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    Dark Energy, Dark Matter, Dark Force: Not Afraid Of The Dark
    By Sascha Vongehr | August 19th 2011 12:51 AM | 7 comments | Print | E-mail | Track Comments
    About Sascha

    Dr. Sascha Vongehr [风洒沙] studied phil/math/chem/phys in Germany, obtained a BSc in theoretical physics (electro-mag) & MSc (stringtheory)...

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    One problem with the acceptance of dark energy and dark matter is that they are presented as if they are brothers, as if presenting merely the start of a new line of cheap ad hoc fudge factors: Dark energy, dark matter, dark force, what next? However, dark energy is not a fudge factor and not new.

    dark energy dark matter


    Similar has been pointed out before, namely that dark energy is much less weird than usual pressure in Einstein’s general relativity:

    The crazy point about dark energy is that something that pulls itself together expands, right? But that is not the contribution of dark energy. That part is usual general relativity. It has been accepted for almost a century: Pressure, although it pushes and "wants to expand", contributes to gravity, which pulls and leads to contraction.


    Let me try a different way:

    Take a usual pendulum, like a massive bob hanging from a string, and watch it swinging. Somebody writes down the equation that describes this pendulum: It has mass M and the length of the string L and also a term for the gravity of the earth for example. You can see how simple these formulas are at Wiki's Harmonic Oscillator. Now say that you see the swinging slowly damp down. No problem: There is a damping term D in the equation that describes that. Now somebody opens the window. The wind turbulences due to the window’s shape render the air flow around the pendulum somewhat periodic. You see the pendulum swinging more again – it accelerates. It accelerates just slightly and you yourself do not even feel the wind at all. Mysterious?



    Of course not. And the equation that describes this is easily modified: Add an applied force term F(t). These terms in the equation of the harmonic oscillator, the damping D and the driving F(t), are completely natural. Especially, if you have another pendulum, say a vibrating string or a mass hanging from a spring, you apply the same or a very similar formula.



    If you enter your observations of the oscillation, the formula will tell you whether there is damping going on: the damping D turns out to be non-zero. It also tells you whether there is a driving force that feeds the oscillation, namely if F(t) is non-zero.

    You would be a fool to look at the data of a pendulum and start to reject physics just because the observations tell you that F(t) is non-zero. You might not yet know what exactly is driving the system, the wind or a magnet or whatever, but something pushes it.


    Dark energy comes from the classical theory of relativity known for a century now. You have the usual equation and you look at a particular system to which the equation is applicable, the universe, and put your observational data, here from the observed expansion of space, into the formula. There is a driving term Λ (lambda) in there. Lambda was not put in there by hand much later. It should be in there, just like there should be a damping D and a driving F(t) in your equation if you want to describe an oscillator that you come across somewhere.


    Here is Einstein’s equation Guv + Λ guv = Tuv with the pressure-energy density tensor T on the right hand side and gravity, the Guv, on the left side. Observation tells us that Λ is non-zero. This is called dark energy or, if it turns out to be constant over time, also cosmological constant.


    Does dark energy exist? Does the driving term in the formula of the harmonic oscillator exist? It is right there on the paper where I wrote it down, and it would even exist if it turned out to be zero - but guess what, it is bigger than zero.


    You may ask what the fundamental nature of dark energy is: Is it quantum, vacuum energy for example? There are many unknowns, but it is not valid to say that dark energy does not exist or has not been observed. Of course it is observed. It is the observed data that leave the dark energy term non-zero! And that is dark energy – the lambda term does have the units of energy – just like the pressure-energy density tensor T right next to it (g has no units).


    Some do not "believe" in dark energy. Imagine I measure the damping term D of a pendulum to be non-zero and then people blabber about that "friction" cannot possibly be because what on earth is "friction" supposed to be.


    Everybody who throws dark energy and dark matter into one pot is highly suspect. Dark matter can be called a hypothesis, a good one by the way, although dark matter does not fit well to galaxy rotation curves – Modified Newtonian Dynamics (MOND) fits better (See: Begeman, Broeils, &Sanders 1991; Sellwood&McGaugh 2005). The driving term F in the formula describing an oscillator is not a hypothesis.

    Comments

    MikeCrow
    What's your thought on MOND? Data does seem to line up better with MOND than CDM.

    I will note that MOND can have it's coefficients adjusted to whatever they'd like, making it match better. So that in itself, IMO doesn't necessarily make it more correct.
    Never is a long time.
    vongehr
    MOND adjustable terms are less arbitrary than MOND-enemies portray. What is generally misunderstood about MOND: The model does not claim that the universe is just working according to a modified, classical mechanics like Newton's. Such would be silly and MOND guys are not silly! It is not correct to say that MOND just explains rotation rates and no more. MOND is simply an indication of that there is an as yet neglected term, possibly due to emergent gravity (Eric Verlinde also now models rotation rates!). Once we figure out the actual nature of the MOND model acceleration term, this may very well be responsible for all the other stuff that dark matter and its adjustable parameters now more or less successfully model, even the bullet cluster.
    The Stand-Up Physicist
    I had the impression that the data about one galaxy passing through another was a bullet in the head for MOND. The center of the gravitational mass is in a different place than the center of the inertial, visible mass.

    When the good Swiss teacher Balmer came up with his equation for the spectral lines of hydrogen, it just worked. It took Bohr's work quantizing angular momentum to justify the form of the equation. Do you think we may be in a similar situation with MOND? Whatever proposal supersedes MOND to be at peace with the bullet cluster data would also have to have the same math properties as MOND to match the first graph in your blog (1/R dependence for the force law). This constraint is one reason I remain skeptical of dark matter.
    vongehr
    Yes, the bullet cluster and dwarf galaxies argue strongly for dark matter, but a mechanism from emergent gravity, say entropic gravity, could conceivably explain these. Dark energy is different. It does not matter whether your different formula for a pendulum reshuffles and renames terms or maybe discover that the pendulum is slowed down by dancing octonions. If a pendulum slows down, there is damping, period.
    "Not Afraid Of The Dark"

    "When you walk through the storm hold your head up high and don't be afraid of the dark..." :)

    So, the next article will be on the possible nature of dark energy?

    A theory (MOND) which reproduces actual structure in hard data, vs a smooth curve that hardly fits, should not be ignored out of hand. An example comes to mind, that is, trying to fit the theory of UV molecular absorption to an actual IR spectrum of the same molecule. While the theory of UV electronic absorption is correct, it will only produce a smooth curve through the structured IR spectrum. However, the theory of IR ro-vibronic absorption will fit the structure exactly. I realize many bright minds have tried to explain why MOND fits the data so well and have failed. Still, I belive MOND is telling us something important, which we are missing completely. While I do not believe in String Conjecture (it is hardly a 'theory' by any metric), perhaps, on large scales, the universe is only 2D (a brane), which is what MOND is suggesting. Perhaps the 3D we see is only the finite thickness of a larger 2D 'landscape.' Sorry, had to sneak that in there for fun. As for dark energy, as a hyper-meticulous, Virgonian experimentalist, I believe time will show the perceived acceleration, to be an artifact. Granted, the supernovae data set is impressive, but the amount of massaging that goes into the data reduction is suspect. Sort of like all of the, up to now, hints of Higgs. Final prediction, SUSY and Higgs bosons, do not exist.

    James Ph. Kotsybar
    DARK ENERGY QUESTION  -- James Ph. Kotsybar 

    Could inflation have done more than we know, 
    shortly after the Big Bang’s first salvo, 
    and created a dense matter halo 
    beyond the horizon where we can go? 

    Beyond the horizon that we can see, 
    is there a remote possibility 
    of a most massive field of gravity 
    that pulls the strings of our reality? 

    Perhaps it’s just dense matter that’s the source, 
    accelerating expansion perforce, 
    and not some new and mysterious force, 
    or change of gravity’s attractive course, 
    as though we are bound by a black hole’s skin, 
    that stretches space to surface dimension.