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Thread: Dark matter/Dark energy

  1. #1 Dark matter/Dark energy 
    New Member ceic02120's Avatar
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    I'm no scientist. So i would like someone to explain why space isn't cosidered dark matter and time not dark energy? Space seems to bend light around galaxys for some strange reason. and they say that over time,lots of it, matter itself will degrade. Time being the force behind the eventual, "Big Rip", i mean.


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  3. #2  
    Forum Radioactive Isotope MagiMaster's Avatar
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    When scientists calculate the gravitational forces required the spin galaxies the way we see them spinning, and then measure how much matter can be seen in those galaxies, there's a fairly big difference. The difference must be some matter that we can't detect at the moment, and it's called "dark matter" just because someone called it that and it stuck.

    When scientists calculate the energy needed to cause galaxies to fly apart the way we see them flying apart, then measure all the energy (including mass-energy equivalence), there's a difference. Whatever makes up this difference is called "dark energy" because someone called it that and it stuck.


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  4. #3 Re: Dark matter/Dark energy 
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    Quote Originally Posted by ceic02120
    I'm no scientist. So i would like someone to explain why space isn't cosidered dark matter and time not dark energy? Space seems to bend light around galaxys for some strange reason. and they say that over time,lots of it, matter itself will degrade. Time being the force behind the eventual, "Big Rip", i mean.
    If you look at the rotation rates for many galaxies you find that there is not enough observed matter in the galaxies to provide the gravity necessary for the centrepital force needed to hold the galaxies together. That is the origin of "dark matter'. There is more recent observational data of gravitational lensing that also shows that more matter is required than what is observed. Dark matter is hypothesized to provide the matter necessary for the gravitational fields that are observed. No one knows the source of this additional gravitation or matter, so in the tradition of physics, lacking an explanation, the problem is given a name -- "dark matter".

    Dark energy is an entirely different thing. About 1998 observations in involving type 1A supernovas provided evidence that the rate of expansion of the universe was increasing. Up until that time it was generally believed that the combined gravitation of the matter in the universe was causing the expansion to decelerate, and the open question was whether or not there was enough matter to eventually halt the expansion and start a contraction. The news that the expansion is actually accelerating was rather startling.

    In order to account for an accelerating expansion, within general relativity, one includes in the model a positive cosmological constant (prior to the 1998 data, the cosmological constant was thought to be zero). This cosmological constant provides what is in effect a repulsive force correctly models the observed accelerating expansion. The big problem is that no one has a good explanation as to why the cosmological constant should be the small positive value that is observed. Lacking a good explanation, it too has been given a name -- "dark energy".

    There is one possible explanation for dark energy. The zero point energy of the quantum vacuum can in principle produce a cosmological constant. The only problem is that the value calculated using quantum electrodynamics overpredicts the observed value by a factor of which is a huge discrepancy. So the problem remains "dark" because no one really has a clue as to the source.
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    Moderator Moderator Dishmaster's Avatar
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    I only wanted to mention that there are also tracks in astrophysics that try to avoid inventing "Dark Matter" and use alternative explanations to match the observations like modifications of the gravitational law. The need for additional "Dark Matter" is only justified, if the 3rd Keplerian law (or the relativistic analogue) is assumed to be true for the entire parameter space of distances and accelerations. To me, the "Dark Matter" invention smells a bit like the infamous aether.

    As for the "Dark Energy", the last results I remember show that the possible accelerated expansion of the universe is not a solid detection. If there aren't any newer results, the expansion rate determined from those SN measurements (all uncertainties included) is still within the measuring accuracy of a normal expansion.

    So, if I am not mistaken, it is not even fully established that these "Dark" things actually exist. I know that public press releases often tend to overestimate the implications of the results presented there. The professional publications linked to those are mostly much more humble in their conclusions.

    But, I don't want to start a discussion on the side, whether or not these things really exist. One answer might come from the LHC experiments.
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  6. #5  
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    Quote Originally Posted by Dishmaster
    I only wanted to mention that there are also tracks in astrophysics that try to avoid inventing "Dark Matter" and use alternative explanations to match the observations like modifications of the gravitational law. The need for additional "Dark Matter" is only justified, if the 3rd Keplerian law (or the relativistic analogue) is assumed to be true for the entire parameter space of distances and accelerations. To me, the "Dark Matter" invention smells a bit like the infamous aether.

    As for the "Dark Energy", the last results I remember show that the possible accelerated expansion of the universe is not a solid detection. If there aren't any newer results, the expansion rate determined from those SN measurements (all uncertainties included) is still within the measuring accuracy of a normal expansion.

    So, if I am not mistaken, it is not even fully established that these "Dark" things actually exist. I know that public press releases often tend to overestimate the implications of the results presented there. The professional publications linked to those are mostly much more humble in their conclusions.

    But, I don't want to start a discussion on the side, whether or not these things really exist. One answer might come from the LHC experiments.
    I don't see how the LHC is going to tell us much except perhaps for the crowd that thinks dark matter might be supersymmetric particles. That strikes me as pretty far out even if they can be shown to exist.

    Your statement that there is observational evidence suggesting that the expansion of the universe is not accelerating is interesting. All that I have heard for about the last decade indicates that it is accelerating. I have not looked hard at the observational data, and probably will not as it is a huge job. But I have always wondered just how reliable the conclusions really were. So any reliable references that you might have professing contrary opinions would be of interest.

    I am also a bit skeptical about "dark matter", but the evidence there seems pretty good as it includes anamolous gravitational lensing. But I like the dark matter idea a lot better than MOND theories (Modification of Newtonian Dynamics) if only because MOND seem so contrived. It also fails to handle the graviational lensing as far as I know.
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  7. #6  
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    Quote Originally Posted by DrRocket
    I don't see how the LHC is going to tell us much except perhaps for the crowd that thinks dark matter might be supersymmetric particles. That strikes me as pretty far out even if they can be shown to exist.
    I agree. The downside of this LHC project is that it has been conveyed to the general public as a tool to find "Dark Matter". And now almost all the money at CERN is put in there, while all the other - scientifically at least as important - projects have severe funding problems.
    Quote Originally Posted by DrRocket
    Your statement that there is observational evidence suggesting that the expansion of the universe is not accelerating is interesting. All that I have heard for about the last decade indicates that it is accelerating. I have not looked hard at the observational data, and probably will not as it is a huge job. But I have always wondered just how reliable the conclusions really were. So any reliable references that you might have professing contrary opinions would be of interest.
    From the top of my head, I remember a graph with large error bars and some curves representing different cosmological expansion models. But I will try dig something out. Might take a while. But I can already say that it is heavily debated, whether Type Ia SNe are really the standard candles as they have been used so far. There could be ambiguities with the metallicity (abundance of higher elements) of the SNe in earlier epochs of the universe.
    Quote Originally Posted by DrRocket
    I am also a bit skeptical about "dark matter", but the evidence there seems pretty good as it includes anamolous gravitational lensing. But I like the dark matter idea a lot better than MOND theories (Modification of Newtonian Dynamics) if only because MOND seem so contrived. It also fails to handle the graviational lensing as far as I know.
    MOND cannot do that, because it is classical mechanics. It's not MOND's fault, but Newton's. But there are a few promising projects going on trying to create something like "Relativistic MOND", also including gravitational lensing.

    http://en.wikipedia.org/wiki/Tensor-...scalar_gravity
    http://en.wikipedia.org/wiki/Scalar-...vector_gravity
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  8. #7  
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    The hypothesis that dark matter exists seems, in itself, not all that surprising. Why should the majority of massive particles in the universe interact with the photon?

    If you tacked on a bunch of massive quantum fields into the standard model lagrangian that didn't interact with the known gauge fields, it seems like what you'd end up with is a particle that (1) contributes to gravity, and (2) is impossible to detect via the electromagnetic spectrum.
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  9. #8  
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    So, it seems that my previous remark on the accuracy of SNe Ia measurements was based on the knowledge of 2003. In the meantime, it seems that this particular branch has evolved a lot, and the existence of an accelerating universe and some force that is called "Dark Energy" is well established. I found a pretty good review paper on the topic. I haven't read it thoroughly yet, but it contains all the important questions and answers. There is also some advanced math involved, but it doesn't seems to be very crucial to the understanding of that review.

    http://de.arxiv.org/abs/0803.0982

    The graph I had in mind with my previous post is on page 13. But newer results are presented on page 16, leaving hardly any doubt on the validity of the conclusions.

    Enjoy reading. I surely will.
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  10. #9  
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    Quote Originally Posted by Dishmaster
    So, it seems that my previous remark on the accuracy of SNe Ia measurements was based on the knowledge of 2003. In the meantime, it seems that this particular branch has evolved a lot, and the existence of an accelerating universe and some force that is called "Dark Energy" is well established. I found a pretty good review paper on the topic. I haven't read it thoroughly yet, but it contains all the important questions and answers. There is also some advanced math involved, but it doesn't seems to be very crucial to the understanding of that review.

    http://de.arxiv.org/abs/0803.0982

    The graph I had in mind with my previous post is on page 13. But newer results are presented on page 16, leaving hardly any doubt on the validity of the conclusions.

    Enjoy reading. I surely will.
    Interesting.

    The trick is now to figure out the fundamental reason for the cosmological constant.

    There is at least one camp that thinks that the source is the nature of the energy of the vacuum that is predicted by quantum field theories. That vacuum energy results in a "negative pressure" term that, in the stress-energy tensor of general relativty, results in a positive cosmological constant. The only problem is that the best available theory, quantum electrodynamics, results in a prediction that is high by a mere factor of . As I recall, if the cosmological constant were as predicted, the universe would have expanded into oblivion within a small fraction of a second following the big bang. There seems to be something amiss in that theory.
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