Sky map and dark energy

  1. wolram

    wolram 3,730
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    In this cosmic cacophony, one particular note was louder than the rest, and it survives to this day as a characteristic wavelength in the clustering of galaxies.

    "Galaxies are slightly more likely to be separated by 500 million light years than by 400 or 600 million light years," Eisenstein told New Scientist. By plotting the positions of millions of galaxies, the Sloan team expects to see this pattern over a wide swath of the universe.

    What sound wave was this and how did the scientists deduce it?
  2. jcsd
  3. I think that one has to be careful when interpreting 3-D or even 2-D largescale images of the observable universe. If you take a "snapshot" of galaxy distribution going out over 8 billion light years, you have to take into account the fact that the position and distribution of matter has been greatly altered by the passage of time and the expansion of space. Using images like this to justify a hypothesis about clustering could be faulty.
  4. LowlyPion

    LowlyPion 5,337
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    I read the article a couple of days ago and looked at the mapping picture and in a bit of serendipity today while making pancakes I observed something that actually looks quite similar to the pictures and it occurred to me that the browning pattern that occurs on the bottom of a pancake, owes in large part the size and noticeable texture of the browned areas to the lighter areas to the heat of the griddle. The hotter the griddle the larger the bubbles and hence the vapor pattern that formed and trapped to create a lighter area. With slight adjustment I could see that I could refine the pattern finer - pouring to pouting - by just a minor heat adjustment of the pan.

    The point is maybe a long way around the barn, and not in any way to offer a primer in making pancakes, but rather to observe that thinking there would be some "fundamental wave" that happenstance may have statistically spread distributions of inter galaxy distances, might rather be associated with a phenomenon that has little to do with any wavelengths, and more to do with the heating and cooling processes in the earliest stages of the universe. Just an observation.
  5. HallsofIvy

    HallsofIvy 40,297
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    It was no "sound" wave. Of course there can't be a sound wave in vacuum. The "note" spoken of is an electromagnetic frequency. If I remember correctly it is actually in the microwave spectrum.
  6. HallsofIvy,
    There are sound waves in space because space is not a vacuum (there's dust and hydrogen and such, even in intergalactic space). The note spoken of is not in the microwave and it's not electromagnetic. You may have thought it's in the microwave because we detected these waves by looking at the temperature anisotropies in the CMB (which is in the microwave).

    The wavelength spoken of relates to sound waves in space when the universe was very young (~100,000 yrs old). The first part of this paper gives a non-technical introduction to what these waves are and might give you the idea for how they are calculated. You can also look at this section of the wikipedia article on structure formation.
    Last edited: Aug 21, 2008
  7. Neutralseer,
    Do you think that expansion of space would cause the apparent "wavelength" to expand over billions of years? If so, wouldn't the more distant expanses of the universe show a shorter wavelength than the closer regions, because of the billions of years difference? My point being that if we are seeing periodicity near us that matches periodicity 8 billion light years away something is not right.
  8. wolram

    wolram 3,730
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    It seems that first link is for a pay site but thanks any way.
    I see from responses that this paper is not one of the top ten favorites:smile:
  9. George Jones

    George Jones 6,363
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    The eprint is here

    I've been meaning for a while to add a few words of explanation to this thread, but I don't know when (or even if) this will happen.
  10. wolram

    wolram 3,730
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    Thank you George, i hope you will help with the explanation, i do worry about theories that have to have Dark stuff in them.
  11. George Jones

    George Jones 6,363
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    To set the stage, I first have to write a few words about the early universe.

    Early on, the universe was much smaller than it is now, and the stuff in it was much more tightly packed than it is now. The universe was a hot, dense mixture of dark matter, neutrinos,and a plasma of neutrons, protons electrons, and photons. The photons had enough energy that they prevented the formation of atoms. The photons scattered around again an again off the electric charges, never managing to propagate very far with respect to their local patch of matter. Effectively, matter and photons were locked together.

    About 380,000 years after the Big Bang, the universe had expanded and cooled enough that most photons had insufficient energy to stop electrons from becoming bound to protons. Atoms formed and photons stopped scattering and started streaming freely through the universe. Although this didn't happen instantaneously, the transition was fairly fast.

    Back to the time before the transition. The mixture that made up the stuff in the universe was an *almost* homogeneous mixture. Some places were , however, slightly more dense than others. These place that were over-dense in everything, dark matter, protons, neutrons, photons.

    Consider one such over-dense region. The pressure of the photons bouncing around prevents the region prevents from collapsing gravitationally. In fact, in such a region the photon pressure can be so high that the photons drive a spherical wave (ripple) of normal matter outwards (more than just normal cosmic expansion) from the centre of the region. Dark matter stays at the center, since photons don't interact with dark matter. This spherical sound wave in the early universe is like a circular ripple in a pond that propagates out from where a pebble plopped in, but this wave is driven by photons.

    When 380,000 year transition occurs, matter and photons become unlocked, i.e., photons stream away freely and the spherical matter wave, without its driving force, stops propagating. It doesn't die completely in amplitude, it just becomes locked into the expansion of the universe.

    So now we have an over-dense dark matter at the centre (remember, the photons didn't drive the dark matter), and a (very slightly) over-dense region of normal matter where the spherical wave froze into the fabric of the universe. The dark matter at the centre gravitationally attracts normal matter and the spherical ripple of normal matter gravitationally attracts dark matter. The centre becomes a region of above average galaxy formation and the spherical ripple become a region of (slightly) above average galaxy formation.

    What is the distance between these regions of above average galaxy formation? 380,000 years ago, the ripple froze in position with respect to its center, and the expansion of the universe has since expanded this distance to about 500 million light-years.

    This story is repeated for many over-dense regions, and so should be noticeable statistically in galaxy surveys. Universe models without dark matter make different (acoustic oscillation) predictions than universes with dark matter.

    I have mentioned dark matter, but I have yet to mention the connection to dark energy. Maybe in another post. Also, if anyone is interested, I think I can find links to explanations of the above.
  12. wolram

    wolram 3,730
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    I am reading with some joy and follow so far so more would be even better.

    Thanks George.
  13. jimgraber

    jimgraber 189
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  14. wolram

    wolram 3,730
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