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Is it possible that the 'edge' of universe reflects light? Have scientists been able to conclusively rule that out somehow? Does the universe even have an edg

by cwkoss 4y ago

Is it possible that the 'edge' of universe reflects light? Have scientists been able to conclusively rule that out somehow? Does the universe even have an edge? What is our current understanding of what the boundaries of the universe might be like?

eganist 4y ago
> 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.
- pwatsonwailes 4y ago
  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.
  - oblio 4y ago
    I was thinking about something weird right now. 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.
    - micromacrofoot 4y ago
      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
      - chrisweekly 4y ago
        wait, no, a big bang doesn't happen IN spacetime, it IS spacetime per se. that's part of what makes it so confusing.
        godshatter 4y ago
        What is spacetime expanding into?
        micromacrofoot 4y ago
        as far as we know spacetime is everything that exists
        NineStarPoint 4y ago
        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.
    - deleted 4y ago
      [deleted]
    - GrantS 4y ago
      Eternal inflation: https://en.m.wikipedia.org/wiki/Eternal_inflation https://en.m.wikipedia.org/wiki/Eternal_inflation A more reader-friendly explanation: https://medium.com/amazing-science/if-inflation-is-true-then-we-live-in-a-multiverse-d2555178d062 https://medium.com/amazing-science/if-inflation-is-true-then...
  - SkyMarshal 4y ago
    > 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. That's mindboggling.
    - boppo1 4y ago
      What do I have to study to understand all this in mathematical terms? Is an undergrad text good enough?
      - pc86 4y ago
        Depends what you mean by "in mathematical terms." There are YouTube videos that cover the concept with real numbers.
      - hnuser123456 4y ago
        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. https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Comoving_coordinates https://en.wikipedia.org/wiki/Comoving_and_proper_distances#... https://public.nrao.edu/ask/inconsistency-between-the-age-and-diameter-of-the-universe-2/ https://public.nrao.edu/ask/inconsistency-between-the-age-an... However, the local group/cluster of galaxies is close enough to remain gravitationally bound, and we're still gonna merge with Andromeda.
        Maursault 4y ago
        > 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.
  - make3 4y ago
    how can that work if nothing is faster than light in the referential of the objects being compared? is it that space expands faster than light moves?
    - seanw444 4y ago
      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.
      - seszett 4y ago
        When I read this, it seems to me like this is more or less the same as saying that the speed of light is decreasing, but I am probably wrong?
        jhoechtl 4y ago
        Is this something anybody ever thought out to look at things this way? Sounds like an intriguing Gedankenexperiment
        eigenket 4y ago
        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.
        dpkirchner 4y ago
        Like if you calculated the speed not in meters per second but fraction-of-the-universe per second?
        cout 4y ago
        What if space isn't expanding and we are just shrinking?
- deleted 4y ago
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- 8note 4y ago
  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
- afro88 4y ago
  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?
  - ivalm 4y ago
    > 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?).
    - afro88 4y ago
      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?
      - ivalm 4y ago
        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.
- soulofmischief 4y ago
  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. https://en.m.wikipedia.org/wiki/Particle_horizon https://en.m.wikipedia.org/wiki/Particle_horizon https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone
- jacobr1 4y ago
  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.
- colordrops 4y ago
  How does this work with the big bang happening at a specific point in the past?
soiler 4y ago
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.
teraflop 4y ago
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.
- chrisweekly 4y ago
  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.
  - eigenket 4y ago
    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.
ben_w 4y ago
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 galaxies we're seeing are in front of that. [0] 3000 K, https://en.wikipedia.org/wiki/Recombination_(cosmology) https://en.wikipedia.org/wiki/Recombination_(cosmology)
- lostmsu 4y ago
  This is hard to internalize. Why do we continue seeing CMB? My understanding is that the early galaxies still produced light after recombination.
  - cbracken 4y ago
    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.
  - ben_w 4y ago
    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.
twawaaay 4y ago
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.
- 0xbadcafebee 4y ago
  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?
  - twawaaay 4y ago
    Or wait until shrooms wear off.
  - pwatsonwailes 4y ago
    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.
xcskier56 4y ago
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
jhoechtl 4y ago
My theory: It's not an edge but we look onto ourselves back in time. As space and time evolves, it's hard to identify as such. One day we will recognize that we essentially were looking onto space to see ourselves while starring. The last thing is meant metaphorical.
deleted 4y ago
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CuriouslyC 4y ago
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.
_8j50 4y ago
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?

by cwkoss 4y ago

eganist 4y ago

> 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.

pwatsonwailes 4y ago
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.
- oblio 4y ago
  I was thinking about something weird right now. 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.
  - micromacrofoot 4y ago
    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
    - chrisweekly 4y ago
      wait, no, a big bang doesn't happen IN spacetime, it IS spacetime per se. that's part of what makes it so confusing.
      - godshatter 4y ago
        What is spacetime expanding into?
        micromacrofoot 4y ago
        as far as we know spacetime is everything that exists
        NineStarPoint 4y ago
        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.
  - deleted 4y ago
    [deleted]
  - GrantS 4y ago
    Eternal inflation: https://en.m.wikipedia.org/wiki/Eternal_inflation https://en.m.wikipedia.org/wiki/Eternal_inflation A more reader-friendly explanation: https://medium.com/amazing-science/if-inflation-is-true-then-we-live-in-a-multiverse-d2555178d062 https://medium.com/amazing-science/if-inflation-is-true-then...
- SkyMarshal 4y ago
  > 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. That's mindboggling.
  - boppo1 4y ago
    What do I have to study to understand all this in mathematical terms? Is an undergrad text good enough?
    - pc86 4y ago
      Depends what you mean by "in mathematical terms." There are YouTube videos that cover the concept with real numbers.
    - hnuser123456 4y ago
      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. https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Comoving_coordinates https://en.wikipedia.org/wiki/Comoving_and_proper_distances#... https://public.nrao.edu/ask/inconsistency-between-the-age-and-diameter-of-the-universe-2/ https://public.nrao.edu/ask/inconsistency-between-the-age-an... However, the local group/cluster of galaxies is close enough to remain gravitationally bound, and we're still gonna merge with Andromeda.
      - Maursault 4y ago
        > 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.
- make3 4y ago
  how can that work if nothing is faster than light in the referential of the objects being compared? is it that space expands faster than light moves?
  - seanw444 4y ago
    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.
    - seszett 4y ago
      When I read this, it seems to me like this is more or less the same as saying that the speed of light is decreasing, but I am probably wrong?
      - jhoechtl 4y ago
        Is this something anybody ever thought out to look at things this way? Sounds like an intriguing Gedankenexperiment
        eigenket 4y ago
        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.
      - dpkirchner 4y ago
        Like if you calculated the speed not in meters per second but fraction-of-the-universe per second?
      - cout 4y ago
        What if space isn't expanding and we are just shrinking?
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8note 4y ago
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
afro88 4y ago
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?
- ivalm 4y ago
  > 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?).
  - afro88 4y ago
    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?
    - ivalm 4y ago
      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.
soulofmischief 4y ago
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. https://en.m.wikipedia.org/wiki/Particle_horizon https://en.m.wikipedia.org/wiki/Particle_horizon https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone
jacobr1 4y ago
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.
colordrops 4y ago
How does this work with the big bang happening at a specific point in the past?

soiler 4y ago

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.

teraflop 4y ago

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.

chrisweekly 4y ago
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.
- eigenket 4y ago
  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.

ben_w 4y ago

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 galaxies we're seeing are in front of that. [0] 3000 K, https://en.wikipedia.org/wiki/Recombination_(cosmology) https://en.wikipedia.org/wiki/Recombination_(cosmology)

lostmsu 4y ago
This is hard to internalize. Why do we continue seeing CMB? My understanding is that the early galaxies still produced light after recombination.
- cbracken 4y ago
  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.
- ben_w 4y ago
  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.

twawaaay 4y ago

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.

0xbadcafebee 4y ago
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?
- twawaaay 4y ago
  Or wait until shrooms wear off.
- pwatsonwailes 4y ago
  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.

xcskier56 4y ago

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

jhoechtl 4y ago

My theory: It's not an edge but we look onto ourselves back in time. As space and time evolves, it's hard to identify as such. One day we will recognize that we essentially were looking onto space to see ourselves while starring. The last thing is meant metaphorical.

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CuriouslyC 4y ago

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.

_8j50 4y ago

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?