On Dark Energy.

  1. quasar987

    quasar987 4,770
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    Another one of the physics teacher working at the college I attended (see my previous post) wrote a book on basic Astronomy and Astrophysics. In this book it says that the expansion of the Universe is due to the expansion of space itself and that this expansion of space is entirely predicted by Einstein's theory of GR. Now, if this is so, where does the DARK ENERGY (suposedly a MYSTICAL force of repulsion between matter) fit it ?!
     
  2. jcsd
  3. this dark energy is a result of a constant that Einstein added to his equation. A positive cosmological constant predicts that there must be a sort of anti - gravity in the universe.
     
  4. pervect

    pervect 8,015
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    Here's a very brief history of cosmology post-Einstein:

    1) Einstein comes up with GR.
    2) Einstein proposes a static universe, with a cosmological constant to keep it from collapsing from its own gravity
    3) Hubble observes that the universe isn't static, that it's expanding
    4) Einstein recans the cosmological constant.
    5) Varioius people make more and more observations of the rate of expansion, to compare it with the model predicted by General Relativity, called the Einstein-Friedmann cosmology. The cosmological constant goes in and out of fashion
    6) Recent measurements of the rate of expansion put the cosmological constant back "in fashion" again, because certain experimental results are not fitting otherwise. The cosmological constant is given a new popular name, 'dark energy" (which could also include mechanisms that are like Einstein's original cosmological constant in effect, but different in origin).
     
  5. marcus

    marcus 24,742
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    in the einstein equation (1915-1917) the cosmological constant Lambda was just some additional curvature built into space over and above that caused by matter.

    we are used to thinking of all the kinds of matter and energy there are in the world being summed up in an overall "energy density" function----some fraction of a joule per cubic meter.

    if you know SI metric units you will recognize the unit "joule per cubic meter" as a Pascal (it is the same unit as saying "newton per square meter")

    in the 90 years theyve had the einstein equation people have gotten used to thinking of all curvature (on the LHS of the eqn) as resulting from the Pascals of matter on the RHS of the eqn----the energy density and pressure. that is:"Matter shapes space and the shape tells matter how to flow." (Bingo. Cuckoo. whatever)

    the cosmological constant might be some extra curvature on the LHS which is not caused by any matter on the RHS it might just be some intrinsic curvature space was born with.
    but this is not the habit.
    the habit is to attribute the extra curvature to the presence of some postulated energy which also has a postulated pressure, and which has the observational effects associated with the extra curvature.
    we are obliged by 90 years of habit to imagine the existence of dark energy because energy density is where curvature ordinarily comes from

    keep skeptical and have fun
     
    Last edited: Aug 23, 2004
  6. marcus

    marcus 24,742
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    if you have any interest in dark energy at all then you should get
    familiar with the Friedmann equations

    they are what cosmologists use instead of the raw Einstein equation
    because they are much much easier to work with

    and they show the energy density and the pressure explicitly on the RHS

    and there should be a link to them, or a post about them, in the
    Astronomy Cosmology Reference Sticky at PF General Astronomy forum.
    I'll see if i can find it. Otherwise just google Friedmann equation.
     
  7. could dark matter be "strings" joined together to become very long and dense hence the reason we can't see them as they are only planck length wide but anywhere up to infinitely long ???

    ...dark strings
     
  8. Garth

    Garth 3,488
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    Actually this post is about Dark Energy, but no matter, we cannot see Dark Energy or Dark Matter (it is too dark) or 'strings' for that matter (they are too small). Dark Energy and Matter are only observed in the depths of space where the observations are theory dependent. That theory being GR, if the theory changes then the observations will change, or disappear altogether.

    Unless we come up with something that actually is observed, not only in the depths of space but also in the laboratory, then perhaps all we are doing is 'adding extra epicycles' to make the standard theory work.

    See my post #10 https://www.physicsforums.com/showthread.php?p=284031#post284031
     
  9. thanx Garth...

    so where do you propose this paradigm will shift to...can any variant of string/brane theory currently account for dark matter or energy, eg a dark string speculation ???

    and isn't it true that inflation requires early universe matter to be flung out at speeds exceeding c making it another contradiction of GR ???

    cheers
     
  10. Garth

    Garth 3,488
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    You may be interested in my thread "Self Creation Cosmology - a new gravitational theory" https://www.physicsforums.com/showthread.php?t=32713 because SCC does not require Inflation, unknown Dark Matter or Dark Energy. There is Dark Matter but it is ordinary baryonic matter such as hydrogen and helium, it is dark in the sense of being non-luminous and not being an unknown substance. The theory may or may not stand up but it is falsifiable and is at present about to be tested by the Gravity Probe B satellite.
    No - the restriction on not going faster than light only applies within space-time. Inflation is talking about space-time itself expanding at an enormous rate, objects (not that there were any then!) within space-time would have been carried along with that general expansion. At present with the Hubble flow, or expansion of the universe, the galaxies are not moving within space-time but it is space-time itself which is expanding and we, and everything else, are simply being carried along with it. Distant galaxies beyond our event-horizon are moving away from us at velocities greater than the speed of light, that is why we cannot see them.
     
    Last edited: Aug 26, 2004
  11. The thing is, if you look at the CMB as consistant, at what point would this be so?

    If you have consider how pearls and chains are formed in a universe that is cooling, and you need some framework in which to comprehend the interlinking capability.

    So in the beginning we look at how supersymmetrical states would have existed and how the expansitory universe, would neuronically connect. :smile:

    http://astro.uchicago.edu/~andrey/soft/p3d/p3d_evol.gif

    http://www.gsfc.nasa.gov/gsfc/spacesci/pictures/20020812gamma/denset.jpg

    http://www.gsfc.nasa.gov/topstory/20020812gamma.html
     
  12. Nereid

    Nereid 4,014
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    This question is about theory; at least as important - since this is science, arguably more important - is experiment and observation.

    AFAIK, there are two classes of observations (no experiments :cry: ) which support 'dark energy', distant supernovae and 'all cosmologically relevant observations'

    1) Distant supernovae. Ever since Hubble first published obsevational results which pointed to an expanding universe, astronomers have been keen to characterise that expansion as accurately as possible - over all distance scales, and in all directions. To do this, they need independent measures of both distance and recession speed. The latter is relatively easy to measure, with great precision - the 'redshift' of galaxies and quasars; the former has proven immensely difficult, and only in the last decade has a broad consensus emerged (there is still considerable 'observational error', and there are still some dissenters). Distance can be measured by a number of different types of observations, and the 'distance ladder' is now reasonably well established, out to perhaps as much as 10 billion light years.

    While redshifts are relatively easy to measure, their interpretation as 'recession due to the expanding universe' has had its challenges and upsets. Perhaps the biggest was 'The Great Attractor'; apparently there is an awful lot more mass in the local part of the universe (~100 Mpc) than can be 'seen'.

    So, the 'expanding universe' hypothesis predicts that the universe is, and has been, expanding uniformly - in all directions, and at all times (at least, after any inflation ended; observationally we can't directly see any earlier than ~300,000 years after the BB, well after any inflation finished), over sufficiently large chunks of the universe (superclusters have sufficient mass so 'expansion' of objects within a supercluster may be small compared with net gravitational attraction from masses within the supercluster).

    What do observations show? The best distance indicator for very distant objects is Type 1a supernovae - we think we understand their behaviour sufficiently well so good observations of their lightcurves can be turned into estimates of distance (this 1998 poster gives more details). Plotting distance against redshift shows that the data are not on the curve predicted by 'uniform expansion' (it's not quite as clear-cut as this; different models of the universe - e.g. with different amounts of dark matter - give different curves, but no model that is consistent with other observational data - e.g. WMAP - goes near the data). What model curves will go through the data? Those in which the rate of expansion of the universe is increasing!

    'Dark energy' is the shorthand that is used for whatever might be causing the observed acceleration of the universal expansion.

    But not so fast! How can astronomers be so certain that there aren't other effects involved? Initially, quite a bit of work had to be done to nail these down, e.g. do we understand Type 1a SN sufficiently well? what if the distant SN are partly obscurred by dust? and so on. Many of these 'systematic errors' have now been characterised and their effects on the data taken into account. However, some feel that at least an OOM more of good data are needed (SNAP to the rescue?)

    2. All 'cosmologically relevant observations': As Garth correctly points out, this is a bit of chicken and egg. Basically, you take all observations that have relevance to cosmology - WMAP and others on the CMBR, large scale structure (e.g. 2dF, SDSS), primordial nuclide abundances, the Hubble relationship, the distant SN data, ... - and see what sort of cosmological model is most consistent with it all. The idea is that there is more than enough data to constrain models, so if the models are wrong, there will be obvious inconsistencies. The good news is that there are models which are consistent with all the data ... but only those which have 'dark energy' in them!

    Further reading: Wikipedia
    APOD
    Snowmass 2001, Yellow Book on Dark Energy
    short article - model-independent dark energy
    nice, bite-sized physicsweb summary
     
    Last edited: Aug 26, 2004
  13. Garth

    Garth 3,488
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    Not quite "only those with Dark energy in them"!

    As we have discussed in a previous thread an Indian team, consisting of Gehaut, Lohiya et al., have been looking at the strictly linearly expanding or "freely coasting" universe. It fits exactly all the constraints!

    Their papers can be found at:
    A Concordant "Freely Coasting Cosmology"
    http://arxiv.org/abs/astro-ph/0306448

    A "Freely Coasting" Universe
    http://arxiv.org/abs/astro-ph/0209209

    They say "A strictly linear evolution of the cosmological scale factor is surprisingly an excellent fit to a host of cosmological observations. Any model that can support such a coasting presents itself as a falsifiable model as far as classical cosmological tests are concerned." (Taken from the abstract of the first paper above)

    One theory that does indeed "support such a coasting" is SCC - in its Jordan Frame formulation- without requiring Dark Energy. [See my paper "Self Creation Cosmology - An Alternative Gravitational Theory" http://arxiv.org/abs/gr-qc/0405094 (to be published in 'Progress in
    General Relativity and Quantum Cosmology Research', Nova Science
    Publishers, Inc. New York.)]
     
  14. marcus

    marcus 24,742
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    Hi Nereid, at one time I thought you made a nuanced distinction between
    the cosmological constant and dark energy. Does this "whatever might be causing" definition get rid of any earlier distinction you may have made and simply lump the two? I'm not certain I understand why you have dark energy in quotes, and consistently write 'dark energy'. I don't recall your doing this before (perhaps you did and I simply failed to notice). It would be interesting to know if your thinking has been changing about this. Would you care to clarify your own view?
     
    Last edited: Aug 26, 2004
  15. Maybe she is softening like Peter Woit? :biggrin: Maybe one day, I will convert you too, Marcus. :rofl:

    Oh Marcus, could you correct your quote? :smile:
     
    Last edited: Aug 26, 2004
  16. marcus

    marcus 24,742
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    he?

    Who do you mean sol?

    Nereid is a she. Both she and Peter Woit are crisp thinkers, in my view, and soften rarely. On the other hand, even when you might be joking you have intuitions about people that are worth listening to, so I will consider it a possibility :smile:
     
  17. Nereid

    Nereid 4,014
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    Thanks for the clarification marcus!

    In these discussions it can be quite difficult to write something brief that also reflects all the major aspects in play ... let alone ALL aspects which can be said to fit (somehow). Garth's post is a good example of this - a cosmological model which the authors claim to be consistent with all the observational data, and one which differs markedly from the 'concordance model' (no doubt there are others out there too ... with relatively weak observational constraints, theoreticians can have the most marvellous fun imaginable :wink:).

    From the observational perspective, at the current level of constraint, both a cosmological constant and some kind of 'dark energy' are pretty much indistinguishable, though we are all looking forward to the day when that ceases to be! Since "cosmological constant, 'dark energy', or something else; anyway, whatever gives rise to the observed acceleration of expansion" is anything but shorthand, I think you'll find that 'dark energy' is more convenient :smile: (Of course, when talking 'theory', cc and DE are certainly distinguised!)
     
  18. Nereid

    Nereid 4,014
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    Message from the global collective of Nereids, their partners, friends and families: "On the Internet, nobody knows you're a dog" Woof! Woof!!

    (and no, lady dogs do NOT reveal their breeds :shy: )
     
  19. marcus

    marcus 24,742
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    that's my good old Nereid :smile:


    see sol? crispness :tongue:
     
  20. I made correction Marcus in previous post.

    Yes if you cook the bacon to long it can become crisp:)

    The very foundational principals have to have a basis in which to move from. I scream loudly scientific verification is the only road too :cry: :cry: and anything less is theoretical speculation. There, I feel better :biggrin:

    When one sends out it's tentacles from the mind, it can flop quite freely if you do not have control. So wild speculation would have been loosening a grip to what we have known for certain, and what shakes our foundation.

    I often compare it to the view I had of the Grand Canyon and the wide open expanse, yet I held firmly to the rail :rofl:
     
  21. marcus

    marcus 24,742
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    but a guy dog with a good nose can tell if its a lady---forget about breeds we are talking about what matters, nuff said, and WOOF WOOF to you too
     
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