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Expanding universe and gravity





ocalhoun
I was reading another thread and had a very interesting idea:
What if the reason, or at least part of the reason, that things with mass attract to each other by gravity is because of the universe's expansion?

Suppose that anything with mass creates a zone around it where the universe does not expand. If that were the case, then the further you were away from a massive object, the more you would seem to be pulled away from it by the expansion of the universe creating more space in between it and you. This is very much like the effect of gravity getting weaker as you go away from what is attracting you.

It's not even really a completed pondering yet, but it seems very interesting to me, so I thought I'd share it.
Bikerman
Well, in a back to front way it is correct.
Mass does indeed create a 'field' around itself. Classically this is known as a distortion in spacetime (general relativity).
Space does indeed appear to expand in the absence of mass (gravity) and so distant galaxies are receding from us at a high 'speed' because the space between them and us is expanding.
This does not, however, mean that expansion causes gravity - rather the lack of gravity allows expansion.
DoctorBeaver
Bikerman - you raise an interesting point there.

Is space expanding faster the further you get from a gravitational field? If gravity holds the expansion in check in some areas, that would seem to have to be the case but I have heard nothing about that.
Bikerman
DoctorBeaver wrote:
Bikerman - you raise an interesting point there.

Is space expanding faster the further you get from a gravitational field? If gravity holds the expansion in check in some areas, that would seem to have to be the case but I have heard nothing about that.

Yes, I think this is well established as the basic 'expansion' theory. Thus we see that the local cluster of galaxies are gravity bound (no expansion). Thus we see other galaxy clusters where the galaxies are close enough to be gravity bound, but we also see that the further 'out' you look to distant galaxies, the higher the redshift (hubble's law).
The two main proposed 'expansion mechanisms' use a slightly different explanation (quintessence and dark energy/cosmological constant) but the essence is the same - that expansion is a property of empty space and has a very low value (much much lower than gravity), so it only 'kicks in' in very deep intergalactic space where spacetime is essentially flat.
DoctorBeaver
I think I didn't explain what I meant particularly well. I know what I mean, but explaining it isn't easy.

If you take a mathematical point anywhere in the universe, is the "pressure" of expansion the same no matter where that point is? I appreciate that where there is more matter, the greater is the force holding expansion in check; but does every point in space have the same amount of repulsive energy?

I'm still not happy with the way I've phrased that, but I tend to think things like this in patterns not words Confused
Bikerman
DoctorBeaver wrote:
I think I didn't explain what I meant particularly well. I know what I mean, but explaining it isn't easy.

If you take a mathematical point anywhere in the universe, is the "pressure" of expansion the same no matter where that point is? I appreciate that where there is more matter, the greater is the force holding expansion in check; but does every point in space have the same amount of repulsive energy?

I'm still not happy with the way I've phrased that, but I tend to think things like this in patterns not words Confused

Yes, I understand what you mean.
I think the answer is perhaps. (classic eh? Smile ).
Basically the reason for saying so is that there are competing views of what the expansive force actually is. If the notion of a cosmological constant is correct (vacuum energy, dark energy, call it what you will) then the answer is yes - the expansive force is uniformly pervasive.
If, however, the notion of quintessence is correct then the answer is no. Quintessence is posited as time and spatially dependant, rather than uniform.
I'm certainly no expert on it - the current maestro seems to be Steinhardt (who it should be said has a pretty good track record in physics). The best I can probably do is point you towards a 'primer' on the matter and leave you to do some googling;
http://www.astronomytoday.com/cosmology/quintessence.html
DoctorBeaver
1 point I note from that article you linked to is that for the cosmological constant to actually be constant it must have always had the same value. From what I can gather, that is probably not the case; the epoch of inflation seems to point to it having a variable (or, at least, varying) value. That, of course, is assuming that the force that drove inflation is the same force that is causing the acceleration in the rate of expansion. Although that appears to be so, there is no proof as of yet.

Something that nags at me about all this concerns the known forces and their unification. It is theorised that at sufficiently high energy the known forces (with the possible exception of gravity) become indistinguishable. It is only when the energy level drops sufficiently for symmetry-breaking to take place that these forces become discrete.

The universe is still expanding and cooling. How can we say for certain that 1 of the currently-known forces will not diverge into 2 separate forces when the energy level drops further? Could that already have happened? Could the accelerated rate of expansion be the manifestation of yet another symmetry-breaking event giving rise to a 5th force of nature that priorly was not apparent?

Unfortunately I don't know enough about forces to know the implications of this thought or whether it is even possible.
Bikerman
Well I think we need to consider inflation and expansion as separate phenomena.
Inflation theory only covers that very brief period post BB where there was an exponential growth in size. Expansion theory is different. If the cosmological constant IS constant then one would expect expansion to increase as time goes on and the amount of 'flat' spacetime increases..
Of course this is a major simplification because I don't fully understand the theory....but who does ? Wink
DoctorBeaver
In the book I'm currently re-reading - "On The Shores Of The Unknown" by Joseph Silk, Savilian Professor of Astronomy, Oxford university - it states that the current acceleration in the rate of expansion could well be caused by the same force that drove inflation but 10^-100 as powerful.

He expains it in terms of positive and negative pressure (the same as causes the Casimir effect).
Bikerman
DoctorBeaver wrote:
In the book I'm currently re-reading - "On The Shores Of The Unknown" by Joseph Silk, Savilian Professor of Astronomy, Oxford university - it states that the current acceleration in the rate of expansion could well be caused by the same force that drove inflation but 10^-100 as powerful.

Well, I have to admit that I am out of my depth here, but I understand the basic inflation theory (based on the de-Sitter solution to GR) and I understood that this required an Inflaton field in the very early universe, and the inflaton particles then quickly decayed into normal radiation...
I may have it wrong, of course, and I will seek wiser council before saying more...
DoctorBeaver
I've come across that also, but the positive/negatie pressure explanation seems more plausible to me as it is based on a known effect - vacuum energy.
Bikerman
Hmm..the problem with that explanation (zero point energy) is that it doesn't work when you slot the figures into quantum field theory. The numbers you get out, for the cosmological constant, are too big - by an enormous amount (about 10^120 time too big). That's a big problem...
DoctorBeaver
What's 120 orders of magnitude here & there? Laughing
dreulet
The Big Bang theorist believe in a finite universe and that next will be the big rip, The End. Hogwash! Therir fallable math proves infinty exists it's just hard to wrap your head around that it has always existed is dynamic and inflating to posiblly to other forms of gravity where if you were there it would contracting on you. Einsteins equation left out the speed of dark and everyone believes that dark is the abscence of color when it is not. Light and dark are only byproducts of energy wich is simply movement and comes in many forms, gravity, magnetism, electricity. These are all just byproducts of the mavement that infinitely existed along with the two matters dense Space "Dark Matter" and Particulate Matter both equivalint with just seperate properties when movement makes them act differently in different densities. My theory (what Einstein forgot) plugs in E= Space times the speed of Dark squared treating space as matter allows you to predict the energy of a black whole or any dark matter for that case once you calculate it's mass. Turn off a light source to see the speed of dark. Black holes don't swallow anything they just spit energy taken from the particles it comes in contact with through gravity and sucks the dark matter to it gravitationally and spits out the particulate matter that it cannot share space with, (they never mix they are seperated always at the transitional points where light magnetism gravity electricity are all just simple byproducts of the movement between the matter Dark and Particulate) and we percieve the light we say it swallowed and now notice it emits it out in two incredible cones of energy which we measure using lust light. The Dark is not emitting wavelengths because it can't escape the gravity of the energy of very dense dark space which is truly an object and should be considered matter. The two never mix and the density is the key to which one wins. Particulate matter wins in less dense space and you have nuclear fission and a star ignites. When the space is denser the star super novas into a black hole where now the denser space exists. No it is not a hole and litterally doesn't swallow anything into the event horizon other than gravitational dark matter. You can't travel into it and wormholes aren't real either. You also can't go back in time or travel to alternate universes. Nice imagination. Have you ever met anyone from the future? You can't go back that way but there is a real way and once it is started we will be able to go forward as fast as we can into the future and when we come back we will be visiting the past we left which would be our future but the old people weve left behind will be traveling into the past as soon as they talk to us but since we only experienced a shorter time then them they would actually being travelling into the past and thats the only way it can work but it is theoretically proven.
Bikerman
Dreulet,
I'm not sure what medication you are taking, but it clearly isn't working. I presume you just string words together to make meaningless but important sounding sentences? It might fool the very very credulous, but anyone at all familiar with science will quickly recognise it for the gibberish that it is...
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