Oscillating Universe: QG Theories on Gravity & Black Holes

[wolram]

If the possibility of an expanding, contracting cycle of our universe is valid, it
seems that gravity will have to be switched off at a certain point in the contracting phase, to avoid becoming a black hole, do QG theories tell us at
what point gravity stops being a, "force", "metric" ?
If this rubbish please say so.

 

[rtharbaugh1]

Hi Wolram

I don't know which model you are interested in, but it happens I have been thinking about what it means to say that the universe is collapsing or expanding. I hope you won't mind if I develop the thoughts here.

First, the idea of "the universe" is in need of some definition. Usually I think physicists and astronomers mean "the observable universe", but I don't know of anyone who thinks we have already seen all of it that there is to see. So the universe, in general, includes some things we cannot, at least yet, observe. This opens the door to all kinds of idle speculations.

Now if we are talking about the observable universe, the most recent estimates I have seen suggest we can see about eighty percent of what is out there. It is assumed in general that the remaining part is more or less like what we do see locally, since nothing that we can see suggests otherwise. This uniformitarian view has some problems.

For one thing, the most distant things we can see should be much younger than what is immediately around us. Galaxies and stars forming in the early universe, very far away from us now, in theory should look different than what we have now. For one thing, there is the metal content of stars.

Current theory suggests that early stars would not have much in the way of heavy elements, since these elements are not thought to be present in the big bang, but only to appear much later when stars have had time enough to burn off their lighter fuels and collapse violently, thus triggering events energetic enough to cause considerable fusion. However, it seems there are heavy metals in the stars and galaxies all the way out to the edge of our vision.

When people talk about the universe collapsing, they are usually as far as I know referring to a part of the theory of Hubble expansion. The astronomer Edward Hubble showed by evidence of the red shift of celestial objects that, in general, everything in the observable universe seems to be moving away from us. This lead to the question of whethor the expansion should go on forever, or if gravity should eventually slow it down, even stop it, and cause the universe to collapse again.

All evidence we have today, as far as I know, is that the universe is continuing to expand, and in fact is expanding faster as time goes on, which seems to suggest that it will not be overcome by its own gravity and collapse again. This dissappoints some people, who prefer the end of the universe be a big crunch rather than the big freeze which would seem to be the likely outcome of a universe which continues to expand forever.

In fact, the idea of the big bang was also born of the observation of the expanding universe, based on the idea that if you follow the expansion backwards in time, you must inevitabley come to a place and time where everything in the universe was in one place...the big bang singularity. There is nothing absolute about this logic. The big bang is not a proven fact, and there are other ways to imagine how the early universe may have formed.

It seems to me that the big ideas, bang crunch and freeze, are probably overly simplistic. It could be (here comes some idle speculation) that the observable universe is really only a tiny portion of the whole universe, and the little bit we see is undergoing some expansion, while in other adjacent areas some contraction may be going on. In this case, our observable universe can be expanding all around us as far as we can see, and even be expanding faster and faster, but eventually the expansion may slow down again and a contraction phase could set in locally. The accelerating expansion we observe would not then result in an inevitable big freeze, nor would it be realistic to follow the expansion backward in time to a singularity.

In fact, such a universe could be rather like the effect of sound waves in air, where local areas of increased and decreased density of air molocules transmit the energy we hear as sound. One molocule, in analogy our observable universe, could observe all of the molocules around it moving away, and then some time later, observe all of the molocules around it moving back together. This oscillation would not, of course, rather hardly ever, result in the molocule being left entirely alone in the room, nor would it necessarily imply that all of the molocules in the room had once occupied the singular position.

Our observable universe seems to be in a period of expansion. We have known about this for about a hundred years. In that time we have had enough data to take a further derivative and discover that in fact the expansion seems to be accelerating. But acceleration is not the end of the derivative series. We may in future be able to take a derivative of the acceleration data and discover that the universe is accelerating faster and faster also. This would be the result not of a big bang, but of a big jerk.

I suppose the creationists may take offense at my suggestion that the observable universe may be the result of a big jerk. In my experience, the God of Creation has been called worse things. Looking at the state of affairs in the world, I would have to say at best that God has seemingly developed a deplorable taste for practical jokes.

Be well,

Richard

 

[wolram]

I am sorry rtharbaugh1, allthough i find your observations educational, they do
not ansewer my question, if anyone is interested i will try to explain it better,
but i doubt if i can.

 

[mccrone]

"In this case, our observable universe can be expanding all around us as far as we can see, and even be expanding faster and faster, but eventually the expansion may slow down again and a contraction phase could set in locally. The accelerating expansion we observe would not then result in an inevitable big freeze, nor would it be realistic to follow the expansion backward in time to a singularity."

JM: Is this not Linde's eternal fractal inflation?

Generally speaking, the assumption that the Big Bang starts from a fundamental smallness (a Planckian singularity) is suspect. Linde is a good source for the alternative - that our local universe is a small area of "drop-out" from a vast and eternal landscape of "expanding" spacetimes. So instead of starting from the infinitely small, start from (and perhaps return to?) the infinitely large.

I would go a step further and say this Linde vision is too crisp - it demands a specific collection of spacetimes. I prefer a vaguer starting point - which would be neither large nor small as distinctions such as these would be the kind of distinctions that would get "thermalised".

But generally speaking, Linde is right I feel when he calls for us to detach from the assumption that beginnings must be small and singular (though that is how they may appear once you find yourself inside a particular branch of an infinitely branching system).

To comment on Wolram's initial question, the whole idea of a contraction back to a singularity seems "impossible" to me given the above kind of cosmological view. It depends on the idea that history is reversible (as seems to be suggested by equations - equations developed by chucking out history as a factor...see Atiyah).

But say the universe did contract under its own gravity, it would make a rather large black hole - I can't remember the figure offhand, but the visible universe alone would make a black hole the size of a galaxy, or something. It would hit the Planck energy density limit, but not be Planck sized.

This would be one factor to consider. There are others. But as I say, I'm much more interested in eternally expanding cold void (or asymptotic heat death) scenarios as they seem the more likely fate.

 

[hellfire]

If the possibility of an expanding, contracting cycle of our universe is valid, it
seems that gravity will have to be switched off at a certain point in the contracting phase, to avoid becoming a black hole

I may be wrong, but this seams not to be the case in classical general relativity. The existence of a singularity in the FRW models does not imply a black hole, even in oscillating universes (for example the classical Tolman model). A black hole is a static solution to Einstein's equations and the FRW solutions are dynamic. May be you are concerned with the "singularity" at t = 0. Quantum cosmological models based on quantum gravity try to remove it. There are lots of proposals to remove the initial singularity in the FRW models, e.g. based on old quantum gravity theories (the Hawking and Vilenkin proposals) or based on new theories (LQG cosmology or string cosmology), etc.

 

[wolram]

I may be wrong, but this seams not to be the case in classical general relativity. The existence of a singularity in the FRW models does not imply a black hole, even in oscillating universes (for example the classical Tolman model). A black hole is a static solution to Einstein's equations and the FRW solutions are dynamic. May be you are concerned with the "singularity" at t = 0. Quantum cosmological models based on quantum gravity try to remove it. There are lots of proposals to remove the initial singularity in the FRW models, e.g. based on old quantum gravity theories (the Hawking and Vilenkin proposals) or based on new theories (LQG cosmology or string cosmology), etc.

Thanks Helfire, that is why i put this is in strings and QG, i don't no why it was moved, it seems to me that to keep this model alive it needs some unintuitive
tricks, to remove the singularity surly one would have to turn off gravity, i am
not very well informed, but it just seems logical to me.

 

[hellfire]

I think it is widely accepted that gravity does also exist at Planck scales. At this sacale gravity is not described by the classical theory but by quantum theory of gravity. Thus I am not sure whether "turn off gravity" makes sense at all. There must be a transition to a regime in which quantum effects are dominant and this will result in a removal of the singularity.

 

[wolram]

I think it is widely accepted that gravity does also exist at Planck scales. At this sacale gravity is not described by the classical theory but by quantum theory of gravity. Thus I am not sure whether "turn off gravity" makes sense at all. There must be a transition to a regime in which quantum effects are dominant and this will result in a removal of the singularity.

Thanks Hellfire, i think this is beyond my level of understanding, it seems to invoke some physics that are illogical to me

 

[dubmugga]

the most recent estimates I have seen suggest we can see about eighty percent of what is out there...

It could be (here comes some idle speculation) that the observable universe is really only a tiny portion of the whole universe, and the little bit we see is undergoing some expansion, while in other adjacent areas some contraction may be going on...

Be well,
Richard

how about 80 percent of what we can see from our patch embedded in the hyperspherical skin of the soccer ball model...

...expansion/contraction takes place much like the swirling on a bubble skin but looking at it from trapped in the bubble skin we only see a small percentage of what is there before our line of sight curves around the inside and outside of the membrane

if gravity is to spacetime what surface tension is to bubbles at both the Planck size and universal size then what happens if it pops ?

imagine a bubble in your foamy bathtub popping and getting absorbed into the next 3 bubbles it connects to but the membrane of which the whole foam is made is still intact and retains it surface tension but is slightly different according to the size of the bubble

on a side note i imagined the oscillating universe to be oscillating between interchangeable spatial dimensions refreshing and reconstituting the universe at superluminal speed allowing for time and creating the illusion of motion...

:yuck: :tongue: