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Does the number of galaxies change with time?

  1. Jul 21, 2012 #1
    As the universe age, the distance to the cosmic horizon increases, and previously hidden galaxies beyond the horizon should become visible.

    Due to the expansion of the universe though, the distance between the galaxies increases. This has the effect that a galaxy that we can see today, would be past our cosmic horizon in the future.

    Right now in the cosmic history, do the number of galaxies in the visible universe increase or decrease with time (not counting new galaxies formed within the visible universe)?
     
  2. jcsd
  3. Jul 21, 2012 #2
    It decreases. After enough time, in the distant future, we will be the only galaxy in our observable universe.
     
  4. Jul 21, 2012 #3

    andrewkirk

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    In addition to the accelerating expansion of the universe pushing now-visible galaxies outside the observable universe, there is the fact that some galaxies collide and, presumably, merge. I think we are destined to collide with Andromeda some time aren't we?
     
  5. Jul 21, 2012 #4

    Chronos

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    With the JWST, galaxies as remote as z~15, or about 275 million years after the big bang, may be detectable. This is the era during which the first stars in the universe are believed to have formed.
     
  6. Jul 22, 2012 #5

    marcus

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    Andrew you are right about Andromeda merging with Milkyway. there was recent confirmation: more accurate measurement of the sideways motion. It's approaching and according to the best data so far, the sideways motion is not enough to cause a miss. This will not necessarily disrupt the solar system. The sky might be extra pretty for a while but it won't necessarily hurt, it is believed.

    It sounds like JMD doesn't want to count new galaxies being formed (out of gas) or mergers diminishing the count either (as you suggest). He may be thinking in terms of amount of stuff, in whatever form.

    It sounds like the idea is to imagine the number of galaxies remaining in fixed proportion of total matter within sight. So the question could be rephrased: is the visible portion GAINING OR LOSING matter in any form? That would correspond to the visible portion gaining or losing galaxies because the proportion doesn't change.
    If it were losing it would be by stuff receding out beyond our socalled "particle horizon" (the radius of the observable region). If gaining it would be by the "particle horizon" extending out to include more stuff.
    If it's gaining stuff, then its galaxy count is rising, if it's losing stuff then its galaxy count is falling. Because galaxy count corresponds approximately to amount of stuff.

    Our present particle horizon is at a COMOVING distance of about 46 billion light years. That's where the stuff is that we see when we observe the CMR (cosmic microwave radiation). We see it as it was a long time ago when it hadn't formed galaxies yet---still hot gas. But

    BUT IF THE PARTICLE HORIZON SHOULD INCREASE TO comoving distance of 47 billion light years THE VISIBLE UNIVERSE WOULD INCLUDE MORE MATTER AND WE WOULD SEE MORE GALAXIES.

    Essentially or roughly speaking the same COMOVING volume always contains the same amount of matter, and averaged over time the same number of galaxies.

    That's the nice thing about comoving distance, which simply means PROPER distance measured at the present moment. Worldlines of galaxies stay at approx the same comov. distance from us and from each other.
    =====================

    I'm interested in what you think, because I know you are learning this stuff and are good with math too.

    Is the particle horizon radius of 46 Gly increasing in comoving terms?

    Is the comoving volume of the observable universe (which means the number of galaxies we can see in the sky, because it's roughly proportional) increasing?
     
    Last edited: Jul 22, 2012
  7. Jul 22, 2012 #6

    marcus

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    I would say right now the visible galaxy count is increasing.

    Over the next 10 billion years I expect the particle horizon to extend out from present 46 Gly to about 55 Gly (comoving distance).

    So the comoving volume of the observable will increase by a factor of the cube of 55/46.
    That is by a factor of 1.4.

    So the number of galaxies in the visible universe will increase by 40%.

    At the same time they will gradually get HARDER TO SEE. But not not impossibly hard (yet). The scalefactor will be twice what it is today. So stuff will accordingly be more REDSHIFTED.
    That means that the galaxies will be dimmer and redder and take better and better telescopes to spot them. But there will be 40% more speckles in the sky, representing those 40% more galaxies, if your telescope is good enough to see them.
    ===========================

    When you read popular stuff like by Krauss which says that in far future ours will be the only galaxy, what I think that means is the images of other galaxies will still be there in the sky, but redshifted so much they are practically impossible to see.

    I could be wrong of course, but that's what I think the popularizers really mean.
     
    Last edited: Jul 22, 2012
  8. Jul 22, 2012 #7

    marcus

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    In the very very long term everything except our galaxy (which has merged with Andromeda and members of the local group) passes out thru the CEH cosmic event horizon (this is not the particle horizon or limit of the observable universe it is the CEH, different).

    Every galaxy except ours that today is within the CEH distance of about 15 Gly will eventually pass thru but we will never see it actually get thru
    It will seem to get stuck there and gradually become redder and redder. Because of redshifting the time slows to a standstill. So it stays within view but becomes too redshifted to see.

    But that is in the very long term and the OP asked a question about RIGHT NOW.

    Right now the number of galaxies in the observable universe is increasing (not decreasing) and I guess it will increase by about 40% over the next 10 billion years or so.

    Have a look at Lineweaver Davis: http://arxiv.org/abs/astro-ph/0310808/
     
  9. Jul 22, 2012 #8

    andrewkirk

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    That's a very interesting thought. I had assumed they would disappear with a 'pop' but what you say makes more sense.
     
  10. Jul 22, 2012 #9

    marcus

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    I know :biggrin: it is a curious thought that we continue to see them (but just in ever-longer wavelengths) after they are beyond 16 Gly CEH limit. A bit like a black hole EH.

    Of course the objects that make up our observable universe are the objects we can SEE, and we never see them as or where they are today.

    But the main point here is, I think, to correct the mistaken notion that RIGHT NOW the number of galaxies in the observable universe is decreasing. It is actually INCREASING at quite a substantial rate!

    The observable region is getting bigger rapidly in terms of the amount of matter (and the number of galaxies) it contains. That is, increasing in real, comoving volume, terms. Comoving distance is a measure which is not subject to expansion, so the comoving distance from us of any bunch of matter essentially does not change. Only changes due to small random local motions, not with Hubble law expansion. So when the comoving volume increases 40% that is a real increase in the amount of galaxies and overall matter that is observable.

    This redshift fade-out problem we were talking about has not yet become a problem, at least not a fatal one. Technicaly one can still in principle detect the light from any galaxy in the observable universe, even if resolving and focusing an image are beyond present capability. This is because we can still see the CMB, which was radiated at 3000 Kelvin before the gas had formed stars! If you can detect that light, the most ancient light coming from near the very edge of the observable, then in principle you can detect light from any galaxy in the observable region! Galaxies are all hotter, younger, and closer than the ancient 3000K gas.
     
    Last edited: Jul 22, 2012
  11. Jul 23, 2012 #10
    Marcus: wonderful explanations...I think I finally understand those pieces, due to you and Lineweaver and Davis!!!!!....[lol]

    BUT!!!!!!!!!!!!!!

    I think you forgot to multiple by the 'third' 55/46.....I calculate about 1.7....??????????

    55/46 x 55/46 = 1.43 x 55/46 = 1.71
     
  12. Jul 23, 2012 #11

    marcus

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    Hah! did I square the number instead of cube it? Thanks for catching that, Naty!
    (55/46)^3 = 1.71
    Indeed I did :redface:
     
  13. Jul 23, 2012 #12

    Chronos

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    Based on current models, Gott calculates the current comoving distance to our particle horizon as 14,300 Mpc and a maximum possible future distance of 19,000 Mpc (re: http://arxiv.org/abs/astro-ph/0310571) [Broken]. This gives a ratio of about 2.36 times the current vs future possible volume of the observable universe. Whether or not the density of galaxies in the universe will remain constant over time is uncertain.
     
    Last edited by a moderator: May 6, 2017
  14. Jul 24, 2012 #13
    Chronos:

    What do we think are the uncertainties? Seems like with the unending expansion of space and a rather fixed amount of common matter, galaxy formation will gradually slow and eventually galaxy formation would stop. Is is not expected the universe will end up empty and cold?

    [Seems like I just described the inflationary era as well: matter density falls with expansion while dark energy density seems to be constant......would that be different now or the future.]
     
    Last edited: Jul 24, 2012
  15. Jul 24, 2012 #14
    Chronos:
    Wow, I did not realize that we can observe now, in a 13.7B year old universe, such a large proportion of the ultimate 'visible' universe [in the 'infinite future'].

    That helps put in perspective Marcus' figure: in 10B years volume increases only 1.71 and then to the infinite future an ultimate 2.36...
    it's still a bit 'weird'!!
     
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