relativity coupled to a fluid of strings: "When interpreted in the context of a cosmological model with a string fluid, the new solution naturally explains why the critical acceleration of Milgrom is of the same order of magnitude as the Hubble parameter." http://link.springer.com/article/10.1007/BF02107935

    Bianchi Type-V Bulk Viscous Fluid String Dust Cosmological Model In General Relativity
    http://link.springer.com/article/10.1007/s10509-005-0152-8

    Turbulent Mixing, Viscosity, Diffusion, and Gravity in the Formation of Cosmological Structures: The Fluid Mechanics of Dark Matter http://fluidsengineering.asmedigitalcollection.asme.org/article.aspx?articleid=1429292

    Bianchi type-V perfect fluid space-time models in general relativity
    http://link.springer.com/article/10.1007/s10509-008-9811-x
     
  13. Dec 12, 2016 #12

    phinds

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    OK, this gets out of my depth but it sounds like a combination of string theory to describe spacetime and then fluid dynamics describing dark matter, neither of which is convincing to me that there is a general "fluid" in spacetime. What am I missing?
     
    Last edited: Dec 12, 2016
  14. Dec 12, 2016 #13

    PeterDonis

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    Thread closed for moderation.
     
  15. Dec 12, 2016 #14

    Drakkith

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    Hi HyperStrings. Unfortunately none of those models are accepted by the mainstream community as a likely replacement for the standard model given by GR (which doesn't speak of space-time as a fluid or even a fabric) or the model currently used for dark matter. It's important to understand the difference between speculative models like these that get generated all the time, and the base model that is currently accepted. And, unfortunately, it's not always easy to determine what's considered speculative and what isn't, especially when it comes to dark matter, where we still haven't conclusively figured out just what it is yet.

    Since this thread is currently locked, please feel free to send me a private message if you wish to discuss this matter further.
     
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