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#38
Nov2212, 12:31 AM

Astronomy
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PF Gold
P: 23,120

They have a different word for the present day distance to the farthest galaxy that we will eventually see as it is TODAY. That distance is called the CEH (cosmic event horizon). It is about 16 billion ly. 16 Gly is the distance today of matter which we WILL in our far distant future be able to observe (as that matter is today.) This is also shown in figure 1. 


#39
Nov2212, 12:39 AM

P: 15

Do observables cross the CEH at less than light speed? Wheeler told me that I needed to tell my teacher David Layzer that he should revisit his calculations. IIRC, Wheeler was wrong. 


#40
Nov2212, 12:54 AM

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#41
Nov2212, 12:56 AM

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#42
Nov2212, 02:45 AM

P: 398

You observe your keyboard as it was in the past...
The distance to the edge of your observable universe is 47 billion years, the universe is 13.7 billion years old. This is not a paradox: space has expanded. Objects at the edge of our observable universe are moving faster than light. That is ok, because no information is actually traveling faster than light. Considering the "observable universe", no matter how big it is, is a perfect sphere around you, based on the distance you can see in all directions due to the finite speed of light, then the observable universe is always centered on you, and every point in the universe has a different observable universe. The ENTIRE universe is infinite, and has always been infinite, even at the big bang. Imagine you had an infinite plane expanding arbitrarily quickly for points separated by an arbitrarily large distance. What is the center of mass of this unverse? There is none, and never was. 


#43
Nov2212, 02:53 AM

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#44
Nov2212, 09:35 AM

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PF Gold
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#45
Nov2212, 09:42 AM

PF Gold
P: 6,123

Try this http://en.wikipedia.org/wiki/Observable_universe Which includes the following: The age of the universe is about 13.75 billion years, but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.75 billion lightyears distance.[2] The diameter of the observable universe is estimated at about 28 billion parsecs (93 billion lightyears),[3] putting the edge of the observable universe at about 46–47 billion lightyears away.[4][5] OOPS ... I see that I missed an entire page of responses on this thread, so this really wasn't necessary. 


#46
Nov2212, 10:45 AM

P: 15

I'm not saying you are wrong. But consider this. If we draw geodesics on a Euclidean plane, parallels remain at a constant mutual distance. Now the world lines^{*} of the local universal rest frames are geodesics which are growing further apart in an expanding universe. If spacetime were "flat" I would expect geodesics to remain at constant mutual distance. *I mistakenly used the term "time lines" in a previous post. 


#47
Nov2212, 10:51 AM

P: 15

I guess this really begs the meaning of "observable". To my way of thinking something that is observable is something that I can observe, IOW "see". What is being called "observable" in the above is really conjecture. It may be reasonable conjecture, but it is a poor choice of wording to call objects which were not 47 billion light years away when they emitted what we can currently observe as "observable" at 47 billion light years. 


#48
Nov2212, 11:45 AM

Astronomy
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PF Gold
P: 23,120

It is a common misconception that this means that light emitted today by anything farther than that will never reach us. Lineweaver and Davis explained in a SciAm article ("charley" link in my sig) how light can reach us even if emitted from a galaxy the distance to which is increasing somewhat faster than c. It's just a minor point but might interest you. The distance which is currently increasing at exactly c is called the Hubble radius, commonly estimated to be about 14 Gly. The Hubble radius is smaller than the CEH, commonly estimated at about 16 Gly. You might be interested in a cosmology calculator (constructed by a PF member, Jorrie) which is online here" http://www.einsteinstheoryofrelat...oLean_A25.html The row of the table labeled S=1 is the present. The precise numbers depend on what parameter values, like present Hubble expansion rate, you plug in and they are measured only to some finite accuracy, so in conversation one rounds off and says "about". The calculator does not round off for you. It uses recent values of the model parameters. It says the Hubble radius (distance which is expanding at c) is 13.9 Gly or about 14 Gly. It says the CEH is currently about 15.9 or 16 Gly. 


#49
Nov2212, 12:57 PM

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#50
Nov2212, 01:04 PM

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#51
Nov2212, 01:12 PM

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#53
Nov2212, 02:06 PM

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PF Gold
P: 9,386

Distance is a very messy thing in cosmology. Figuring out where things are at in the universe relative to one another can be very confusing. For example, the most distant observable thing in the universe is the CMB at z~1100. When those photons were emitted, the source of the CMB was a mere 42 million light years from our current position in the universe. At the same time, photons from a galaxy at z~3 were 5.7 BILLION light years distant when emitted by that galaxy. At seems rather illogical that a foreground galaxy at z~3 can emit photons at better than 10 times the distance of the background CMB, but, that is the way it is with expansion. It also provides us with seemingly exotic concepts like luminosity distance and angular diameter distance. If all this does not confuse you, I've done a poor job explaining it.



#54
Nov2212, 05:10 PM

P: 398




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