Can We Determine the Center of the Universe Using Comet Orbits?

[shivakumar06]

can we find the center of the universe by measuring the deviation from theoretical value of the shape of comet's orbit when compared to real shape of comet's orbit.

 

[phinds]

There is no center to the universe.

The OBSERVABLE universe has a center, and you are it.

EDIT: you would likely find it informative to read the FAQ in the cosmology section

 

[Hetware]

There is no center to the universe.

The OBSERVABLE universe has a center, and you are it.

EDIT: you would likely find it informative to read the FAQ in the cosmology section

Are we moving relative to the observable universe?

 

[phinds]

Are we moving relative to the observable universe?

Wherever you are, you are the center of the observable universe, so no, we are not by definition.

EDIT: you would likely find it informative to read the FAQ in the cosmology section

 

[shivakumar06]

we know that universe was created after big bang occured. then the universe would have been a point and started expanding in all direction then it have looked like a spherical ball whose surface is expanding a center. so it is meaningful to talk of center of universe isn't it

 

[Jimmy]

we know that big bang occured. then the universe would have been a point object. so then the universe would have had a center.

Where's the center of a point?

 

[shivakumar06]

Where's the center of a point?

sorry. i have edited my posting now.

 

[phinds]

we know that universe was created after big bang occured. then the universe would have been a point and started expanding in all direction then it have looked like a spherical ball whose surface is expanding a center. so it is meaningful to talk of center of universe isn't it

Absolutely not. The big bang was most emphatically NOT a point. It happened everwhere and there is no center and no edge.

This is a bit hard to get your head around when you first hear it, but it is the case. You should read more cosmology. Try the FAQ in the cosmology section.

 

[Chronos]

The premise the universe began as an infinitesimal point is illogical on two counts:
1] If the universe is infinite, it was infinite from the beginning. Only the observable universe would be point-like at its inception.
2] Unless the big bang occurred in some kind preexisting space, the size of a point is undefined in the absence of external spatial coordinates.

 

[phinds]

The premise the universe began as an infinitesimal point is illogical on two counts:
1] If the universe is infinite, it was infinite from the beginning. Only the observable universe would be point-like at its inception.2] Unless the big bang occurred in some kind preexisting space, the size of a point is undefined in the absence of external spatial coordinates.

I agree w/ what you are saying, except that I see the bolded statements as mutually contradictory. If the current observable universe started off as a dimensionless point, then it seems to me that so would have the entire universe. The observable universe likely started off REALLY small, but not as a point. Perhaps that's what you intended by "point-like" ?

 

[litup]

Maybe it would help to think of the universe as a big balloon. Right now we think the balloon is about ~50 billion LY 'across'. Now, going back in time, the universe gets progressively smaller and so does the balloon. The thing is, we have to think of ourselves as being on the surface of the balloon so we don't really have a center, do we? But as the universe goes back further in time, at some point in time it was say, 1 cm across. It is still that same balloon and on the surface that same point looks the same as it does if it is 50 billion light years across, the angles all go out at exactly the same angle from a point on the surface, so it doesn't matter if the universe were one micron across or 50 billion LY across, we are still on the surface so there is no center we can find.

The idea of the balloon shows us the universe has a curvature so if we had a spacecraft that could do say a trillion times the speed of light and travel in what we think is a straight line, we would come back to the same place after X amount of time, whatever that is, say at one trillion c, assuming the universe is actually 50 E9 ly across, we would come back to our solar system in about 20 minutes even though we thought we would be traveling in a straight line. Our straight line is really a very slow curve not even noticeable if we only went from here to the closest star, Alpha Centauri or thereabouts.

 

[Hetware]

Wherever you are, you are the center of the observable universe, so no, we are not by definition.

EDIT: you would likely find it informative to read the FAQ in the cosmology section

I had this conversation with John Archibald Wheeler. How would you determine the center of mass of the observable universe, at least approximately?

 

[Chronos]

I agree w/ what you are saying, except that I see the bolded statements as mutually contradictory. If the current observable universe started off as a dimensionless point, then it seems to me that so would have the entire universe. The observable universe likely started off REALLY small, but not as a point. Perhaps that's what you intended by "point-like" ?

That is essentially the issue addressed by my second assertion. An infinite ensemble of infinitesimal points would collectively, and individually, be spatially undefined.

 

[phinds]

That is essentially the issue addressed by my second assertion. An infinite ensemble of infinitesimal points would collectively, and individually, be spatially undefined.

Ah ... an interesting way to look at it. I had not considered it that way before. Thanks for the clarification.

 

[phinds]

I had this conversation with John Archibald Wheeler. How would you determine the center of mass of the observable universe, at least approximately?

The approximate center of mass of the observable universe is wherever you are. Given homogeneity, I'd say the approximation is quite accurate.

 

[Pasha582]

we know that universe was created after big bang occurred. then the universe would have been a point and started expanding in all direction then it have looked like a spherical ball whose surface is expanding a center. so it is meaningful to talk of center of universe isn't it

Not really. You are imagining the universe as an expanding ball, which is probably in error. The Poincare Conjecture yields possible hints about the topology of our universe. You would be better off imagining our universe mapped to the surface of a ball, like the surface of the earth. Some people used to think Vatican City, Rome, was the center of the world. Others thought it was in Ireland. Bad points could be made for either. If the Earth were a perfect sphere, no matter where you were the horizon would appear equidistant in all directions, giving you the illusion you were at the center. But if you were to travel far enough in a straight line, you would return to your starting point.

The topology of our universe is probably much like that. Also, as the sphere expands, objects appear to recede from you--just as they do in our universe at large. No point on the surface of this sphere may rightly be considered the center. But we COULD possibly use the word "center" to describe a point in time, roughly 13.7 billion years ago...

 

[FreeMitya]

This is a good explanation supplemented with a visual example: (Skip to 8:20)https://www.youtube.com/watch?v=3pAnRKD4raY

 

[phinds]

This is a good explanation supplemented with a visual example: (Skip to 8:20)


https://www.youtube.com/watch?v=3pAnRKD4raY

Nicely done video but I am at a complete loss to see how it has ANY bearing on this thread. What am I missing?

 

[FreeMitya]

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?

 

[phinds]

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.

 

[FreeMitya]

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.

 

[Thermate]

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.

 

[Thermate]

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.

 

[phinds]

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.

 

[Thermate]

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?

 

[Drakkith]

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.

 

[phinds]

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)

 

[Thermate]

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?

 

[Drakkith]

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.

 

[Thermate]

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.

 

[phinds]

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.

 

[Thermate]

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.

 

[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.

 

[Thermate]

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.

 

[marcus]

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.

 

[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.

 

[Drakkith]

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]

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]

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]

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]

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]

You observe your keyboard as it was in the past...

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]

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]

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]

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]

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]

"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]

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-relativity-4engineers.com/CosmoLean_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]

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]

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?