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> 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 u

> 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.
    - reptation 4y ago
      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.
      - pmayrgundter 4y ago
        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: https://news.ycombinator.com/item?id=32938153 https://news.ycombinator.com/item?id=32938153
        deleted 4y ago
        [deleted]
      - bee_rider 4y ago
        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.
  - hnuser123456 4y ago
    Better wikipedia link: https://en.wikipedia.org/wiki/Observable_universe#:~:text=The%20comoving%20distance%20from%20Earth,8.8%C3%971026%20m https://en.wikipedia.org/wiki/Observable_universe#:~:text=Th...).
Twisol 4y ago
> 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.)
- shagie 4y ago
  That rate is "only" approximately 68 kilometers per second per megaparsec. 68 kps isn't that fast (it's about the same speed as the Helios 2 solar probe) and a mega parsec is big distance (3.2M light years). Its that there's a lot of megaparsecs between here and there and the sum of all of those 68 kps is more than the speed of light. The relevant Kurzgesagt : TRUE Limits Of Humanity – The Final Border We Will Never Cross - https://youtu.be/uzkD5SeuwzM https://youtu.be/uzkD5SeuwzM
  - Twisol 4y ago
    Yes, true -- the distance between two points doesn't grow linearly, but rather proportionally to itself :D
  - sfifs 4y ago
    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)
    - 4y ago
kibwen 4y ago
> 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. Relevant Kurzgesagt video: https://www.youtube.com/watch?v=uzkD5SeuwzM https://www.youtube.com/watch?v=uzkD5SeuwzM
- sampo 4y ago
  Lawrence Krauss to Joe Rogan: "Nothing can travel though space faster than light, but space can do whatever the hell it wants." https://www.youtube.com/shorts/WGSYKoUqvps https://www.youtube.com/shorts/WGSYKoUqvps
- dalbasal 4y ago
  So galaxies are falling off the edge of the universe never to be seen again?
  - Iwan-Zotow 4y ago
    Yes, that is exactly what predicted to happen in the future - dark cold universe
  - zelphirkalt 4y ago
    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.
    - alisonatwork 4y ago
      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] [0] https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth
deanCommie 4y ago
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.
- thom 4y ago
  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.
ryanSrich 4y ago
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.
- davidvarela_us 4y ago
  Isn't this just the anthropic principle[0]? e.g. "conditions that are observed in the universe must allow the observer to exist" [0]: https://en.wikipedia.org/wiki/Anthropic_principle https://en.wikipedia.org/wiki/Anthropic_principle
Rury 4y ago
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.
- SkyMarshal 4y ago
  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. https://youtu.be/hps-HfpL1vc https://youtu.be/hps-HfpL1vc

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.
  - reptation 4y ago
    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.
    - pmayrgundter 4y ago
      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: https://news.ycombinator.com/item?id=32938153 https://news.ycombinator.com/item?id=32938153
      - deleted 4y ago
        [deleted]
    - bee_rider 4y ago
      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.
- hnuser123456 4y ago
  Better wikipedia link: https://en.wikipedia.org/wiki/Observable_universe#:~:text=The%20comoving%20distance%20from%20Earth,8.8%C3%971026%20m https://en.wikipedia.org/wiki/Observable_universe#:~:text=Th...).

Twisol 4y ago

> 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.)

shagie 4y ago
That rate is "only" approximately 68 kilometers per second per megaparsec. 68 kps isn't that fast (it's about the same speed as the Helios 2 solar probe) and a mega parsec is big distance (3.2M light years). Its that there's a lot of megaparsecs between here and there and the sum of all of those 68 kps is more than the speed of light. The relevant Kurzgesagt : TRUE Limits Of Humanity – The Final Border We Will Never Cross - https://youtu.be/uzkD5SeuwzM https://youtu.be/uzkD5SeuwzM
- Twisol 4y ago
  Yes, true -- the distance between two points doesn't grow linearly, but rather proportionally to itself :D
- sfifs 4y ago
  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)
  - 4y ago

kibwen 4y ago

> 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. Relevant Kurzgesagt video: https://www.youtube.com/watch?v=uzkD5SeuwzM https://www.youtube.com/watch?v=uzkD5SeuwzM

sampo 4y ago
Lawrence Krauss to Joe Rogan: "Nothing can travel though space faster than light, but space can do whatever the hell it wants." https://www.youtube.com/shorts/WGSYKoUqvps https://www.youtube.com/shorts/WGSYKoUqvps
dalbasal 4y ago
So galaxies are falling off the edge of the universe never to be seen again?
- Iwan-Zotow 4y ago
  Yes, that is exactly what predicted to happen in the future - dark cold universe
- zelphirkalt 4y ago
  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.
  - alisonatwork 4y ago
    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] [0] https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth

deanCommie 4y ago

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.

thom 4y ago
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.

ryanSrich 4y ago

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.

davidvarela_us 4y ago
Isn't this just the anthropic principle[0]? e.g. "conditions that are observed in the universe must allow the observer to exist" [0]: https://en.wikipedia.org/wiki/Anthropic_principle https://en.wikipedia.org/wiki/Anthropic_principle

Rury 4y ago

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.

SkyMarshal 4y ago
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. https://youtu.be/hps-HfpL1vc https://youtu.be/hps-HfpL1vc