This one goes out to... chroot

Discussion in 'Physics & Math' started by GundamWing, Feb 25, 2003.

  1. GundamWing Registered Senior Member

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    Hey Warren,

    Can you please briefly describe the expected properties of "Dark Matter" and "Dark Energy" for the rest of us in the light? All jokes aside, i'm completely interested.

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  3. Persol I am the great and mighty Zo. Registered Senior Member

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    From the general description usually given 'dark matter' sounds like matter which we just don't/can't detect yet. I'd assume that this includes space dust that isn't glowing for one reason or another... and any other material that happens to be floating around and not emitting radiation.

    Every reference I see to dark energy refers to it as being anti-gravitational. It seems to have been created to explain how the universe's expansion could be accelerating.
     
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  5. voltron Registered Senior Member

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    Even for someone of Warren's intelligence, it's a tad difficult to tell you the (expected) properties of something that has yet to be even detected. All ideas mentioned thusfar have been speculation due to the odd/unexpected/bizarre/[insert synonym for unexplainable here] occurrences in the galaxy.

    If you're still interested in knowing "expected" properties:
    http://www.ask.com/main/metaAnswer....dark matter?&dt=030225002739&amt=&pg=1&qsrc=0
    http://www.ask.com/main/metaAnswer....dark matter?&dt=030225002739&amt=&pg=1&qsrc=0
    http://hepwww.rl.ac.uk/ukdmc/ukdmc.html
    http://www.ask.com/main/metaAnswer....dark matter?&dt=030225002739&amt=&pg=1&qsrc=0

    All of those links were found by asking Jeeves: "What is dark matter?"
     
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  7. chroot Crackpot killer Registered Senior Member

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    People used to be perplexed with the idea of dark matter. The problem with dark matter is that is it specifically non-baryonic. Whatever it is, it's not protons and neutrons. We believe we understand big-bang nucleosynthesis (BBN) very well, since these temperature regimes are routinely investigated in our particle accelerators. We also believe we understand stellar nuclear physics very well, and BBN is a cousin of stellar physics.

    The bottom line is that we think we understand very well why the universe is 75% hydrogen and 25% helium by mass. We have fully-developed theoretical explanations, and loads of experimental evidence to convince us.

    So the dark matter is not protons and neutrons. What is it? Some people have speculated on WIMPs (weakly interacting massive particles) and MACHOs (massive compact halo objects). WIMPs might include such things as massive neutrinos, or magnetic monopoles. Unfortunately, our observational cosmology experiments are slitting our own throats -- we now have very strong upper bounds on both the neutrino masses and the number of neutrino species. There is one experiment (LSND) that disagrees, however, and predicts a fourth "sterile" neutrino, which does not interact at all with electrons, muons, or tau particles, and which might have mass. Physicists are currently trying to figure out what's wrong with LSND, because it disgrees violently with other experiments, such as WMAP, which lead us to believe there are exactly three neutrino flavors, with a total summed mass of about 0.38 eV. The jury's still out on that one.

    If it's not a massive neutrino, it might be magnetic monopoles, or even gravitons. A few experiments have been conducted to detect monopoles, which have to be extraordinarily massive. There was one event, detected on Valentine's day in (IIRC) 1986, but no one has yet conclusively found a magnetic monopole. The magnetic monopole hypothesis has fallen into disfavor. Graviton experiments like LIGO have not yet conclusively found any gravitons yet, either.

    The bottom line is that, well, we don't know what the dark matter is.

    However, astrophysicists have gotten comfortable enough with the concept that there's stuff in galaxies that we can't see, largely because it combines many different problems (rotation curves of galaxies, matter fluctuations that seed galaxy formation, etc.) into the single problem of understanding what the dark matter itself is. A host of problems are all solved by assuming a density of dark matter that is measured by many experiments -- this agreement in itself suggests we're on the right track.

    I should more specifically say that astrophysicists have gotten comfortable with dark matter because it throws the ball out of their court and into the particle physicist's court. In other words, the astrophysicists tell the particle physicists quite confidently "we've predicted it and measured it. You tell us what it is." As crazy as it sounds, this is historically a common state of affairs. Theorists predict a particle or phenomenon, and wait for experimentalists to develop the apparatus to detect the particle. It was even said that the particles were "on order" by the theorists, to be whipped up by the experimental line-cooks. Antimatter particles, hadron resonances, and the top quark are just a few examples of this process. Though it sounds crazy that astrophysicsts have happily decided that dark matter exists and have begun using it to solve problems -- without even knowing it what it is -- it's not an uncommon policy.

    Now we have this entirely new problem, the dark energy, that has taken center stage. Dark energy is most simply referred to as a cosmological constant -- it's a positive vacuum energy term that exerts negative pressure.

    A so-called 'equation of state' is an equation that relates density and pressure. For radiation, the proportionality constant (<font face=symbol>w</font>) is 1/3; for normal matter, it's zero. For dark energy, it looks more and more like it's -1. Experiments like WMAP have been placing strong bounds on this parameter. If it's less than -1, the universe is not stable, and cannot exist. WMAP asserts that it's less than -0.79, and theoretically it must be greater than or equal to -1. If this proportionality constant is -1, then the dark energy is a cosmological constant, just like the one that Einstein first inserted into his field equations. The constant (with <font face=symbol>w</font> = -1) does not affect the other terms in the equations. It's like an "offset."

    So dark energy is not usually considered to be particulate -- you're not going to find any 'darkons'

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    flying around. The dark energy may really just be a characteristic of our universe -- it has a positive vacuum energy density, and the expansion of space is energetically favored to reduce this energy density towards zero.

    Which is more mysterious, dark matter or dark energy? To me, it's dark matter.

    - Warren
     
    Last edited: Feb 25, 2003
  8. hlreed Registered Senior Member

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    245
    Is dark stuff another aether, to justify our notions of motion?

    If we have to have a certain M to make the equations go with observation, then why is that mysterious? We do not know if light has mass, or if there are massive neutrinos. What is the fuss?
     
  9. chroot Crackpot killer Registered Senior Member

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    2,350
    Er, no, it has nothing to do with an aether.
    M = mass, I assume. It's mysterious because we believe only 4.4% of the mass in the universe is protons and neutrons (baryons). Another 22% is dark matter, but it ain't protons and neutrons. That's a mystery.
    Of course we do. Light has no mass.
    Of course we do. Can't you read? Our experiments tell us there are no more than 3 neutrino species, with a summed mass of no more than 0.38 eV. This is many, many orders of magnitude too small to explain dark matter. Whatever dark matter is, it ain't neutrinos either.

    - Warren
     
  10. hlreed Registered Senior Member

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    245
    No, I can't read, I depend on you.

    So do we have an exact count of all the neutrinos?
    Have enough of them with any weight and you have dark matter covered.
     
  11. chroot Crackpot killer Registered Senior Member

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    2,350
    The number density of neutrinos is well-understood theoretically (weak decoupling occured about 118,000 years after the big bang and set the neutrino characteristic temperature and number density) and experimentally (Super Kamiokande, SNO, etc. etc.). You can't just make shit up.

    - Warren
     
  12. hlreed Registered Senior Member

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    What am I making up you little boy?
     
  13. chroot Crackpot killer Registered Senior Member

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    Seemed you were trying to make up the neutrino number density. Little boy?

    - Warren
     
  14. lethe Registered Senior Member

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    very nice, chroot. i had actually been wondering what in the hell dark energy is for a while.

    lemme ask you something. is there any reason to think that the dark energy is not the same as the dark matter? are there very large differences in the required densities?

    as i understand it, vacuum energy is not well understood. QFT basically predicts infinite, or at best, very large vacuum energies, while cosmology measures very small vacuum energies. QFT doesn t really care about it s embarrassing infinite vacuum energy, because it can just be subtracted off, and the energy differences are all that is measurable, and the theory works quite well, so OK. but GR says gravity couples to all energy. we don t worry too much, since we already know that GR and QFT are incompatible.

    so we think that the dark energy is some kind of poorly understood vacuum energy, and we are waiting for quantum gravity to come along and resolve the issue?

    while dark matter has nothing to do with vacuum energy, it is in fact real matter (i.e. not a vacuum, real particles, albeit of unknown type)?

    but i wonder if there is a particular reason that the weird vacuum energy (dark energy) could also be responsible for the galactic rotation rates and such, and account for the dark matter as well...

    maybe they can t be the same thing because of the negative pressure issue? i guess, i m not exactly sure what "negative pressure" means.

    can you comment?


    one more thing:
    i think the LIGO experiment is looking for classical gravitational waves, coming from extremely powerful sources (pulsars falling into black holes, or something crazy, i forget). they haven t seen any yet, but they aren t even expected to see any until a year or so, when they ramp up the sensitivity another couple of orders of magnitude. but they do expect to eventually see them.

    however, the prospect of detecting a graviton is much farther in the future, if at all. to detect gravity at a particle physics level, you probably need to go to the planck energy. in other words, don t count on it happening in your lifetime, or your great grand children s.

    also, some physicists believe that the problem with renormalizing quantum gravity lies with the perturbative approach (LQG people). but the virtual exchange particle is a mechanism of the perturbative formalism. so it is not clear that the concept of a graviton will ever be useful.

    string theorists, however, are more than happy to do perturbative expansions. so they talk about gravitons.

    even so, if gravitons do exist, then they are certainly massless, spin 2. massless, for much the same reason that photons must be massless.
     

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