What is the size of the observable Universe?

timmdeeg
Gold Member
it looks like the dumbing-down of some of this information for public consumption has caused me to build up an incorrect understanding.
The size of the observable universe is given by the proper distance light has since the very early universe been traveling away from us up to now. As you can see here (described as particle horizon) this light is now about 46 billion light years away from us. This definition clarifies that we can't see a galaxy which is at the horizon of the observable universe. We just can calculate the distance now to the horizon from our knowledge of how the universe was expanding since then.

kimbyd
Gold Member
2018 Award
The size of the observable universe is given by the proper distance light has since the very early universe been traveling away from us up to now. As you can see here (described as particle horizon) this light is now about 46 billion light years away from us. This definition clarifies that we can't see a galaxy which is at the horizon of the observable universe. We just can calculate the distance now to the horizon from our knowledge of how the universe was expanding since then.
I think this description misses a step: why is the size of the universe given by light which traveled from our location? If we're observing, why aren't we using a definition that relies upon what the receiver sees instead?

The definition you gave works because the situation is symmetric: because the universe is homogeneous on large scales, the light which was emitted from our location billions of years ago and traveled to some other galaxy out there behaves in the same way as light from that other galaxy which traveled to meet us. It's just that it's a little easier mathematically to work in coordinates where the origin of the coordinate system is the emitter rather than the receiver.

phinds
Gold Member
The most distant would be the cosmic microwave background radiation. That's just the most distant galaxy, whereas there is quite a bit of space seen further away, at the time when galaxies hadn't yet formed (but filled with gas).
Meant to respond to this earlier but forgot. This is incorrect; it's backwards. The CMB formed about 400,000 years after the singularity and galaxies didn't start forming until about 100 million years after that, so the distance to the CMB includes the time before galaxies started forming.

There IS more space "behind" the CMB that we can't see but it's not very much because of the extreme density back then (everything was closer together)

timmdeeg
Gold Member
I think this description misses a step: why is the size of the universe given by light which traveled from our location? If we're observing, why aren't we using a definition that relies upon what the receiver sees instead?
I can't see the the difference because of the symmetric situation as you said. Why shouldn't I prefer the proper distance now to the particle horizon? This is easy to be seen from the space-time diagram which shows how the distance to the particle horizon and hence the size of the observable universe depends on the expansion of the universe till now.
Perhaps I didn't get your point.

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

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Bandersnatch
Meant to respond to this earlier but forgot. This is incorrect; it's backwards. The CMB formed about 400,000 years after the singularity and galaxies didn't start forming until about 100 million years after that, so the distance to the CMB includes the time before galaxies started forming.
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.

Bandersnatch
For the sake of completeness the size of the observable universe is larger than shown in the diagram because neutrinos are travelling 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
Gold Member
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
Gold Member
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 occured 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
Gold Member
'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.

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 ;
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
Gold Member
... 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.

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mfb
Mentor
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.

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.

mfb
Mentor
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.
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
Gold Member
The author confuses light travel time with distance.

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?

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.

It does say on the website:
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). [. . .]

Bandersnatch
Yes. It's still wrong.

I love NASA!

phinds
Gold Member
Gold Member
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