At the Edge of the (theoretically visible) Universe

Whatever the age of the universe is, measured in years, take that number, multiply it by light years, and you have the radius of the in-theory-visible universe.

My question is, what would it look like right around that boundary?

The occasion for my question is something I’ve wondered about since I was a kid.

As we know from pop-science books, as you look further and further out, you’re also looking further and further back in time so to speak. Things that are 13 billion light years away, we are seeing as they were 13 billion years ago.

So, say the universe is 17 billion years old. Say we could see things that are 17 billion light years away. (Or if this is for some reason logically impossible, say we can see things that are as close to 17 billion light years away as you like.) So when I look way out there, as far as I can look, I can see, basically, the big bang, or at least, its immediate aftermath. But I run into a snag when I try to formulate what I should see around that boundary. For at the Big Bang, everything was accelerating away from the Bang. But from my point of view, everything is on average accelerating away from me. Yet, in the scenario I’m describing, seeing the Big Bang is basically seeing the boundary of the universe. From my point of view, it (or its immediate aftermath), or its image anyway, is spread out across the boundary of the visible universe. Everything, then, should be accelerating away from that boundary and towards me.

So this is a great teaching moment, isn’t it?

-FrL-

Are you asking what it would look like today if we could magically transport ourselves 13 billion light years away?

It looks just like us. We know that because to them, we are 13 billion light years away and we know what we look like.

What they see at their boundary is the spread out light (or, technically, microwave background) that has taken 13 billion years to arrive. Just like we do.

This has to be true from the premise that there are no privileged reference frames. Everything looks the same as everywhere else at the current time.

No. I’m asking what it would look like to us as we observe the boundary, from here, where we are now.

-FrL-

There is no boundary.

As you say, looking far away is looking back in time. If you look far enough back in time, you see the universe when it was hot enough and dense enough that it was glowing and opaque to photons; that is, you see blackbody radiation emitted from the plasma that filled the universe shortly before all of the electrons found their protons and became neutral hydrogen. This is called the “surface of last scattering,” and it is the source of the Cosmic Microwave Background radiation (the source of the quoted temperature of the universe, ~2.73K) that is the subject of such recent interest. Because of the expansion of the universe, this radiation has been redshifted all the way into the microwave.

The reason the CMBR is so interesting is that fluctuations in its temperature reflect the density fluctuations of the universe at the time of last scattering, so they tell cosmologists something about how quickly structure formed from the nearly-uniform plasma existing in the early universe.

If you imagine using something besides photons to “look” back in time–neutrinos, say, or gravitons–you will (according to current cosmological models) see the same thing: a nearly-uniform “surface of last scattering” corresponding to the time after the Big Bang at which the universe became transparent to the radiation you’re looking for. For neutrinos this is somewhere around the time of weak symmetry breaking when neutrinos become weakly-interacting; for gravitons, if they exist, presumably it would be somewhere around the Planck temperature.

I’m talking about the visible universe. There’s a boundary around that, determined by the age of the universe and the speed of light. (Also determined by the powers of our measuring instruments, of course, but I was trying to abstract away from that.)

-FrL-

In addition, if at a certain point, the expansion of the universe exceeds the speed of light, we can know no information about anything beyond this point.

No, there isn’t.

There’s a limit to what we see, because we are limited by the speed of light. So when we look farther out we see light that was emitted in the past. And we can only do that up to a point.

The farthest thing we can “see” is the CMB (Cosmic Microwave Background) left over from the big bang. That’s everywhere, but it doesn’t constitute a boundary. If we were anywhere in the universe it would look exactly the same.

Younger than that, from the first billion years, we can see some very young galaxies and quasars and other weird constructs as pinpoints in the background everywhere, at a variety of wavelengths.

The universe itself is unbounded. There is the same stuff, in essentially the same overall proportions on the large scale, everywhere. No middle, no edge, no boundaries.

When asking questions like this you have to carefully differentiate between asking what, theoretically, is there today 13 billion years away from us, and what can we see that existed 13 billion years ago that isn’t any direction “away” from us except in time. Talking about boundaries just confuses the issue because they aren’t any and never were. (And, yep, getting that is just one of those mental leaps that you have to make to absorb the rest of it.)

We do not know this to be the case.

Yes, there is. You talked about that boundary later in the post I just quoted.

Check my posts–I’ve been talking about the visible universe, not the universe. There is an upper limit concerning what in the universe could possibly be detected by us, and that upper limit is determined by the speed of light and the age of the universe. That upper limit constitutes the boundary around the visible universe.

-FrL-

Thanks for the response, Omphalaskeptic.

So as one comes closer and closer to seeing the big bang (or its immediate aftermath), does one somehow start to see things accelerating away from that event? The reason I’m asking is because it seems like something’s accelerating away from that event would have to appear to us as though it were that thing’s accelerating towards us. Because the image we have of the immediate aftermath of the event appears to us to lie all around the boundary of what I’ve been calling the “visible universe,” meaning anything moving away from that event should appear to us to be moving away from that boundary–and away from that boundary is, basically, towards us.

-FrL-

I always thought of the “balloon” example as a model of the universe.

please tell me if I got the following correct:

  1. There is no “boundary” to the universe, it’s curved back around upon itself.

  2. You can’t look towards the direction the universe is expanding towards, because no such direction exists. The universe itself is inflating in all directions at once. (I am not sure what the rate is.)

Take an uninflated balloon, and with a Sharpee, draw two spots on it. Inflate the balloon, and the two spots appear farther apart, even though they have no means of moving themselves. From their viewpoint (assuming that the balloon is the entire universe), the universe is expanding, but they could not point “out” from the balloon, as they do not have the tools to describe, understand, or affect the volume outside of their little universe-balloon.

You can’t look back at the Big Bang itself because for several hundred thousand years afterwards the universe was, for all practical purposes, opaque. It was filled with hot plasma at sufficient density that light couldn’t propagate a significant distance.

So if you look far enough back in time all you see is the leftover glow of that plasma – the cosmic background radiation.

It sounds like you might be thinking of the Big Bang as a point in space which is surrounded by an expanding spherical boundary, the surface of last scattering (for whatever radiation you’re looking at). But this is not right; the Big Bang is a time, not a place. Moving forward in time from the Big Bang, the universe expands, and everything not gravitationally or otherwise bound therefore starts moving away from everything else.

I think that what you’re suggesting is that very old quasars, for example, are moving away from the surface of last scattering and so they should be moving toward us (and if it’s not true for quasars, maybe if we look back farther we should see this). This is not true, though; the surface of last scattering is moving away from us fast enough that the quasars are also moving away from us.

This is what you should expect to see basically as far back as you are able to look. Wherever you look (cosmologically speaking) you see things like galaxies and quasars moving (“acceleration” is not speed) away from the Big Bang, which is to say away from just about everything else in the universe. They are all receding from us (and everything else) because of the cosmological expansion, and this recession has (in the Big Bang models) been going on ever since the Big Bang, so they have always been receding from us (though at the surface of last scattering they were still just small density fluctuations in the plasma).

Whether the expansion is accelerating or decelerating is a more complicated question and depends on how far back you are looking, whether inflation happened, and whether the universe is open or closed, but I don’t think this is the question you were asking.

You’re not thinking in the right number of dimensions. You’re thinking of a big 3D sphere, and wondering what the boundary of that sphere is like. But the edge of the universe is not spatial, it’s temporal. So the edge that you’re looking for is: NOW. We are constantly, every second, at the edge of the universe.

Let me try to clarify something about this boundary. The “surface of last scattering” which is observed as the CMB is not physically a spherical surface. Physically, it is a radiation that filled the entire universe at a given time, about 400000 years after the Big Bang. We see it as a sphere centered on our location, because as we look in any direction we are seeing light that last scattered off plasma in the early universe just before recombination and has been traveling ever since then in a straight line from wherever it last scattered until now, 13.7Gyr later, where it finally hit a COBE detector. Because it’s been moving in a straight line, we know it started somewhere on a spherical surface centered on COBE, but at the time it left that surface there were similar blackbody photons being radiated from everywhere in the universe, which are now forming similar spherical surfaces around anyone else in the universe who happens to be looking.

Right. What’s been puzzling me is what results when I try to focus on what things should look like from where we are, as abstracted from what things are actually like “out there”. In other words, bracketing the fact that what we’d actually be looking at is a billions-of-years-old radiation filling the entire universe, the way it appears to us (I think, right? even after what you’ve said…) is as though it were a kind of “shell” of radiation. Following similar reasoning, it has always seemed to me that whatever the actual physical facts of the case are, it should appear to us as though we can see the actual big bang (or its immediate aftermath) as though it were a shell surrounding the entire universe. And that screwed with my intuitions because if that’s how things should look, then it seemed to me, it should look like things are moving towards us and away from the “shell” “surrounding” the visible universe. (Attention, Exapno Mapcase. Attention Exapno Mapcase. Please note the scare quotes. Please note the scare quotes. Thank you for your attention.)

I believe it is possible, though, that if I ponder your previous post, I may get some clarity about this. I’m not quite there yet, though. :frowning:

-FrL-

It’s common to imagine the Big Bang as an explosion – there’s a BANG and then a bunch of things fly away from each other in all directions.

But the Big Bang wasn’t an explosion. Rather it was (and continues to be) a stretching of space. Distant objects aren’t really flying away from us. They’re more or less stationary. And we’re more or less stationary. Distant objects just look like they’re moving relative to us because space is stretching.

So even if we could magically look through the cosmic background radiation all the way back to the Big Bang we still wouldn’t see things exploding toward us. They don’t have any significant momentum either toward us or away from us. It’s just an illusion caused by the stretching of space.

Might I clarify the issue with the “Yes there is/No there isn’t” issue?

No, there is no objective “edge of the universe” somewhere six or seven gigaparsecs out there.

There is, however, an end to the observable Universe from any given frame of reference, including our own, produced by the maximum distance there has been time for light to travel since the Big Bang. This does not imply a privileged frame of reference; as noted above by others, Xgwff posting on the Eqderwff Gmdg Message Board on a planet around a star in a galaxy 17.8 billion light-years away is referencing the “microwave haze” that is all he can observe of the Milky Way Galaxy and Local Group and making exactly the same point.

If you stand looking out to sea, or on the roof of a house in Kansas, you see the horizon. The horizon is “real” in that it marks the maximum extent of what is visible to you. It is however an artifact of your location and altitude, not a thing with objective physical reality. “The edge of the observable universe” is a similar visual horizon.

Yes, that’s right. But keep in mind that this shell is also redshifted, which is to say that it is also receding from us. Not because it’s actually moving, mind you, but because we can look farther back in time, as time goes on, and so we see portions of the plasma universe that are further away.

No and yes, respectively. Quasars are redshifted, but not by as much as the surface of last scattering, which means that quasars are moving away from us but the shell is moving away even faster. Which is another way of saying that everything is moving away from everything else, because of the cosmological expansion. (And on preview, as Pochacco notes, calling this “motion” is somewhat misleading; it’s an expansion of the intervening space which causes the distances to grow with time.)