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Dark Fluid: serious MOND-like theory unifying DM-DE or crackpottery?

  1. May 16, 2010 #1
    Dark Fluid: serious MOND-like theory unifying dark energy-matter

    Reading Saul's thread on dark matter on the ropes,

    I wondered whether some sort of theory involving a fluid, not particle, of inhomogenous density could explain observation, and perhaps be related to dark energy, and lo and behold, there are proposals of dark fluid.

    Instead of two unaccounted theories, dark matter and dark energy, there is only one, a dark fluid that has attractive gravitational attraction within galaxies, and repulsive positive cosmological constant like effects at extragalactic scale.



    The two biggest mysteries in cosmology may be one. A new theory says that dark matter and dark energy could arise from a single dark fluid that permeates the whole universe. And this could mean Earth-based dark matter searches will come up empty.

    Dark matter, as originally hypothesized, is extra hidden mass that astrophysicists calculate is necessary for holding together fast-turning galaxies. The most popular notion is that this matter is made of some yet-to-be-identified particle that has almost no interactions with light or ordinary matter. Yet it seems to be everywhere, acting as a scaffolding for galaxy clusters and the whole structure of the universe.

    On the other hand, dark energy is needed to explain the more recently-discovered acceleration of the universe's expansion. It supposedly exists all throughout space, delivering a pressure that counteracts gravity.

    It's counterintuitive that one substance could be both a gravitational anchor for galaxies and anti-gravity force for the universe. However, HongSheng Zhao of the University of St Andrews in Scotland claims that a fluid-like dark energy can act like dark matter when its density becomes high enough.

    "Dark energy is a property of the vacuum — of fields that we do not easily see," Zhao told Space.com. "From it, we can derive the dark matter effect."

    Zhao compares this dark fluid to Earth's atmosphere. Atmospheric pressure causes air to expand, but part of the air can collapse to form clouds. In the same way, the dark fluid might generally expand, but it also could collect around galaxies to help hold them together.


    Zhao is not the first theorist to try to bring dark energy and dark matter under the same framework.



    Efforts are currently underway to hunt for very massive dark-matter particles with a variety of experiments. The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Geneva is a particle accelerator that amongst other objectives, could potentially detect dark matter particles.

    According to Dr Zhao, these efforts could turn out to be fruitless. He said, "In this simpler picture of universe, the dark matter would be at a surprisingly low energy scale, too low to be probed by upcoming Large Hadron Collider.

    "The search for dark-matter particles so far has concentrated on highly-energetic particles. If dark matter however is a twin phenomenon of dark energy, it will not show up at instruments like the LHC, but has been seen over and over again in galaxies by astronomers."

    However, the Universe might be absent of dark-matter particles at all. The findings of Dr Zhao are also compatible with an interpretation of the dark component as a modification of the law of gravity rather than particles or energy.​


    Dark Fluid: Towards a unification of empirical theories of galaxy rotation, Inflation and Dark Energy
    Authors: HongSheng Zhao (SUPA, St Andrews) Baojiu Li (DAMTP, Cambridge)
    (Submitted on 10 Apr 2008)

    Abstract: Empirical theories of Dark Matter like MOND gravity and of Dark Energy like f(R) gravity were motivated by astronomical data. But could these theories be branches rooted from a more general hence natural framework? Here we propose the natural Lagrangian of such a framework based on simple dimensional analysis and co-variant symmetry requirements, and explore various outcomes in a top-down fashion. Our framework preserves the co-variant formulation of GR, but allows the expanding physical metric be bent by a single new species of Dark Fluid flowing in space-time. Its non-uniform stress tensor and current vector are simply functions of a vector field of variable norm, resembling the 4-vector electromagnetic potential description for the photon fluid, but is dark (e.g., by very early decoupling from the baryon-radiation fluid). The Dark Fluid framework naturally branches into a continuous spectrum of theories with Dark Energy and Dark Matter effects, including the $f(R)$ gravity, TeVeS-like theories, Einstein-Aether and $\nu\Lambda$ theories as limiting cases. When the vector field degenerates into a pure Higgs-like scalar field, we obtain the physics for inflaton and quintessence. In this broad setting we emphasize the non-constant dynamical field behind the cosmological constant effect, and highlight plausible corrections beyond the classical MOND predictions. Choices of parameters can be made to pass BBN, PPN, and causality constraints. The Dark Fluid is inspired to unify/simplify the astronomically successful ingredients of previous constructions: the desired effects of inflaton plus quintessence plus Cold DM particle fields or MOND-like scalar field(s) are shown largely achievable by one vector field only.
    Last edited: May 17, 2010
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