B What is the size of the observable Universe?

[Bandersnatch]

For the sake of completeness the size of the observable universe is larger than shown in the diagram because neutrinos are traveling since the very beginning, about 380000 years before last scattering. But I have no idea how much the correction would amount.

The diagram goes all the way back to the singularity.
It's better visible on conformal time vs comoving distance version:

Where the CMB emission is just below the mark of scalefactor = 0.001. The difference in proper distance is not large, some hundred million light-years.

The point in @kimbyd 's post is that it's not immediately obvious that the particle horizon and past light cone are symmetrical reflections, especially not if one thinks in terms of proper distance (first graph above), so it requires clarification as to why we care about outgoing signals instead of incoming.

 

[timmdeeg]

The diagram goes all the way back to the singularity.
It's better visible on conformal time vs comoving distance version:

Ah I see, thanks. Yes the conformal time diagram clearly shows the symmetry.

 

[phinds]

You seem to be repeating what I wrote, so I'm guessing you don't think it's backwards after all. Maybe it's about wording - the 'that' in my post refers to the galaxy in the OP.

You said specifically "The most distant would be the cosmic microwave background radiation. That's just the most distant galaxy " but the CMB is not the most distant galaxy. It occurred long before galaxies formed. Don't see how this can just be a matter of wording.

 

[Bandersnatch]

'That' is referring to the distance from OP's post in the quote box. One would hope it's obvious that CMB is not a galaxy.

 

[phinds]

'That' is referring to the distance from OP's post in the quote box. One would hope it's obvious that CMB is not a galaxy.

OK.

 

[Rohan]

Here is another approach to the problem of the relation of the "observable size of the universe"
to it's 'possible actual size'
I think it worth while to go back to the beginning because we will uncover;
- certain limitations of the human mind and resulting incorrect assumptions
- a really interesting anomaly concerning the speed of light and "inflation"

1/ Hubble discovered that all galaxies are receding with velocities proportional to
their distance from our galaxy , the Milky-Way
-so far so good; but rather than accept that we have a privileged position (at the centre);
astrophysicists came to consider that the situation was "analogous " to points on the surface,
of an expanding sphere; in mathematical jargon a "2-sphere".

2/Now a "2-sphere" exactly fits the conventional notion of a sphere;
ie a closed 2D surface with uniform curvature;
but a "3-sphere" does not fit any conventional idea of a sphere and
is certainly not the 3D space enclosed by the 2-sphere.
Rather their relation is that if a 3-sphere is intersected by the surface;
which is the 3D analogue of a great-circle it will define it's maximal 2-sphere !

3/ But to make matters worse (conceptually) the universe is now
not considered the 3 dimensions of space and 1 of time of Newtonian physics;
but the inseparable 4D space-time of General Relativity(GR) .

4/ Thus the 'shape' of the universe is considered to be some expanding '4-space';
whose 4D analogue of volume is limited yet unbounded !
...a 4-sphere if the curvature is every where the same ;
but I have the impression there is no certainty about this and
some have even considered topologically more complex spaces eg a toroid

5/Now the most distant observable galaxies would be expected to be no more than 13.6 x10^9 light-years;
from the Milky-Way ;if the 'separation velocity'
ie the change in distance per unit time due to expansion of space-time between the galaxies;
is less than the velocity of light (C) (perhaps it would be better if it were called Maxwell's Velocity ?)

6/ Now this might be the end of the story ie "The observable size of the universe is it's actual size";
if it weren't for certain statistical measures of the average radius of curvature of the universe;
suggesting something greater than 13.6 x10^9 light-years perhaps 93x 10^9 light-years !

7/ Now the problem is if the universe (whatever it's actual shape');
is populated with galaxies out to this distance from the Milky-Way,
then during some period of the "Big Bang" their separation velocity must have exceeded C;
yet GR tells us velocity in any frame of reference can't exceed C !

8/ Thus I am led to the curious anomaly that somehow C is the maximum velocity "through" space;
but not the maximum velocity due to inflation; the separation velocity of two masses due to expansion of space-time ?

9/ The idea occurs that when a sub-atomic particle eg a proton passes through a region of space-time;
the quantum vacuum is a sea of virtual particles hopping in and out of existence inside the Heisenberg limits;
now since no proton is distinguishable from any other; really we can have no certainty that our proton hasn't;
annihilated with a virtual anti-proton so that the previous virtual proton becomes real ?
Extrapolating; how can we know that macroscopic masses eg galaxies are composed of their original protons and electrons ?
So how could we know that macroscopic masses traveling 'through' the space-time of the quantum vacuum;
are not just a 'reality' wave in a sea of virtual particles; a wave whose maximum velocity is C? ?

 

[phinds]

... during some period of the "Big Bang" their separation velocity must have exceeded C; yet GR tells us velocity in any frame of reference can't exceed C !

Yes, GR tells us that PROPER motion cannot exceed c. Recession velocity has nothing to do with proper motion and has no limit. Currently, objects at the distant reaches of our observable universe are receding from us at about 3c but have a miniscule proper motion relative to us so no speeding tickets are issued.

8/ Thus I am led to the curious anomaly that somehow C is the maximum velocity "through" space;
but not the maximum velocity due to inflation; the separation velocity of two masses due to expansion of space-time ?

That's just the way cosmological geometry works. Receeding things aren't "moving" in the proper motion sense of that word.

 

[mfb]

9/ The idea occurs that when a sub-atomic particle eg a proton passes through a region of space-time;
the quantum vacuum is a sea of virtual particles hopping in and out of existence inside the Heisenberg limits;
now since no proton is distinguishable from any other; really we can have no certainty that our proton hasn't;
annihilated with a virtual anti-proton so that the previous virtual proton becomes real ?
Extrapolating; how can we know that macroscopic masses eg galaxies are composed of their original protons and electrons ?
So how could we know that macroscopic masses traveling 'through' the space-time of the quantum vacuum;
are not just a 'reality' wave in a sea of virtual particles; a wave whose maximum velocity is C? ?

That makes no sense at all.

See the previous reply by phinds for the difference between expansion of space and moving in space.

 

[Mary Conrads Sanburn]

I hope this helps. I found this article:
How big is the Universe?
Nobody really knows how big the Universe is because we cannot see to the edge of it. We don't even know if it has an edge. We can only see out to a distance of about 14 billion light years from Earth. This means that the size of the Universe that we can see is about 28 billion light years in diameter (across). Light has not reached us from beyond this distance. In addition, the size of the Universe is changing and gets larger with time.
http://coolcosmos.ipac.caltech.edu/ask/237-How-big-is-the-Universe-

 

[weirdoguy]

the quantum vacuum is a sea of virtual particles hopping in and out of existence inside the Heisenberg limits;

Oh no it's not:
https://www.physicsforums.com/insights/physics-virtual-particles/https://www.physicsforums.com/insights/misconceptions-virtual-particles/

 

[mfb]

I hope this helps. I found this article:
How big is the Universe?
Nobody really knows how big the Universe is because we cannot see to the edge of it. We don't even know if it has an edge. We can only see out to a distance of about 14 billion light years from Earth. This means that the size of the Universe that we can see is about 28 billion light years in diameter (across). Light has not reached us from beyond this distance. In addition, the size of the Universe is changing and gets larger with time.
http://coolcosmos.ipac.caltech.edu/ask/237-How-big-is-the-Universe-

Unfortunately that is wrong. Probably a simplification the author wanted to make, but it is too oversimplified.
The URL doesn't work so I can't check the context.

 

[timmdeeg]

The author confuses light travel time with distance.

 

[Mary Conrads Sanburn]

Unfortunately that is wrong. Probably a simplification the author wanted to make, but it is too oversimplified.
The URL doesn't work so I can't check the context.

Hi mfb

The URL does work for me. I just tested it.

At the end of the page it states:
ipac JPL Calteck Nasa
Cool Cosmos is an IPAC website. Based on Government Sponsored Research NAS7-03001 and NNN12AA01C.

Does anyone here see it besides me?
\\http://coolcosmos.ipac.caltech.edu/ask/237-How-big-is-the-Universe-//

 

[Bandersnatch]

It didn't work for me earlier, but it does now.

Anyway, it's still wrong, for the reasons already stated in this thread.

 

[Mary Conrads Sanburn]

It does say on the website:
About This Site
Cool Cosmos at IPAC
Cool Cosmos is a NASA education and outreach website for infrared astronomy and related topics, with information on all NASA-involved infrared missions, including the Spitzer Space Telescope, the Wide-Field Infrared Survey Explorer (WISE), Herschel, Planck, the 2-Micron All-Sky Survey (2MASS), the Stratospheric Observatory for Infrared Astronomy (SOFIA), the James Webb Space Telescope (JWST), the Wide-Field Infrared Survey Telescope (WFIRST/AFTA), and Euclid. This site is hosted at IPAC (Infrared Processing and Analysis Center), and funded by NASA's Spitzer Science Center, at the California Institute of Technology in Pasadena.
IPAC was founded in 1985 to support the Infrared Astronomical Satellite (IRAS) mission, which provided the first space-based survey of the infrared sky. Subsequently IPAC's role expanded to include science operations, data archives, and community support for ten astronomy and planetary science missions, with a special emphasis on infrared-submillimeter astronomy and exoplanet science. IPAC also operates several data archives, including those enabling research in infrared astronomy (IRSA), exoplanets (NASA Exoplanet Archive), and extragalactic astronomy (NED). [. . .]
http://coolcosmos.ipac.caltech.edu/page/about_this_site

 

[Bandersnatch]

Yes. It's still wrong.

 

[Mary Conrads Sanburn]

I love NASA!

 

[phinds]

I love NASA!

Sure, but it's still wrong.

 

[HankDorsett]

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.

 

[phinds]

I am reducing what I believe the size of the observable universe to a diameter less than 3 billion light years.

I don't see how you arrived at that. It's certainly true that at some point in the past that volume of space that is the current observable universe was that size, but so what? A while after that it was 5 billion LY in diameter and the later still it was 10 billion, and on and on until it got to the current size of about 97 billion LY. What's the point of quoting one of the earlier figures?

 

[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.