- #1
jonmtkisco
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Gravitational Expansion -- NOT
A brief followup is necessary regarding my post "Gravitational Expansion - Flat Universe".
As best I can recall, chronon is the only person who ever responded directly to the substance of the "gravitational expansion" proposition I described. So I must congratulate him for so succinctly nailing the explanation of why the proposition can't possibly be consistent with physical reality. He said:
"Furthermore, if you are trying to say that mass/energy has repulsive effects as well as attractive effects, and that these are responsible for the acceleration (rather than deceleration) of the expansion of the universe, then the same effects should apply to the earth, and so we should be accelerating away from its surface"
As I explained elsewhere in the thread, I wasn't attributing repulsive effects to gravity, instead I was suggesting that gravity might simply be the direct cause of spatial expansion. Nevertheless, chronon correctly points out why the predicted results of gravitational expansion cannot be reconciled to everyday physical reality on planet earth.
If spatial expansion operated at "escape velocity" on earth, then we would observe all local objects not fastened down to be moving away from us at a very high recession velocity. If you dropped a rock, it would appear to fly away upwards (so would you!), accelerating quickly to supersonic speed. Gravity eventually would slow its recession, but never halt it. Localized spatial expansion would cause all matter to become dispersed as far away from other matter as possible.
I considered the idea that increasing the "gross" strength of gravity (without any corresponding increase in the expansion rate) might avoid this problem, but it only tempers it. If gravity were doubled, objects would still appear to fly away from the Earth's surface for some distance, then slow down and eventually crash back to the surface. This is true no matter how strong gravity is, because it takes time for an object in free-fall to accelerate. There is no way to avoid it from moving upwards before it begins to descend.
I suppose all of this seems obvious in retrospect, it just didn't occur to me. Oh well.
The learning I take away from this is that neither gravitational effects, nor anti-gravitational effects, in any local region are likely to have any differentiating effect at all on the local rate of expansion. Thus, for the same reason, local gravity cannot cause local space to contract at an "above-average rate" either. The expansion rate must be determined entirely holistically, that is, for the universe as a whole, without regard to any non-homogeneous localized events. I think that is what the 'standard model' says, although this subject usually is not articulated very clearly. So space must be expanding at exactly the same rate within a supervoid as it is within a supercluster (although gravitational binding obscures the detectability of spatial expansion within a bound structure such as a supercluster).
In that respect, it seems rather strange that the summation of localized homogeneous events, such as the redshifting of free radiation, can dynamically affect the expansion rate of the universe as a whole, to a rather dramatic degree. But if a particular region experienced an unusually strong (i.e., non-homogeneous) amount of radiation redshifting, that region apparently would experience no deviation from the average expansion rate of the universe, no matter how slight.
It is interesting to note that the summation of many occurrances of one particular localized event, the conversion of matter to energy through stellar fusion, must in aggregate have a tiny but definite effect on the overall expansion curve. The release of free radiation increases local "pressure" which increases "gravitational density" which increases the Friedmann deceleration rate; but the released radiation will dwindle relatively quickly due to expansionary redshifting, and the long-term expansion rate will eventually be faster, due to the lower total matter density.
Conversely, supernovae fusion converts some energy into matter, in the form of elements heavier than iron. This conversion of energy into matter has the opposite effect on the long-term expansion rate as what I described above.
Gravity's "now you see it, now you don't" effect on spatial expansion is quite curious. It's easy to neglect this topic because the lack of detectable local effects.
Jon
A brief followup is necessary regarding my post "Gravitational Expansion - Flat Universe".
As best I can recall, chronon is the only person who ever responded directly to the substance of the "gravitational expansion" proposition I described. So I must congratulate him for so succinctly nailing the explanation of why the proposition can't possibly be consistent with physical reality. He said:
"Furthermore, if you are trying to say that mass/energy has repulsive effects as well as attractive effects, and that these are responsible for the acceleration (rather than deceleration) of the expansion of the universe, then the same effects should apply to the earth, and so we should be accelerating away from its surface"
As I explained elsewhere in the thread, I wasn't attributing repulsive effects to gravity, instead I was suggesting that gravity might simply be the direct cause of spatial expansion. Nevertheless, chronon correctly points out why the predicted results of gravitational expansion cannot be reconciled to everyday physical reality on planet earth.
If spatial expansion operated at "escape velocity" on earth, then we would observe all local objects not fastened down to be moving away from us at a very high recession velocity. If you dropped a rock, it would appear to fly away upwards (so would you!), accelerating quickly to supersonic speed. Gravity eventually would slow its recession, but never halt it. Localized spatial expansion would cause all matter to become dispersed as far away from other matter as possible.
I considered the idea that increasing the "gross" strength of gravity (without any corresponding increase in the expansion rate) might avoid this problem, but it only tempers it. If gravity were doubled, objects would still appear to fly away from the Earth's surface for some distance, then slow down and eventually crash back to the surface. This is true no matter how strong gravity is, because it takes time for an object in free-fall to accelerate. There is no way to avoid it from moving upwards before it begins to descend.
I suppose all of this seems obvious in retrospect, it just didn't occur to me. Oh well.
The learning I take away from this is that neither gravitational effects, nor anti-gravitational effects, in any local region are likely to have any differentiating effect at all on the local rate of expansion. Thus, for the same reason, local gravity cannot cause local space to contract at an "above-average rate" either. The expansion rate must be determined entirely holistically, that is, for the universe as a whole, without regard to any non-homogeneous localized events. I think that is what the 'standard model' says, although this subject usually is not articulated very clearly. So space must be expanding at exactly the same rate within a supervoid as it is within a supercluster (although gravitational binding obscures the detectability of spatial expansion within a bound structure such as a supercluster).
In that respect, it seems rather strange that the summation of localized homogeneous events, such as the redshifting of free radiation, can dynamically affect the expansion rate of the universe as a whole, to a rather dramatic degree. But if a particular region experienced an unusually strong (i.e., non-homogeneous) amount of radiation redshifting, that region apparently would experience no deviation from the average expansion rate of the universe, no matter how slight.
It is interesting to note that the summation of many occurrances of one particular localized event, the conversion of matter to energy through stellar fusion, must in aggregate have a tiny but definite effect on the overall expansion curve. The release of free radiation increases local "pressure" which increases "gravitational density" which increases the Friedmann deceleration rate; but the released radiation will dwindle relatively quickly due to expansionary redshifting, and the long-term expansion rate will eventually be faster, due to the lower total matter density.
Conversely, supernovae fusion converts some energy into matter, in the form of elements heavier than iron. This conversion of energy into matter has the opposite effect on the long-term expansion rate as what I described above.
Gravity's "now you see it, now you don't" effect on spatial expansion is quite curious. It's easy to neglect this topic because the lack of detectable local effects.
Jon
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