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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

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.
  - Dylan16807 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. The image isn't enough to demonstrate the difference. But the speed of light is, along with the way acceleration and redshift work. > Of course, we can figure out what it would look like, but if that is your argument, then it is circular. A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. You might have some specific circular argument in mind, but there are non-circular arguments for what GP is saying.
    - mannykannot 7y ago
      > The image isn't enough to demonstrate the difference... Precisely. > ...But the speed of light is, along with the way acceleration and redshift work. Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.) > A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)

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.
- Dylan16807 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. The image isn't enough to demonstrate the difference. But the speed of light is, along with the way acceleration and redshift work. > Of course, we can figure out what it would look like, but if that is your argument, then it is circular. A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. You might have some specific circular argument in mind, but there are non-circular arguments for what GP is saying.
  - mannykannot 7y ago
    > The image isn't enough to demonstrate the difference... Precisely. > ...But the speed of light is, along with the way acceleration and redshift work. Are you referring to the cosmological component of redshift? I suppose that, at least in principle, we could determine the cosmological component by first measuring the Hubble constant from the change in distance, over time, of those stars whose distance can be measured without any assumptions about redshift, but has that actually been done? (or some other experiment that directly calculates the cosmological component of redshift?) OP's claim is that we already have sufficient experimental results, not that we could, in principle, get them. AFAIK, experimental error in these stellar distance measurements is too large, and they are over too short a period of time, for the rate of expansion of the universe to be taken directly from them (note that the anomaly that is the subject of the article itself brings into question the accuracy of the cosmic distance ladder.) > A chain of logic that starts with the evidence we have, goes on to what telescopes show elsewhere, and then concludes about expansion, is not circular. It seems pretty clear that OP was thinking that if we were to observe the universe from a distant galaxy, it also would seem to be at the center of the expansion - but that would be the case in both scenarios, so it does not actually distinguish between them. If, however, OP had in mind some other set of measurements that, when combined with those from Earth, distinguish between the scenarios, then the question becomes, can you deduce what those far-away measurements would be without choosing between the two scenarios, either explicitly or implicitly? If that choice is being made, then you are begging the question. What set of measurements and calculation do you have in mind that avoid this circularity? (the above-mentioned redshift measurements are a separate issue, both because they are clearly not what OP had in mind, and because they do not depend on any calculation of what observations would be made from a distant galaxy.)