B What is the size of the observable Universe?

[PeterDonis]

I am reducing what I believe the size of the observable universe to a diameter less than 3 billion light years. My reasoning behind this conclusion is based off of the farthest galaxies we have observed. I realize cosmic background radiation should be used but unfortunately I haven't looked into how we are able to observe it.

If that's your conclusion, then you are moving backwards, not forwards, in your understanding.

First, the observations of the CMB are not in question. The fact that you personally have not yet looked into them does not justify you refusing to include them in what you based your belief on. If you want your beliefs to be accurate you need to look at all the available evidence. If you haven't looked at all the available evidence, the correct thing to do is not state a belief based on incomplete evidence, but to just say you haven't formed a belief yet because you haven't yet looked at all the available evidence.

Second, even ignoring the CMB and just looking at the farthest galaxies we have observed, you evidently have not looked at all the available evidence:

https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects
Third, note that that Wikipedia article says "light travel distance", which is just the light travel time multiplied by the speed of light. But as a number of posts in this thread have pointed out, that is not the same as either (a) the distance the object that emitted the light is from us now, or (b) the distance the object that emitted the light was from us when it emitted the light. So you need to make up your mind which of those distances you are interested in.

 

[HankDorsett]

If that's your conclusion, then you are moving backwards, not forwards, in your understanding.

First, the observations of the CMB are not in question. The fact that you personally have not yet looked into them does not justify you refusing to include them in what you based your belief on. If you want your beliefs to be accurate you need to look at all the available evidence. If you haven't looked at all the available evidence, the correct thing to do is not state a belief based on incomplete evidence, but to just say you haven't formed a belief yet because you haven't yet looked at all the available evidence.

Second, even ignoring the CMB and just looking at the farthest galaxies we have observed, you evidently have not looked at all the available evidence:

https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects
Third, note that that Wikipedia article says "light travel distance", which is just the light travel time multiplied by the speed of light. But as a number of posts in this thread have pointed out, that is not the same as either (a) the distance the object that emitted the light is from us now, or (b) the distance the object that emitted the light was from us when it emitted the light. So you need to make up your mind which of those distances you are interested in.

I don't think I was confusing this. The light we are currently seeing was from when that Galaxy was 2.7 billion light years away. After 13.3 billion years Universe expansion has moved that Galaxy out to approximately 35 billion light years away. even though that light started 2.7 billion light years away from Earth Universe expansion between these two points resulted in that light needing to travel 13.3 billion light years to reach Earth.

 

[phinds]

I don't think I was confusing this. The light we are currently seeing was from when that Galaxy was 2.7 billion light years away. After 13.3 billion years Universe expansion has moved that Galaxy out to approximately 35 billion light years away. even though that light started 2.7 billion light years away from Earth Universe expansion between these two points resulted in that light needing to travel 13.3 billion light years to reach Earth.

and yet you conclude "I am reducing what I believe the size of the observable universe to a diameter less than 3 billion light years ". I still don't see how you get there.

 

[Bandersnatch]

The most distant Galaxy we have observed had a light travel distance of 13.3 billion light years. the light that we are currently observing which had that 13 billion light year travel distance was produced when this galaxy was only 2.7 billion light years away.

So you insist on using distance at emission. Which will:
1) mean you will be talking about a different distance than anyone else who's talking about the size of the observable universe;
2) confuse you, since the oldest observable galaxy was not the farthest at emission - every known galaxy that had emitted its currently-observed light earlier than 3 billion years ago was farther at emission than the 13.3 Gyr old one. This is another good reason to stick to distance at reception, since in that case you always get older=farther.
For example, should you include CMB (makes no sense to exclude it), your personal idea of how large the observable universe is would have to be corrected down towards 44 million light-years of radius. Even though, using the same measure of distance you adopted, it'd make nearly every other observable object, other than the nearest galaxies, to be ostensibly farther than the size of the observable universe.
Even if you'd insist on counting galaxies only, just another discovery of an even older galaxy would cause the size of the observable universe to shrink.
Does this make sense to you?

Before I go. I would really like to see what steps they took that got them to the 93 billion light year observable universe. I'm also interested if there is a map of the universe that shows where these galaxies were when the met at the light we are seeing.

The steps for CMB are the same as with any other object, including the 13.3 Glyr old galaxy, whose redshift is used as a proxy of distance - you plug the redshift into the model of expansion and see what distances it outputs.
You can use a cosmological calculator to do it for you, such as this one. I don't remember if it was mentioned here already.
The output for distance at emission vs. cosmic time - discussed above - is shown below (early cutoff at z=11.09 is to indicate the galaxy from the OP):

Below is the output for distance now vs. cosmic time, all the way to the CMB:

 

[PeterDonis]

I don't think I was confusing this.

Perhaps you weren't confused about what you meant by "the size of the observable universe", but everyone else in this thread certainly was.

The light we are currently seeing was from when that Galaxy was 2.7 billion light years away.

And if that's how you personally want to define "the size of the observable universe", that's fine as far as you personally are concerned. But as @Bandersnatch has pointed out, you are using a different definition from everyone else, which means every time you read anything at all about distances in cosmology, you are going to need to do the work of translating back and forth between your definition and everyone else's. Not to mention the other issues @Bandersnatch raised.

What you are not going to be able to do is just throw out numbers in a PF thread using your definition, without saying so, and expect everyone else to agree with your numbers, since everyone else is using a different definition.

 

[mfb]

It is generally a bad idea to use expressions that have a fixed meaning in cosmology (like radius of the observable universe) and then use them for something else. Why would you do that? It just leads to confusion whenever you use your non-standard definition of that expression.

 

[russ_watters]

It is generally a bad idea to use expressions that have a fixed meaning in cosmology (like radius of the observable universe) and then use them for something else.

I think this is true of anything, not just cosmology.

 

[kimbyd]

After some help from various members on this forum I see where I went wrong on some of my assumptions. With this information and sticking within the definitions of scientific observation and observable universe I am reducing what I believe the size of the observable universe to a diameter less than 3 billion light years. My reasoning behind this conclusion is based off of the farthest galaxies we have observed. I realize cosmic background radiation should be used but unfortunately I haven't looked into how we are able to observe it.

The most distant Galaxy we have observed had a light travel distance of 13.3 billion light years. the light that we are currently observing which had that 13 billion light year travel distance was produced when this galaxy was only 2.7 billion light years away.

if you see anything wrong with my information please let me know. I've had to change my understanding many times before and obviously I'll need to change it many times in the future.

Before I go. I would really like to see what steps they took that got them to the 93 billion light year observable universe. I'm also interested if there is a map of the universe that shows where these galaxies were when the met at the light we are seeing.

Why would you be using the past size to mark the size of the observable universe?

Also, the distance you are using, which is the angular size distance, peaks at a certain point in the past, then decreases later. The peak angular size distance is just under 6 billion light years.

 

[HankDorsett]

I finally was able to find an explanation on the observable universe that allowed me to get over my narrow view. I have been limiting my view based off the definition of scientific observation which I now see is incorrect.

Here's what I found.
The word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself.

my apologies if I caused any aggravation.

 

[kimbyd]

I finally was able to find an explanation on the observable universe that allowed me to get over my narrow view. I have been limiting my view based off the definition of scientific observation which I now see is incorrect.

Here's what I found.
The word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself.

my apologies if I caused any aggravation.

No worries. Though the distance we can actually observe is, in absolute terms, extremely close to the speed of light limitation. Getting earlier/further than current observations, which stop at the cosmic microwave background (before which our universe was opaque to light), we need to observe using other things than light. Possible examples are neutrinos and gravitational waves. Neutrinos may not be feasible, because they are impacted by local structure to a significant degree. Gravitational waves are possible, but nobody knows how strong the background gravity wave signal is just yet.