Gizmodo has an interesting article that someone asked my thoughts on. Part of their “Giz asks” series, it asks various physicists what’s at the edge of the universe? The physicists polled include Sean Carroll, Jo Dunkley, Jessie Shelton, Michael Troxel, Abigail Vieregg, and Arthur B. Kosowsky.
They all give similar answers, that space isn’t known to have any edge. It may be infinite, or it may curve back on itself in an extra-dimensional sphere or torus (donut) shape, meaning that if you travel in one direction long enough, you might end up where you started. The best measurements to date imply that space is flat, although we can’t rule out that within the uncertainty of our current measurements that it doesn’t curve in a way that eventually leads one of the other shapes.
Many of the physicists mention the observable universe, and that the actual universe is thought to continue well beyond it, although one pointed out that we can’t rule out major variations just beyond the boundary of our observations.
If you think about the universe as all that we can causally interact with, then the observable universe could be considered our universe, with the “edge” being the edge of what we can observe. Although the edge of observations may change in the future. Currently the furthest thing we can see is the cosmic microwave background radiation. In terms of electromagnetic radiation, it’s hard to imagine we’ll ever see farther than that.
However, if cosmic inflation is correct, and a lot of physicists are convinced it is, then the causal universe might be far larger than the currently observable universe. We can currently detect gravitational waves. If we could ever detect such waves from the period during inflation in the 10-32 seconds after the big bang, when the universe was thought to have expanded to 1030 to 10100 its previous size, then we might be able to infer things about the universe far beyond the limits of electromagnetic observation.
Of course, that range refers to things from the past that could causally affect us. If we only think about what we can causally affect from here on, then due to the ongoing expansion of the universe, the cosmic horizon has a radius of about 14-16 billion light years, which would be the limit of what we could ever conceivably have any causal influence on. As I’ve written about before, this means that most of the universe we can observe is already forever beyond our reach.
But it’s interesting to speculate what might happen if we’re ever able to travel FTL (faster than light). What might we see beyond the observable universe? Would it just be the same kind of stuff we can currently see going on into infinity? Or would we eventually find regions of the universe where things are very different?
I mentioned cosmic inflation. A variation of that idea is eternal inflation, where inflation is the natural state of spacetime, but that due to a random quantum fluctuation, a bubble of low inflation was created, aka our universe. There are different conceptions about what the edge of this bubble might look like. Some see it as a bubble of time as well as space, which we can’t leave because the edges of the bubble are the beginning of our universe in time, the big bang.
Other physicists have speculated that we could travel until we reached regions where the expansion of the universe was faster and faster, until we approached inflationary space. Unless our method of FTL protected us in some manner, we could never enter inflationary space. Aside from it perpetually receding from us, the expansion rate would overcome the nuclear forces holding our atomic nuclei together, not to mention the electromagnetic forces, and we’d be instantly ripped apart.
So travel to another bubble, even with an FTL drive, would probably never be a thing. Even if it was, other bubbles are thought to have different laws of physics. If we ever made it to another bubble, we might find its physics hostile to our form of life.
There have been measurements lately indicating that dark energy, the force driving the current expansion of the universe, may be increasing in strength. If so, then within a few tens of billions of years, the universe as we know it might end in a “big rip.” If we are in a bubble, then that bubble might eventually come to a violent end, perhaps dissolving into inflationary space.
I sometimes wonder if information of any kind, not to mention any form of life, could be preserved in inflationary space. Based on the description, it doesn’t seem so. I don’t know what’s bleaker, the long slow heat death of the universe under constant dark energy, or a big rip in a few tens of billions of year.
So in pondering the edge of the universe, we have edges in observability (currently 13.8 billion years), in inward causality (depends on cosmic inflation), on outward causality (14-16 billion light years), and edges in time including the big bang and one of the possible endings (heat death, big rip, and an increasingly unlikely one we didn’t discuss, the big crunch), as well as possible curving loopbacks and infinite expanses.
Did I miss anything?
SelfAwarePatterns 
Mike.
the original picture is much better. Change, please.
As for inflation, even Sean Carroll is skeptical.
Of course, I exclude it completely.
But we’re talking about classic theory (Guth),
because of course they’ve come up with super inflation (sc, ethan, ……..).
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Stan,
I hate doing major updates to a post after publishing. The next time I used that image, I’ll try to use the full thing. I used that one because it was largely an afterthought for this post and it was already in my WordPress library.
Inflation is definitely not as settled as the big bang overall, but it apparently has a lot of independent lines of logic converging on it, from particle physics, general relativity, and overall cosmology. As I understand it, if it’s wrong, there will need to be another theory at least as radical to solve all the issues it does.
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The idea that the universe curves back on itself to form a multi-dimensional sphere has always appealed to me. I have no sciency reasons to think that’s how it really is. I just don’t like the idea that a finite universe would have an edge, or that there would be some sort of barrier around it, or something like that. So if the universe is finite, the curved space-time multi-dimensional sphere makes the most sense to me. Of course if the universe is truly infinite, then never mind.
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If there is an edge, the question is, “what is beyond this edge?” But having a universe that curves back onto itself in some other dimension is problematic as well. Closed linear shapes appear limited in a 2D space that is larger than these shapes. Closed 2D shapes appear limited in a 3D space larger than these shapes. So, if we assume that our 3D or 4D space is closed, we must also assume the existence of the 4th, 5th, 6th, etc. dimensions larger than our universe. Then, there is a question whether those are infinite, finite, or closed themselves. And, moreover, how many dimensions are there? Where do we stop adding dimensions and why?
As H.L.Mencken said, “We are here and it is now. Further than that, all human knowledge is moonshine.” Actually “here and now” seem to be a good choice of where to stop. 🙂
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Turtles all the way down 😀
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I’ve found that to be a good enough answer. I’m dead serious. Think about it – infinite process occurring ‘as needed’.
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By “as needed”, do you think they only come into being when accessed or interacted with?
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Sorry, for some reason can’t reply to your reply. “By “as needed”, do you think they only come into being when accessed or interacted with?” To put it simply, yes: You will find what you seek – I think of it as a kind of ‘video game’ where the landscape is rendered as you approach it – the ‘frayed end’ of which is forever out of reach (since it resolves as you approach). It’s a bit like that, mixed with a ‘good-enough principle’, where, to reuse agrudzinsky’s point of the ‘here and now’ is good enough and will always be good enough (thus it’s ‘the best’ since it’s ‘the only’). Displacement is an illusion since it’s just the transformation of the boundary (‘un-fraying’ as you approach, ‘fraying’ as you leave). Words are inadequate and describing it all here is inappropriate, but let’s just say I have a pretty complex idea that makes sense when fully understood. More here: https://wp.me/pamtHT-o8
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Thanks Tom. I’ll check it out. Just subscribed. (Re-subscribed?)
BTW, if you ever want to reply when the commenting gets this indented, you can do it by hitting the Reply link in the notice email (if you’re subscribed to the thread) or if you’re logged in with a WordPress account, use the notice bar at the top, or at https://wordpress.com/read/conversations . I wish I knew why WordPress doesn’t let people keep replying in the blog itself, but for some reason that’s the way they have it laid out.
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Thanks for the tip! Using it right now 😉 and btw, yes you kind of ‘re-subscribed’ since my taomath.org domain went down I moved the content to a free wordpress subdomain (sorry if you get ads!). Being broke sucks ^^’.
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No worries at all. The vast majority of WordPress users use the free version. Glad you were able to move the content over! Hope your financial situation gets better soon.
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Similar to agrudzinsky, it seems like every scenario eventually presents problems.
Infinite space leads to truly bizarre scenarios, like the exact configuration of the atoms in our observable universe eventually repeating an infinite number of times, and with an infinite number of variations. In other words, in infinite space, there are an infinite number of you, as well as an infinite number of copies with every possible variation. The many worlds interpretation of quantum mechanics and infinite space end up looking very similar. (Max Tegmark actually somehow reconciled them into the same theory mathematically.)
And of course, as agrudzinsky pointed out, curved space just seems to delay the reckoning.
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The question about boundaries of the universe is not scientific. It’s a philosophical one. In terms of language, we can define concepts, essentially, by drawing a line between what a concept *is* and what it *is not*.The more we define what it *is not*, the more specific is the definition. But when you deal with “omni-” things like universe, omnipotence, omniscience, or infinity, you can’t really say what these concepts do *not* include and you end up with paradoxes. In my view, the universe includes everything, seen and unseen, known and unknown. The only requirement seems to be “…everything that exists”. So, the question “what is beyond our universe” is silly because we defined the universe as something that includes everything. Even if we conclude that our “universe” is one of multiple universes in something called “multiverse” – that does not make sense because now we artificially created something that exists, but is not a part of our “universe” which was supposed to include everything that exists. A “multiverse”, therefore, is simply an extension of our old “universe” concept.
Also, science deals with “falsifiable” things and concepts – those that can be proven wrong or non-existent, at least, in theory. Boundaries separate things from what these things are *not*. E.g. my skin is the boundary of my body. My body is inside my skin. What is outside my skin is *not* my body. Water usually has a surface. Water is under the surface and what is above the surface is not considered water. When you have a “boundary” there must be something on both sides of this boundary. Otherwise, the concept of the “boundary” does not make sense like the classic “one hand clapping”. But if the universe includes *everything*, what can be on the other side of the “boundary of the universe”? Something that is not a part of “everything”? Like, what, … nothing? What is this “nothing”? How do you even define it? And, then, how do you detect it scientically? The “edge of the universe” cannot be detected by scientific methods even in principle.
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You can drive yourself crazy thinking about this too much. Both a finite reality with any kind of edge, and an infinite reality, are bizarre. Reality is absurd.
On falsifiability, I think we have to be careful with that criteria. What is non falsifiable today may someday become falsifiable. 18th century astronomers thought discussing what the stars were made of was hopeless speculation, until spectrum absorption lines were discovered. And many physicists saw Einstein’s quantum entanglement paradox as pointless metaphysical navel gazing, until John Bell came up with a way to test it.
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We need to distinguish between “falsifiable in practice” and “falsifiable in principle”. Once a phenomenon has a clear definition, it is also clear what is needed to falsify it. It was fairly clear what a star is and what it means to find out its composition. So, the composition of stars was always falsifiable in principle. It was just not falsifiable in practice for a while, until the science and technology for it became available.
On the other hand, there is no and cannot be in principle a definition of “nothing” (not universe) as opposed to “something” (universe). Once we define “nothing”, e.g. as “vacuum”, it instantly becomes “something”. Vacuum defined as space void of matter implies the existence of matter and existence of space, so “vacuum” is not “nothing”. Same with “silence” or “darkness”. You can’t define a “lack of everything”. It means a lack of definition as well. It’s a self-refuting concept. “Nothing” (non-existence) does not exist by definition (it even lacks a definition) and cannot be detected in principle.
So, all we can talk about is the boundaries of the *known* universe which is currently the background radiation as those physicists point out. Once they are reached, we find something beyond those boundaries and that expands our knowledge. I think, expanding our knowledge of the universe is as good as expanding the boundaries of the universe itself. In this sense, I think, there are no boundaries to the universe.
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As I said, I don’t have firm scientific reasons for my opinion here. It is just an opinion. But a curved-space universe appeals to me, at least on an aesthetic level.
A truly infinite universe would start repeating itself eventually, and so we’d have infinite copies of each of us, which starts to seem a bit absurd (in my opinion). But a finite universe, if it’s also flat, would have an edge, which seems even more absurd to me. Of course the universe is already known to do lots of things that might seem absurd, so what do I know?
But a curved-space universe elegantly avoids the absurdities of both an infinite universe and a flat finite universe. I think that’s pretty neat. I guess it still passes the buck to other, higher level dimensions, which is a bit less neat, a bit less elegant than I’d like. But as far as the three spatial and one temporal dimension that we humans experience, I feel like a curved-space universe is the least weird option.
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I agree it’s a neat solution for the universe we can perceive. Although if we ever reach the point where we can traverse the whole thing, we might become frustrated with living in a closed system.
An interesting question is how seriously we should take the analogy of a warped plane for spacetime. It’s very useful for thinking about general relativity, but is this warping actually happening in higher dimensions? Or is “warping” just a metaphor we use to describe a reality that is more “warp like”? If the latter, then a lot of concepts like wormholes or a universal sphere or torus seem like they go out the window.
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That’s an interesting notion. I never really thought about it before, but we use so many metaphors to explain general relativity… we may not even have a word for whatever spacetime is actually doing… “warp” is probably the best approximation our language provides. But this could be a case where the words we use give us the wrong impression… I’ll have to ponder this more. Very interesting thing to think about.
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Or, it may repeat itself infinitely in time, just like one day follows the other day and one year follows the other year. Even as you travel around the globe, when you come back to the place from which you started, you don’t find things exactly the same as they were. They changed. That’s the wonderful dualism of “staying the same” and yet changing all the time.
Another dualism is between certainty and uncertainty. Things distant in time or space appear uncertain to us. But as they approach, they become increasingly certain until they reach “here and now”. But then they go away in distance and time again and become uncertain.
Read Roger Penrose “Cycles of Time” on this topic. He believes in a cyclic universe that goes from birth to death and rebirth again. It can go through cycles of big bangs, expansions, collapses, and big bangs again. Or even, the thermal death may somehow become equivalent to a great crunch just like one end of the stick is opposite, but equivalent to the other end of the same stick. Most of the processes in this world are periodic. I don’t see why universe and space-time shouldn’t be.
I don’t believe in a “flat” space. If we don’t see the curve, it’s probably because we don’t look on a large enough scale.
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Here. Take a look.
https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/
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I’m with you about flat space. We probably can’t see the universe on a large enough scale, or maybe our measuring techniques aren’t sensitive enough yet. Thanks for the recommendation. I’ll look into Cycles of Time.
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Right now I’m real interested in whether LIGO will be able to use its data to resolve the growing discrepency between two methods of measuring the expansion of the universe. As the error bars get smaller, the difference becomes more apparent. It’s gotten to be a real vexation for cosmologists.
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It’s these inconsistencies that are always interesting, because resolving them sometimes leads to breakthroughs. There was a recent study that attempted to help resolve the issue by measuring the distance to quasars. What they found implied that dark energy may be increasing in strength. If so, it would explain the discrepancy. But it also might mean the universe is heading for a big rip.
http://chandra.si.edu/blog/node/711
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It’s a flat earth, ergo a flat universe 😉
What boggles me is how we call something space, but there’s still the capacity to have material in that space. What is outside of a place where you can have material in it? Is the universe surrounded by non space? Or spookily, is the universe threaded through with non space time and time again? We just can’t perceive it for it’s absence not only of matter but of dimension as well?
It’s like common perception is that empty space, as it the physically fillable zone, is what the big bang exploded into. But to me it seems more like the big bangs explosion actually made there be space at all. Outside of physically fillable space, I dunno.
Yes, what if other bubbles have their own physics rules? What if every bubble is basically taking some kind of raw fabric and its physics are just that bubbles ‘opinion’ on how the fabric should be weaved? Much like a god?
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Sometimes I think our conception of a curved space is heavily influenced by our discovery that the earth wasn’t flat. We naturally tend to think that space might turn out to be the same way. Of course, there’s no guarantee that space will be like higher dimensional land.
Personally, I find the prospect of infinite space or the idea of any kind of edge to reality, to both be absurd. A curved spacetime implies higher dimensions, which just delays the reckoning since those higher dimensions must be infinite or have their own edge. Any way you go, reality is absurd.
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Sometimes I think our conception of a curved space is heavily influenced by our discovery that the earth wasn’t flat.
Interesting suggestion of bias there – not implausible. 🙂
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If the universe has “edge”, so it’s not endless. I don’t believe that, because this “edge” is somehow a sequel to the universe, at least in terms of space.
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I personally struggle both with the idea of an edge and with an infinite universe. Yet one or the other must be true in some form. Even a curved space that loops back on itself has to do so in some higher dimensional environment, which we then have to ask the same questions about.
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Exactly!
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