Big bang at the edges of the Universe?

[Bombu]

TL;DR Summary

If telescopes could see, in every direction, ~14 billion years back to the big bang, why would that not include all of space?

Watching reports about the Webb telescope I see that they can see nearly back to the big bang, within a few hundred million years. Better telescopes are yet to come and soon I would expect that they will be able to see even closer to the big bang.

This leads me to feel that that must be all of space in one direction and when we eventually have observations back to the reionization era (and earlier with new techniques not based on photons) in every direction that we will must have seen all of space.

I'm confident that this is a misunderstanding on my part. I expect the answer will refer to the expansion but even then it still seems like everything has to be confined in that space.

 

[Ibix]

Everything we can see is confined within a region called the observable universe. Stuff can be further away but not visible because light hasn't had time to reach us from that far away.

Note that I haven't mentioned expansion. That just adds to the above; if the universe is infinite there are regions we can never see even if we wait forever, which would not be the case in a static universe.

 

[Bombu]

But how does that reconcile with being able to see back so close to the big bang? Where did all that space come from? I can imagine that, because of expansion, that something like what you say is true. The current real state of distant objects may be unknown and unknowable but mustn't they all have developed in the space between us and the big bang?

And if I look far enough in any direction I'll look back to the time of the big bang.

Thank you for your response. I'm looking forward (in time) to finding out what I am misunderstanding.

 

[Hill]

all have developed in the space between us and the big bang

The big bang is not a place. It happened everywhere in space some 14 billion years ago. It happened where you are now as well as 100 trillion light years away. There is no such thing as "space between us and the big bang."

 

[DAH]

TL;DR Summary: If telescopes could see, in every direction, ~14 billion years back to the big bang, why would that not include all of space?

Watching reports about the Webb telescope I see that they can see nearly back to the big bang, within a few hundred million years. Better telescopes are yet to come and soon I would expect that they will be able to see even closer to the big bang.

This leads me to feel that that must be all of space in one direction and when we eventually have observations back to the reionization era (and earlier with new techniques not based on photons) in every direction that we will must have seen all of space.

I'm confident that this is a misunderstanding on my part. I expect the answer will refer to the expansion but even then it still seems like everything has to be confined in that space.

When we observe the most distant galaxies, then we are seeing them at the time of emission when the radius of the observable universe was much smaller. A galaxy with a redshift of 20 will have a proper distance at emission of about 2 billion light year, however the co-moving distance (the distance it will be now since the universe has expanded during the time the light takes to reach us) is >30 billion light years.

 

[Bombu]

So a galaxy with a redshift of 20 was 2 billion light years away when it emitted the light that

The big bang is not a place. It happened everywhere in space some 14 billion years ago. It happened where you are now as well as 100 trillion light years away. There is no such thing as "space between us and the big bang."

So are observers 100 trillion light years away also looking back 14 billion years at galaxies only a few hundred million years old?

we're seeing today. Since then, the expansion has put it 30 billion light years away. Is that a correct understanding of what you said?

With no expansion, light from that 2 billion light year distant source would take 2 billion years to get to the telescope. As expansion continues it takes longer and longer. It occurs to me that stated distances in light years don't reflect the time it takes light to make the trip because expansion increases the distance during the trip, I presume, so the light leaving the 30 billion light year away source would take even longer to get here, right? But that's a bit of a digression, I suppose.

In my original question I mentioned expansion; I wondered that if we could look back to the big bang in every direction then that would encompass all of space. I expected that expansion would make things farther away and farther apart, perhaps even to the point of unobservability, but still, all the stuff in existence would be in that bounded area. I know I must be mistaken, because this is contrary to the way people talk about space.
I remember, as a kid, speculating about a universe where you could look, with a sufficiently powerful telescope, at the back of your head. This is somewhat kindred to that, except incorporating the fact that you look back in time. If you can look back to the beginning of time in every direction what else could there be?

 

[PeroK]

So a galaxy with a redshift of 20 was 2 billion light years away when it emitted the light that

So are observers 100 trillion light years away also looking back 14 billion years at galaxies only a few hundred million years old?

Yes, if indeed the universe is infinite, or at least that large.

we're seeing today. Since then, the expansion has put it 30 billion light years away. Is that a correct understanding of what you said?

Yes.

With no expansion, light from that 2 billion light year distant source would take 2 billion years to get to the telescope.

Yes.

As expansion continues it takes longer and longer. It occurs to me that stated distances in light years don't reflect the time it takes light to make the trip because expansion increases the distance during the trip, I presume, so the light leaving the 30 billion light year away source would take even longer to get here, right? But that's a bit of a digression, I suppose.

Yes. Distances are usually given in so-called comoving coordinates. Not the eventual light travel time.

In my original question I mentioned expansion; I wondered that if we could look back to the big bang in every direction then that would encompass all of space. I expected that expansion would make things farther away and farther apart, perhaps even to the point of unobservability,

Yes. Some galaxies are too far away ever to be observed on Earth. Although, that depends on the cosmological model.

but still, all the stuff in existence would be in that bounded area.

The universe may be finite. But, current measurements show that it must be at least 50 to 100 times the size of the current observable universe.

I know I must be mistaken, because this is contrary to the way people talk about space.
I remember, as a kid, speculating about a universe where you could look, with a sufficiently powerful telescope, at the back of your head.

There's currently no evidence for a closed, finite universe where that would be the case.

If you can look back to the beginning of time in every direction what else could there be?

You can never see back to time zero in the Big Bang model, because that's where the mathematical model breaks down. This is aka the "singularity".

In fact, with light we can only see back to the last scattering surface. Although, further back with gravitational waves.

In any case, we can only see back to a finite part of the universe.

For further reading, try this excellent insight on cosmological horizons:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

 

[Jaime Rudas]

The universe may be finite. But, current measurements show that it must be at least 50 to 100 times the size of the current observable universe.

Actually, recent measurements show that the maximum circumference of the universe must be at least 10 times the size of the maximum circumference of the current observable universe.

 

[DAH]

With no expansion, light from that 2 billion light year distant source would take 2 billion years to get to the telescope. As expansion continues it takes longer and longer. It occurs to me that stated distances in light years don't reflect the time it takes light to make the trip because expansion increases the distance during the trip, I presume, so the light leaving the 30 billion light year away source would take even longer to get here, right? But that's a bit of a digression, I suppose.

When observing galaxies there are three distances to consider, the proper distance at emission, the co-moving distance and the distance the light has traveled to reach us. The light travel distance is always somewhere between the emission distance and co-moving. All these can be calculated using $\frac{\text{d}z}{\text{d}t}$ and the cosmological parameters for a FRW universe.

 

[Bombu]

Yes, if indeed the universe is infinite, or at least that large.

Yes.

Yes.

Yes. Distances are usually given in so-called comoving coordinates. Not the eventual light travel time.

Yes. Some galaxies are too far away ever to be observed on Earth. Although, that depends on the cosmological model.

The universe may be finite. But, current measurements show that it must be at least 50 to 100 times the size of the current observable universe.

There's currently no evidence for a closed, finite universe where that would be the case.

You can never see back to time zero in the Big Bang model, because that's where the mathematical model breaks down. This is aka the "singularity".

In fact, with light we can only see back to the last scattering surface. Although, further back with gravitational waves.

In any case, we can only see back to a finite part of the universe.

For further reading, try this excellent insight on cosmological horizons:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

Thanks, everybody. I'm working on getting through that article. I may take me awhile. I found another great resource for explaining "last scattering surface" at http://ned.ipac.caltech.edu/level5/Glossary/

 

[Hornbein]

Actually, recent measurements show that the maximum circumference of the universe must be at least 10 times the size of the maximum circumference of the current observable universe.

Note that this means the volume is at least 1000 times that which we can observe.

On second thought, the volume of the observed Universe -- since we can see back to denser eras, just what is this volume? A question not worth considering, think I.

 

[Jaime Rudas]

Note that this means the volume is at least 1000 times that which we can observe.

Actually, in volume, it would be at least 4700 times larger than the observable universe, but what I wanted to highlight is that it isn't "at least 50 to 100 times the size of the current observable universe" as stated.

 

[PeroK]

Actually, in volume, it would be at least 4700 times larger than the observable universe, but what I wanted to highlight is that it isn't "at least 50 to 100 times the size of the current observable universe" as stated.

To set the record straight, I thought I'd read that the radius of the universe was at least 50 to 100 times the radius of the observable universe. But, when I looked it up, it seemed that your figure of 10 times was closer to current research. I was going to reply, but I thought a "like" of your post was sufficient.

 

[Hornbein]

Actually, in volume, it would be at least 4700 times larger than the observable universe, but what I wanted to highlight is that it isn't "at least 50 to 100 times the size of the current observable universe" as stated.

Let's say the true circumference is 10 times that which we observe. That means the radius is 10 times that which we observe. The volume is the cube of that with some constant factors that cancel out. So the volume is 1000 times greater than what we observe. Unless you want to factor in the expansion of the Universe, which I'm not going to attempt.

 

[PeroK]

Let's say the true circumference is 10 times that which we observe. That means the radius is 10 times that which we observe. The volume is the cube of that with some constant factors that cancel out. So the volume is 1000 times greater than what we observe.

I think we all see that.

Unless you want to factor in the expansion of the Universe, which I'm not going to attempt.

This is already factored in. The estimated size of the observable universe is an extrapolation based on the estimated expansion of the universe and the recession of the most distant galaxies that we can see over the past 13 billion years or so.

 

[pinball1970]

This is already factored in. The estimated size of the observable universe is an extrapolation based on the estimated expansion of the universe and the recession of the most distant galaxies that we can see over the past 13 billion years or so.

This was the part that confused me, the observable universe is 90 billion ly wide but is less than 14 billion years old. The light we can see from the most distant objects are about 30 gly distant but they themselves are less than 14 billion years old, say a few hundred million years after the BB.
Put in the expansion and those distances (and ages) make more sense.

 

[PeroK]

This was the part that confused me, the observable universe is 90 billion ly wide but is less than 14 billion years old. The light we can see from the most distant objects are about 30 gly distant but they themselves are less than 14 billion years old, say a few hundred million years after the BB.
Put in the expansion and those distances (and ages) make more sense.

The furthest/oldest galaxies we can see are estimated to be about 33 billion light years away now, with the light having taken about 13.5 billion years to reach Earth (using comoving coordinates). They would have been less than 13.5 billion light years away when the light was emitted, but are now more than 13.5 billion light years away - based again on the model of the expanding universe.

The last scattering surface (that we see from the CMBR) is even further and older. That is now more than 40 billion light years away. And that surface was a lot closer when the light was emitted about 13.8 billion years ago. We also infer that there are galaxies there now! We see that surface at the time when atoms were first able to form. And we infer the same evolution of the universe at that distance as we see for the regions closer to us.

The edge of the observable universe is a theoretical maximum distance for any light speed information to reach us today. For example, we can see further back than the last scattering surface by detecting gravitational waves.

 

[Jaime Rudas]

Let's say the true circumference is 10 times that which we observe. That means the radius is 10 times that which we observe. The volume is the cube of that with some constant factors that cancel out. So the volume is 1000 times greater than what we observe.

No, because the volume of the observable universe is the volume of a sphere, that is, 4πR³/3, while, if the universe is closed, that is, if it is a 3-sphere, its volume would be equal to 2π²R³

 

[Hornbein]

No, because the volume of the observable universe is the volume of a sphere, that is, 4πR³/3, while, if the universe is closed, that is, if it is a 3-sphere, its volume would be equal to 2π²R³

Ha! I guess you're right.

 

[Bombu]

Thanks, everybody. I'm working on getting through that article. I may take me awhile. I found another great resource for explaining "last scattering surface" at http://ned.ipac.caltech.edu/level5/Glossary/

I've been reading that if the universe is flat and infinite now then it was always flat and infinite. If that's the case then wouldn't it be meaningless to talk about the size of the universe? One often hears that the universe was very small immediately after the big bang, like comparisons to common objects. Small as a walnut, ect.

What are they referring to? Some have written that that refers to the "observable universe" but that seems odd to me.

In other developments I had never heard of the "last scattering surface." That is the concept that really addressed my original question so thank you for that.

 

[Ibix]

What are they referring to?

Either a closed universe, which really was small, or the observable universe, which is a finite region of a much larger (possibly infinite) universe.

 

[phinds]

To set the record straight, I thought I'd read that the radius of the universe was at least 50 to 100 times the radius of the observable universe. But, when I looked it up, it seemed that your figure of 10 times was closer to current research. I was going to reply, but I thought a "like" of your post was sufficient.

Over the years I've seen estimates that the universe is X times larger than the Observable Universe, with X being 3, 10, ~100, 10E500, etc. Is the figure of 10 anywhere near solid (meaning that the U is at least that much bigger than the OU) or is it just another swag?

 

[Jaime Rudas]

Over the years I've seen estimates that the universe is X times larger than the Observable Universe, with X being 3, 10, ~100, 10E500, etc. Is the figure of 10 anywhere near solid (meaning that the U is at least that much bigger than the OU) or is it just another swag?

According to what was calculated here, the radius of curvature of the universe is at least about 10 times larger than the radius of the observable universe, which means that its volume is at least about 4700 times larger.

 

[Ibix]

According to what was calculated here, the radius of curvature of the universe is at least about 10 times larger than the radius of the observable universe, which means that its volume is at least about 4700 times larger.

Indeed - ten times the radius of the observable universe is the tightest curvature the universe could have if it is actually closed, given the error bars on our measurements. It could be larger than that, including being a flat or open universe which would be infinite.

 

[Jaime Rudas]

Indeed - ten times the radius of the observable universe is the tightest curvature the universe could have if it is actually closed, given the error bars on our measurements. It could be larger than that, including being a flat or open universe which would be infinite.

Yes, that's precisely why I wrote "at least" in bold.

 

[Ibix]

Yes, that's precisely why I wrote "at least" in bold.

Sure. I was just adding the bit that the "at least" stems from the error bars on the measurements, more for the OP's benefit.