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Finding center of universe

by shivakumar06
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Drakkith
#37
Nov22-12, 12:27 AM
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Quote Quote by Thermate View Post
But you really CAN'T see that far, because you only see the past for that timeline that is 47 billion light years away. So it is NOT observable.
What isn't observable?
marcus
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Nov22-12, 12:31 AM
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Quote Quote by Thermate View Post
But you really CAN'T see that far, because you only see the past for that timeline that is 47 billion light years away. So it is NOT observable.
You see we are just quibbling about words. We see distant matter as it WAS, not as it is today. But we nevertheless OBSERVE that matter TODAY. So it consitutes what we call the observable portion of the universe. that is how cosmologists use the word. You seem to want them to speak differently. 46 Gly is the distance today of matter we are observing today (as it was along time ago)

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.
Thermate
#39
Nov22-12, 12:39 AM
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Quote Quote by marcus View Post
You see we are just quibbling about words. We see distant matter as it WAS, not as it is today. But we nevertheless OBSERVE that matter. So it consitutes what we call the observable portion of the universe. that is how cosmologists use the word. You seem to want them to speak differently.

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.

This is also shown in figure 1.

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.
Thermate
#40
Nov22-12, 12:54 AM
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Quote Quote by Drakkith View Post
What isn't observable?
The universe 47 billion light years away according to the center of mass plane of natural simultaneity. IOW wristwatch time since the big bang.
Drakkith
#41
Nov22-12, 12:56 AM
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The universe 47 billion light years away according to the center of mass plane of natural simultaneity. IOW wristwatch time since the big bang.
Well, nothing is. Everything is observed as it was in the past.
soothsayer
#42
Nov22-12, 02:45 AM
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You observe your keyboard as it was in the past...

Quote Quote by Thermate View Post
Really? Has the global curvature of the universe changed in the past 13 billion years?
No, it hasn't, not so far as we can tell. Ω = 1 which means Ω has always equaled one, unless maybe you believe inflation.

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.
soothsayer
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Nov22-12, 02:53 AM
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Quote Quote by Thermate View Post
Do observables cross the CEH at less than light speed?
Essentially, yes. What it's really saying is that this is the boundary at which space is expanding faster than light, such that light emitted from objects RIGHT NOW at this boundary will never reach us, just as light emitted inside a black hole event horizon will never reach us. Anything outside of this CEH (ones that we will eventually see as they are today) are necessarily moving slower than light. I think this horizon is about 12 billion light years away, if I'm not mistaken?
Chronos
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Nov22-12, 09:35 AM
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Quote Quote by Thermate View Post
Please provide a specific reference. ISBN and page number.
"Today the diameter of the observable universe is estimated at about 28 billion parsecs (93 billion light-years). This diameter is increasing by 1.96 million km/s, which is about 6.5 times faster than the speed of light in empty space." Extra Dimensions in Space and Time, Bars & Teming, ISBN 978-0-387-77637-8, p27
phinds
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Nov22-12, 09:42 AM
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Quote Quote by Thermate View Post
Please provide a specific reference. ISBN and page number.
There is this really nifty thing on the internet called "Google Search". You should learn how to use it.

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 light-years distance.[2] The diameter of the observable universe is estimated at about 28 billion parsecs (93 billion light-years),[3] putting the edge of the observable universe at about 46–47 billion light-years away.[4][5]


OOPS ... I see that I missed an entire page of responses on this thread, so this really wasn't necessary.
Thermate
#46
Nov22-12, 10:45 AM
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Quote Quote by soothsayer View Post
You observe your keyboard as it was in the past...



No, it hasn't, not so far as we can tell. Ω = 1 which means Ω has always equaled one, unless maybe you believe inflation.

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.
It's been well over a decade since I seriously thought about cosmology, so I'm a bit rusty. Something, however, seems wrong with the idea that the curvature is effectively zero.

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 space-time were "flat" I would expect geodesics to remain at constant mutual distance.

*I mistakenly used the term "time lines" in a previous post.
Thermate
#47
Nov22-12, 10:51 AM
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Quote Quote by Chronos View Post
"Today the diameter of the observable universe is estimated at about 28 billion parsecs (93 billion light-years). This diameter is increasing by 1.96 million km/s, which is about 6.5 times faster than the speed of light in empty space." Extra Dimensions in Space and Time, Bars & Teming, ISBN 978-0-387-77637-8, p27

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.
marcus
#48
Nov22-12, 11:45 AM
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Quote Quote by soothsayer View Post
Essentially, yes. What it's really saying is that this is the boundary at which space is expanding faster than light, such that light emitted from objects RIGHT NOW at this boundary will never reach us, just as light emitted inside a black hole event horizon will never reach us. Anything outside of this CEH (ones that we will eventually see as they are today) are necessarily moving slower than light. I think this horizon is about 12 billion light years away, if I'm not mistaken?
If a galaxy is NOW at distance 14 Gly, then the distance to it is now increasing at rate c.
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.einsteins-theory-of-relat...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.
Drakkith
#49
Nov22-12, 12:57 PM
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Quote Quote by Thermate View Post
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.
What makes this different than everything else you see? If a car is moving past you it is actually very slightly further along its path than you see it due to the finite speed of light. The only difference I see is the magnitude of the difference.
phinds
#50
Nov22-12, 01:04 PM
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Quote Quote by Drakkith View Post
What makes this different than everything else you see? If a car is moving past you it is actually very slightly further along its path than you see it due to the finite speed of light. The only difference I see is the magnitude of the difference.
I see you keep on doing the same old thing ... being reasonable to people who don't want to be reasonable. Don't you ever get tired of it?
Thermate
#51
Nov22-12, 01:12 PM
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Quote Quote by Drakkith View Post
What makes this different than everything else you see? If a car is moving past you it is actually very slightly further along its path than you see it due to the finite speed of light. The only difference I see is the magnitude of the difference.
I agree with that. The reason that it becomes significant in terms of such huge time and distance scales is because the current state of that part of the universe has no effect on our part, and will not for billions of years. Only events billions of years in the past have any bearing on our current state.
phinds
#52
Nov22-12, 01:37 PM
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Quote Quote by Thermate View Post
Only events billions of years in the past have any bearing on our current state.
Now there's a point of view you don't see very often.
Chronos
#53
Nov22-12, 02:06 PM
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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.
soothsayer
#54
Nov22-12, 05:10 PM
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Quote Quote by Thermate View Post
It's been well over a decade since I seriously thought about cosmology, so I'm a bit rusty. Something, however, seems wrong with the idea that the curvature is effectively zero.

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 space-time were "flat" I would expect geodesics to remain at constant mutual distance.

*I mistakenly used the term "time lines" in a previous post.
The lines separate but remain forever non-intersecting, which means the geometry is always Euclidean. If you take a plane and expand it, the geometry is still Euclidean, even during the expansion. The metric for describing spacetime intervals does not change. This is because the expansion is not due to an "open" cosmic geometry, but due to dark energy. If the universe were open, Ω < 1, or closed, Ω > 1, then the value for Ω would actually change over time, thus the geometry would change over time, but Ω = 1 is constant in time.


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