> Is it possible that the 'edge' of universe reflects light? Have scientists been able to conclusively rule that out somehow?
It's not an edge the way you think. It's the edge of what we can see based on the known age of the universe and how much light has reached us. I.e no real boundaries as best as we can tell.
The best way you can look at this is that early galaxies are being found earlier than expected based on our model of how the universe formed. The further you look, functionally the further back in time you look (not just further away)
This is an oversimplification and an astrophysics expert can give you something better.
Without getting into lots of detail, this is basically correct. It's why we talk about the observable universe. There's lots of stuff we can't see because the light from it won't reach us.
This is one of the most interesting aspects to the universe. It's not, "because the light from it hasn't reached us yet but is on the way and will get here eventually."
Rather it's that the rate of expansion of the universe is accelerating, so that we're moving away from parts of it faster than its light can cover the distance to us. It will never reach us.
> Rather it's that the rate of expansion of the universe is accelerating, so that we're moving away from parts of it faster than its light can cover the distance to us. It will never reach us.
It's actually even worse than that. Because of the accelerating expansion of the universe, over time the part of the universe that we can observe will get smaller, allowing us to see less and less of it. Eventually, all that we'll be able to see is our own local group of galaxies, where gravitational attraction will win out over the universe's expansion. However, this won't really be a problem for a few billion years.
I know it is considered increasingly unlikely, but: What a parallel to flat Earth ideas people had (...) hundreds of years ago, until they realized we are on a sphere. Maybe in a few hundred years people will laugh about our limited ideas of the universe as well.
This is wrong by an order of magnitude. It was already known thousands of years ago that the Earth is more-or-less spherical. The idea that people in recent history believed in a flat Earth is a myth.[0]
>The idea that people in recent history believed in a flat Earth is a myth.
The way you've worded this, you're incorrect. There are people today who believe in a flat Earth, crazily enough.
Of course, the idea that most of society (namely educated people) believed in a flat Earth is a myth. From your link:
"The myth of the flat Earth, or the flat earth error, is a modern historical misconception that European scholars and educated people during the Middle Ages believed the Earth to be flat."
What the typical serf working in the fields thought about the shape of the Earth is probably unknown.
If we had to guess (of course not good research to be guessing): The typical person working on a field probably believed what the church told them the world looks like. If they ever heard about being on a sphere, they would probably laugh and point to the horizon, saying: "How can Earth be a sphere? Don't you have eyes to see? Look into the distance, all is flat!" and any kind of argumentation, that it is simply so huge, that one cannot see it, would seem to them like a made up story, a lie to further some hidden agenda the explaining person has.
That does not really detract from my comment though. Thousands of years are also hundreds of years. Just more hundreds. So if we are getting technical about that, I don't think it is wrong what I wrote. Still, thanks for putting the time frame a bit clearer.
> Rather it's that the rate of expansion of the universe is accelerating, so that we're moving away from parts of it faster than its light can cover the distance to us.
From my understanding, it's not that we're moving away from parts of the universe, but that the distance between us is growing so fast that light sent from one part will find that after traveling toward us for some amount of time, the remaining distance to travel is actually more than than when it started.
One way for the distance between two objects to increase is indeed for those objects to literally be moving in opposite directions through space. But the expansion of the universe itself causes the distance between two otherwise-stationary points to nonetheless increase. Put differently, it's the cosmic yardstick that's shrinking, not the entities that must necessarily be moving.
(This is also why two points can be "moving apart" faster than light speed, the cosmic speed limit.)
So such an expansion phenomenon should affect everything uniformly right? Are we (people /animals) then getting "bigger"? Over time innthe extreme will it cause problems in signal transmission in our own nervous systems (since apparently we have a problem seeing light beyond the observable limit)
The strong nuclear force, electromagnetic force, and even gravity are strong enough to overwhelm the expansion within individual galaxies and even local groups of galaxies.
In other words the expansion of spacetime very very very slightly tries to shift the atoms of your body apart but we can't even detect it because ordinary forces like chemical (electromagnetic) bonds are exponentially stronger - enough to pull things back where they should be. Actually in the current epoch I'm not sure if the expansion is strong enough to shift an electron by 0.01% of its own width let alone move an atom.
Space is really really big so that tiny amount of expansion adds up over long distances.
I don't think that's true. Expansion is not very weak around matter, it just doesn't exist. It sounds a little counter intuitive to think that expansion "shutdowns" as we get closer to galaxy. But that's because expansion and gravity are a little more complicated that just a force.
We over simplify what expansion really is, which leads to this type of thinking.
Expansion and gravity are results of the same equations.
Einstein equations are difficult to use, so we usually split the results in two models, FLRW (empty space) and Schwarzschild metrics (around matter). Computing the equations leads, respectively, to expansion and gravity. And it's not like there's one and the other, with gravity fighting expansion. It's "one or the other".
Around matter (in Schwarzschild metric), solving Einstein questions, we see zero expansion drifting. If there is matter, there is gravity, and no expansion.
Quoting Wikipedia [1]:
> Once objects are formed and bound by gravity, they "drop out" of the expansion and do not subsequently expand under the influence of the cosmological metric, there being no force compelling them to do so
> Einstein equations are difficult to use, so we usually split the results in two models, FLRW (empty space) and Schwarzschild metrics (around matter). Computing the equations leads, respectively, to expansion and gravity. And it's not like there's one and the other, with gravity fighting expansion. It's "one or the other".
Arguably splitting a complex model of reality in two for convenience and saying that it’s “one or the other” is also an over simplification.
By the way, the same wikipedia entry also says things like “gravity binds matter together strongly enough that metric expansion cannot be observed on a smaller scale at this time.”
Gravitational attraction still dominates on even fairly large scales, and chemical bonds absolutely dominate over anything expansion could possibly do -- it would contribute an incredibly light force opposing any bonds. So no, we (and our galaxy) are in no danger of being inflated from the inside by space itself.
> So such an expansion phenomenon should affect everything uniformly right?
Apparently not. As far as I can tell (IANAC) cosmic expansion affects the empty space between galaxies, but not concentrations of mass. Galaxies (and everything in them) are immune to cosmic expansion.
I understood that cosmic expansion is a consequence of General Relativity; DE is supposed to explain accelerating expansion. Is that right? But wouldn't expansion result in there being more empty space and less nearby stuff; and therefore in accelerating expansion?
> Want to understand what we know about Dark Energy: the hypothetical form of energy that exerts a negative, repulsive pressure on the universe that effects the energy on the largest scales? Then enjoy this Dark Energy playlist!
And the question is "is the rate of acceleration accelerating?" If so, then make sure you watch "Could the Universe End by Tearing Apart Every Atom?"
If that question is of interest to you, PBS Space Time - Could the Universe End by Tearing Apart Every Atom? https://youtu.be/gEyXTQ9do-c gets into the "what if" of dark energy and its influence on matter.
You'll note that it isn't until the very end of the instant before the Big Rip that that make it so that the expansion of the universe that it overcomes the strength of chemical bonds.
With the current rate of expansion, no. We're still bound together more tightly than the rate. This extends all the way out to a fair distance (galactic distance).
If the rate does get to the point where it is noticeable the "galaxies can't hold together" you get into the Big Rip end of the universe situation.
You don’t get larger - just like you don’t dissolve in water and wind doesn’t spread parts of you all over the land. There are interactions of matter keeping you together.
> but that the distance between us is growing so fast that light sent from one part will find that after traveling toward us for some amount of time, the remaining distance to travel is actually more than than when it started.
Good clarification, that's what I meant, I guess I didn't say it accurately. I don't actually think of us as moving, but more like the scale of the entire universe is increasing while the ability to traverse it - light speed - remains a constant.
You have it right. My cosmology teacher at UVA a couple decades ago used the analogy of points marked on the surface of a balloon that's being inflated. (maybe more intuitive than other shortcuts to understanding?)
The universe is 13.7b yo, but due to acceleration of expansion, if you could "teleport", it's actually currently 93b ly across. So we're already in a bubble within a greater universe that we'll never be able to escape even if you could instantly reach lightspeed right now, and due to the expansion continuing to increase, the fractional size of this bubble relative to the rest is shrinking. Faraway galaxies that are "currently" on the edge of the bubble but expanding away are becoming forever unreachable as you read this.
> The universe is 13.7b yo, but due to acceleration of expansion, if you could "teleport", it's actually currently 93b ly across. So we're already in a bubble within a greater universe that we'll never be able to escape
This number, the age of the universe, has changed a few times since I learned to read 45 some years ago. What are the chances that this isn't really "the" universe, but what we know as the observable universe is really a mind-bogglingly massive black hole that was sucked out of the actual universe, and the actual age of "the" universe is incalculably old, trillions of quadrillions of years old, and it's only our baby universe is what is roughly 13.7Byo? Maybe the Great Attractor hides the mother of all singularities. I'm sure there could be a way to explain the CBR and what seems like the Big Bang and Inflation. Maybe this baby universe only appears to be expanding, when it's just a growing black hole.
It's worth pointing out that there is a minority of physicists who don't accept the Big Bang as proven beyond doubt. An alternative theory would be a 'steady-state' universe which, as you suggest, would be much older than the ~14 BYO age. If the medium of space itself dispersed light for instance, red shifts might be observed that explain the astronomical data.
Just for any future historians: all us normal people are aware that the idea of the Big Bang seems a little fantastical, but relativity or whatever is, like, way complicated. Most of us just have to trust the physicists. Of course now that you have the Theory of Everything, notation and thought-experiments developed to make it obvious, and relativity is just a special case, we look pretty dumb. But if you look at the operators that your undergrads pull out to solve Theory of Everything equations and try to somehow derive them with ancient 21’st century math, they are actually really complicated!
Actually, having a bit of sympathy now for the folks who believed in the Luminiferous aether.
Eric Lerner is one of them and he's advocating for the idea that the BB never happened and that we'd see plenty of old galaxies with JWST. He's since updated his thoughts and there was a bunch of controversy:
The number hasn't really changed, it's just been measured with increasing precision. It is rather unlikely to be wrong given measurements via various methods are all in agreement. Summaries about these various experiments are available: https://imagine.gsfc.nasa.gov/science/featured_science/tenye...
It's black holes all the way down. Fun to note that, from the reference of someone outside a black hole, the singularity contained within hasn't happened yet, and never will.
So mindboggling that frankly I refuse to accept it.
The likelyhood that we'll figure this out in my lifetime is zero, but I simply can't comprehend a universe that accelerates without cause (dark energy) infinitely.
Just as we don't understand the root causes of dark energy, I believe it's just as logical to believe that something will eventually slow it down.
I have to believe that because only a cyclical theory of the universe makes sense to me. It's my faith, I suppose.
The more depressing part to me isn't that I'll never know, but rather it's entirely possible HUMANITY can never know, any more than an ant can know about General Relativity.
I think it’s actually quite a simple model to think of the universe as a network of discrete nodes, rather than a spatial thing that’s expanding and within which various things are accelerating. It makes the Big Bang simpler - we start from a small number of nodes and they grow in number, so we don’t need to think about what the universe is expanding ‘into’. And it explains expansion - nodes can divide like mitosis, so we don’t need to explain some force pushing things apart.
Something that’s even more mind boggling than that, at least to me is that the universe is still within the first few seconds (or minutes) of its life in terms of relative time we can comprehend. As stars collapse and black holes consume everything, the universe will spend 99% of its life in complete desolate darkness. An endless sea of black holes for billions and billions of years. The fact that we exist, in the split second that life can exist is unimaginable.
I'm still not convinced that the expansion of the universe is accelerating based on current evidence.
That argument largely comes from supernovae appearing dimmer than we think they should, and more distant things appear more red and it looks that way in any direction we look. But the thing is, we sometimes calculate how distant things are based on redness, so it's kind of circular reasoning.
Sure we have other things which help gauge distance, like brightness and periods of Cepheids, but if you look into history on Cepheids, the association that brightness is directly related to periods was built upon an assumption that the Cepheids in a galaxy were roughly all the same distance away. That may seem probable, but it isn't a given, as galaxies can be at various angles to our perspective, as well as being different size in various dimensions. It also assumes that it is impossible for fake Cepheids to exist, which might even confer a reverse association. How could we know if we're looking at false Cepheids vs real Cepheids, and that there's not multiple types of Cepheids with different causes for pulsations at various brightnesses?
Next you have to consider movement is relative. It's entirely possible a brighter galaxy is moving 2x faster away from us, than a dimmer galaxy that is actually closer to us. Yet this is hardly considered from distance calculations.
Lastly, people also say the Big Bang is not an explosion of matter moving outward to fill an empty universe, but rather an expansion of space between things. IMO, this is mostly just a model, a way of viewing things. The thing is, you can still look at things from normal intuition (of say an explosion), and it still conforms that definition (ie it's objects moving in space over time, vs it's space filling in between objects over time). And so, if looking further into the galaxy, means looking further in time, the dynamics of an explosion suggests that those galaxies will be moving faster away from us. As the outmost debris of an explosion, is the fastest moving debris of an explosion, and speed between two pieces of debris, is highly associated to their relative positions and tends to increase as distance between them increases, even regardless of where they are in an explosion. So even if further galaxies are indeed moving faster away from us, and it is faster the further we look, and looks that way every which way, I don't see why this would necessarily mean the expansion of the universe is accelerating. As it appears to me, it can be predicted by conventional (non-accelerating) explosion dynamics.
Considering the two methods for measuring the rate of the universe’s expansion differ by about 10%, the question seems unsettled at best. You’re not wrong to question it.
We don't (can't?) even know if there weren't multiple Big Bangs, right?
I.e we're just in a specific "universe" we can observe, but maybe several of these are just side by side, not necessarily parallel as in parallel realities.
assuming light from a big bang travels in all directions, then another big bang's light could be heading in our direction and potentially observable — the reason we can't observe the entirety of our universe is that spacetime is expanding in a way where light at the beginning isn't traveling fast enough to outpace expansion
Unless we figure out a way to take measurements of areas outside of space-time to compare against (which seems quite impossible), it’s probably a question humanity will never answer.
It might be a question with no meaning: The universe interacts with nothing else. It spawned out of nothing, and its expansion is only meaningful if you’re inside the universe to see it happening.
It could be a question with a lot of meaning: perhaps the universe exists on top of some higher dimensional substrate that is conducive to big-bang style expanding universes. Maybe the reason the universe expands is only possible to answer by having access to the information of what it is expanding into.
It could be question impossible to wrap our heads around: Maybe the area outside the universe runs on metaphor, and our universe expands in a sense that would make more sense to a writer than a physicist.
Basically, endless sci-fi can be written about that question. But given we are (probably) restricted to staying within our universe’s laws of physics, it’s quite likely we’ll never really know.
Correct, as I understand it. No matter can travel at the speed of light, and no light can travel faster than the speed of light. But the rule doesn't extend to the rate of expansion of the "field" on which those things exist.
I vote we bring back “aether” as a valid term. The aether is stretching everywhere, and in so doing it spreads distant things away faster than light can overcome.
Yeah, ideas like this are usually called "tired light" models, they have been extensively explored for the last hundred years or so. A lot of these models have been shown to be false by experiments, but I guess if you try you can probably cook up models which haven't been falsified by anything yet.
Think of the universe as some sort of information system. The speed of light caps the rate at which information propagates, which is a desirable or at least necessary property at scale. But there’s some discrete substrate underlying that propagation of information. That substrate can multiply, like inserting nodes in a graph or empty spaces in a linked list. That makes it longer to traverse from one end to the other. The rate of propagation stays the same but the map keeps changing.
If we had some absolute zero reference outside the universe - let's call it a great alien petri dish - we probably could find something moving faster than the speed of light, in reference to that absolute, out-of-universe observation point? But measuring that might be hard.
And on the other hand, we might be able to find two objects which are static with reference to the universe, but actually increasing the distance from each other at a speed beyond c or rather 2c, which should be impossible, because the universe between them expands?
This is very weird to think about, but accepting your reference framework - the universe - changes makes it easier.
> is it that space expands faster than light moves?
Yes. Picture an ant walking on the surface of a balloon. You could conceivably blow up the balloon faster than the ant could walk across it. If you were blowing up an infinitely stretchy balloon with an ant at the far end, you could conceivably blow it up fast enough that the ant could never reach you.
We have a concept of the Particle Horizon, the max distance light could have traveled in the universe, and that is our boundary, our effective edge of the universe.
There's the concept of the light cone, which is the total volume of observable light which can ever reach an observer, or inversely, the total volume ever traveled by a given point source. The expansion of the universe means that there is a certain boundary, a horizon where the universe expands too much for light to ever travel the required distance.
I guess it's reasonable to assume that there are parts of the universe that are far older and much further away (ie, their light hasn't reached us yet). Which would mean we can never really guess the age of the universe. We can only guess the age of our local area?
Which kind of sounds a bit like the whole "everything revoles around earth" transitioning to "everything revolves around the sun". The universe is what light has reached us transitioning to the area of light that has reached us is just a small spec of the actual universe?
> I guess it's reasonable to assume that there are parts of the universe that are far older and much further away (ie, their light hasn't reached us yet). Which would mean we can never really guess the age of the universe. We can only guess the age of our local area?
But older parts of the universe would emit light that would have more time to travel. So unless space is not continuous, we can confidently say that no older light exists. The main counterfactual is that there is an older universe that is discontinuous with the observable universe (but in what sense is that older universe part of "ours" then?).
But they can still be far away that the light hasn't arrived at us yet (or maybe never will?). It's entirely reasonable that space may not be continuous. There may be groups of galaxies far enough away from each other that they burn out before receiving the other's light.
How can we confidently assume this isn't the case?
No, there was a time that all space was hot, and we know from microwave background radiation that it is homogenous. So it’s not just “galaxies are far from each other”, it would take “space time is discontinuous somewhere and this discontinuity is just on the edge of our observable universe”. There is also no older light that can ever reach us, as the amount of universe that is observable is actually decreasing (beyond that, CMB will always be the oldest light as it predates any astronomical object formation). Anyways, CMB really constraints the age/homogeneity of the early universe.
I wouldn't be surprised if there's a hawking radiation equivalent for that edge, but the wavelength is on the order of the size of the universe, so basically impossible to measure
An analogy might be the horizon. There is no fixed horizon, it is just the boundary of how far your can view, given both the curvature of the earth and the quality of your eyes. It is relative to where you are on the earth, and by your altitude. So while it is calculable, it isn't a fixed boundary like a river, or a wall.
No, we have no evidence that the universe has any kind of "edge" that is topologically different from its interior.
There is a boundary to what we can see. As the early universe cooled, it changed from an opaque plasma to transparent gas. So as we look farther away, and also backward in time, we see the last point at which it was opaque; this is the cosmic microwave background. But this isn't a "real" boundary that something could hit. And it long predates the formation of galaxies, so it couldn't have reflected images of galaxies.
also (citation needed, this is from memory from university cosmology classes over 20y ago, maybe misremembering or info out of date) the shape of the universe may be less like a sphere and more like a toroid, or a multidimensional moebius strip.
so (hand-wavy, impossible IRL but maybe illustrative / fun to think about) if you could freeze time and look far enough in one direction, you'd see the back of your own head.
There are some reasons why cosmologists don't like the idea of a universe that loops back on itself, one is that such a universe can't be both isotropic (the same in every direction) and homogeneous (the same at every point) [edit: while also being flat, as observations seem to indicate it is].
To get a rough idea of why, imagine taking a 1km by 1km square and identifying the opposite sides, you now have a homogeneous space (every point is equivalent to every other point), but it isn't isotropic because some directions are special. If you put a rock on the ground and walk due east, you'll have to walk 1km to reach the rock again (assuming you start in what used to be the center of the square it takes 500m to where the edge used to be, and another 500m from the edge back to your starting point). On the other hand if you walk south east you have to walk sqrt(2) km to get back to your rock (sqrt(2)/2 km to get to what used to be the south east corner, and another sqrt(2)/2 km to get back to where you started).
So although the torus space is homogeneous there are traces of the fact that it used to be a square, embedded in the fact that some directions are special (the 4 cardinal directions have the shortest distance to get back to where you started and the 4 intercardinal directions have the longest). Cosmologists think this lack of isotropy is essentially ugly, and don't like the idea of living in a universe where some directions are special.
Aside from just not liking the idea (which isn't very scientific) its also relevant that the universe looks pretty isotropic when do observations, we emphatically don't see traces of the sort of anisotropy you'd see in a toroidal universe anywhere.
Theres nothing massively wrong with it, it's a perfectly plausible model.
The one thing that isn't particularly nice is that spheres have intrinsic curvature, essentially if you draw two parallel lines on a sphere they will eventually touch. We can go and look at astronomical data and see if the universe has any intrinsic curvature that we can see.
People did this and it turns out that from all the astronomical data we have the universe looks incredibly, spectacularly flat. No curvature at all that we can detect. This doesn't mean it isn't a sphere, but it means that if it is a sphere it's a really big one. Much much bigger than the observable universe.
Ahhh, yes parts of that sound familiar. So, the reason we can (at least pretty much) ~~rule out~~ a 3-sphere [EDIT: I didn’t read carefully, and missed the “it could be a really really big 3-sphere so that the curvature is close enough to zero” part], is because you can’t have a flat sphere,
But you can have a flat torus (or some other shapes that “wrap around”), but we have different reasons to disbelieve those shapes (the “looks the same in any direction” and “looks the same in every position” expectations).
The far limit of the visible universe is the cosmic microwave background, which is the heavily red-shifted view of when all of space was filled with an opaque plasma of similar temperature to the surface of a star[0].
The process that caused the light we see as the CMB happened everywhere in the universe roughly all at once. At the time, from any given point, you'd just have seen blinding light but as things cooled, you'd see a sphere expanding around you where this light was dimming, as less and less, and finally none was being emitted around you, but due to the finite speed of light, light from outside that sphere would still continue arriving in your eyeballs. That's the situation we're in today; that sphere's just really big now.
During recombination epoch ~400,000 years after the Big Bang, this light would have been visible. Due to expansion, over time that light has stretched to longer and longer wavelengths, and we currently see it as microwaves.
Note: It's been ~30 years since I was actually studying physics & astronomy; others may be able to offer better explanations or correct me.
The CMB happened everywhere in the universe at the same time[0], so what we're seeing is the light which took 13-point-whatever billion years to get here[1] from some part of the universe in that direction at that younger age.
> My understanding is that the early galaxies still produced light after recombination.
I'm not sure what you're imagining about recombination, because I've not heard any suggestion of any galaxies existing before it, so they only produced light after recombination.
[0] for some definition of the concept, even though relativity is formulated with the assumption that there isn't any good concept of simultaneousness.
[1] from our point of view. From light's point of view, time isn't defined.
The edge of visible universe is Big Bang. Or more concretely, the time couple hundred thousand years after Big Bang when the universe became translucent to light.
Things cannot "reflect off of the edge of visible universe" because that would require that the light travel back in time which is nonsense.
As of this moment we cannot exclude possibility of discontinuities in the universe which would be cause for example by inflation. But we also have not observed any.
if time is a dimension, and we can only see the past and the present, could it not be that, if one's gaze were shifted, one could see the future? effectively looking at the "other side" of the dimension of time? looking straight at one "side" of time, you can only see the past and present. but if you had a galactic set of mirrors set up around "time", couldn't you "see" the other side where the future lies? effectively you'd be looking at a reflection of the future.
or, time being malleable (relative to things like mass and movement), wouldn't it be possible to refract or bend time the way light is, such that you could see things (that already happened in the past) sooner, even if they are really far away? maybe like how bending a race track can allow a vehicle to exert more force or go faster, but with light?
Time doesn't exist as a thing in the way you're thinking about it. It's why in physics we talk about spacetime. (The following is not entirely accurate and an oversimplification.)
Space and time are one thing, which have to be thought about together. Your current thinking imagines that you're in a box, with x, y and z coordinates, and that time is a thing passing inside it. Instead, it'd be more accurate to talk about that you're in a frame of reference with x, y, z and a, and all are tied together. There's no sense in which you can talk about space and not also be talking about time, and vice versa. For a similar idea, a 3d volume is not a plane plus a z axis, where you can talk about moving through just the x and y axis without the z axis mattering. You can talk about a view of that, but it doesn't mean the z axis isn't relevant. Ask two planes not colliding whilst viewed from above how important a z axis is. (Maths jokes are the worst.)
The actual physics involved for discussing this gets absurdly complex very quickly, but this is about as simple as I can think how to explain it whilst still being in the bounds of accurate.
What is the edge of the universe? How would you define that? In most models, the universe doesn't have anything that could be called an edge. Unless you are referring to the boundary of the local universe, which is defined as the sphere around us at which objects are too far away for their light to ever reach us. It's not a physical structure that could reflect light, it's more like the opposite of that.
Talking about the edge of the universe implies space is a thing. Physics works just fine if you imagine the x, y and z of particles describes an arbitrary set of coupled attributes on the particle itself. It is more likely that space is an emergent property of the entanglement between information, and in that case the idea of a boundary is nonsensical. Under that model, "spacetime" would behave like a foam - if one bit of it pulled far enough away from the rest it'd shoot off as a bubble.
Space and time are intertwined interstingly that way. If time somehow loops back or reverses (circular time) the it is not unreasonable to suggest space also loops back. Is it possible that at a certain boundary of space and point in time you loop back and start over at 0:0:0/0:0:0 space/time coordinates. At least in this current realm of reality, is it like one big space-time movie on a loop?
I have a memory from when I was a kid and learning about the universe. In my head it definitely had an edge, and it resembled a really really big hockey rink with boards. But it was also only 2d at that time, which I think I remember knowing that wasn't right but not being able to conceptualize a 3d universe. So yes, in the universe of my 8yo mind the "edge" might reflect light
Does the universe even have an edge? What is our current understanding of what the boundaries of the universe might be like?