7 ms·

Thank you, went to the comments to learn this. So the universe actually expands pretty fast then! Faster than the speed of light? Strangely...

jhoechtl 8y ago
Which makes it then effectivelly infinite?
- mirimir 8y ago
  IANAP, but I think so. I vaguely recall that early inflationary expansion was very fast. Like several times the speed of light.
  - knappe 8y ago
    No, not really. One theory is that eventually things will slow and then collapse again. https://en.wikipedia.org/wiki/Big_Crunch https://en.wikipedia.org/wiki/Big_Crunch Taken to the extreme, this expansion and collapse has been happening over and over again in a cyclical fashion. https://en.wikipedia.org/wiki/Cyclic_model https://en.wikipedia.org/wiki/Cyclic_model
    - pixl97 8y ago
      The shape of the universe currently prevents this. For what we can tell the universe is flat, which means at no point will it collapse on itself, unless something major changes. That said, major changes have happened, around 8 billion years ago expansion appears to have sped up.
      - earenndil 8y ago
        I'm no expert (so correct me if I'm wrong), but from what I'm read, flat, concave, and convex universes are all possibilities and they would all look flat to us because our measurements take place on such a small scale.
        sandworm101 8y ago
        Our measurements occur across billions of light-years. By comparing redshift agaist the standard candles we measure relative speeds of vastly distant objects. And given the speed of light, we are also measuring what those speeds were billions of years ago. It is a very big ruler.
        roywiggins 8y ago
        We've measured the flatness of the universe on the scale of the cosmic microwave background, which is essentially as big a ruler as could possibly exist. https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.html https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.htm... https://wmap.gsfc.nasa.gov/universe/uni_shape.html https://wmap.gsfc.nasa.gov/universe/uni_shape.html
      - rkagerer 8y ago
        Those darn flatspacers
  - mordero 8y ago
    Space itself is expanding. At small scales, it isn't really that "fast" (gravity can/does overcome the effects within our galaxy and the Local Group). However, at large scales, it can exceed the speed of light (google "Hubble volume") so at some point, light emitted from sources outside this range will not longer reach us because the space between here and there expanded faster than light travels.
  - mirimir 8y ago
    Belated edit: More like ~10^81 times the speed of light. > According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light [0] It seems that the universe was on the order of ~1 meter in diameter then.[1] So the "'edges' of the Universe were expanding away from each other" at ~10^90 meters per second. 0) http://curious.astro.cornell.edu/physics/109-the-universe/cosmology-and-the-big-bang/inflation/664-how-can-the-universe-expand-faster-than-the-speed-of-light-during-inflation-advanced http://curious.astro.cornell.edu/physics/109-the-universe/co... 1) https://www.forbes.com/sites/startswithabang/2017/03/24/how-big-was-the-universe-at-the-moment-of-its-creation/ https://www.forbes.com/sites/startswithabang/2017/03/24/how-... Edit: Upon reflection, I get that "~1 meter in diameter" makes no sense. Maybe just say that the "edges" were ~1 meter apart.
    - jandrese 8y ago
      So the universe is expanding at greater than 10^90 m/s now, since the expansion is accelerating?
      - improbable22 8y ago
        No, "inflation" refers to a specific episode in the very early history of the universe. Or rather, in some models of this. Much slower expansion continued and continues today, and if you look really carefully is slightly accelerating not decelerating right now. m/s, or comparing to the speed of light, isn't really a great way to measure this. Points far away from us are moving away, and faster the further you go; we know of no limits to this.
    - monkeydreams 8y ago
      > Belated edit: More like ~10^81 times the speed of light. I have no grounding in physics on which to place this hunch but... doesn't this sound like a wee fudge factor to make the Big Bang theory fit evidence? I am reminded of the concept of "aether" and "land bridge theory" where people had a workable hypothesis except for the niggly little problem that the hypothesis was contraindicated by some of the evidence.
ryanseys 8y ago
I think it means that it's expanding and traveling in the opposite direction of us. If we start at the same spot, then both travel in opposite directions at 1 mile/hour, at the end of an hour we will be 2 miles away from each other.
- teekert 8y ago
  There is not really a center, galaxies are more like raisins in a rising bread. They all move away from each other with the same relative speeds. At least... Any physicists here to tell me whether or not space-time density get less closer to the edge? I mean... Is there an Edge?
  - plazmaphyujin 8y ago
    There is no edge. It defies the normal way of thinking but every point in the universe is also the center of the universe. The Big Bang happened at every point and space-time is expanding at every point.
    - whatshisface 8y ago
      There could be an edge, but it's definitely not within the distance we can see out. The main mind-bender is that there's no need for an edge.
      - plazmaphyujin 8y ago
        Well, not really. The distance we can see out is measured in time and we can very nearly see the Big Bang. We can observe the Universe's Recombination event and it is in every direction we look as the Cosmic Microwave Background. This happened everywhere in existence. If there is an edge to the Universe it is in the fourth dimension when the Universe was a singularity.
btilly 8y ago
This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other. If you understand manifolds and general relativity, this analogy is surprisingly exact.
- mehrdadn 8y ago
  I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.
  - jandrese 8y ago
    Cupcakes have a center.
    - mehrdadn 8y ago
      That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.
    - jmts 8y ago
      This is physics, so it is best to assume an infinite cupcake.
  - btilly 8y ago
    I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.
    - mehrdadn 8y ago
UlisesAC4 8y ago
Space still expands lower than light speed, the moment expansions goes faster night skies will be completely dark because other objects' light won't be able to reach our field of vision. I do not remember how that phenomenon is called.
- sandworm101 8y ago
  Not really. The far edge of our perception can be at lightspeed, but then everything closer would remain slower. So night wont suddenly become dark. We will first loose distant galaxies, but 99.9999 of starlight comes from our own galaxy. By the time that is moving away at lightspeed, the view will be the least of our concerns.
- caprese 8y ago
  Or the night skies light up completely because there was another big bang somewhere else and the light from that universe cluster just reached us our observable universe may be around an unobservable area whose light hasn't reached us yet
  - martin1975 8y ago
    so a parallel universe somehow interferes with ours? I don't think even most enthusiastic multiverse theorists will be able to grasp that.
    - caprese 8y ago
      not really parallel like alternate dimensions, maybe adjacent to not get distracted by colliqualisms and scifi tropes? just another area in the vast void that also had a big bang which is outside of our observable area right now, but whose light may reach us one day.
- 6nf 8y ago
  You might be referring to the Big Rip?
PopeDotNinja 8y ago
Yup! There is light moving towards us from the other side of the universe, but we'll never see it because the space between that light is stretching faster than thr light can travel (or something like that). Check out this page on the Observable Universe... https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe.
waterhouse 8y ago
I think it expands uniformly, so that, after time t, a distance of x becomes a distance of x * k, with k being very slightly greater than 1. Then there will always exist large enough x to outstrip the speed of light. To illustrate, imagine the expansion factor is, say, 10^-20 per second. Then a distance of 10^30 meters will increase to a distance of 10^30 + 10^10 meters in one second, while light travels at 3 * 10^8 meters per second, so expansion would outstrip lightspeed at that distance. If the expansion factor were 10^-40 per second, then a distance of 10^50 meters would be increasing faster than lightspeed.

jhoechtl 8y ago

Which makes it then effectivelly infinite?

mirimir 8y ago
IANAP, but I think so. I vaguely recall that early inflationary expansion was very fast. Like several times the speed of light.
- knappe 8y ago
  No, not really. One theory is that eventually things will slow and then collapse again. https://en.wikipedia.org/wiki/Big_Crunch https://en.wikipedia.org/wiki/Big_Crunch Taken to the extreme, this expansion and collapse has been happening over and over again in a cyclical fashion. https://en.wikipedia.org/wiki/Cyclic_model https://en.wikipedia.org/wiki/Cyclic_model
  - pixl97 8y ago
    The shape of the universe currently prevents this. For what we can tell the universe is flat, which means at no point will it collapse on itself, unless something major changes. That said, major changes have happened, around 8 billion years ago expansion appears to have sped up.
    - earenndil 8y ago
      I'm no expert (so correct me if I'm wrong), but from what I'm read, flat, concave, and convex universes are all possibilities and they would all look flat to us because our measurements take place on such a small scale.
      - sandworm101 8y ago
        Our measurements occur across billions of light-years. By comparing redshift agaist the standard candles we measure relative speeds of vastly distant objects. And given the speed of light, we are also measuring what those speeds were billions of years ago. It is a very big ruler.
        roywiggins 8y ago
        We've measured the flatness of the universe on the scale of the cosmic microwave background, which is essentially as big a ruler as could possibly exist. https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.html https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.htm... https://wmap.gsfc.nasa.gov/universe/uni_shape.html https://wmap.gsfc.nasa.gov/universe/uni_shape.html
    - rkagerer 8y ago
      Those darn flatspacers
- mordero 8y ago
  Space itself is expanding. At small scales, it isn't really that "fast" (gravity can/does overcome the effects within our galaxy and the Local Group). However, at large scales, it can exceed the speed of light (google "Hubble volume") so at some point, light emitted from sources outside this range will not longer reach us because the space between here and there expanded faster than light travels.
- mirimir 8y ago
  Belated edit: More like ~10^81 times the speed of light. > According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light [0] It seems that the universe was on the order of ~1 meter in diameter then.[1] So the "'edges' of the Universe were expanding away from each other" at ~10^90 meters per second. 0) http://curious.astro.cornell.edu/physics/109-the-universe/cosmology-and-the-big-bang/inflation/664-how-can-the-universe-expand-faster-than-the-speed-of-light-during-inflation-advanced http://curious.astro.cornell.edu/physics/109-the-universe/co... 1) https://www.forbes.com/sites/startswithabang/2017/03/24/how-big-was-the-universe-at-the-moment-of-its-creation/ https://www.forbes.com/sites/startswithabang/2017/03/24/how-... Edit: Upon reflection, I get that "~1 meter in diameter" makes no sense. Maybe just say that the "edges" were ~1 meter apart.
  - jandrese 8y ago
    So the universe is expanding at greater than 10^90 m/s now, since the expansion is accelerating?
    - improbable22 8y ago
      No, "inflation" refers to a specific episode in the very early history of the universe. Or rather, in some models of this. Much slower expansion continued and continues today, and if you look really carefully is slightly accelerating not decelerating right now. m/s, or comparing to the speed of light, isn't really a great way to measure this. Points far away from us are moving away, and faster the further you go; we know of no limits to this.
  - monkeydreams 8y ago
    > Belated edit: More like ~10^81 times the speed of light. I have no grounding in physics on which to place this hunch but... doesn't this sound like a wee fudge factor to make the Big Bang theory fit evidence? I am reminded of the concept of "aether" and "land bridge theory" where people had a workable hypothesis except for the niggly little problem that the hypothesis was contraindicated by some of the evidence.

ryanseys 8y ago

I think it means that it's expanding and traveling in the opposite direction of us. If we start at the same spot, then both travel in opposite directions at 1 mile/hour, at the end of an hour we will be 2 miles away from each other.

teekert 8y ago
There is not really a center, galaxies are more like raisins in a rising bread. They all move away from each other with the same relative speeds. At least... Any physicists here to tell me whether or not space-time density get less closer to the edge? I mean... Is there an Edge?
- plazmaphyujin 8y ago
  There is no edge. It defies the normal way of thinking but every point in the universe is also the center of the universe. The Big Bang happened at every point and space-time is expanding at every point.
  - whatshisface 8y ago
    There could be an edge, but it's definitely not within the distance we can see out. The main mind-bender is that there's no need for an edge.
    - plazmaphyujin 8y ago
      Well, not really. The distance we can see out is measured in time and we can very nearly see the Big Bang. We can observe the Universe's Recombination event and it is in every direction we look as the Cosmic Microwave Background. This happened everywhere in existence. If there is an edge to the Universe it is in the fourth dimension when the Universe was a singularity.

btilly 8y ago

This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other. If you understand manifolds and general relativity, this analogy is surprisingly exact.

mehrdadn 8y ago
I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.
- jandrese 8y ago
  Cupcakes have a center.
  - mehrdadn 8y ago
    That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.
  - jmts 8y ago
    This is physics, so it is best to assume an infinite cupcake.
- btilly 8y ago
  I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.
  - mehrdadn 8y ago

UlisesAC4 8y ago

Space still expands lower than light speed, the moment expansions goes faster night skies will be completely dark because other objects' light won't be able to reach our field of vision. I do not remember how that phenomenon is called.

sandworm101 8y ago
Not really. The far edge of our perception can be at lightspeed, but then everything closer would remain slower. So night wont suddenly become dark. We will first loose distant galaxies, but 99.9999 of starlight comes from our own galaxy. By the time that is moving away at lightspeed, the view will be the least of our concerns.
caprese 8y ago
Or the night skies light up completely because there was another big bang somewhere else and the light from that universe cluster just reached us our observable universe may be around an unobservable area whose light hasn't reached us yet
- martin1975 8y ago
  so a parallel universe somehow interferes with ours? I don't think even most enthusiastic multiverse theorists will be able to grasp that.
  - caprese 8y ago
    not really parallel like alternate dimensions, maybe adjacent to not get distracted by colliqualisms and scifi tropes? just another area in the vast void that also had a big bang which is outside of our observable area right now, but whose light may reach us one day.
6nf 8y ago
You might be referring to the Big Rip?

PopeDotNinja 8y ago

Yup! There is light moving towards us from the other side of the universe, but we'll never see it because the space between that light is stretching faster than thr light can travel (or something like that). Check out this page on the Observable Universe... https://en.wikipedia.org/wiki/Observable_universe https://en.wikipedia.org/wiki/Observable_universe.

waterhouse 8y ago

I think it expands uniformly, so that, after time t, a distance of x becomes a distance of x * k, with k being very slightly greater than 1. Then there will always exist large enough x to outstrip the speed of light. To illustrate, imagine the expansion factor is, say, 10^-20 per second. Then a distance of 10^30 meters will increase to a distance of 10^30 + 10^10 meters in one second, while light travels at 3 * 10^8 meters per second, so expansion would outstrip lightspeed at that distance. If the expansion factor were 10^-40 per second, then a distance of 10^50 meters would be increasing faster than lightspeed.