Ask Ethan: Could the Universe be infinite?

If eternal inflation is true, then is there any vantage point in our spacetime where we could see the inflation continuing to happen around it? And, yes, I realize this is just a fancier way of asking if there's an edge to the universe, and that the answer is no, but then...why not? Did the end of inflation turn our spacetime into a discrete, self-contained region?

I did come across an essay awhile back trying to explain how a region of spacetime can look infinite from inside while being finite from outside, but it was too math-heavy for me to follow.

Pardon my simple-minded comprehension, but: in the Forbes version of this post, you say "The nearest galaxy, some 2.5 million light years away, appears to us as it was 2.5 million years ago, because the light requires that much time to journey to our eyes from when it was emitted." That would seem to mean that when we observe the far reaches of the observable universe, 46 billion light years away, we are observing it as it existed 46 billion years ago. Yet the Big Bang is said to have occurred only 13.8 billion years ago.

If we accept eternal inflation, even if the grand-universe (as opposed to our puny big bang universe) started a finite period of time ago, isn't it effectively infinite, as its going to keep growing at an almost inconceivable exponential rate forever?

It's only effectively infinite if it will never, ever stop. If there can exist no possible mechanism that will ever make it stop. 'Never' though, is a very long word, even for the universe.

the first photo, instead of being called "artist's logarithmic scale conception.... " should be called "Top side view of Starbuck's coffee cup" :D

@ Bunny

The answer depends IMO on the definition of Universe in question. If by Universe you mean our observable universe, or even the cosmological horizon universe (meaning all that can ever be causally linked within this spacetime we are in), then no.. no boundary exists, because even at the furthest point, you will just have the same stuff on and on and on. And this is the universe that 99% of the time is considered. But... the WHOLE existing universe, if it's not infinite, must have a limit somewhere. Thus, in an inflation/multiverse scenario, there does exist a coordinate point, which is outside of our universe, or a co-moving coordinate that is just at the edge. But you would have to be a higher-dimensional being, in order to appreciate it. At least that's what my limited understanding of it tells me, i might be very wrong.

p.s.
the part " there does exist a coordinate point, which is outside of our universe," .. should more precisely be outside of our whole existing universe.

I think that we have a mother universe which has a point in the center with a radius with infinite length in all directions or in 360 grades,and a phisical(experimental)universe which apparently has begun with a big bang with a limited inflasion inside the mother universe and all photons of lights of our universe which separate from our universe which must be daily more than the weight of a medium galaxy will be as the bricks for reproducing a new universe in our neighborhoud in accompany with separated photons of other neighbour universes and as our planet is as small as a small sand with respect to our whole limited universe so our universe with its high prabability milliards multy neighbour physical universes fills only the least place inside of our infinite size mother universe.

Thank you, Sinisa. What I've read on the topic of eternal inflation suggested to me that, if you could move at an arbitrarily fast velocity, you could eventually reach the region of exponential inflation and other "bubbles" like ours just by traveling straight ahead. (Of course, you can't move at arbitrarily fast velocities.) That seems to violate the Copernican principle, since you could fly right out of our universe; but I suppose in an eternal-inflation scenario, our universe is actually the rare anomaly in a vast, eternally inflating cosmos anyway. So how can you say it's a violation? But it just seems too naive.

On the other hand, I've also come across material that makes it sound more complex than that, like where you mention higher-dimensional beings. It's confusing, and I'm not equipped to determine if authors are even talking about exactly the same things. So I appreciate your input.

By the way, if anyone has some suggested reading on these questions, I'd love to get recommendations.

@Tom T. #2:

You have to remember that space is expanding, and it's expanding faster the farther away an object is. A great deal of space has formed between us and a galaxy whose light was emitted 10 billion years ago.

Ethan has written about this counterintuitive issue before.

@ Bunny

Like you mentioned, it can be thought of in different ways. If you abide by laws of our Universe, and you are in it already, then there is no possible way of reaching the edge. Speed of light being one of the problems.

Copernican principle, and everything else that's usually considered, is almost always given in relation to our observable universe, since that's the only thing that "exists" for us. Physics will rarely talk about the whole existing unviverse, since we know next to nothing about it's size, and in reality, nothing beyond our cosmological horizon can ever interact with us...so for all purposes, it's like it doesn't exist.

So the only way to "reach" the edge as a thought experiment, is to either discard some law (like speed of light) or to put yourself in a higher dimension (meaning to put yourself outside of our universe... like imagining you are in a soda bottle and bubbles around you are universes).

But in both cases, you can't say after that this breaks some law... of course it does.. we deliberately broke something in order to put an observer "outside" of our own universe.

As for the material. hard to pin a single thing or a paper. I found youtube online classes on cosmology and astrophysics to be of immense help and value. I don't mean public lectures (which are ok for general info, but become very dull and boring frankly, once you have a base), but actual recorded university semester's on these subjects. Susskind's has several classes available (GR, QM, Inflation, String theory, cosmology..) all with 10+ hours of material. You also have Guth's classes on Inflation available. Just look for youtube channel's of big universities like MIT, Stanford, Yalle, Berkley etc.. and look for recorded classes on the topic that interests you.

p.s.
the second most single valued source for information on these topics IMO is wikipedia (but in english)... articles in localized versions are very limited (at least in case of my native language).

just google i.e. "eternal inflation wiki" or any area just ending with "wiki".. and start reading. it gives you 2 major things... crosslinks on important elements if you want to dig deeper, and reference links to papers, books etc. from where the info was gathered. So you can just skim over or really dig into the bone, depending on your time and interest.

"And, yes, I realize this is just a fancier way of asking if there’s an edge to the universe, and that the answer is no, but then…why not? "

Because there's no causal link from here to that point. And, at that point, since at the singularity it was in the same single point as we were, that point too would see an "infinite" universe around it. Else it would not have been at a singularity.

The more general problem is you leave undefined what you mean by "edge" and "universe". Even if there were a point beyond which there were no matter or radiation, there would still be *space*, and since the universe is everything there is, including space, that can't be the edge.

To answer your question would be to make up my own definition of your question, and answer that. And if I don't tell you how I defined your query, you won't know why the answer is right.

"That would seem to mean that when we observe the far reaches of the observable universe, 46 billion light years away, "

We don't observer the matte at that distance. It is only 46 billion light years away now, not when the light it emitted that we now see left it, in which case, that matter is 13 billion light years away at the time at which it emitted the light we are seeing

"we are observing it as it existed 46 billion years ago."

As it existed when the light we are seeing left it, 13 billion years ago.

"started a finite period of time ago, isn’t it effectively infinite, "

Nope, still definitely a long way from infinity.

"as its going to keep growing at an almost inconceivable exponential rate forever?"

Until nearly forever has passed, it won't have reached even virtually infinity.

As with Tom, your problem is the mixing of different timeframes in the same event, which isn't the case.

Thanks for the recommendations, Sinisa. I hadn't considered lectures because of my attention problems, but I can back up and replay a video or audio recording, so that may be feasible.

I appreciate your position, Wow. Unfortunately, asking poorly defined questions is a side effect of my not knowing what I'm talking about. Frustrating!

I was thinking of an "edge" in terms of a place in space that we could fly to with our warp drive and say, "If you go past that boundary, you'll find yourself experiencing exponential expansion like the sort that predated the Big Bang because inflation is still happening there."

I was thinking of an “edge” in terms of a place in space that we could fly to with our warp drive and say, “If you go past that boundary, you’ll find yourself experiencing exponential expansion like the sort that predated the Big Bang because inflation is still happening there.”

The universe could be finite but have no such edges. The ubiquitous balloon analogy springs to mind; the surface area of a sphere is finite, but if you were a 2D being living on it, there would be no point you could fly to and experience the end of the surface. Our universe may be similarly curved. Or it might not be; AIUI our measurements show it to be flat. Extrapolating out how big it would have to be for those measurements to 'miss' the curvature means that even if its curved and finite, it would still need to be orders of magnitude bigger than our causally viewable universe. But other than that, we just don't know the shape.

@ Bunny re: 17

if you start within the universe.. you will find out that the more you're trying to reach the edge, the more laws of physics of this universe you have to throw out the window. the end result being that you will have so much energy within you... that you will become the next inflation point :) if grand unified theory is correct in any sense... even if you pass c multiple times and somehow not become a BH.. by the time you reach the edge.. you'll have so much energy that all the forces and particles have become one unfied field. In a poetic sense, you will become that which you are trying to find :)

If the universe is arbitrary large, maybe this could be a way out of matter-antimatter asymmetry? If there were quantum fluctuations in the matter:antimatter ratio, maybe these were expanded out to so large scales that in our observable universe we see an excess of normal matter, but on the scale of the whole universe, the amount of matter and antimatter would be equal?

@Sinisa Lazarek #19: Interesting. So there is no place that would just start out close to the edge. There's no galaxy where, just by chance, a region of rapid inflation would be within the boundaries of the observable universe. (Obviously, nothing in the inflationary space would itself be observable.)

I hope it's clear that I'm not trying to argue in favor of an edge like this. I've just never understood why there isn't one. I've enjoyed this discussion, and I intend to follow up with the material you've suggested. I may just have to knuckle down and learn some more math, too.

@PH #20: The energy scale for your hypothesis is wrong. Inflation (the expansion of quantum fluctuations to cosmological sizes) happens at energies comparable to the Planck scale (~10^19 GeV).

Baryogenesis and CP violation are processes that happen way down at Standard Model -ish energies, long after inflation, ~1000 GeV or so down below 1 GeV.

Above those energies, even if CP violating processes occur, the particles and photons are still in chemical equilibrium, so you can't sustain an asymmetry.

@Michael Kelsey #22: I see. So, energies way below inflation are sufficient for continuous production and annihilation of matter and antimatter, which would remove any asymmetry produced by inflation. Thank you for explaining.

"I appreciate your position, Wow. Unfortunately, asking poorly defined questions is a side effect of my not knowing what I’m talking about. Frustrating!"

It's why scientists use the language of mathematics. It's a logical system that has very specific meaning, even if it's practically unreadable.

The simplest problem is that if there IS an actual edge to this universe-bubble, then it would not have been causally connected to most of the universe bubble itself, and there would have been no singularity at the big bang event.

The problem is also similar to the hoary old "if a tree fell in a forest" conundrum (and the Xeno paradoxes), where the definition of universe has to be defined. If it's where there's matter and radiation, there may be an answer, but if it's definable WITHOUT requiring content, then it may be inherently impossible to answer, even hypothetically,

But the way out of this is to ask why you want to know, what is it you will understand if it were answered for you with yes, or with no.

If there WAS I appreciate your position, Wow. Unfortunately, asking poorly defined questions is a side effect of my not knowing what I’m talking about. Frustrating!a galaxy from where you would only see other galaxies spread out at less than a hemisphere of space, so what? "What's beyond the edge" is answered with "Nothing: go to that galaxy and look". And you'd see nothing more than if you were on the edge of our galaxy looking away from any of the naked eye galaxies at night, but that would still be *space* out there, so how would you know that there's not just empty space?