finding center of universe

marcus

Offhand you could google "Lineweaver expanding confusion"
or google "Lineweaver inflation cosmic microwave background".

You will get some links that say "arxiv". this is a free archive of scientific articles. Lineweaver is a top cosmologist and a good writer. He explains things clearly. I think
Figure 1 in either of those papers would show the particle horizon to be around 46 billion LY.
this is how far away TODAY the most distant matter is that we could in principle be getting a signal from.
this is normally what people mean by the radius of the observable region.

Of course that matter was much closer when it emitted the light, and the light has had to contend with expansion so it has taken some 13 billion years to get to us. But the matter which emitted the light is NOW about 46 billion LY away, because of expansion.

So that is the present distance of the most distant stuff we can see, and it is the radius of the observable. the figures here are estimates. Something around 46, maybe 47, as was said earlier. Can't be too exact.

I have figure 1 from one of Lineweaver's articles in my signature. You could check the figure without even having to go fetch the article from arxiv. If you want.

You can see where the lightcone touches down, around 46 Gly (billion ly). You can see in the figure how it touches down a little over halfway between the 40 Gly mark and the tick for 50 Gly. So it is roughly 46. Also this is where the particle horizon line crosses the level marking present time.

Googling you get the arxiv copies of both articles and both have the same figure 1 in them. It is basic.

Thermate

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.

Drakkith

What isn't observable?

marcus

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

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

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

Well, nothing is. Everything is observed as it was in the past.

soothsayer

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.

soothsayer

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

"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

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

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

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

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

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

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

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.