# finding center of universe

by shivakumar06
Tags: universe
P: 31
 Quote by phinds Nicely done video but I am at a complete loss to see how it has ANY bearing on this thread. What am I missing?
Admittedly, I was responding less to the original post and more to the first post (your post) about how wherever one is in space, one is always at the centre of the universe, and I thought a visual example of the Cosmological principle would be helpful to laymen like myself. Did you skip to 8:20?
PF Patron
P: 4,966
 Quote by FreeMitya Admittedly, I was responding less to the original post and more to the first post (your post) about how wherever one is in space, one is always at the centre of the universe, and I thought a visual example of the Cosmological principle would be helpful to laymen like myself. Did you skip to 8:20?
I watched the whole thing, thought the emphasis on black holes made it irrelevant to this thread even though there was some discussion about the center.
P: 31
 Quote by phinds I watched the whole thing, thought the emphasis on black holes made it irrelevant to this thread even though there was some discussion about the center.
No worries, it's completely understandable.
 P: 15 There is a set of non-intersecting world lines, each one at rest relative to the the observable universe. In an expanding universe these world lines move farther apart. Relative rest with respect to the observable universe can be determined by measuring the Doppler shift of the most distant radiation sources. If an observer is moving relative to the center of mass of the observable universe he will observe a blue shift in the direction toward which he is moving, and a red shift in the opposite direction. I originally stated this with respect to the cosmic background radiation. That statement is probably still valid, but there's a little catch, that I'm not certain of.
P: 15
 Quote by phinds The approximate center of mass of the observable universe is wherever you are. Given homogeneity, I'd say the approximation is quite accurate.
Isn't the center of mass the sum of the moments divided by the sum of the masses? Surely that location is not following me around. If I step on the gas, the center of mass of the observable universe is not accelerating with me.
PF Patron
P: 4,966
 Quote by Thermate Isn't the center of mass the sum of the moments divided by the sum of the masses? Surely that location is not following me around. If I step on the gas, the center of mass of the observable universe is not accelerating with me.
Do you really think your mass, or the distance you can travel, makes anything other than an infinitesimal difference, compared to the mass of the observable universe and the distance from you to its edge? I stand by my statement.
P: 15
 Quote by phinds Do you really think your mass, or the distance you can travel, makes anything other than an infinitesimal difference, compared to the mass of the observable universe and the distance from you to its edge? I stand by my statement.
So are you, or are you not moving relative the the center of mass of the observable universe?
PF Patron
P: 10,394
 Quote by Thermate So are you, or are you not moving relative the the center of mass of the observable universe?
Of course you are moving with respect to the center of mass of the OBSERVABLE universe. Just keep in mind that your observable universe changes as you move.
PF Patron
P: 4,966
 Quote by Thermate So are you, or are you not moving relative the the center of mass of the observable universe?
As Drakkith said, yes you are, but I would add that

1) Your observable universe is being constantly redefined due to motion
2) this is far more due to the movement of the earth/sun/galaxy than to anything you personally can do here on Earth
3) the distance to the edge of your observable universe is about 47 billion light years and all of the motion in #1 is trivial by comparison.
4) Even a much faster motion would not change the fact that the center of mass of your observable universe, due to homogeniety, pretty much follows along with you (the light-cone center is always exactly where you are, by definition)
P: 15
 Quote by phinds 3) the distance to the edge of your observable universe is about 47 billion light years and all of the motion in #1 is trivial by comparison.
Please provide the sound and valid reasoning that concludes that my observable universe is about 47 billion light years. Does that mean I can see back before the big bang?
PF Patron
P: 10,394
 Quote by Thermate Please provide the sound and valid reasoning that concludes that my observable universe is about 47 billion light years. Does that mean I can see back before the big bang?
That is the radius of the observable universe. The universe has expanded over the last 13.7 billion years and is now the observable universe is much bigger than the 13.7 billion light years one might expect.
P: 15
 Quote by Drakkith That is the radius of the observable universe. The universe has expanded over the last 13.7 billion years and is now the observable universe is much bigger than the 13.7 billion light years one might expect.
This may be some kind of gravitational lensing effect. I will grant that the entire concept of time becomes a bit nebulous in this context; nonetheless, one would expect that the observable universe is no larger than the distance light could travel in the age of the universe.

I am aware that there are three 3-planes of simultaneity with respect to the local universal rest frame. Perhaps one of them will account for your ~47 billion year observable scale. I would like to see your sources on this.
PF Patron
P: 4,966
 Quote by Thermate This may be some kind of gravitational lensing effect.
HUH? I'd say you don't understand gravitational lensing. Certainly it has nothing to do with this discussion.

 I will grant that the entire concept of time becomes a bit nebulous in this context; nonetheless, one would expect that the observable universe is no larger than the distance light could travel in the age of the universe.
If the universe were totally static, you would be rigth, but it isn't, so you are wrong. It's expanding.

 I am aware that there are three 3-planes of simultaneity with respect to the local universal rest frame. Perhaps one of them will account for your ~47 billion year observable scale. I would like to see your sources on this.
I have no idea what you mean by this. As for a reference of the 47 billion years, get any Cosmology 101 text.
P: 15
 Quote by phinds HUH? I'd say you don't understand gravitational lensing. Certainly it has nothing to do with this discussion. If the universe were totally static, you would be rigth, but it isn't, so you are wrong. It's expanding. I have no idea what you mean by this. As for a reference of the 47 billion years, get any Cosmology 101 text.

Please provide a specific reference. ISBN and page number.
 P: 15 OK. I see cosmology has finally caught up with what I was saying decades ago. You are talking about the Universe as treated as simultaneous with the local center of mass of the observable universe. Congratulations to the scientific community! You finally caught up with a high school dropout. As I said, this is a "lensing" effect. One plane of simultaneity is the time that all observers will agree upon when they look at their watches and measure their own time lines relative to the big bang.
P: 15
 Quote by phinds HUH? I'd say you don't understand gravitational lensing. Certainly it has nothing to do with this discussion.
Really? Has the global curvature of the universe changed in the past 13 billion years?

 If the universe were totally static, you would be rigth, but it isn't, so you are wrong. It's expanding. I have no idea what you mean by this. As for a reference of the 47 billion years, get any Cosmology 101 text.
Another second plane of simultaneity is what actually hit's our eyes. The further back in space you look. the further back in time you see.

Those are the easy ones. Sorry. I can't give you a reference on this, because I only have it as an immediate thought. I don't know that it is written down anywhere.
PF Patron
P: 22,349
 Quote by Thermate Please provide a specific reference. ISBN and page number.
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.
P: 15
 Quote by Thermate OK. I see cosmology has finally caught up with what I was saying decades ago. You are talking about the Universe as treated as simultaneous with the local center of mass of the observable universe. Congratulations to the scientific community! You finally caught up with a high school dropout. As I said, this is a "lensing" effect. One plane of simultaneity is the time that all observers will agree upon when they look at their watches and measure their own time lines relative to the big bang.

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.

 Related Discussions Cosmology 16 Cosmology 4 Special & General Relativity 7 General Astronomy 10