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I View from the Void

  1. Aug 23, 2016 #1
    Imagine you could jump in space/time from Earth to a point in deep intergalactic space smack dab in the middle of a void, those regions where dark energies are at their strongest. What would the view be like?

    Of course it's not total dark. You still have the light of distant galaxies. Would the galactic cluster nets become obvious then since there isn't the usual array of a star field?

    Would dark energy create any distortions?
    laniakea-universe-milky-way-map.png
     
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  3. Aug 23, 2016 #2

    Bandersnatch

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    :welcome:
    Isn't it? How many distant galaxies can you see from Earth with a naked eye?
     
  4. Aug 23, 2016 #3

    phyzguy

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    I think you have a misconception here. Dark energy, as accurately as we can measure today, is a constant energy density of empty space that does not vary in time or in space. So it is no stronger in the center of a cosmic void than it is here in the solar system.
     
  5. Aug 23, 2016 #4

    Drakkith

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    We'd be able to clearly see a little more of the sky since we wouldn't have the intervening dust of the milky way blocking our sight, and of course we wouldn't have the low glow of the milky way's background stars nor the bright points from very luminous stars nearby, so that would probably help some. However these would not be visible to the naked eye. They are far too dim, even in a void.
     
  6. Aug 23, 2016 #5
    I understand your reply Phyzguy. I just put the question there because there might be some point I might have overlooked with dark energy being a factor to the visual. I wasn't expecting it would be.

    Basically my question sort of revolves around this meme:
    all-the-stars-you-see-at-night.jpg

    If we're in a space ship outside the milky way spiral in the void of deep space would we be able to see Andromeda or other galaxies with the naked eye?
    Would the network of Laniakea be revealed before us (depending on our distance of course)?

    So Drakkith, are you saying that even in a void of space away from everything, that galaxies are too dim to be visible to the human eye?
     
  7. Aug 23, 2016 #6

    Bandersnatch

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    Under perfect conditions (full dark adaptation), an unaided eye can view objects up to magnitude 8 - for extended objects this is a bit harder, because the contrast is lower for the same amount of light. This is assuming somebody with very good eyesight.
    Take the Andromeda, a typical large, bright galaxy - at 3.5 mag and 2.5 million light years (Mly), it would become invisible at approx. 10-15 Mly*.
    A supercluster like Laniakea has an extent of approx. 500 Mly. As long as you're standing farther away than 15 Mly from any galaxy, you'd see complete darkness.

    *four magnitudes difference is approx 40 times dimmer (2.5^4), which corresponds to approx 6-7 times further away (by the inverse square law)
     
  8. Aug 23, 2016 #7

    phyzguy

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    So I think you got your answer. It would be pitch dark.
     
  9. Aug 23, 2016 #8
    NOW THAT IS WHAT I WAS LOOKING FOR!!!!
    Thank you Bandersnatch.

    So the key factor is distance in determining viewable luminescence. Laniakea could not be seen by a human eye. And in the void of space we could only just make out a galaxy similar to Andromeda as long as we're within 15 Mly. That is super helpful to visualize.

    Silly next question: Wouldn't there be the occasional rouge star wandering the voids?
    Stars that were tossed out of galactic clusters for one reason or another.
     
  10. Aug 23, 2016 #9

    Bandersnatch

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    There do exist rare stars with velocities higher than galactic escape velocity, so you do get those. An event like a galactic collision may result in a larger batch being ejected into intergalactic space. However, individual stars are dim as compared with a galaxy, so you should apply the few thousand light years visibility rule the meme tries to make a point of. This, combined with the vast expanse of empty intergalactic space makes it rather improbable you'd have one in your randomly-chosen sky in a void.
     
    Last edited: Aug 23, 2016
  11. Aug 23, 2016 #10
    Thank you so much Bander!
     
  12. Aug 23, 2016 #11

    Bandersnatch

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    By the way, Hayden planetarium realeased a free 'Digital universe' software for visualising extra- and intra-galactic observational data sets. Most of the Laniakea cluster is covered by the (almost) full-sky Tully survey included in the datasets. You can get a very nice visual intuition on how many galaxies you'd see from where by turning the distance grid on, using the 15 Mly rule of thumb, and navigating around.
    http://www.amnh.org/our-research/hayden-planetarium/digital-universe/

    The software requires a bit of effort to learn to operate (not the most user-friendly interface), but it's worth it, IMO.
     
  13. Aug 23, 2016 #12

    Drakkith

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    Yes I am. You would see nothing but blackness if you looked out the window.
     
  14. Aug 23, 2016 #13
    This is interesting, but wouldn't Worldwide Telescope, Celestia or Digital Universe Atlas (all free software) serve the same purpose while being more user friendly?

    -------------------

    Eh, nevermind... it's nice.
     
    Last edited: Aug 23, 2016
  15. Aug 24, 2016 #14

    Bandersnatch

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    Digital Universe Atlas and Digital Universe are the same thing :)
    Other than that, DU beats the other two in terms of extragalactic visualisation. It has the most extensive data sets, from a number of surveys (whereas the other two rely on Sloan only).
     
  16. Aug 24, 2016 #15
    I installed the app and WOW! Went through the PDF and figured the interface out. So much phenomenal information and useful references. Quite invaluable to any author writing a sci-fi series that wants a vital ring of authenticity. My story's setting would take place in one of those voids. Clearly I'll need to be some distance closer to get a visual frame of reference. Total black would just confuse most readers, not to mention would be the superlative in visual boredom.

    I'm having trouble finding information on "Tully’s smoothed density surfaces" via Google searches. My best guess is this is a map of dark matter density based upon gravimetric lensing. Though I wouldn't gamble a dollar on it. Am I off the mark?

    Thank you so much for sharing your knowledge!
    -Gary T.
     
  17. Aug 25, 2016 #16

    Bandersnatch

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    I think these are isosurfaces. Not representations of any sort of actual physical extent of any type of matter, but rather averaged densities of galaxies - sort of like graphing the distribution of matter in a way that lets you see structure. Similar technique is used in drawing electron orbitals (but with probability instead of density).

    You take a galaxy, which normally is shown as a point-like concentration of matter, and pretend that all its mass is 'smeared' across a volume around it, where the volume is scaled is such a way, so as to keep the density within constant (for example, 1 Milky Way mass per 1 cubic Mly - or anything else, the choice is arbitrary). I.e., a more massive galaxy will produce a larger volume around it. If the volume of constant density overlaps with another, from another galaxy, then it will 'bulge out' where the two intersect, so that the local density encompassed by the two volumes stays constant. If you have a lot of massive galaxies in a line next to one another, you get these elongated blob-like shapes.

    Another way of seeing it is asking 'how can I encompass galaxies with a surface, so that the density enclosed remains at some chosen constant value?'

    There seems to be some cut-off volume employed, so that regions with too little density (i.e., not enough mass per unit volume) are not displayed. There are also two isosurfaces shown, one encompassing higher density than the other.

    As a result you can easily see which regions have a lot of matter in them, and that there's some 'clumping' involved.



    I don't know whether the masses used for calculating these isosurfaces include dark matter, or just luminous matter. In any case, while dark matter content does vary from galaxy to galaxy, it's not a bad first approximation if you assume it's proportional to luminous matter. If you do that, then the isosurfaces will have the same shape with or without DM (but scaled differently).

    My understanding is that the actual DM distribution follows closely galaxies themselves, so that most of it is concentrated in roughly spherical 'halos' around each galaxy, with sizes on the order of a few times the size of the galaxy. If it was included in the visualisation, it'd make the points representing galaxies puff-up somewhat, but it'd be nowhere near in extent to the sizes of those isosurfaces.

    Anyway, that's how I read it.
     
    Last edited: Aug 25, 2016
  18. Aug 25, 2016 #17
    Another thorough, in depth response. Wonderful!
    I've got a good handle now. From the land of the Keck Observatory, mahalo nui loa!
     
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