Event Horizon and Particle Horizon

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  • Thread starter Arman777
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Main Question or Discussion Point

The furthest distance that we can see is defined by the Radius of the Particle Horizon which its nearly 46 Gly. However, the cosmic event horizon is nearly 16 Gly. Is this means the galaxies that further than the 16 Gly are just will stay the same in the sky? Since their light can never reach us, in other words, their images on the sky will never change?

For example an object at 20 Gly, we will never see its "future" since its light cannot reach us due to the expansion of the universe ?

And after the Event horizon becomes stable at 17.6 Gly, every galaxy that crosses that distance will stay on that horizon and we will see them as getting redshifted to infinity?

How can we calculate the time needed for clusters in our supercluster to pass the cosmic event horizon ?,

More important I dont understand something. If the horizon is the horizon that is furthest distance that can communicate then how it can be growing ?
These horizon things makes me so confused can someone help me, to understand them better, I now just the definitions but I can not grasp the main idea (even I read and watched many things about them)
 

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  • #2
Bandersnatch
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Can you read a lightcone graph in comoving coordinates? Such as this one:
upload_2019-1-8_19-20-20.png

Those make it so much easier to see and explain.
 

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  • #3
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Can you read a lightcone graph in comoving coordinates? Such as this one:
View attachment 236990
Those make it so much easier to see and explain.
I can read it somehow. Hubble Sphere is kind of confusing. Whats the difference between light cone and event horizon ?

For example at comoving distance between 40 Glyr and 20 Glyr at ##a(t_0)##. We cannot recieve their light. But we see their light redshifted to infinite?
 
  • #4
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Is this statment true:
at ##a(t)=1.5## we will be no longer to recieve new information from a galaxy, that has a comoving distance ##10Glyr##
 
  • #5
Bandersnatch
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Never mind the Hubble sphere, it's going to be confusing in those coordinates.

The event horizon is the past light cone of an observer in infinite future. If you imagine the apex of the light cone going up with time (with its base encompassing more and more comoving distance), it reaches the top of the graph in infinite future, at which point the cone will be coincident with the event horizon.
For example at comoving distance between 40 Glyr and 20 Glyr at ##a(t_0)##. We cannot recieve their light. But we see their light redshifted to infinite?
Remember, you only ever see what's on your past light cone.
We can't ever receive the light they emit now, but we're seeing the light they emitted in the past (where their comoving coordinate crosses our current light cone) just fine.
The last state we'll ever see is where the coordinate crosses the event horizon. It's a past event, but light emitted at that point will reach the observer only after infinite time.
The currently observed redshift of a galaxy at approx 20 Glyr is z~3. It will gradually increase to infinity, as the future observer's light cone approaches the event horizon.
Is this statment true:
at ##a(t)=1.5## we will be no longer to recieve new information from a galaxy, that has a comoving distance ##10Glyr##
The galaxy crosses the horizon sometime before that, so we won't be able to receive any information it emits after the crossing. But we'll keep receiving new (in the sense of never before seen) information about the past state of the galaxy, until forever. From the time before it crossed the event horizon. Where, again, the moment of crossing will become observable only after infinite time.


Tell you what, check out this post:
https://www.physicsforums.com/threa...increase-bc-of-expansion.912881/#post-5754083
where I give a shoddily-written tutorial on how to read those graphs. See if it clarifies anything, and I'll get back to you tomorrow.
 
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  • #6
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Okay thanks, If I still dont understand I ll write you again.
 
  • #7
kimbyd
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The galaxy crosses the horizon sometime before that, so we won't be able to receive any information it emits after the crossing. But we'll keep receiving new (in the sense of never before seen) information about the past state of the galaxy, until forever. From the time before it crossed the event horizon. Where, again, the moment of crossing will become observable only after infinite time.
Though eventually the light will be redshifted to the point that wavelengths will be longer than the horizon, making detection impossible. Apparently all galaxies that aren't gravitationally bound to us will become undetectable in about 2 trillion years or so due to this.
 
  • #8
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Though eventually the light will be redshifted to the point that wavelengths will be longer than the horizon, making detection impossible. Apparently all galaxies that aren't gravitationally bound to us will become undetectable in about 2 trillion years or so due to this.
How did you calculated the 2 triliion year period..?
 
  • #11
kimbyd
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... and, for this number, Wikipedia references
https://arxiv.org/abs/astro-ph/9902189

See the paragraph that contains equation (4).
Thanks for following the links there! That paper looks like a great read for later. One fun quote: "On the bright side for astronomers, funding priorities for cosmological observations will become exponentially more important as time goes on."
 
  • #12
jbriggs444
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funding priorities for cosmological observations will become exponentially more important as time goes on.
As if importance has a linear scale. Or is numeric. Or is a total order. Or is even a partial order. Or is even well defined.
 
  • #13
kimbyd
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As if importance has a linear scale. Or is numeric. Or is a total order. Or is even a partial order. Or is even well defined.
I'm sure the statement was meant as a joke rather than anything approaching a realistic appraisal.
 

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