13bn+ lightyears in all directions?

[PrestonMcCall]

OK, We can see the universe out to about 13 or 14 billion light years. Can we see it in all directions, or are some directions shorter?

If we were in the middle, it would make sense, but surely we are not the 'center' of the universe. Could it be that light or photons only can travel that far? Could it be that beyond that, the universe is expanding faster than the speed of light? Perhaps if it is shorter in some directions, that area is expanding at a smaller rate? Still, I am curious. does it appear that we are in the middle?

 

[Ich]

does it appear that we are in the middle?

Yes. But the most important point is that, according to the cosmological principle, we believe that this applies to every position in the universe. The universe appears isotropic for every comoving observer, so it appears for everyone that they are in the middle, even if the universe has no center or boundary and is the same everywhere (homogeneous).

What is limiting our sight is that -due to the light travel time - we're seeing older and older objects the further out we look, but we can't see beyond time zero, the "Big Bang".

 

[PrestonMcCall]

So Ich. Are you saying we can see the same distance in all directions?

 

[Ich]

Yes. We see out to the CMB, and it looks the same in every direction.

 

[DaveC426913]

Yes. We see out to the CMB, and it looks the same in every direction.

And additonally, if we flew to a planet 5 billion light years away, and took our measurements, we would still conclude that we are in the centre of our universe.

 

[PRDan4th]

This is a very hard concept to understand. The halo around our visible universe (CMB) moves with us as we move away from our spot in the universe and stays 13 + BLYs distant, like a dog with a bone on a stick on his head. But are we seeing a new universe, are new galaxies coming into vision in the direction we move while others disappear?

 

[DaveC426913]

This is a very hard concept to understand. The halo around our visible universe (CMB) moves with us as we move away from our spot in the universe and stays 13 + BLYs distant, like a dog with a bone on a stick on his head. But are we seeing a new universe, are new galaxies coming into vision in the direction we move while others disappear?

Well, one of the problems is that we cannot travel as fast as the universe is expanding. So, by the time we've moved that 5Gly, the universe has expanded more than 5Gly.

However, it's not just about travelling. The alien race on the planet (who didn't have to wait 5Gy to do their observing) also see a 13Gly radius universe around them.

 

[Ich]

The halo around our visible universe (CMB) moves with us as we move away from our spot in the universe and stays 13 + BLYs distant

That's not how it works.
Forget about expansion for the moment and imagine the universe being filled everywhere with a thick, glowing mist. Suddenly, the mist ceases to exist everywhere.
What do you see, given that light has a finite speed?

 

[Calimero]

Yes. We see out to the CMB, and it looks the same in every direction.

We see out to the surface of last scattering. It is important to straighten terminology especially when explaining things to novice.

Well, one of the problems is that we cannot travel as fast as the universe is expanding. So, by the time we've moved that 5Gly, the universe has expanded more than 5Gly.

However, it's not just about travelling. The alien race on the planet (who didn't have to wait 5Gy to do their observing) also see a 13Gly radius universe around them.

Dave I don't get it. In the same post you are talking about expanding universe, and then about 13.7 Gly radius?

 

[DaveC426913]

We see out to the surface of last scattering. It is important to straighten terminology especially when explaining things to novice.



Dave I don't get it. In the same post you are talking about expanding universe, and then about 13.7 Gly radius?

Yes. Light has only been traveling for 13.7Gy, yet recent estimates suggest our universe is around 78Gly in radius.

 

[zewpals]

Yes. Light has only been traveling for 13.7Gy, yet recent estimates suggest our universe is around 78Gly in radius.

Haha I've been begging for someone to answer this. You actually answered two questions for me...
1) That we can only see 13.7 Gly because light has only been traveling for 13.7 Gy.
2) Since the Universe is approximately 78 Gly in radius (assuming it's a sphere), that means that it expands at a rate faster than light can travel. <<<<not sure if this is logically correct

Thank you very much Dave :D

 

[Calimero]

Haha I've been begging for someone to answer this. You actually answered two questions for me...
1) That we can only see 13.7 Gly because light has only been traveling for 13.7 Gy.
2) Since the Universe is approximately 78 Gly in radius (assuming it's a sphere), that means that it expands at a rate faster than light can travel. <<<<not sure if this is logically correct

Thank you very much Dave :D

Ok zewpals, stop right there. Dave is talking about observable universe being 78 Gly in diameter. Only thing that you get it right is that light is traveling for 13.7 Gy.

The most distant 'object' we can see is Surface of last scattering (point of origin of CMB photons we are seeing now). It is boundary of our observable universe.

Surface of last scattering is NOW 45 Gly from us.

 

[DaveC426913]

2) Since the Universe is approximately 78 Gly in radius (assuming it's a sphere), that means that it expands at a rate faster than light can travel. <<<<not sure if this is logically correct

It is true. The most distant parts of the universe that are receding from us faster than the speed of light. And before you ask: no this does not violate relativity.

The term for this is "superluminal recession".
http://en.wikipedia.org/wiki/Faster-than-light#Universal_expansion

 

[PRDan4th]

We must see the CMB as a point in space-time that was produced 13.7 BLY ago, near the "big bang" time. Those objects that we see near the 13.7 BLY range must be much farther away now since space has been expanding ever since they emitted the light we see now.

We and everyone else in the universe can only see 13.7 BLY and are all seeing the same CMB point regardless of where they are. Is this right?

 

[friend]

Could the slight differences in the CMB be caused by slightly different rates of expansion in various areas of the universe? We already know that the expansion is not constant in time. So could it also not be constant in space? Perhaps the more dark matter, the less space expands? Do we know?

 

[Chalnoth]

Could the slight differences in the CMB be caused by slightly different rates of expansion in various areas of the universe? We already know that the expansion is not constant in time. So could it also not be constant in space? Perhaps the more dark matter, the less space expands? Do we know?

The differences in temperature we see in the CMB are due to the very slight differences in density of those areas at those times. Those slight differences in density do in fact have an effect on the local expansion rate.

 

[Dmitry67]

no, because it would courve space differently in different areas. Based on the observations, space is nearly flat.

 

[Chalnoth]

no, because it would courve space differently in different areas. Based on the observations, space is nearly flat.

Well, the differences in density also do curve space differently as well. This is the motivation behind weak lensing surveys.

 

[PiTHON]

I too have been trying to visualize what the universe "looks like", and I think this thread might have cleared some things up for me. I have a few questions to try and confirm my theories though.

1) The universe was completely opaque until ~380,000 years after the BB, at which time, nearly instantly across the universe, recombination happened. Shortly afterwords, photons decoupled from matter and were able to travel freely through the universe. This is the CMB that we can currently see. We see it as a surface of last scattering.

2) There is matter (free electrons/protons etc..) behind the point of last scattering, correct? So every year, we see matter (decoupled photons from the CMB) that is 1 light year more distant from our current location right? --in an ideal situation that ignores expansion of space, any time dilation effects etc..-- If true, that means there is a shell of opaque matter "behind" (from our view) this last scattering event that hasn't recombined yet. So even if you move 5Gly away, you can still only see 13.7Gly in any direction because you are just peering farther into the shell in the direction you moved, and less far into the shell in the opposite direction. This is why changing viewpoints still results in a sphere of visible universe being the same (neglecting travel time, expansion etc..)

3) If this is the case, then couldn't we ignore the cosmological principle for a moment and select a special frame of reference that is much much closer to outside the edge of the "shell" of opaque matter? Wouldn't a person in this special frame see 13.7Gly in one direction (assuming they are there "now"), but see to the edge of where matter resides in the universe in another direction, assuming a flat/infinite universe and a finite amount of matter, even *with* expansion?

Everyone with knowledge on the subject seems extremely reluctant to say yes to the last question. Is it because the expansion of space is higher than the speed of light once you look out far enough? So using the "shell of opaque matter" analogy, *within* the shell, but *outside* the current CMB location from our perspective, space is moving faster than light, so we will *never* get to see the true (if it exists) edge of the universe? And because of this fact, there is no point in discussing an edge of the universe? So the only response to said questions by the extremely reluctant physicists are "since it's impossible to know the physics/properties of said reference frame don't even try to think about it" instead of "yes, it is possible there is a special frame of reference within our universe that would create the viewing conditions you set forth"?

Or is there a much better reason to say no to part 3) of my post that I do not know about? The only caveat I know of currently is if the universe is closed, there would be no "edge" to speak of.

 

[Chalnoth]

The point of the cosmological principle is that there are observers for which the universe is homogeneous and isotropic. This doesn't mean that every choice of observer sees a homogeneous, isotropic universe. Just that there is such a choice available. Typically observers that are moving or accelerating quickly with respect to the CMB won't see a homogeneous, isotropic universe (in actuality, we don't, due to our orbital motion and the motion of our galaxy...the CMB only appears nearly isotropic when we have removed the effect of our own motion from it).

Also bear in mind that this "shell" of opaque matter is a shell in time, not in space. An observer "closer" to this shell would be an observer in the past, and would see the shell as being much closer in every direction because the transition to a transparent universe happened much more recently.

Remember that as we look far away, we are also looking backward in time. Thus, if we moved to the current location of the matter that emitted some of the CMB we see today, which would be about 48 billion light years in any direction, we wouldn't see the CMB being emitted (that happened 13.7 billion years ago). Instead we would see galaxies and galaxy clusters much like the ones around us. We would see different galaxies and galaxy clusters, of course, but we would see them nonetheless.

 

[Chronos]

This is why I object to talking about how 'BIG' the universe is 'NOW'. Who cares? It is model dependent and impossible to confirm observationally. We only see the part whose photons have reached us from the surface of last scattering. We have plenty of puzzles left to solve before we can frame that question in less than philosophical terms.

 

[PiTHON]

I agree that the existence of such a special place in the universe means little to zero in terms of solving current cosmological problems. I just want to make sure I have a fairly correct grasp on the basic concepts of what a simple universe could/may look like.

I guess the problem came up because I started my question from the frame of our universe. For a general exercise in thinking about a universe, is it not logical to think that there exists an edge to the distribution matter inside a flat, unbounded, finite matter universe, with our without expansion? Or is there a critical flaw in my thinking?

I hope that one day I'll be decent enough to help solve some of those cosmological puzzles, however insignificant they may be. But I know I should be able to pin down the basic properties of a very simplified universe before I even attempt something as complex as ours. And I'll remember to keep these theoretical questions in the context of a hypothetical, simplified model universe instead of asking if our universe would work a certain way given certain constraints.

 

[Chalnoth]

I guess the problem came up because I started my question from the frame of our universe. For a general exercise in thinking about a universe, is it not logical to think that there exists an edge to the distribution matter inside a flat, unbounded, finite matter universe, with our without expansion? Or is there a critical flaw in my thinking?

Well, there's a little contradiction in your wording there in that "unbounded" means no edge.

That aside, observationally we know that if there is an edge, it must be far beyond what we can currently see (because we would expect that any such edge wouldn't be hard, and thus would show itself as a strong anisotropy in the CMB). There may be an edge, there may not be. But mathematically it's easier to just leave it out, especially as it obviously has no effects upon what we can actually measure even if there is one out there.

I hope that one day I'll be decent enough to help solve some of those cosmological puzzles, however insignificant they may be. But I know I should be able to pin down the basic properties of a very simplified universe before I even attempt something as complex as ours. And I'll remember to keep these theoretical questions in the context of a hypothetical, simplified model universe instead of asking if our universe would work a certain way given certain constraints.

If you want to get your feet wet, a good idea is to start by understanding the Friedman-Robertson-Walker universe. Unfortunately, really getting a grasp of this universe requires at least a rudimentary understanding of General Relativity, which isn't terribly easy. But in any case, this universe is easy to write down because it is a perfectly-homogeneous, perfectly-isotropic universe (which ours isn't: it has galaxies). This is a fairly good approximation to our universe, however, and so stands as a good first step to understanding it.