How Vast Is the (Non-Visible) Universe?

Factual Questions
1

We know the observable universe is a small fraction of the actual universe.

The visible area of it is a spherical region of space from which light has had enough time to reach us since the Big Bang.

Obviously, there is a lot of universe that is invisible to us, impossible to observe, and completely unknown.

For some reason, I’ve always thought about the non-observable universe. When I was little, one of my greatest fears was that aliens would land outside my house and abduct me. So I would go to sleep with the lights on, thinking about places where aliens could come to Earth from, and many other places beyond that, including the non-observable universe, a concept that hadn’t yet formed in my mind and for which I had no name either.

But I’ve always wondered how large the non-visible universe is. It’s not an imperious question, but I cannot refrain from thinking about it. I’ve also read about it, including Kant’s argument that human reason cannot prove whether the universe is infinite or finite, or Einstein’s position that the universe is finite but unbounded.

What is the actual size of the Universe?

I’ve always felt it is an unanswerable question. But apparently someone has already answered it.

This morning I watched a presentation-like video, where the ratio between the observable universe and the total universe is the same as that between a lightbulb and Pluto.

You can watch the video and the argumentation (which is way toward the end) here:

Scientists believe the universe is 150 sextillion times bigger than we thought | Watch

I wonder if this is, indeed, considered to be the real size of the universe.

Misread thread titles
2

Universe, by definition, means it contains everything. Therefore the Universe is the everything part, and the other parts are what the everything expands into. That part is not anything, until something is in it, and then it’s the Universe.

3

There is, indeed, a theory of cosmic inflation where the early universe expanded by… a lot. There are a number of models for inflation, and it is not ruled out.

There are a bunch of proposed ground- and space-based experiments; “who knows?” is not the scientific approach to such questions :slight_smile:

4

Compared to an infinite universe, a universe 150 sextillion times the Hubble Volume would be minute.

5

It is not actually known whether the universe is or is not infinite (its global topology/geometry if you think about taking a spatial slice). There is not enough data yet to decide the question. Unless there has been some news?

6

Given our current understanding of physics the answer is almost certainly a huge “no idea”.

As noted above it isn’t clear there is a bound. Theories of everything are as numerous as people coming up with them. It isn’t just the size of that part of our universe that is receding beyond the observable boundary. Lots of pundits talk of infinite variations in other universes with different physics and reality. Maybe an infinite number. An infinity of infinities.

I am wont to misquote Arthur C Clarke.

The universe is either finite in size or infinite in size. Contemplating either is equally terrifying.

7

The question is, what’s on the ‘other side’ of the universe. ‘Nothing’ is not a good answer for me. If the universe can expand into ‘nothing’ that makes nothing, something.

At least to me.

8

Do we?

9

The universe is not expanding into a bigger universe/aether/nothing.

Many cosmological models imply that the universe is really big, but I do not know that it is absolutely ruled out that it be “small”.

10

I think so - the edge of the observable universe is indistinguishable in characteristics from the rest of the visible universe, so we can be confident that there is no significant change in space-time just beyond the visible horizon. If there was an empty void, or a solid wall, just outside the visible universe, this would show up in the distribution of galaxies.

But what lies beyond that no-one can tell.

11

There is, so far as it has meaning, no “other side”.

When we talk of the “observable universe” we mean that bit we can see.

Imagine the Earth has no dry land - just a huge single ocean. You are on a ship somewhere on the sea. You look out, you see water in all directions, but you also see a horizon. Past that horizon you cannot see what is there. Imagine one day you see another ship appear on that horizon. What does the other ship see looking in your direction? They see your ship on the horizon. In the other direction they see stuff you cannot. Neither of you is on the edge or in any sense on the “other side” of anything other than two arbitrary lines, each dependant upon the location of the observer in question.

Because the universe we can see is expanding, and is expanding faster the further away it is, eventually the speed things are receding at exceeds the speed of light. At that point we can never see what is past that boundary. But an entity sitting at the edge we observe is not sitting on any sort of physical boundary. Looking back at us, they see us sitting on the edge of their observable universe, and if they look the other direction they see universe we cannot.

Now a planet has a finite size, and the horizon we observe is dependant upon the size of the planet - as witnessed by the curvature of the ocean under you. (Height above the sea also matters, but we will ignore that.) For the universe there is a similar question. Is the universe itself curved in such a manner that it might be finite? Like a sphere with no boundaries, but in every dimension, not just 2. The current answer is that, as far as anything we can measure, the universe is flat. Doesn’t mean it isn’t curved, but if it is, it is curved by an amount that so far has defied detection. This also has implication about the future of the universe.

So there is some suggestion that the universe is infinite. But it isn’t especially strong evidence.

12

Exactly – could be that the observable universe is a small fraction of the total, could be that it’s a big fraction.

13

This is not strictly the case; we can recieve light from beyond the light-speed-distance, because it passess through a lot of other expanding spacetime before it gets here. So we can see galaxies which are currently expanding away from us at speeds much faster than light.

However, if we tried to actually go and visit those galaxies we could never get there, even at the speed of light, because of expansion that will occur in the future. Distances are tricky.

14

Sure I get that. Speed of light and all. That’s sort of meaningless in what I’m trying to question.

I guess my question is - How can the universe be finite, and infinite at the same time?

It’s sort of like pi. An infinite number when represented by a decimal. There is always a ‘next’ number. I see the universe the same way. There is always something more. To put it another way, there is no such thing as ‘non-existence’ at least in ‘space’. By definition, non - existence exists.

15

Hence my pithy remark / misquote.

Consider a universe that is closed in some sense. You travel in one direction apparently in a straight line, and discover you arrive back where you started. You could do this in two ways, you approach where you started in from the opposite direction from how you left, or you could arrive back travelling from the direction you set off in - all the while travelling in a laser perfect straight line. Either way you would conclude that the universe was of finite size, but also that there was no such thing as an outside. No outside because there is no edge - anywhere.

Same universe could also be getting bigger. Next time it takes longer to arrive back. But still no edge.

No reason why the universe might not behave like this. No reason why it would either. Personally I think the idea has a certain satifsactory neatness to it. But that isn’t a reason it could be true.

I have a certain dislike of throwing infinities about willy nilly. Although it is a little trite to start to invoke Cantor, there is a case to be made for being a little more precise about what sort of infinity we are talking about. There is a lot to be said to be careful whenever infinity is mentioned. Whilst space is the topic at hand, you can’t avoid time. And that can change your viewpoint.
If the universe is bounded, the Poincaré recurrence theorem might suggest that we eternally repeat over unimaginable but finite cycles. Philosophers from the stoics to Nietzsche were than by this.

Asside:

Pi has an infinite representation in any base. It also has countably infinite many digits. There are more numbers than there are digits of Pi.

16

We have no idea. For a long time, it was even possible that the visible universe was actually larger than the entire universe. That’s been ruled out by observation, but it’s still possible that the entire thing is only slightly larger than the visible part.

First of all, the Universe might be infinite. We have no way of ruling that out. If it’s not infinite, there are basically two ways that it could be finite: It could be hyperspherical, or it could have a nontrivial topology. If it’s hyperspherical, then space is curved on large scales. So far as we can determine, the large-scale curvature of the Universe is either zero, or very very close to it. This suggests that, if the Universe is hyperspherical, then it must be very, very large, much larger than the observable portion, in much the same way that Kansas appearing to be flat proves that the Earth must be much larger than Kansas.

But if the Universe has nontrivial topology, then it might be perfectly flat, but still wrap around on itself. The classic analogy is the game Asteroids, though there are a number of other video games whose map works the same way. Go too far to the west, and you end up in the “eastern” portions of the map. Go too far north, and you end up in the “southern” portions. There are, IIRC, about a dozen different ways that flat space could be arranged to produce an effect like this (the exact number is known; I just don’t remember it). In this case, if you travel far enough, everything is just repetitions of a single “tile”, and so that one tile can be considered to be the whole Universe.

So how big is that tile? Well, if it were small enough, then it could actually be smaller than what we see. Look out far enough, and you’d see the repetitions. This would be most easily seen from the most distant things we can see, the cosmic microwave background radiation. If the “tile” were smaller than the distance to the CMB, then we’d see patterns of circles on the CMB where circles in different directions match up to each other. But extensive analysis of the CMB data has not found those matching circles. So if the Universe is tiled, then the tile must be larger than the sphere of the CMB. But it might be just slightly larger, or it might be incomparably greater.

Or, it’s also possible that the Universe is both curved and topologically nontrivial. There are tilings that would work both for positive and negative curvature. In any of them, though, the curvature would be significant over the extent of a single tile, so if any of those are the case, then we’re back to saying that the Universe must be much larger than what we can see, to be consistent with our observed near-zero curvature.

Or, as mentioned above, it might be infinite. There’s no upper bound on how big it might be, so we can’t rule out infinity, except perhaps on philosophical grounds (but different philosophies will disagree).

17

I guess the “estimate” in the video is based on the observation that space seems very flat. A scientist would probably say: “Observations imply that if the universe is finite, it must be at least about hundreds of times larger than the observable universe.” Something like that.