7 ms·

Correct me if any of the following is wrong, though: when we say the universe is expanding, we mean that matter is spreading out. The further apart the matter i

Correct me if any of the following is wrong, though: when we say the universe is expanding, we mean that matter is spreading out. The further apart the matter is, the faster it's spreading out. If we back-trace the position of stars, they all coincide at roughly the same point in time and space. This led us to believe that at some point in time all of this matter was together, incredibly dense, and in a very "hot", high-energy state. This led to the prediction that has since been confirmed, that from the furthest reaches of the observable universe, we would see cosmic background radiation. And as time goes, that cosmic background radiation is getting further and further away, so the observable bubble is expanding. Maybe I have some of that wrong, but that's my understanding. None of that means that the fabric of space (as in space-time) is actually expanding. And even if it did, none of that means that nothing could have existed outside of the observable bubble billions of years ago, and could have since been passed by the observable edge and now be inside that observable bubble. Unless maybe the source of the background radiation was still dense / high-energy enough to absolutely obliterate everything in it's path. And maybe it was? But I just don't see how we can reason about what was outside of the observable bubble.

ecnahc515 7y ago
> when we say the universe is expanding, we mean that matter is spreading out It's been a while, but I recall reading in a book that's actually the space between everything that's expanding, much like you state in your second paragraph. For example, if you blow up a balloon, two points on it will appear to be moving apart from each other even though they're stationary. I believe this Forbes article is in agreement: https://www.forbes.com/sites/startswithabang/2018/02/10/ask-ethan-how-do-we-know-space-is-expanding/#7356826349e6 https://www.forbes.com/sites/startswithabang/2018/02/10/ask-...
- wahern 7y ago
  It's also been described like a loaf of bread expanding, where stars are like raisins, except the loaf might be infinitely big. See https://en.wikipedia.org/wiki/Inflation_(cosmology) https://en.wikipedia.org/wiki/Inflation_(cosmology) and https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe But that's also somewhat misleading as it gives the impression that it's just interstellar space (reinforcing the non-scientific connotation of space) that inflates. At some point even atoms may not be able to hold together. See https://en.wikipedia.org/wiki/Big_Rip https://en.wikipedia.org/wiki/Big_Rip Whether the Big Rip is true or not, it's useful as a supplemental illustration.
  - Dylan16807 7y ago
    To be clear, the big rip is only if expansion gets massively faster. At the current rate, the disturbance on matter is ridiculously negligibly small.
mannykannot 7y ago
Perhaps a big bang that's 'just' an explosion in spacetime, rather than of spacetime, would not work with known physics because that concentration of that much matter would be a black hole? Until now, I never thought to ask anyone.
- amelius 7y ago
  I'm not a physicist but I suspect this comment hits the nail on the head. You can only "undo" a black-hole by expanding the space it is embedded in.
- tomrod 7y ago
  I think string theory approaches this idea with the holographic principle: https://en.m.wikipedia.org/wiki/Holographic_principle https://en.m.wikipedia.org/wiki/Holographic_principle
- swebs 7y ago
  It wouldn't work out because it wouldn't match what our telescopes are observing. This image has a good example of what's happening https://upload.wikimedia.org/wikipedia/commons/c/c7/Space_Expansion_%28no_center%29.png https://upload.wikimedia.org/wikipedia/commons/c/c7/Space_Ex... You can see that from any given point: 1. Every other point is moving away from you 2. Points further away are moving away at a faster rate This could only work out if space were expanding. If it were an explosion instead, everything would be moving away from a fixed central point.
  - mannykannot 7y ago
    That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. Furthermore, our telescopes have only observed from the vicinity of the sun - we have not actually seen what the universe looks like from another star. Of course, we can figure out what it would look like, but if that is your argument, then it is circular.
ganonm 7y ago
Space itself is actually growing as well as celestial bodies having some relative motion that on average moves away from a common point. The growth in space itself is called inflation. Last time I studied cosmology (university) this was an accepted fact although the precise mechanism that explains this inflation was not agreed upon (scalar field, dark energy etc). I think there is some debate about whether inflation is speeding up or slowing down or how long it has occurred for but AFAIK the concensus is that it is a real thing. To make an analogy, it is as if you had an elastic sheet with objects moving on its surface. The elastic sheet can be expanded which causes the distance between any two fixed points to grow. (it's been many years since I studied physics so any corrections to my comments are appreciated)

ecnahc515 7y ago

> when we say the universe is expanding, we mean that matter is spreading out It's been a while, but I recall reading in a book that's actually the space between everything that's expanding, much like you state in your second paragraph. For example, if you blow up a balloon, two points on it will appear to be moving apart from each other even though they're stationary. I believe this Forbes article is in agreement: https://www.forbes.com/sites/startswithabang/2018/02/10/ask-ethan-how-do-we-know-space-is-expanding/#7356826349e6 https://www.forbes.com/sites/startswithabang/2018/02/10/ask-...

wahern 7y ago
It's also been described like a loaf of bread expanding, where stars are like raisins, except the loaf might be infinitely big. See https://en.wikipedia.org/wiki/Inflation_(cosmology) https://en.wikipedia.org/wiki/Inflation_(cosmology) and https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe But that's also somewhat misleading as it gives the impression that it's just interstellar space (reinforcing the non-scientific connotation of space) that inflates. At some point even atoms may not be able to hold together. See https://en.wikipedia.org/wiki/Big_Rip https://en.wikipedia.org/wiki/Big_Rip Whether the Big Rip is true or not, it's useful as a supplemental illustration.
- Dylan16807 7y ago
  To be clear, the big rip is only if expansion gets massively faster. At the current rate, the disturbance on matter is ridiculously negligibly small.

mannykannot 7y ago

Perhaps a big bang that's 'just' an explosion in spacetime, rather than of spacetime, would not work with known physics because that concentration of that much matter would be a black hole? Until now, I never thought to ask anyone.

amelius 7y ago
I'm not a physicist but I suspect this comment hits the nail on the head. You can only "undo" a black-hole by expanding the space it is embedded in.
tomrod 7y ago
I think string theory approaches this idea with the holographic principle: https://en.m.wikipedia.org/wiki/Holographic_principle https://en.m.wikipedia.org/wiki/Holographic_principle
swebs 7y ago
It wouldn't work out because it wouldn't match what our telescopes are observing. This image has a good example of what's happening https://upload.wikimedia.org/wikipedia/commons/c/c7/Space_Expansion_%28no_center%29.png https://upload.wikimedia.org/wikipedia/commons/c/c7/Space_Ex... You can see that from any given point: 1. Every other point is moving away from you 2. Points further away are moving away at a faster rate This could only work out if space were expanding. If it were an explosion instead, everything would be moving away from a fixed central point.
- mannykannot 7y ago
  That actually demonstrates the opposite: despite the fact that the images themselves show the expansion of a grid in a two-dimensional space, each of the dots appears, from its perspective, to be at the center of the expansion, unless you either specify an absolute frame of reference, or (which amounts to the same thing) can identify the edge of the expanding region. Furthermore, our telescopes have only observed from the vicinity of the sun - we have not actually seen what the universe looks like from another star. Of course, we can figure out what it would look like, but if that is your argument, then it is circular.

ganonm 7y ago

Space itself is actually growing as well as celestial bodies having some relative motion that on average moves away from a common point. The growth in space itself is called inflation. Last time I studied cosmology (university) this was an accepted fact although the precise mechanism that explains this inflation was not agreed upon (scalar field, dark energy etc). I think there is some debate about whether inflation is speeding up or slowing down or how long it has occurred for but AFAIK the concensus is that it is a real thing. To make an analogy, it is as if you had an elastic sheet with objects moving on its surface. The elastic sheet can be expanded which causes the distance between any two fixed points to grow. (it's been many years since I studied physics so any corrections to my comments are appreciated)