Uncovering the Truth: Our Position in the Universe & Cosmic Background Radiation

[remmeler]

This may be a stupid question but:
If we can see Cosmic Background radiation in all directions, would that not indicate that we were in the unlikely position of being in the exact center of the universe (the place where the big bang happened). If we were closer to one edge or the other, we would not be able to see all the way back in one of the directions, because it would be further than 13 billion years away.

 

[bcrowell]

FAQ: What about the edge of the universe?

Standard cosmological models do not have edges. They come in two flavors, open and closed. The open type has negative spatial curvature and infinite volume. The closed one has positive curvature and finite volume; spatially, it is the three-dimensional analog of a sphere. Neither has an edge. The open type has no edges because it extends to infinite distances. The closed type has no edges because it wraps around on itself. Current observations of the cosmic microwave background's anisotropy show that our universe is very nearly spatially flat (on the cosmological scale). If it is exactly flat, then it is a special case lying between the more general open and closed cases. The flat case has infinite volume and no edges.

Sometimes people use the word "universe" when they really mean "observable universe." The observable universe does have an edge, which simply lies at the maximum distance that light would have been able to travel since the universe became transparent shortly after the Big Bang. We are at the center of our observable universe, and its edges are expanding outward as time goes on, because in the future light will have had more time to travel to us. An observer billions of light years away from us is at the center of their observable universe, which has different edges than ours. All of these edges are boundaries of the availability of information, not places where anything physically special happens.

 

[remmeler]

Sorry to press, but I don't exactly follow your answer. example: If the leading edge of the background radiation (not the edge of the universe) is , let's say 13 billion light years away. That is a finite distance. If I can see that finite distance in any direction, then it stands to reason, in my mind, that, unless the background radiation is something else, you would have to not be able to see the leading edge of the background radiation in at least one direction unless you were in the unlikely spot as being at the center of where the big bang happened.

 

[BillSaltLake]

Even though a "normal" explosion is not a good analogy, if you were somewhere inside a normal explosion, the fireball would look the same in all directions if you were more than a few optical extinction depths inside from the edge, regardless of whether you were at the center.

 

[bcrowell]

Sorry to press, but I don't exactly follow your answer. example: If the leading edge of the background radiation (not the edge of the universe) is , let's say 13 billion light years away. That is a finite distance. If I can see that finite distance in any direction, then it stands to reason, in my mind, that, unless the background radiation is something else, you would have to not be able to see the leading edge of the background radiation in at least one direction unless you were in the unlikely spot as being at the center of where the big bang happened.

FAQ: Where did the Big Bang happen? Would that be the center of the universe?
According to standard cosmological models, which are based on general relativity and are found to agree well with observations, the Big Bang was not an explosion that happened at a particular point in a preexisting landscape of time and space. Time and space did not exist before the Big Bang -- or even *at* the Big Bang, which is a point where the theory breaks down because things get infinite. The high temperatures and high densities associated with the early universe existed everywhere at once and were very nearly uniform. In these models, which are constructed so as to be perfectly uniform, no point in space has different properties than any other. This is in good agreement with observations of the universe, which shows that that there is a nearly complete lack of structure on very large scales.

 

[remmeler]

Let me know where I am going wrong in my example:
Let's say you look out 12.5 Billion Light Years and you see some clumping, then you look out 12.9 billion years and you start seeing the cosmic background radiation. These are finite distances. It may be true that you and only see 13.5 because of the speed of light, but I am talking about that point where your start seeing an even distribution. If I am in a galaxy that is 1 billion light years away from our present galaxy in one direction, won't I be closer to that background radiation in that direction and that distance further away in the other direction. So if that is the case when I look in that other direction 1 billion light years, I will see our present galaxy and when I look 12.9 billion light years in that direction I will not see the background radiation, because I now would need to look and additional 1 billion light years, because I am that much further away from the background radiation.
I hope someone can explain why my finite distance idea is wrong.

 

[bcrowell]

If I am in a galaxy that is 1 billion light years away from our present galaxy in one direction, won't I be closer to that background radiation in that direction and that distance further away in the other direction.

At any given position, CMB radiation is constantly flowing past you from all directions. Some CMB flowed past Earth in the past, and some will in the future. Say an observer lived in galaxy G, 1 billion light years away from us. Then some of the CMB that is our past CMB is their present CMB, some of our future CMB is their present CMB, and so on.

 

[remmeler]

A2065 Galaxy in Corona Borealis Supercluster is about 1 billion light years away. Is the problem that I am dealing with because I am dealing with the thought of miles in my finite example and we are actually dealing with time in light years. If I could use the Comoving Distance that tells where the galaxies are now rather than where they were when they emitted the light, would that make any difference. It just doesn't seem logical that wherever I am in the observable universe, whether it is a billion or 2 billion miles from earth, I would still see the cosmic radiation start at the exact same distance even though I am further in the same direction that I was when I looked from the Earth.

 

[remmeler]

Let me try a simpler point. I see a galaxy or planet 10 light years away. I travel through a wormhole immediately 6 trillion miles x 5 = 30 trillion miles (5 light years distance but immediately). I then look at that same galaxy or planet. Won't it now be 5 light years away. If true, can't that be extended to any distance, even to the very edge where clumping turns into just background radiation.

 

[petm1]

I see a galaxy or planet 10 light years away.

You are not seeing a galaxy that is 10 light years away, you are seeing an image formed within your eyes by photons emitted 10 years ago. This view is nearly the same where ever you stand within our visible universe and is nothing more than the photons as they pass your "now". As for the CMB it will always be passing you in your same "now" as the light from all galaxies but it will appear older.

 

[bcrowell]

It just doesn't seem logical that wherever I am in the observable universe, whether it is a billion or 2 billion miles from earth, I would still see the cosmic radiation start at the exact same distance even though I am further in the same direction that I was when I looked from the Earth.

Why does it not seem logical to you?

 

[cosmik debris]

Remmler I think your problem is that you think the big bang was at the centre of a sphere. The universe is modeled in 4 dimensions we are on a 3d surface. If you think a dimension lower i.e. the universe is a sphere then we would be living on a 2d surface, like we do on the surface of the earth. The big bang which is the centre of the sphere is not in our view of the universe.

/disclaimer
This is only an analogy.
/end disclaimer

 

[bapowell]

remmler -- this was already adequately answered in the other thread in which you asked this question:
https://www.physicsforums.com/showthread.php?t=131570

 

[remmeler]

I hate to beat a dead horse, but I think I am getting answers to a question I have not asked.

Let's remove the speed of light for the moment. I see a planet which I have determined is 6 trillion miles x 10 =60 trillion miles from me. I travel instantly 30 trillion miles and look. I would assume that that planet is now 30 trillion miles away.

Now, I now look at the spot where clumping just starts in the Universe which is 6 trillion miles x about 13 billion. If I instantly move half that distance and look I should be able to calculate the remaining distance. I realize what happens with light and light years. What I am trying to get at is that, if you move closer to something by whatever calculation you use, then you should not have to look as far either in actual miles or light years to see that same spot.

An Earth example might be: If I look toward the horizon on a clear flat plane with poles of different colors set every mile. I should see a certain distance and a certain number of colors. Let's say the last color was red. If I move closer then the red pole is closer and a see other colors that I couldn't see before. I am asking a question purely about distance.

The part about background radiation obscures my question. Let's replace it with looking at the farthest galaxy. If I move half the distance to that galaxy shouldn't that galaxy now be calculated at a shorter distance from my new vantage point. Since that galaxy was say 12 billion light years away, then if I instantly cover half of that distance shouldn't I be 6 billion light years away in my new vantage point.

If the above is true, then if I look back to the place and time when we perceive that the clumping has just started and looking further only to see background radiation and calculate how far that would be in all directions. It seems from what I have read, the distance given in light years would be equal in all directions no matter what direction we look (I am trying to get the term background radiation out of the question). If the distance is equal in each direction, that to me would mean that we would have to be located at the center of the observable universe that was expanding in all directions at either the speed of light or faster than the speed of light. I am not suggesting that we are in the center. I think the answer does have to do with the fabric of Space/Time and the fact that we are looking back in time. I just don't understand the answers that I have been given. It seems to me to be a paradox.

 

[marcus]

Let me try a simpler point. I see a galaxy or planet 10 light years away. I travel through a wormhole immediately 6 trillion miles x 5 = 30 trillion miles (5 light years distance but immediately). I then look at that same galaxy or planet. Won't it now be 5 light years away. If true, can't that be extended to any distance, even to the very edge where clumping turns into just background radiation.

...The part about background radiation obscures my question. Let's replace it with looking at the farthest galaxy. If I move half the distance to that galaxy shouldn't that galaxy now be calculated at a shorter distance from my new vantage point. Since that galaxy was say 12 billion light years away, then if I instantly cover half of that distance shouldn't I be 6 billion light years away in my new vantage point.

If the above is true, then if I look back to the place and time when we perceive that the clumping has just started and looking further only to see background radiation and calculate how far that would be in all directions. It seems from what I have read, the distance given in light years would be equal in all directions no matter what direction we look (I am trying to get the term background radiation out of the question). If the distance is equal in each direction, that to me would mean that we would have to be located at the center of the observable universe that was expanding in all directions at either the speed of light or faster than the speed of light. I am not suggesting that we are in the center. I think the answer does have to do with the fabric of Space/Time and the fact that we are looking back in time. I just don't understand the answers that I have been given. It seems to me to be a paradox.

We certainly are in the center of our observable universe---that is we are in the center of the portion of the universe that we are currently able to observe. That does not seem to me to be a paradox, though. It seems natural and obvious.

Tell me what is wrong or paradoxical about this: We take a picture of some matter just beginning to form stars and small proto-galaxies, say redshift z = 9. We plug that into the calculator and find that at the present moment that matter is some distance X and that the light took time T to get here.

Now that matter continued to form stars and heavier elements and eventually stars like the sun and planets, and there is somebody with a telescope on one of the resulting planets. He is in the center of his observable universe, and we are on the edge of it. He sees our matter the way it was long before it became us. He sees our matter when it was just clumping, beginning to make starlight.

From the spectral lines he measures that we are at redshift z = 9 and he calculates that we are at a distance X and that the light from our matter (when it was first primitive stars and protogalaxies) took time T to get to him.

"Edge of the observable universe" is a kind of reciprocal relationship. Our matter forms part of the edge of his O.U. and his matter forms part of the edge of ours.

You can see that kind of reciprocal relation if you watch the balloon model (but in one lower dimension). You will see how it works out.

 

[marcus]

As a side issue, you said:

If the distance is equal in each direction, that to me would mean that we would have to be located at the center of the observable universe that was expanding in all directions at either the speed of light or faster than the speed of light.

But when people talk about the speed of expansion they do not mean the speed that the radius of the observable is expanding. Nobody needs to know that speed.

I can show you how to estimate that speed. But it is more of a curiosity. The real rate that people use is either a'(t) the time derivative of the scale factor, or they use the fractional rate of increase of the scalefactor, namely
a'(t)/a(t) which is the technical definition of the Hubble rate H(t).

Right now a'(t)/a(t) is 1/140 of one percent per million years, and that is H(t=now).

It doesn't matter what the radius of the observable is, that does not enter. What matters is the fractional or percentage rate of increase of distances between stationary observers (observers at rest relative to background.).

We are talking about change in geometry (not ordinary motion, like when you are going somewhere, traveling to some destination.) Another name for dynamically changing geometry is spacetime curvature.
=========================

But if you want to know how fast the matter at the edge of our observable is receding that is another business. The way you find out (nobody would bother to know the number, it isn't important, but we can find it out) is to google "cosmos calculator" and put in standard parameters .27, .73, 71, and then the redshift z = 1100.
That is the redshift of the current edge of our observable. The matter that made the oldest light we can see. The most distant matter we currently can detect light from.
That light has been redshifted by z=1100.

So put that in the calculator and you will get the distance back THEN when the light was emitted (about 41 million LY) and the distance NOW today (45 billion LY, about 1100 times farther) and it will also tell you the speed at which that distance of 45 billion LY is currently expanding (I checked, it says 3.3 c)

Here is the link for "cosmos calculator"
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html
It rounds off and does not show as many decimal places as Ned Wright's cosmo calculator, but it does give expansion rates as multiples of c.

 

[Chronos]

The age of the universe when photons were emitted by your destination galaxy is younger by the light travel time of those photons - e.g., the universe was only 12.7 billion light years of age when photons we now view were emitted by a galaxy a billion light years distant.

 

[fillipeano]

The sound is a bit off, but Professor Filippenko gives a great short explanation on how hubbles law works, and why no galaxy can claim to be the center of the universe.
Hope it helps.
http://www.youtube.com/watch?v=ucXNP2O9bAE&feature=related#t=09m48s