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Author Topic: Will crossing the Hubble sphere cause a gravitational catastrophe?  (Read 3257 times)

Offline jeffreyH

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If enough mass crosses the Hubble sphere, leaving a critically low mass behind, will there be a gravitational catastrophe? That is will the balance of all the interacting gravitational fields be so disturbed that there will be a catastrophic change.


 

Offline JohnDuffield

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I wouldn't quite put it that way, but I would say that IMHO if the universe keeps on expanding there may be a "dark energy" catastrophe of sorts. I rather thought it might be something like this article by cosmologist Phil Plait:  Prince Rupert’s Drops. Exploding. 

 

Offline PhysBang

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If enough mass crosses the Hubble sphere, leaving a critically low mass behind, will there be a gravitational catastrophe? That is will the balance of all the interacting gravitational fields be so disturbed that there will be a catastrophic change.
What do you mean?

If there is a sizable inhomogeneity in a spacetime region, then, yes, there will not be the expansion (or other gravitation phenomena) there that one expects from a homogeneous spacetime. This is why we see galaxies and solar systems.

Because of the action of a cosmological constant, mass can continue to leave any given horizons, but do so in an isotropic fashion. That means that there could be less and less gravity to create a "drag" on expansion for any given region.
 

Offline jeffreyH

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If enough mass crosses the Hubble sphere, leaving a critically low mass behind, will there be a gravitational catastrophe? That is will the balance of all the interacting gravitational fields be so disturbed that there will be a catastrophic change.
What do you mean?

If there is a sizable inhomogeneity in a spacetime region, then, yes, there will not be the expansion (or other gravitation phenomena) there that one expects from a homogeneous spacetime. This is why we see galaxies and solar systems.

Because of the action of a cosmological constant, mass can continue to leave any given horizons, but do so in an isotropic fashion. That means that there could be less and less gravity to create a "drag" on expansion for any given region.

Your last paragraph appears to answer the question. However wouldn't this require that at a certain point in time the Hubble sphere must start to shrink.
 

Offline Bill S

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Objects leaving the Hubble sphere do not pass a physical boundary, beyond which they no longer influence anything within the sphere.  The boundary of the HS is everywhere in the cosmos, its perceived position is relative to any observer, so any changes would be extremely slow.

If expansion is homogeneous the only (very) long term effect would be reduced drag, as PhysBang points out.  I assume this would lead to accelerated expansion, unless “dark energy” is a gravitational effect, in which case, might not that diminish as more matter left the HS?
 

Offline PhysBang

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Reduced drag does not in itself accelerate expansion, since the expansion as an inertial element to it: it continues at a steady rate unless something interferes with it. Less drag means less interference, so less to slow down the rate of the expansion.

The cosmological constant (or dark energy or some similar cause) does accelerate expansion (as a part of the working of gravity). So the less drag there is, then the more efficiently that the acceleration can work.
 

Online chiralSPO

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We assume that the universe is essentially the same everywhere (on a large scale), and is boundless; but we have no direct evidence of anything outside of the Hubble sphere (Hubble bubble?).

Perhaps everything in the universe really did expand from one specific point (limited by c, except as space expands), and there is a boundary beyond which there is no matter, no light and no evidence of either (no gravity). This boundary would expand continuously, and nothing especially interesting should happen at the vanguard (leading edge) but anything that somehow managed to be beyond that boundary would never know that the universe existed until the boundary expands to/beyond that point.
 

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