The observable universe

[FireGarden]

I'm not entirely sure how to phrase my question...

The observable universe expands, like the rest of the universe. But as it does so, is there more and more of the universe in it? Specifically, if there's a galaxy currently outside of the visible universe, will we ever see it?

As we look further into space we see events that occured further back in time. Let's say today is t=0 and that decoupling (when the universe became transparent) happened at t=-15. So we can see 15 into the past and no further. At t=1, we'll be able to see 16 (Or, atleast, more than 15) into the past, but are we seeing the same event (decoupling) as it happened in a more distant part of the universe than when we saw it at t=0, or is all that extra distance due to expansion? IE: are we still seeing the same point in space when labelled with co-moving co-ordinates? (Imagine an expanding chessboard. a2 is always 1 square up from a1. Let a2 be the limit of the observable chessboard at t=0. At t=1 the chessboard is bigger, but is a2 still the limit of observability?)

Another attempt is,
Suppose at t=0 we see no stars before t=-14 but see stars after t=-14, then at t=1 we should see a similar "landmark", but these cannot be the same "first" stars. Has the observable universe expanded to include points that were not in it at t=0? Are the "first" stars that we see at t=1 merely marking a point in space that was visible to us at t=0 but was merely dark to us because the light hadn't reached us yet from the stars that were ignited at the same time as those we saw at the t=0 "landmark"?



I hope that one of the formulations of the question makes sense.

Google is not being helpful today...

 

[epepke]

I'm not entirely sure how to phrase my question...

The observable universe expands, like the rest of the universe. But as it does so, is there more and more of the universe in it? Specifically, if there's a galaxy currently outside of the visible universe, will we ever see it?

Unknown and still the subject of controversy.

My view is that when we see the background radiation, we aren't seeing "echoes" of the Big Bang; we're seeing the Big Bang (or some time shortly thereafter) itself. Only, from our frame of reference, it's so big that it's very dim. But in its frame of reference, it's very small. So, when you look north and see the background radiation, and you look south and see the background radiation, you're seeing photons emitted from events that happened (pick a number from a hat) maybe the size of a basketball away from each other. So, if you took all that radiation and imagined it concentrated into a basketball, there would be a lot of it. There would still be a red shift, but it would be a big bunch of red.

As far as I have been able to tell, this is not a common view amongst cosmologists, and I've had cosmologists tell me I'm an idiot from time to time. However, it's the one that makes sense to me.

 

[Cecil]

Let's call ourselves point A (age 15.5), and some other point B such that the distance from A to B is 15 billion light-years.

If we point a telescope at the location of B, we see the light it emitted at age 0.5, right? Now suppose we wait a billion years, so we are age 16.5. If B was stationary wrt us, then it would still be 15 billion light years away and we would see it at age 1.5. If, however, it is moving away at 0.5c, say, then it is 15.5 billion light-years away, and we see it at age 1. If B were receding at the speed of light, then we still see it as it was at age 0.5.

If the universe decoupled at time 0.5 (approximation for the purposes of expalanation) then we will observe the decoupling at some point C such that the distance to C is our age minus 0.5.

So, the rate at which some galaxy in the deep sky ages over time depends on its recession speed, which in turn depends on its distance (Hubble's Law).

Does that answer your question?

 

[Stimpson J. Cat]

If an object moving away from us at less than the speed of light is going to be visible to us at some time in the futute, then that implies that light from that object at some point in the past has already reached us. This implies that the only way something which is outside of the observable universe now, could enter it, is if it did not come from our Big Bang. I am not sure whether there are any Cosmological models which allow this, or not.


Dr. Stupid

 

[TillEulenspiegel]

"The observable universe expands, like the rest of the universe. But as it does so, is there more and more of the universe in it? Specifically, if there's a galaxy currently outside of the visible universe, will we ever see it?"

The universe expands, the observable horizon of the universe expands. There is indeed "more" of the universe, actually called the Hubble Volume . There can be no galaxy outside of the Hubble volume , there can be things that we cannot see that are within the horizon but even if we have the technology to detect objects that far away ( see the latest Hubble DF telescope findings ) there is a point where you would not see a galaxy or stars because the farther out you look the closer you get to the beginnings of our universe and at toward the point where stars or even matter existed.

 

[Eos of the Eons]

[B... there is a point where you would not see a galaxy or stars because the farther out you look the closer you get to the beginnings of our universe and at toward the point where stars or even matter existed. [/B]

See, this is just theory. It states that at one time there was no matter. How can we be so sure of this? That bang had to come from somewhere, and must have been at least one thing of matter to 'bang/explode' right?

What if each galaxy is a big bang? What if we never see this beginning of the universe (the point at which we can no longer ever get visible light from)? And even then, who is to say there isn't matter floating around out there that doesn't emit light?

Why can't we look at other possibilites other than our universe having one big bang x number of years ago?

What is this 'space' that matter exists in (may even say 'floats', but that isn't quite right)? Every particle of matter exists in 'space'. What is space? It's nothing. It's not air, not water, not anything. How can there be a beginning or end to it? Or is this space what has always existed? The univers went 'bang' into this space (this is what I always assumed). So there is no end of space, but an end to our universe, the same as there is an 'end' to our galaxy. So we could look at our univers as a huge galaxy of galaxies? Thus, other universes exist, just not in the same 'space'. Or maybe just one big universe of neverending galaxies exist, thus there not being a beginning or end?

This beginning. Would it not be the center of our universe? Thus, where we can no longer see any light emitting from would be the center with our universe stretching in every direction from it. Then there would be an end to where we are expanding outwards to, where again we would see no visible light.

So where is the middle/center? A bang always has a middle from where everything was pushed out from. Like when a rock is thrown in a puddle.

Or was everything thrown in one direction? This would be like water spilling from a bottle. Still a beginning point where everything is moving away from, and an area it is moving into.

So, would matter be our universe in space? Where there is no longer matter, there is no more universe, but still space?

And what if this matter does not ever have an 'end'? Just stretches forever in every direction because there was no beginning? Just big bangs here and there making up galaxies and/or universes of galaxies?

That is just how my quite uneducated (in matters of space and universe) mind understands it.

It would be great if someone could let me know how any of my perceptions are in fact impossible and why, it would help soooo much.

 

[espritch]

IIRC, beyond a certain Z value (2.5 comes to mind but that seems a little low), objects are actually moving away from us faster than the speed of light. This doesn't violate relativity because it isn't inertial movement but rather a result of expansion of the space between us and the object. The reason we can see things beyond this Z value is that when they light we are seeing left them, they were closer so they were not moving away as fast (the farther away things get, the faster they move away), consequently the light was able to reach us.

Anything beyond our observable horizon will never been seen because it is moving away faster than the observable horizon is expanding. Over time, the number of things we are able to see will actually decrease even though the observable horizon is expanding at the speed of light.

Of course my thinking on this could be a little muddled.

 

[espritch]

What is this 'space' that matter exists in (may even say 'floats', but that isn't quite right)? Every particle of matter exists in 'space'. What is space? It's nothing. It's not air, not water, not anything.

Space is not "nothing". According to relativity, it can be curved by gravity and it can expand (spreading out the matter of the universe in the process). According to quantum mechanics, it is full of zero point energy that is constantly producing and destroying sub atomic virtual particles. Whatever space is, it is certainly not nothing.

 

[epepke]

See, this is just theory. It states that at one time there was no matter.

No, actually it doesn't. Because there was no time.

How can we be so sure of this? That bang had to come from somewhere, and must have been at least one thing of matter to 'bang/explode' right?

Some theories have the universe popping off as a bubble from some other structure. Others don't.

What if each galaxy is a big bang?

According to some theories, every black hole is a big bang for a universe "inside" the black hole.

What if we never see this beginning of the universe (the point at which we can no longer ever get visible light from)? And even then, who is to say there isn't matter floating around out there that doesn't emit light?

Light itself wasn't possible until the universe got big enough. As for matter floating around that doesn't emit light, there may be some. It's called "dark matter."

Why can't we look at other possibilites other than our universe having one big bang x number of years ago?

No reason we can't. Come up with one that is consistent with the available evidence, and you might win a free trip to Sweden.

What is this 'space' that matter exists in (may even say 'floats', but that isn't quite right)? Every particle of matter exists in 'space'. What is space?

Good question. Nobody knows. Except that it's spacetime, not just space.

So there is no end of space, but an end to our universe, the same as there is an 'end' to our galaxy.

Some cosmological theories hold this. Some don't. Some hold that the question is like, if you go off in an airplane, when you get to the "end" of the Earth?

This beginning. Would it not be the center of our universe?

According to most theories, everwhere and everywhen is the center.

So where is the middle/center? A bang always has a middle from where everything was pushed out from. Like when a rock is thrown in a puddle.

That's because the puddle already exists. The splash is "embedded" in the puddle. (That's the word topologists use.) But in cosmology, it isn't necessarily true that the universe is embedded in anything.

And what if this matter does not ever have an 'end'? Just stretches forever in every direction because there was no beginning? Just big bangs here and there making up galaxies and/or universes of galaxies?

It probably doesn't, because if the universe were infinite, and matter were roughly distributed in the universe, and roughly the same amount of matter were stars (which there's good reason to believe just happen due to gravity when there is enough matter), the night sky would be as bright as day.

 

[sorgoth]

It probably doesn't, because if the universe were infinite, and matter were roughly distributed in the universe, and roughly the same amount of matter were stars (which there's good reason to believe just happen due to gravity when there is enough matter), the night sky would be as bright as day.

But what if they were so far away (I'm thinking trillions of light years here) that, by the time the light reached us, it would be beyond dim, and unnoticeable? Like how distances between galaxies are enormous, perhaps the distance between universes are much, much more so.

Of course, the problem with all that is that it is unproveable.

 

[epepke]

But what if they were so far away (I'm thinking trillions of light years here) that, by the time the light reached us, it would be beyond dim, and unnoticeable? Like how distances between galaxies are enormous, perhaps the distance between universes are much, much more so.

Of course, the problem with all that is that it is unproveable.

Dimness is not a problem. Assuming flat space and roughly uniform distribution, for a given solid angle, the farther you look away, the dimmer things get, but there are more of them, in the same proportion.

Of course, space is not flat, but the way in which it is not flat makes distant objects appear larger and therefore brighter than they would if it were flat. So that doesn't help.

Red shift might be a problem, but we have radios these days.

In any event, everwhere we look, we can see the Big Bang. Perhaps there is stuff beyond that. Some cosmological theories hold that there is. I'm not betting on it, though.

 

[Cecil]

It probably doesn't, because if the universe were infinite, and matter were roughly distributed in the universe, and roughly the same amount of matter were stars (which there's good reason to believe just happen due to gravity when there is enough matter), the night sky would be as bright as day.

Except for the big dust clouds that block light from getting through.

 

[FireGarden]

Cecil
I agree that there are some galaxies that are too far away to be seen because they are receding too quickly. But will all galaxies outside the currently observable universe always be unobservable?

Agreeing with epepke, I would say that when we look back as far as possible we are seeing the time when the universe first became transparent.

Stimpson

If an object moving away from us at less than the speed of light is going to be visible to us at some time in the futute, then that implies that light from that object at some point in the past has already reached us.

Why? Surely we do see new points everyday, so we do see more and more of the universe. (That isn't enough to answer whether we would eventually see a galaxy that is currently outside the observable universe.)

Look at it this way. We have to always be able to see the time of decoupling, when the universe became transparent.

Today we see light in the CBR that left on its way to us at t=-15 (the time of decoupling). We can label the point of origin (0). Tomorrow we will see light in the CBR that left at the same time (since the universe became transparent, for our purposes, everywhere at t=-15), but tomorrow's light will have left from a point further away from us. We'll call that point (1). These points will retain their co-ordinates as the universe expands because we are using co-moving co-ordinates. The distance between the points labelled (0) and (1) will increase, but we don't move the labels.

We can't have seen (1) before tomorrow, because (effectively) it began to shine when the universe became transparent at t=-15.

Similarly, we will label the points of origin of the light that reaches us day after day as (2), (3), etc.

What is the distance, today, between (0) and (1)?
If the universe were not expanding it would be a light day. But the universe is expanding, and a light day is too far. Assume that the distance from (0) to (1) was a light day. By tomorrow, the light from (1) will have travelled an extra light day compared to the light from (0). But that wouldn't be enough to reach us. So the distance from (0) to (1) must be less than a light day, let's call it X.

Is the distance from (1) to (2) also X?
It can't be since (2) is further from us than (1) and so it must be receeding from us at a greater rate. In that case, for it's light to reach us the day after tomorrow, the distance from (1) to (2) has to be less than X.

By the same argument, if we call X[n] the distance from to (n-1) then X[n] must be a strictly decreasing but positive sequence. And so it must tend to a limit.

But how far into space does that infinite number of points (0), (1), (2), ... reach? Do they go beyond a galaxy that's currently outside our observable universe? Does that depend on how far outside the currently observable universe the galaxy is?

In other words...
Does the Sum(X[n]) for n=1,2, ..., infinity have a limit?
Not necessarily, even if the limit X[n] tends to is zero.
EG: 1+1/2+1/4+1/8+.... does have a limit.
But 1+1/2+1/3+1/4+.... does not.

espritch
IIRC, beyond a certain Z value (2.5 comes to mind but that seems a little low), objects are actually moving away from us faster than the speed of light.

The CBR has a redshift of about 1000. So the point where todays CBR light was emitted would be receding too quickly for us to ever see it again. (I suppose it's infinitely red-shifted) But we can always see the CBR.

Does that mean that tomorrows CBR radiation came from some point that was closer to us when it emitted its light? How could that be? I'm confused.

Over time, the number of things we are able to see will actually decrease even though the observable horizon is expanding at the speed of light.

Do you mean the "number of points" (measured in co-moving co-ordinates)? This ties in with my previous confusion.

epepke again
Light itself wasn't possible until the universe got big enough.

I'm not sure what you mean here, but there was always light. It's just that at the beginning the light couldn't travel very far before it was absorbed, so the universe was opaque.

 

[epepke]

I'm not sure what you mean here, but there was always light. It's just that at the beginning the light couldn't travel very far before it was absorbed, so the universe was opaque.

That's what I meant. The point is that when looking at the cosmic background radiation, we can't see any light from events at the time of the actual big bang. None of those photons exist any more. We can only see light from some time afterward.

 

[FireGarden]

This backs up espritch

http://curious.astro.cornell.edu/question.php?number=575
Therefore, any galaxy with a redshift greater than 1.4 is currently moving away from us faster than the speed of light.

[...] You might be wondering how we could possibly see a galaxy that is moving away from us faster than the speed of light! The answer is that the motion of the galaxy now has no effect whatsoever on the light that it emitted billions of years ago. The light doesn't care what the galaxy is doing; it just cares about the stretching of space between its current location and us. So we can easily imagine a situation where the galaxy was not moving faster than the speed of light at the moment the light was emitted; therefore, the light was able to "outrun" the expansion of space and move towards us, while the galaxy moved away from us as the universe expanded.

Keeping in mind what we learned above -- that farther objects recede faster in a proportionally stretching universe -- we can immediately see that right after the light is emitted, the galaxy is moving away from us faster than the point at which the light is located, and that this disparity will only increase as time goes on and the galaxy and light separate even more. Therefore, we can easily have a situation where the galaxy keeps on moving away faster and faster, eventually reaching or exceeding the speed of light relative to us, while the light which it emitted billions of years ago leisurely coasts on, never having to move across a region of space that was stretching faster than the speed of light, and therefore reaches us eventually.

[...] Astronomers now have strong evidence that we live in an "accelerating universe," which means that the speed of each individual galaxy with respect to us will increase as time goes on. If we assume that this acceleration continues indefinitely, then galaxies which are currently moving away from us faster than the speed of light will always be moving away from us faster than the speed of light and will eventually reach a point where the space between us and them is stretching so rapidly that any light they emit after that point will never be able to reach us. As time goes by (billions of years in the future), we will see these galaxies freeze and fade, never to be heard from again.

As this would also apply to the CBR, then we'll see the CBR fade and (presumably) become infinitely redshifted. But all the CBR we see will always have come from places that were never moving away from us at the speed of light.


I think this also changes what I told Stimpson.
But I need to think about it some more!

 

[Badly Shaved Monkey]

If an object moving away from us at less than the speed of light is going to be visible to us at some time in the futute, then that implies that light from that object at some point in the past has already reached us. This implies that the only way something which is outside of the observable universe now, could enter it, is if it did not come from our Big Bang. I am not sure whether there are any Cosmological models which allow this, or not.


Dr. Stupid

The thing you need to get your head around with this is that under the inflationary hypothesis (Try this one) of the Big Bang the very fabric of space itself expanded at many many times the speed of light for a short period. The speed of light presents an absolute limit on the rate of transmission of information and so the speed of light and its time since transmission gives you a sphere with a radius within which you can, in principle, communicate. This defines our observable universe (hence the name): a sphere with a radius of approx 13.7Gly. However, under the inflationary hypothesis there is 10^20 to 10^30 times as much universe beyond that radius which is forever beyond our sphere of potential communication, whether it be us looking now for previously emitted light or us sending out a light beam.

Stuff outside our observable universe is forever beyond our sphere of communication so he following is not possible: "the only way something which is outside of the observable universe now, could enter it, is if it did not come from our Big Bang."

The relevance of the recently discovered cosmic acceleration is that it will progressively remove the periphery of our sphere of observation. Ultimately we will be outside the feasible light travel time of our own Sun (were it to last so long, which it won't) and electrons would leave the observable universe of the nucleus of their own atom (though strictly speaking, by that time I think all the neutrons and possibly all the protons will have decayed and there won't be atoms). I think, subject to correction by a clever person, the result of this is that our observable universe gets larger over time, but there is less in it to observe.

The other thing to remember is that it is also wrong to say that the Big Bang happened then, and we now are in the aftermath of it. The Big Bang is continuing all around us. The cosmic microwave background is radiation that was emitted from points very near our position, but it has taken 13.7Gy to reach us because the fabric of space has been stretching like elastic between us and that formerly adjacent emission source. That stretching pulls out the wavelngth of the light so we now receive it as micirwaves representing a black body emission of 2.7K instead of ultrashort wavelength e-m radiation representing an almost incomprehensibly higher temperature.

 

[FireGarden]

I can't see my mistake in my reply to Stimpson.
Tomorrow's CBR must have originated at a point more distant than today's. Even if all the co-ordinates I gave (0), (1), (2) etc were all very close together when they emitted their light and today they are light years apart and moving away from us faster than light.

So every day we are seeing light, all of which began it's journey towards us at t=-15, from further out in the Universe. Points which previously we couldn't see.

So eventually we may see what happened at co-ordinate (1000) at t=-15. But we may not see what happened at (1000) at t=-14, because, in the intervening time, (1000) accelerated its recession and no light from it can reach us. In particular, we would not see a galaxy that happened to form at (1000) at t=-14.

That's the counter-intuitive catch.
The more recent event is the one that it takes us longer (possibly infinitely longer) to find out about.

Or am I still wrong?

 

[FireGarden]

Well I've seen one mistake!
My argument for today's distance between (0) and (1) is totally meaningless in light of what I learned from the article I quoted.

On the other hand,

But how far into space does that infinite number of points (0), (1), (2), ... reach? Do they go beyond a galaxy that's currently outside our observable universe?

I think the answer is "yes".
But only in the sense that you could see the location of the galaxy at the time of decoupling, before the galaxy existed.

 

[Eos of the Eons]

Whao epeke my head is spinning, and I'm trying to digest all this, thanking for answering my questions, this may take a while for all this settle in!

No, actually it doesn't. Because there was no time.



Some theories have the universe popping off as a bubble from some other structure. Others don't.



According to some theories, every black hole is a big bang for a universe "inside" the black hole.



Light itself wasn't possible until the universe got big enough. As for matter floating around that doesn't emit light, there may be some. It's called "dark matter."



No reason we can't. Come up with one that is consistent with the available evidence, and you might win a free trip to Sweden.



Good question. Nobody knows. Except that it's spacetime, not just space.



Some cosmological theories hold this. Some don't. Some hold that the question is like, if you go off in an airplane, when you get to the "end" of the Earth?



According to most theories, everwhere and everywhen is the center.



That's because the puddle already exists. The splash is "embedded" in the puddle. (That's the word topologists use.) But in cosmology, it isn't necessarily true that the universe is embedded in anything.



It probably doesn't, because if the universe were infinite, and matter were roughly distributed in the universe, and roughly the same amount of matter were stars (which there's good reason to believe just happen due to gravity when there is enough matter), the night sky would be as bright as day.

 

[Eos of the Eons]

Some cosmological theories hold this. Some don't. Some hold that the question is like, if you go off in an airplane, when you get to the "end" of the Earth?

When you go straight out into space. That is why I can't wrap my head around most of this. There is an 'end' to earth, an 'end' to our galaxy, but not our universe?

At least I'm learning something, but it's tough when I really don't know much about all of this.