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Are we at the center of the universe and whats at the edge?

by YoungDreamer
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YoungDreamer
#1
Jul7-11, 10:02 PM
P: 61
I am wondering about the size of the universe. I have read that we can only "see" 13.7 billion light years away in all directions and that we are in the center.
To my mind this presents a problem. The problem is what are the odds that we are in the center of the universe?
Then again even with astronomical odds...
But I have always assumed since I was younger that the universe went on forever and our observable universe was caused by a big bang, and that other big bangs could be going on septillions of light years away.
Is that a current theory or am I just dreaming.
And second.
Could it be possible that our technology is only capable of seeing exactly 13.7 billion light years away for whatever reason. Not because that is where the CMB is found, but assume that the matter in our universe goes on for 20 billion light years and if our technology could only see 13.7 billion light years away it would sweep around looking that far back in all direction and we would appear to be at the center. I don't know if I have explained my question well enough, I hope that you will get my point.
One last question.
What would happen if we took a space craft to the edge of our observable universe and just kept going.(I know we cant do that.) Would the gravity of the entire universe stop us from moving further. Or would we loop around or just go on into complete darkness?
Thank You
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bcrowell
#2
Jul7-11, 10:26 PM
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Wow, lots of good questions! You might find it helpful to read this article: http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf

The edge (as referred to in the title of the thread) of the *observable* universe is simply the distance from which light has had time to travel since the big bang. Nothing physical happens there. There is no physical edge : http://www.astro.ucla.edu/~wright/co...y_faq.html#XIN

Quote Quote by YoungDreamer View Post
The problem is what are the odds that we are in the center of the universe?
There is no center. We have a FAQ on this: http://www.physicsforums.com/showthread.php?t=506991

Quote Quote by YoungDreamer View Post
But I have always assumed since I was younger that the universe went on forever
We have a FAQ about this too: http://www.physicsforums.com/showthread.php?t=506986

Quote Quote by YoungDreamer View Post
and our observable universe was caused by a big bang, and that other big bangs could be going on septillions of light years away.
The big bang is not an explosion that happened at a point in a preexisting empty space (see the first FAQ above). Therefore it doesn't make sense to imagine multiple explosions happening at different points in preexisting empty space.

Quote Quote by YoungDreamer View Post
Is that a current theory or am I just dreaming.
Current theory is as described in the second FAQ above. Either the universe is infinite and has always been infinite, or it's closed and has always been closed. Either way, there is no physical edge, and no empty space into which matter is expanding.


Quote Quote by YoungDreamer View Post
Could it be possible that our technology is only capable of seeing exactly 13.7 billion light years away for whatever reason.
The edge of the observable universe isn't set by technology, it's set by the distance that light has had time to travel since the big bang. The radius of the observable universe is actually about 46 billion light years, not 13.7: http://www.physicsforums.com/showthread.php?t=506987

Quote Quote by YoungDreamer View Post
Not because that is where the CMB is found, but assume that the matter in our universe goes on for 20 billion light years and if our technology could only see 13.7 billion light years away it would sweep around looking that far back in all direction and we would appear to be at the center. I don't know if I have explained my question well enough, I hope that you will get my point.
Current cosmological models do not allow light to "wrap around" like this, even if the universe is closed. If it's closed, and light sets off around its circumference, it will never get all the way around, because the circumference is expanding in the meantime.

Quote Quote by YoungDreamer View Post
One last question.
What would happen if we took a space craft to the edge of our observable universe and just kept going.(I know we cant do that.) Would the gravity of the entire universe stop us from moving further. Or would we loop around or just go on into complete darkness?
Nothing special happens there. It has the same properties as any other place in the universe. By the time a spacecraft got there, at a fraction of the speed of light, the edge of the observable universe as seen from earth wouldn't be there anymore. It would be farther out. We can expand our knowledge of the observable universe far more rapidly just by waiting than we could by sending probes. Every year, the observable universe's radius increases by several light-years, both because light has more time to travel and because space is expanding.
YoungDreamer
#3
Jul7-11, 10:50 PM
P: 61
So then the fact the universe is ever expanding combined with the fact that the light has now travelled further points right to the answers of the universes size and can be confirmed constantly?

YoungDreamer
#4
Jul7-11, 10:52 PM
P: 61
Are we at the center of the universe and whats at the edge?

Shouldn't say "points right to the answers of the universes size" but should have said
Show the telltale signs of the universes size.
bcrowell
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Jul7-11, 11:06 PM
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Sorry, I'm not quite following what you're asking in #3 and #4.
marcus
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Jul7-11, 11:22 PM
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Hi Ben, YD,

I recall around 10 days ago YD started a thread with a related question. We tried to answer in that other thread. As I recall, I didn't do a very good job on that one. Pitched it at the wrong level or something, so it didn't take.


Quote Quote by YoungDreamer View Post
I have a question sort of related the size of the observable universe.

I know that we are limited by the speed of light in how far we can see, but what I am wondering is...

In the early universe when the first light could finally break free and travel in straight lines did it travel in all directions like the light given off by a star, and if so could there be light 13.7 billion light years beyond the horizon and we just cant see it because there is nothing to reflect it back to us?

Would this have any importance or relevance of any kind?

And is 13.7 billion light years the distance across the entire universe or just from our position outwards in all directions, making the universe around 28 billion light years across?
YoungDreamer
#7
Jul8-11, 12:25 AM
P: 61
Hello marcus, i'm glad you reminded me of that post.
For some reason no matter how much I read and think I just cant seem to see the universe in the way you do. The only other thing that gives me such trouble understanding is black holes. I am not a scientist of any sort but I have always had a deep yearning to understand the universe, so I do alot of reading on the subject. Obviously I know relatively little about the universe but the ideas and theories seem to be logical and I have a general sense of understanding of things.

However I am still trying to understand the "edge" of the universe. It always changes distance because it is constantly moving away. But if we could somehow travel to the farthest known point in the universe and pulled right up beside the last piece of matter or light and then, travelling in the same direction the universe is expanding what if we suddenly accelerated to light speed.
I know thats alot of extreme hypotheticals but its how I make sense of things. I visualize going there. Probably why I cant make sense of black holes.
Anyways if we approached the the edge of the universe at light speed once we were passing that boundry, because obviously you would have to pass all the matter that is travelling at less than light speed, would space just be created and no matter how far you went it would always just be encompassed by our observible universe therefore always being a part of our universe and never being "outside" of our universe to begin with?

I hope i'm on the right track or that someone can refer me to a good article somewhere.
Thank you for your patience.
YoungDreamer
#8
Jul8-11, 12:40 AM
P: 61
OK after reading a little more maybe I shoule be saying that we would need to travel 45 billion years not 14 billion years?
Not the look back time but proper distance?
But other than that error my question remains the same.
YoungDreamer
#9
Jul8-11, 12:47 AM
P: 61
Or maybe it will be easier to answer if I use my scenario from the other post.

If there is a star close to the edge of the universe and its light is reaching us, it must be shining light at least the same distance in the opposite direction, the direction of the edge of the universe.
Since the light is moving away from us we cannot see it but wouldn't the light still occupy spacetime past the observable universe.
marcus
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Jul8-11, 01:18 AM
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YD, the most basic principle in standard cosmo is called the Cosmological Principle. Matter and space are coextensive. Matter is evenly distributed throughtout all space (roughy speaking on average). Of course the matter is locally clumped and clustered into galaxies and such, but at a larger scale assume it averages out to be even.

If space is infinite then there is an infinite amount of matter, because it is uniformly evenly spread throughout.

If space is finite then there is a finite amount of matter. They are co-extensive.

That is the first rule. I guess it goes back to Einstein around 1920 give or take a few years. To begin to understand you have to teach your mind to picture like that: no big spaces without matter, no matter without space. Later you might come to explore ways of breaking that rule, or testing it, or challenging it. But that is the first step in understanding.

How are you with that? Can you for starters assimilate the Cosmological Principle? Make it a rule to think of the universe that way?

One way of saying this ( http://en.wikipedia.org/wiki/Cosmological_principle ) is that REGARDLESS OF WHERE YOU ARE IN SPACE THINGS LOOK APPROXIMATELY THE SAME as they do here or anywhere else, averaged over large enough scale so individual shapes of galaxies get washed out.

If you think this way, no "edge" or "boundary" is imaginable. Such a thing would contradict the main working assumption we all share---the cosmo principle. Because at an edge things would look different.

Have you watched the Wright balloon model? If not, try it. there is a 2D universe with no edges. It meets the requirements of the Cosmo Principle. Google "wright balloon model" and see if you get it.

I'm assuming you WANT to get so you can think enough like a professional cosmologist that you can understand the basics of what they're are talking about.
YoungDreamer
#11
Jul8-11, 01:32 AM
P: 61
Ok the way I have been thinking of it is space existed and the big bang created matter into the space and has just been filling an increasing volume of space as it expands but the space it is expanding in is infinite.

I know that is not the way the current models work.

Am I too understand it like this, as the universe expands larger and larger it is creating more spacetime, and even if something were to travel past the furthest matter it would still be a part of this universe so there can never be an outside of the universe.
YoungDreamer
#12
Jul8-11, 01:32 AM
P: 61
Ok the way I have been thinking of it is space existed and the big bang created matter into the space and has just been filling an increasing volume of space as it expands but the space it is expanding in is infinite.

I know that is not the way the current models work.

Am I too understand it like this, as the universe expands larger and larger it is creating more spacetime, and even if something were to travel past the furthest matter it would still be a part of this universe so there can never be an outside of the universe.
marcus
#13
Jul8-11, 02:11 AM
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Quote Quote by YoungDreamer View Post
Ok the way I have been thinking of it is space existed and the big bang created matter into the space and has just been filling an increasing volume of space as it expands but the space it is expanding in is infinite.
...
That is your trouble! It is the "explosion" misconception. It is one of the common misconceptions people have.

There is a Scientific American article called "Misconceptions about the Big Bang" that is often recommended. But the main thing is you have to WANT to get over it.

If you want to cling to the "explosion" picture of matter exploding out into empty space then it is very hard to communicate.

I have the SciAm article in my signature, it is the "charley" link. the first page is blank so scroll down
=======

The explosion model would give an inferior fit to the observational data, and runs counter to the spirit of our presentday law of gravity, which is Einstein 1915 relativity.

It is more or less useless to try to proceed if you are hung up on thinking of the universe as infinite empty space with an explosion of matter in it.

I'm off to bed. Maybe I;ll see you tomorrow.
Octavianus
#14
Jul8-11, 06:14 AM
P: 12
Quote Quote by bcrowell View Post
There is no center. We have a FAQ on this: http://www.physicsforums.com/showthread.php?t=506991
"We can visualize this using the metaphor in which galaxies are dots on a balloon being blown up. No point on the balloon is the center of expansion. If you want to pick out a center, you have to pick a point in the air, not on the surface of the balloon. But in the balloon analogy, the third dimension is just an aid to visualization. Only points on the balloon's two-dimensional surface represent actual points in space."

No center in space, yes.
But can't we say that the Universe has a center in time - the Big Bang in the center of the balloon, and that the surface of the balloon is the present time, the air is the past and outside the surface of the balloon is the future which does not yet exist?
YoungDreamer
#15
Jul8-11, 08:48 AM
P: 61
Marcus I absolutely want to shed any misconceptions I have, that is why I'm here.
I know you are not suggesting otherwise.
I think that I have thought the same way for too long and it is just hard relearning such a complex thing that isn't even understood in the first place.
I feel that if I dont look at the universe for what the facts say it is then I am not really looking at it at all.
So...
I have re-read the balloon analogy, I feel that the balloon analogy is good for explaining the idea of the Hubble Constant and how the universe expands but how can I view a 2-D object and get a sense of a 3-D universe?
Or is the balloon analogy not gonna do that.
So...
Here is my revised view, correct me where i'm still not seeing it the right way.
The BB started as a singularity, a very mysterious thing that I wont get into cause I really really dont understand that. The BB wasnt really a bang it was an expansion. And it keeps expanding at an ever increasing rate. It is like a 3-D spherical cobweb stretching out in all directions. And there is never an edge because there is no outside of the universe just like there was no outside of the singularity that created all matter.
YoungDreamer
#16
Jul8-11, 08:51 AM
P: 61
"To begin to understand we must first understand that we may never understand"
bcrowell
#17
Jul8-11, 09:51 AM
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Quote Quote by Octavianus View Post
But can't we say that the Universe has a center in time - the Big Bang in the center of the balloon, and that the surface of the balloon is the present time, the air is the past and outside the surface of the balloon is the future which does not yet exist?
In cosmological models constructed using GR, spacetime only exists for t>0. There was never a t=0.

(The other thing to point out is that the balloon metaphor only works for a closed universe. If the universe is open, then you want to visualize it as an infinite rubber sheet.)

Marcus will do much better than I could for theories like LQG that have a bounce rather than a bang, but I think in those cases you would have a time coordinate that is like the real line, with a temporal "center" at t=0. But I believe the universe has nonzero spatial size at the bounce, so there is still no spatial center.
bcrowell
#18
Jul8-11, 09:54 AM
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Quote Quote by YoungDreamer View Post
Am I too understand it like this, as the universe expands larger and larger it is creating more spacetime, and even if something were to travel past the furthest matter it would still be a part of this universe so there can never be an outside of the universe.
When you say "past the furthest matter," it sounds like you're still imagining matter exploding into a preexisting vacuum. Try the balloon metaphor for the case of a closed universe. The farthest matter is at the opposite side of the balloon. If you go past it, you're just coming back closer to home again.


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