The Universe vs Observable Universe

[Tangent87]

After reading the wikipedia article and looking at many other threads on this forum I am still having a hard time understanding the difference between the Observable universe and the entire universe...

Why is the entire universe not observable to us?

The Big Bang happened 13.8 billion years ago. At that point in time, the distance between us and everything else was 0. The universe has been expanding ever since. But we can observe the cosmic microwave background which is the remnants of the big bang and the time when everything was densely packed together. Therefore I do not understand how there can be anything we can't observe if we can observe the beginning of the universe itself?

Please can someone help?

 

[phinds]

The Big Bang happened 13.8 billion years ago.

yes

At that point in time, the distance between us and everything else was 0.

No.

The universe has been expanding ever since.

yes

But we can observe the cosmic microwave background which is the remnants of the big bang and the time when everything was densely packed together

yes, but "densely packed together" does NOT mean everything was 0 distance from everything else. In fact, the universe may well have been (and most likely was) infinite in extent at that time

You have very likely fallen prey to the pop-science canard that everything started at a single point in space and was an "explosion". It did not and was not.

 

[jedishrfu]

This video might help:

 

[Arman777]

After reading the wikipedia article and looking at many other threads on this forum I am still having a hard time understanding the difference between the Observable universe and the entire universe...

Why is the entire universe not observable to us?

The Big Bang happened 13.8 billion years ago. At that point in time, the distance between us and everything else was 0. The universe has been expanding ever since. But we can observe the cosmic microwave background which is the remnants of the big bang and the time when everything was densely packed together. Therefore I do not understand how there can be anything we can't observe if we can observe the beginning of the universe itself?

Please can someone help?

The problem like this arises when we try to imagine that the universe "has a center" or "the universe started from a point in space" type of analogy. It can be hard to imagine the real case of what happened.

I really like one analogy that I want to explain.

Let us think ourselves as 2D creatures embedded on a 2D surface of a positively curved space (simply we are 2D creatures living on the surface of the sphere).

In this analogy, the Universe will correspond to the entire surface of the sphere.

The observable universe will correspond to the part of the sphere of the surface.

In the image, as you can see the Universe is just the surface of the sphere, but each observer on the sphere has its own observable universe.

Respect to the image (analogy), we can ask "Where did the big bang happen?" The answer would be,
"The center of the 3D sphere". Think an expanding sphere but we are only on the surface and we can only move in the surface so there's no way that we can show where the big bang happened. This is the part that we should realize "universe started from a point in space" description (analogy) is wrong.

When we look further away we see the past, because light takes time to come to us. For example, the observer A is the center of its own observable universe and when she looks around, and observers the sky, she will see a point where she can't look further because light hasn't reached her yet.Again, you can see there's no center for the Universe. The Universe is just a surface of the sphere, it doesn't have any center.

İt's simply the balloon analogy actually.

 

[Chronos]

Keep in mind the observable universe only includes photons emitted at a distance no more than the age of the universe at that particular instant. This is obvious due to the finite speed of light/ The trouble starts when you try to force a birds eye [eye of god] view on the universe. Let's imagine the universe was some arbitrary finite size, like a million light years across at the moment of the bitg bang. One second after the bitg bang you can only see a distance of one light second for the simple reason that is all the further light can travele in one second even though the true size of the universe [in this example] would be a million light years plus one light second at t=1 second. Since the universe is expanding superluminally the extra million light years is hidden from our view..

 

[Tangent87]

Thanks all. Think it makes sense to me now.

 

[rootone]

we can observe the beginning of the universe itself?

CMB is emitted some several hundreds of thousand years after whatever the initial event was

 

[Arman777]

CMB is emitted some several hundreds of thousand years after whatever the initial event was

I remember like we are trying to observe beyond CMB.

 

[Chronos]

The CMB only constrains the range of EM observationa. Neutrinos and gravitational radiation are not blocked by the hot plasma that filled the cosmos prior to recombination.

 

[Grinkle]

One mind boggling consequence of expansion and an infinite universe is that our observable universe was once the 'size' of a grapefruit as best we can model - one grapefruit in an infinite universe. Now it is everything we can ever see, and we (must) assume the grapefruit to be the same as the rest of the infinite young universe in order to conclude anything about the entire universe.

Very humbling.

 

[phinds]

One mind boggling consequence of expansion and an infinite universe is that our observable universe was once the 'size' of a grapefruit as best we can model - one grapefruit in an infinite universe. Now it is everything we can ever see, and we (must) assume the grapefruit to be the same as the rest of the infinite young universe in order to conclude anything about the entire universe.

Very humbling.

No, it was considerably smaller than that, but not as pop-science would have it, a point.

 

[Arman777]

The Big Bang happened 13.8 billion years ago. At that point in time, the distance between us and everything else was 0. The universe has been expanding ever since. But we can observe the cosmic microwave background which is the remnants of the big bang and the time when everything was densely packed together. Therefore I do not understand how there can be anything we can't observe if we can observe the beginning of the universe itself?

Actually I think the main reason is the cosmic inflation.

Also you can look here,

http://www.physicsoftheuniverse.com/topics_bigbang_inflation.html

Due to cosmic inflation we have a much large universe then we would ever expect.

 

[Grinkle]

Actually I think the main reason is the cosmic inflation.

View attachment 219576

Also you can look here,

http://www.physicsoftheuniverse.com/topics_bigbang_inflation.html

Due to cosmic inflation we have a much large universe then we would ever expect.

I think this response is misleading because it implies with your words and depicts with your diagram a finite universe. The universe is not bigger than we expect, its bigger than we can make a mental picture of (if it is infinite).

Possibly you are meaning to explain how objects can move beyond our light cone, not really about how the universe can be larger than we expect it to be. It can't be larger than having infinite spatial extent, I think, but I've said other things on this forum before that thought were just as obvious only to be immediately corrected!

 

[Arman777]

I think this response is misleading because it implies with your words and depicts with your diagram a finite universe. The universe is not bigger than we expect, its bigger than we can make a mental picture of (if it is infinite).
Possibly you are meaning to explain how objects can move beyond our light cone, not really about how the universe can be larger than we expect it to be. It can't be larger than having infinite spatial extent, I think, but I've said other things on this forum before that thought were just as obvious only to be immediately corrected!

Well yes, its possible.

After you asked this it also made me think about the situation. If we consider that the universe was infinite in the first place why we encounter such a horizon or a flatness problem. Since the universe is infinite, it must be flat. So why we consider the flatness problem?

If we consider the universe was finite in the beginning, then the inflation model makes sense. In this sense, without the cosmic inflation, the horizon and flatness problem led me to think that the observable universe would be much different than ours.

If there wasn't some kind of inflation how would be the observable universe or the universe? (finite universe case) In the respect of the size or the shape or etc. Maybe I should open a thread for this question. My estimation is that the observable universe would be big as the universe.

 

[phinds]

My estimation is that the observable universe would be big as the universe.

Why? Your scenario does not lead to that conclusion.

 

[Arman777]

Why? Your scenario does not lead to that conclusion.

I didnt want to mean they will be the exact but they will be close in the size. How close, I don't know. As I said it's just my estimation, I also wonder how would be the universe or the observable universe without the inflation.

 

[phinds]

I didnt want to mean they will be the exact but they will be close in the size.

AGAIN, your scenario does not support that conclusion. You seem to conflate the concept of "finite" with the concept of "small". A finite initial state could have been hundreds of orders of magnitude (or more) larger than the part that is now the OU.

 

[Arman777]

A finite initial state could have been hundreds of orders of magnitude (or more) larger than the part that is now the OU.

I see your point, but if that is the case why are we even thinking about the horizon or flatness problem ?

 

[phinds]

I see your point, but if that is the case why are we even thinking about the horizon or flatness problem ?

Personally, I don't think about them at all

 

[Arman777]

Personally, I don't think about them at all [emoji2]

Well that's nice but I don't think it answers my question

 

[phinds]

Well that's nice but I don't think it answers my question

Right. It was a flippant way of saying that I don't have an answer to your question.

 

[Arman777]

I see your point, but if that is the case why are we even thinking about the horizon or flatness problem ?

In flatness problem the main problem is not about the sizes but its about the densities. For a flat universe we need the density parameter exactly equal to 1 and in the early times of the universe this precision increases up to ##10^{-16}. If the density parameter was larger then 1 then the universe would collapse but If it was also smaller then 1 the expansion will be faster so the inflation just tunes density parameter.

But I feel still confused about the horizon problem. In the inflation the comoving Hubble radius is actually decreasing which it leads to OU originate in the inside of the Hubble radius so that the horizon problem will be solved. I guess without the inflation in any case (either finite or infinite universe) there would be a horizon problem.

So in the end, I think both cases the size of the universe is not important. The problems does not depend on whetever we live on finite or inifinite universe.

 

[Chronos]

The homogenity of the observable universe is something worthy of contemplation. In a finite universe, were you to look in the 'short' direction, it would be entirely reasonable to expect it to look different than in the 'long ' [opposite]. Assuming the universe does not look noticeably different in any direction, it is difficult to avoid concluding a] the universe extends far beyond the portion of it we can see, or 2] it is spatially unbounded [infinite] . Were this not true, we would notice there were fewer distant galaxies in the 'short' direction than in the 'long' direction. Since this is not observed, it is logical to assume the homogenity of the universe is evidence that, as a minimum, it extends spatially far beyond the distance observationally accessible to us. Imagine a universe that is finite, Of course the U can be homogenous and finite iif it wraps around on itself. If s finite U were small enough [compared to our observational reach] we would expect to see the same pattern of galaxies recur [on a smaller scale] simply by looking far enough in the same direction. Of course, the U icould be so big that galaxiess die and their photons are absorbed and scattered before they can cricle it - and new galxies are born, randomly supplying an endless stream of photons circling a finite, homogenous U. Occam's razor prefers the infinite universe scenario.

 

[Arman777]

If s finite U were small enough [compared to our observational reach] we would expect to see the same pattern of galaxies recur [on a smaller scale] simply by looking far enough in the same direction.

Due to the expansion of the universe and considering the size of the OU respect to the U, I don't think that's possible. Why on a smaller scale ? I didnt understand that.

 

[Grinkle]

Occam's razor prefers the infinite universe scenario.

Infinite, or just several orders of magnitude larger than our young observable universe? At 100x or 1000x or some OOM, I would think we stop predicting any difference to our observations? And our young OU was smaller than my grapefruit estimate, per @phinds.

 

[Chronos]

The ratio of linear distance is the only factor that matter. The rate of change is irrelevant. Eah time the linear distance beteen consecutive identical images doubles the image size is halved. I

 

[phinds]

The ratio of linear distance is the only factor that matter. The rate of change is irrelevant. Eah time the linear distance beteen consecutive identical images doubles the image size is halved. I

But the area subtended by the image when the distance is doubled is reduced by a factor of 4, not 2.

 

[Chronos]

Area is reducedy by 4 because it goes by distance squared.

 

[phinds]

Area is reducedy by 4 because it goes by distance squared.

Uh ... isn't that what I just said?

 

[Chronos]

I was addressing the 'why on a smaller scale?' question posed in post 24 in linear terms. Of course area is a nonlinear term, so, the only difference in what we are saying is contextual.