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> 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 thi

> 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
[deleted]
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?

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

[deleted]

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?