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Musings on the cosmological event horizon

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#1 Inkswitch

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Posted 09 January 2020 - 04:49 PM

I was reading an article that, among other things, discussed the expansion of the universe.  It described the cosmological event horizon, a quality of our universe of which I was aware, and I considered an exotic phenomenon that may arise from it.

 

Consider a galaxy that is a nice round spiral and it is oriented to us so that one side of the spiral is 5,000ly closer to us than the other side.  Now consider that galaxy's position to be such that the "line" that is the cosmological event horizon bisects that galaxy.

 

Ought there not be a subset of galaxies, admittedly on the ragged edge of what can be observed with the largest instruments, that are part invisible and part visible?  A fraction galaxy if you will.

 

It is not clear to me the speed at which the "disappearing event" will occur from our perspective but I think it would be slow so that in the example galaxy it would require 5,000-ish years from first contact with the horizon to total disappearance.  Even if it was fast, then there should be the possibility of observing a galaxy disappear.


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#2 futuneral

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Posted 09 January 2020 - 05:02 PM

I believe the cosmological horizon lies beyond the radius at which re-ionization happened. I.e. in theory (ignoring the expansion) 13.8 billion years old light, came from 13.8 billion ly away, so the galaxy right on that border would be half visible. But re-ionization happened about 0.15 - 0.5 billion years later. Re-ionization is an event after which the light first gained the ability to travel through the Universe. So I'd imagine if you're able to peek that far into the past and see a galaxy there, it'll be disappearing into a "fog" of the "dark ages", as opposed to be cleanly cut in half by the horizon.



#3 TOMDEY

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Posted 09 January 2020 - 06:09 PM

The ~edge~ in that reasonable simplistic sense is where the speed of recession extrapolates to precisely c. That assumes that the Hubble Constant is indeed constant, and has always been the same (in both space and time), which is the space-time homogeneous isotropic assumption... what cosmologists always start with, and then refine from there. Thing is, if nothing else, the information density arriving here/now from there/then is diluted to zero, because all emitted wavelengths have stretched to infinity. And (not coincidentally!) may sound familiar... because it is identical to what you witness of objects approaching the Event Horizon of Black Holes! That is to say, just when you think you might see a bifurcated galaxy ~crossing the edge~ it refuses to send your telescope any information about the event. And that's why creative cosmologists chose the term ~event horizon~ because knowledge of a remote event requires information. But it is blacked out by the very nature of the beast.    Tom


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#4 Migwan

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Posted 09 January 2020 - 07:39 PM

Not sure I am reading the OP correctly, but I think it concerns spatial expansion, so technically its not recession.   The quotient still equals 0 at the "cosmological" event horizon, but there is the age factor worth considering.  Whatever light we would see originating near that edge, would conceivably be from the time of the big bang.   As supposedly there were no galaxies at that time, there could be no such light. 

 

If the universe did allow us to see something so near the edge, then it would call into question the age of the universe and perhaps, even the big bang itself.

 

My ramblings aside, it is intuitive to think that continuous spatial expansion over time would make further galaxies disappear.  

 

jd



#5 EJN

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Posted 09 January 2020 - 09:50 PM

As stated above, as an object approaches the cosmic horizon, it gets redshifted to infinity.

Also, the concept of distance gets ambiguous with a finite speed of light. Looking at a galaxy with a high redshift only reliably tells us the light travel time, because of the expansion of the universe in the intervening interval. We can infer how far away it was THEN, but it is much further away NOW. How far depends on the cosmological

model used.

Google "comoving distance"


Edited by EJN, 09 January 2020 - 11:50 PM.


#6 llanitedave

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Posted 10 January 2020 - 12:45 AM

Fun question, really inspires thinking.  The answers are good too.

 

But regardless of the details, the whole concept makes me think that somewhere, somehow, there's a spiral galaxy at the ragged edge of visibility, and because it's rotating, the side of the disk that's rotating away from us exceeds c from our perspective, the side rotating towards us (although it's receding much faster than the tangential rotation velocity is approaching) is ever so slightly less than c.

 

So we'd see an extremely red-shifted half of a galaxy, which over time will gradually fade towards the one edge.



#7 Inkswitch

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Posted 10 January 2020 - 09:24 AM

I had not incorporated red shift into my thinking.  But that ought not matter because, as my understanding goes, the cosmological event horizon (a moving target) is the line beyond which information cannot make it to us because the expansion of the space between the observer and the location beyond the line is happening at a rate greater than c.  On this side of the line it is less than c, admittedly close to c when near the line.  The redshift may be very high but shouldn't we then build a specialized instrument to see that information, a super low frequency radio telescope perhaps.



#8 TOMDEY

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Posted 10 January 2020 - 02:19 PM

I had not incorporated red shift into my thinking.  But that ought not matter because, as my understanding goes, the cosmological event horizon (a moving target) is the line beyond which information cannot make it to us because the expansion of the space between the observer and the location beyond the line is happening at a rate greater than c.  On this side of the line it is less than c, admittedly close to c when near the line.  The redshift may be very high but shouldn't we then build a specialized instrument to see that information, a super low frequency radio telescope perhaps.

This is one of those "limiting case" ~thought experiments~ where, at every turn, we get thwarted by Mother Nature. Here, from Martin Harwit's seminal book Cosmic Discovery (highly recommended reading!)... why VLF (Very Low Frequency radio waves) are precluded from getting from there (the edge of the observable universe) to here. Back in 1970 I did VLF Research Work for the DOD... at the tender age of twenty three! Fascinating work. We could tune into/onto such low freqs (long wavelengths) that we could, literally, hear the carrier!    Tom

 

~ click on ~   >>>

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#9 Keith Rivich

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Posted 10 January 2020 - 03:48 PM

Along with redshifting wouldn't time dilation prevent one from seeing the galaxy fall through the horizon? Not to mention length contraction which should render the galaxy incredibly small. All in all it would be interesting to do research along this epoch. Einstein in real time!


Edited by Keith Rivich, 10 January 2020 - 06:47 PM.


#10 Inkswitch

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Posted 10 January 2020 - 06:32 PM

This is one of those "limiting case" ~thought experiments~ where, at every turn, we get thwarted by Mother Nature.

I thought that might be the tree I was barking up, by I'm nothing if not obstinate.  The book by Harwit is on my reading list by your recommendation in a previous thread, I hadn't gotten to it yet.




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