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  1. #1
    LenderBroker's Avatar
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    Default Dark Matter and Dark Energy: One and the Same?



    Dark Matter and Dark Energy: One and the Same?

    Mon Jul 12, 5:02 PM ET Add Science - Space.com to My Yahoo!

    By Robert Roy Britt
    Senior Science WriterSPACE.com

    Dark matter and dark energy are two of the most vexing problems in
    science today. Together they dominate the universe, comprising some 96
    percent of all mass and energy.

    But nobody knows what either is. It's tempting to consider them
    products of the same unknown phenomenon, something theorist Robert
    Scherrer suggests. The professor of physics at Vanderbilt University
    says "k-essence" is behind it all.

    Dark matter was invoked decades ago to explain why galaxies hold
    together. Given regular matter alone, galaxies might never have
    formed, and today they would fly apart. So there must be some unknown
    stuff that forms invisible clumps to act as gravitational glue.

    Dark energy hit the scene in the late 1990s when astronomers
    discovered the universe is not just expanding, but racing out at an
    ever-faster pace. Some hidden force, a sort of anti-gravity, must be
    pushing galaxies apart from one another in this accelerated expansion.

    Separate theories have been devised to try and solve each mystery.

    To explain dark energy, for example, theorists have re-employed a
    "cosmological constant" that Einstein first introduced as a fudge
    factor to balance the force of gravity. Einstein called the
    cosmological constant a great blunder and retracted it. Yet many
    theorists now are comfortable re-employing it to account for the
    effects of dark energy. But it does not reveal what the force is.

    Scherrer agrees two explanations might be necessary, but he's also
    bothered by that complexity.

    'Embarrassing'

    "It is somewhat embarrassing to have two different unknown sources for
    the dominant forms of matter and energy in the universe," he said in
    an e-mail interview. "On the other hand, that may just be the way
    things are. We don't get to pick the universe we live in."

    To explain it all in one fell swoop, Scherrer invokes an exotic form
    of energy called a scalar field. It's a bit like an electric or
    magnetic field, with energy and pressure and a magnitude. But a scalar
    field has no direction. A scalar field is thought to have been behind
    inflation, the less-than-a-second period after the Big Bang when the
    universe expanded many billions of times before settling into a more
    reasonable rate of growth.

    Scherrer borrows from work by Princeton University's Paul Steinhardt,
    V. Slava Mukhanov at the University of Munich and Christian Armendáriz
    Picón of the University of Chicago, relying on a specific type of
    second-generation scalar field they envisioned called k-essence, short
    for kinetic-energy-driven quintessence.

    K-essence changes behavior over time in Scherrer's model, clumping
    early on to help form galaxies, and now forcing the universe apart.
    Right now, dark matter has a density that decreases as the universe
    expands, he explained, while dark energy has a density that stays
    constant as the universe expands.

    "That means that at very early times, the dark matter 'piece' of the
    k-essence is the dominant one," Scherrer said. "As the universe
    expands and the density of the dark matter 'piece' of the k-essence
    decreases, it eventually falls below the density of the dark energy
    'piece,' and the k- essence behaves more like dark energy."

    Scherrer's model -- not the first trying to tie dark energy and dark
    matter together -- was published July 2 in the online version of the
    journal Physical Review Letters.

    Glaring problem

    There is one glaring problem with the idea, which Scherrer admits to.
    It implies that we live at a very special moment in time when the
    energy densities of dark matter and dark energy are roughly equal.

    Scientists hate coincidences.

    The model also "needs some serious fine-tuning, because its 'dark
    matter' is not permanent but transient," said Mario Livio, senior
    scientist at the Space Telescope Science Institute.

    "There have been quite a few attempts to find one explanation to both
    dark matter and dark energy," Livio told SPACE.com. "In particular,
    there have been theories with modified gravity."

    One example, he said, is to confine all the forces of our universe to
    a four-dimensional plane known as a membrane, or "brane," which is
    sandwiched between other branes. Then let gravity escape to a fifth
    dimension that's perpendicular to the plane, Livio explains. The
    effects of dark matter are then the gravitational influence of other
    branes on ours.

    (Branes have also been used by Steinhardt and other colleagues to put
    a twist on the Big Bang, in which another brane collided with ours,
    releasing energy and heat and leading to the expansion of our
    universe.)

    Connections are great. But that doesn't mean they are right. Exotic
    but widely popular "string theories" of the universe explain dark
    matter as "supersymmetric particles" that bear no relationship to dark
    energy, Livio points out. Serious light might be shed on dark matter
    around 2007, when a particle accelerator called the Large Hadron
    Collider will reach the necessary energies to produce supersymmetric
    particles, if they exist.

    Dark energy, which many experts say is likely to remain mysterious for
    decades, might involve an outside "vacuum energy" that acts upon our
    universe, which many theorists suspect is just one amid many.

    "In spite of the appeal of combining dark matter with dark energy,"
    Livio says, "it is quite possible that the two things do not need to
    be related."

    This article is part of SPACE.com's weekly Mystery Monday series.

    --------------------------------------------------------------------------------
    Copyright © 2004 SPACE.com.

  2. #2
    Sam Wormley's Avatar
    Sam Wormley Guest

    Default Dark Matter and Dark Energy: One and the Same?

    LenderBroker wrote:

    Suggestion: Take your argument to news:sci.physics

    Similar to you reasoning we can't see pink elephants traveling
    faster than the speed of light either.

    "The time has come," the Walrus said, "To talk of many things:
    Of shoes, and ships, and sealing wax, of cabbages and kings.
    And why the sea is boiling hot and whether pigs have wings."

    -- L. Carroll

  3. #3
    Greg Crinklaw's Avatar
    Greg Crinklaw Guest

    Default Dark Matter and Dark Energy: One and the Same?

    Sam Wormley wrote:


    What are you going on about? These appear to be articles from
    space.com, not arguments...


 

 

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