What percent of the universe is outside of galaxies?

  • #1

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Summary:: What percent of the universe is outside of galaxies?

Hello,

What percent of the universe is outside of galaxies?

I need this information for a writing piece I am doing. I need specifically to be able to state that if someone was to randomly relocate to somewhere else in the universe, what is the probability that they would end up inside (or outside) of a galaxy.

Thank you for any help you can provide.

[note: this is not an inquiry into teleportation. I am not asking a scifi question. I am asking an astrophysics question]
 

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  • #2
phinds
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What percent of the universe is outside of galaxies?
Pretty much all of it. 99.9%+

EDIT: I see I was way low on how much of the universe is outside galaxies. It's more like 99.999%+
 
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  • #3
Pretty much all of it. 99.9%+

EDIT: I see I was way low on how much of the universe is outside galaxies. It's more like 99.999%+
Thank you for this reply!

Could you please link me to a good source on this?
 
  • #4
phinds
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Doubt if there is one since it's a totally trivial calculation. Just google "average diameter of galaxies" and "average distance between galaxies"
 
  • #5
Vanadium 50
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If you're just looking for a ballpark estimate, the earth and M31 are 10 galactic radii apart. If you look at the Hibble deep field pictures, they are at least 10 galactic radii apart. That gets you 99.9+% without doing much work.
 
  • #6
russ_watters
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Pretty much all of it. 99.9%+

EDIT: I see I was way low on how much of the universe is outside galaxies. It's more like 99.999%+
99.999%+ looks like a good estimate to me.

Looking at a list of the nearest galaxies to ours, there are a whole bunch of a few hundred or a few thousand light years in diameter within 10 million light years, which defines our "local group". But there's only a small handful approaching 100,000 light years in diameter in that group. So let's say it's equivalent of about 10 in that 10 MLY. That's 99.999%.

But again, that's still the density of our local group, so the universe as a whole is going to be less.
 
  • #7
phyzguy
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You also need to define where a galaxy "ends". The density of stars just get smaller and smaller as you move away from the center. if there is still one star gravitationally bound to the galaxy that is further away from the galaxy center than you are, are you still "inside" the galaxy?
 
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  • #8
You also need to define where a galaxy "ends". The density of stars just get smaller and smaller as you move away from the center. if there is still one star gravitationally bound to the galaxy that is further away from the galaxy center than you are, are you still "inside" the galaxy?

It seems like this sort of calculation would have already been done somewhere. Anyway here is how I need it to show up in my blog:

If one were hypothetically to randomly relocate to anywhere in the universe, the chances that they would end up within a galaxy are about 1 in _______________.

Would it be closer to 1 in 1000 or 1 in 1,000,000 or higher?
 
  • #9
russ_watters
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If one were hypothetically to randomly relocate to anywhere in the universe, the chances that they would end up within a galaxy are about 1 in _______________.

Would it be closer to 1 in 1000 or 1 in 1,000,000 or higher?
Do you know how to convert from a percentage to a 1 in X?
 
  • #10
Do you know how to convert from a percentage to a 1 in X?
do you mean percent to decimal? I learned math through calc 3 and bout halfway through DE and some L.A. but I never learned Probability math. I would love to though. Can you recommend a good calc based entry text?
 
  • #11
PeroK
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It seems like this sort of calculation would have already been done somewhere. Anyway here is how I need it to show up in my blog:

If one were hypothetically to randomly relocate to anywhere in the universe, the chances that they would end up within a galaxy are about 1 in _______________.

Would it be closer to 1 in 1000 or 1 in 1,000,000 or higher?
It's about one in a million of finding yourself in the stellar part of the galaxy. Although, you are more likely to end up in the dark matter halo:

https://en.wikipedia.org/wiki/Dark_matter_halo
 
  • #12
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[note: this is not an inquiry into teleportation. I am not asking a scifi question. I am asking an astrophysics question]
Did you mean to ask this in the sci-fi forum, @deltapapazulu? There is an astronomy forum on PF as well. That might be a more suitable place for any follow up as not everyone dips into the SF posts.
 
  • #13
Vanadium 50
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The thread was moved here.
 
  • #15
Vanadium 50
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I don't think so. I used something called "memory", although at my age "forgettery" might be closer to the truth.
 
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  • #16
berkeman
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Did you mean to ask this in the sci-fi forum, @deltapapazulu? There is an astronomy forum on PF as well. That might be a more suitable place for any follow up as not everyone dips into the SF posts.
If you saw his original version of his thread start, you would understand why it ended up in SciFi.
The thread was moved here.
Yup, I did that when he reposted a deleted thread. I was trying to be charitable in allowing it anywhere here. In its re-posted form, it's pretty much okay. But understanding the OP's original intent and story line, it seemed inappropriate to allow it in the technical forums.
Aha, thanks @Vanadium 50. Is there a way to see that such moves occur from the UI?
Sure, just click on the Report link for a related post and ask the Mentors about the history of the thread. :smile:
 
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  • #17
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Sure, just click on the Report link for a related post and ask the Mentors about the history of the thread. :smile:
Walnut, meet hammer :doh:

My question was in case I'd missed something obvious, ,@berkeman, but I'll post a suggestion that moved OPs be flagged somehow and it can be considered with all the other ideas.
 
  • #18
Klystron
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The gist of the OP's question can be answered by science fiction writing standards. By definition characters in a story arrive close enough to a 'galaxy' for something interesting to happen. Even "Waiting for Godot" type characters have their fog.

Cosmological answers seem predicated on the idea that what we see around us more or less repeats across the universe; there is nothing special about our view or location other than (perhaps) our presence. Popular science writers such as Carl Sagan, Kip Thorne and Isaac Asimov describe these concepts succinctly.

See principle of mediocrity vs. anthropic.
 
  • #19
It depends on if you define the size of a galaxy by the volume of normal matter or the dark matter halo. The typical volume of a galaxy by normal matter is ~10^12 ly^3. If you include the DM halo it's a million times larger. The total number of galaxies in the observable universe is ~10^12. The volume of the observable universe is ~10^32 ly^3. So the fraction of the volume occupied by galaxies is ~10^12*10^12/10^32 = 10^-10 or 10^-4 with DM halo.
 
  • #20
PAllen
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For most purposes, the DM halo would be indistinguishable from vacuum. I mean, our best efforts have so far failed to detect it.
 
  • #21
Yeah the main point was that the definition of the "edge of the galaxy" is not clear. Is it defined by the edge of photon emitting matter or some sort of gravitational edge? What we can say is that the size of a galaxy is somewhere between those to definitions.
 
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