# I What is the size of the observable universe relative to CMBR

#### PeterPendragon

Reading the Wikipedia article makes my head spin! Somehow the figure https://en.m.wikipedia.org/wiki/File:Observable_universe_logarithmic_illustration.png doesn't seem quite right. I don't understand how the CMBR doesn't define the edge of the universe. If we can see the CMBR that happened near the beginning of the expansion, how can there be objects beyond that we can't see? It seems the edge defined by the CMBR is within the theoretically observable sphere, therefore we can see the whole universe? Sorry in advance for the dumb questions. Perhaps someone could suggest some better reading on the topic. Thanks.

Related Astronomy and Astrophysics News on Phys.org

#### phinds

Gold Member
Reading the Wikipedia article makes my head spin! Somehow the figure https://en.m.wikipedia.org/wiki/File:Observable_universe_logarithmic_illustration.png doesn't seem quite right. I don't understand how the CMBR doesn't define the edge of the universe. If we can see the CMBR that happened near the beginning of the expansion, how can there be objects beyond that we can't see? It seems the edge defined by the CMBR is within the theoretically observable sphere, therefore we can see the whole universe? Sorry in advance for the dumb questions. Perhaps someone could suggest some better reading on the topic. Thanks.
Google "Surface of Last Scattering". That will explain why there was stuff happening before the CMBR happened and WHY it happened.

#### rootone

The CMB is the light emitted at a time called recombination, that is the time at which the temparture had dropped enough for atoms (predominatly Hydrogen, some Helium, smalll amount of Lithium), to remain stable.
Prior to this event, the condition of the Universe would have been highly ionized - atomic nucleii would exist, and so would electrons, but these would not be bound together, furthermore in this condition light could not travel through it, because as soon as it exists it almost immediately gets absorbed by an electron.
The recombination isn't that close to the big bag event itself, it is at least several hundred thousands of years later, but I suppose that might be considered a short time compared to the several billion years it took to evolve from that through several generations of stars in to it's the present condition.
https://en.wikipedia.org/wiki/Recombination_(cosmology)

#### PeterPendragon

Hi. I knew all this and had read about the Surface of Last Scattering. What I don't understand is where the article talks about galaxies beyond the observation sphere. Are there galaxies beyond the CMBR? How can that be? Thanks for explaining to a Biologist!

#### phinds

Gold Member
Hi. I knew all this and had read about the Surface of Last Scattering. What I don't understand is where the article talks about galaxies beyond the observation sphere. Are there galaxies beyond the CMBR? How can that be? Thanks for explaining to a Biologist!
THE CMBR is not really a surface in space, it is a surface in TIME and there is one at every point in the entire universe, just as there is an "Observable Universe" at every point. From Earth, we see a CMBR. If someone 10 billion light years from Earth looks out, he ALSO sees a CMBR exactly the same distance from him as ours is from us. Likewise, if someone 10 billion light years farther on from him looks out he will ALSO see a CMBR exactly the same distance from him as ours is from us. This goes on forever. If the universe is finite but unbounded, this still happens, it's just that the geometry is different at the cosmological level

Last edited:

#### PeterPendragon

Wow, what an excellent answer phinds. That really made me have a light bulb go off. I think I get it. Thanks!

#### Chronos

Gold Member
The observable universe is always time dependent. An observer who 'now' appears to be 10 billion light years distant from us would have been observing a universe 10 billion years younger than the one we presently observe. That same observer 'now' would observe a universe 10 billion years older than the universe we observe 'now'. Of course, that observer would be nowhere near the position he appears to be to us 'now'. That observer would 'now' view galaxies that do not even yet appear to exist from our present position. Those galaxies would exist beyond the limits of our present observable universe. It that observer tried to communicate his observations to us we would just yawn and say 'so what?' By the time the message reached us so would the photons he was observing.

#### PeterPendragon

So that's almost a different thread. When we look at Mars through a telescope we don't see "now" either. In fact we can argue we don't know anything about what is happening "now" at any distance. What are you up to? :)

#### phinds

Gold Member
So that's almost a different thread. When we look at Mars through a telescope we don't see "now" either. In fact we can argue we don't know anything about what is happening "now" at any distance.
Yes, that is technically correct but for anything very close the time delay is irrelevant
What are you up to? :)
Huh ?

#### Chronos

Gold Member
The universe to a cosmologically distant observer looks very different in their 'now' than it does to us because their observable universe is much older in the direction opposite us than our observable universe. Conversely, it looks much younger in our direction. To an observer residing at out apparent CMB, we are the ones who appear to reside at their CMB. Neither our galaxy nor earth would even yet exist from their vantage point.

#### D H

Staff Emeritus
Hi. I knew all this and had read about the Surface of Last Scattering. What I don't understand is where the article talks about galaxies beyond the observation sphere. Are there galaxies beyond the CMBR? How can that be? Thanks for explaining to a Biologist!
Presumably you are writing about the wikipedia article on the observable universe. (Your original post links to a wikimedia diagram rather than a wikipedia article.)

Evidence of some event that happened very shortly after the big bang is potentially observable if that event occurred in the right place. The cosmological horizon demarcates the boundary between what is potentially observable and unobservable. The observable universe lies within the cosmological horizon; the unobservable universe lies outside of it.

The CMBR does not originate from the big bang. It instead originates from a point in time about 380,000 years after the big bang. There are 380,000 years of events older than the CMBR that are potentially observable. We need to use something other than electromagnetic radiation to "see" beyond the surface of last scattering because the universe was opaque to electromagnetic radiation up until that point in time.

The universe was not opaque to neutrinos at that time. Neutrinos are one way to see beyond the surface of last scattering. Another possibility is gravitational waves. There are surfaces corresponding to the (electromagnetic) surface of last scattering for neutrinos and gravitational waves, before which the universe was opaque to neutrinos, and even earlier, opaque to gravitational waves. Both of these surfaces are very close to the cosmological horizon.

"What is the size of the observable universe relative to CMBR"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving