I have a question...

It's by now become fairly well established that the 'actual' diameter of the universe is considerably greater than the visible universe due to inflation/expansion. Parts of the universe are forever separated by others because of this period of inflation early on during the Bang Bang; as a result, from any given point in the universe, the visible horizon is only about 13.2 bly in any direction.

It seems to me that this effect should also hold for gravitation; there should also be a gravitational horizon matching the visible horizon, and something that's existed since shortly after the big bang's inflationary epoch, but I don't recall it ever being stated in such terms; all i see reference to is an EM horizon.

As a result, I wonder if:

a) is this a reasonable question? or am i missing something obvious (i.e. it's 'naturally' accounted for in other ways)?

b) has it been accounted for in how it would affect expansion (and the apparent acceleration we're seeing in expansion now)?

c) if so, where is it addressed in current theory, and how?

Thanks - just wondering.

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"...wait for the ricochet."

Well, as I understand it, the 'visible horizon' exists because, at previous times during the Big Bang, photons were interacting with matter so frequently (due to matter density and the overall temperature of the universe during that period) that none were able to escape to be observed by present-day observers. As far as the idea of a gravitational horizon goes, I'm not sure what you are getting at. Are you thinking that gravity did not exist until some point after the Big Bang?

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The dose makes the poison--Paracelsus (1493-1541) http://en.wikipedia.org/wiki/Paracelsus

I don't know. That's why I'm asking--Noclevername

Intelligence may not be clearly defined, but you know stupid when you see it--Noclevername

Science is a way of thinking much more than it is a body of knowledge--Carl Sagan (1934-1996)

I am thinking that as the unverse inflated superluminally for a brief period, there would be gravitationally isolated areas of the universe in the same way as there are visually (EM) isolated areas; EM and Gravity both propagate at c.

I'd think this would also affect the dynamics of expansion thereafter, but I am not sure I undestand quite how. It would be fairly complex, I think, too (simple in principle, but the expression would be complex - just as visual horizons can be 'separate' and overlapping (the diameter of an observer's horizon 8 bly from 'us' in some arbitrarry direction would include part of our horizon in one direction, and part of the universe beyond our visible horizon in the other, but he'd not be able to see all of 'ours' (assuming his visible horizon would also have a 13.2 bly radius, for example).

Similarly, the gravity of objects on the distant end of our horizon (relative to the other observer) would not affect/be affected by objects on his extreme edge in the opposite direction, while objects within our horizon near to the observer would be affected by objects within his horizon, but outside of ours).

I realize that continuous expansion and etc. cause this to be not quite so straightforward (eg objects we see 'now' as within our visible horizon are likely now well out of it, even though they were much closer when the light started enroute to us, etc. (the Inverse RVM Theorem)" ), but I think there may be a real effect here that this simplistic decription points too. I want to know if my thinking IS wrong, and if so, how so and why.

If not, then how is it accounted for - if it is?

It seems to me that the superluminal inflationary epoch necessarily had to cause BOTH EM and gravitational separation between disparate regions. This question has arisen in a couple of contexts that caused me to think of the related issues a bit differently than I had previously. As I'm unable to do the math myself, I am just wondering if this basic conceptualization/description is valid or not, and how it's dealt with.




" Inverse RVM Theorem: Objects in your Rear View Mirror are much closer than they appear. Not.

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"...wait for the ricochet."

What I remember from the theory of relativity is that even if two objects move diametrically at .6 c, light emitted from these objects will still have the same speed in either direction to a nonmoving observer. So eventually light from one object will reach the other since only "1 > .6" counts, not "1 > 2 x .6".

That's not the problem, though. Also keep in mind - superluminal (highly so!) inflationary epoch.


[ Also, a side note (and hopefully not getting the discussion off course...I really would like an answer to the question I'm asking), but there have been jets from black holes, each moving in one direction relative TO the black hole at speeds in excess of .6 c - my guess it relative to each other, they'd either appear to be separating a c (highly reshifted, if if one could see the other) or - (barring some lensing effect) neither are visible to the other, which is my guess anyway...there's a black hole between them. ), however the separation of the objects does appear to exceed c. I'd need to double-check that...shooting from the hip here based on memory of some papers I read a couple of years ago. But this is only indirectly related to the question I'm posing here. ]

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"...wait for the ricochet."

Yes, that should certainly be true, and yes, it is part of the equations of cosmology.
It's accounted for in the "cosmological principle", coupled with the Einstein's equations. The former postulates that the universe is basically the same everywhere, though it changes as it ages, and the latter tells you how something that is the same everywhere evolves dynamically under the action of gravity. To get things like inflation, you just need to add terms to Einstein's equation, and the full ramifications follow from that. That's the power of a very good theory.
Certainly, that would be the expectation.

Sigh. That still doesn't address the question. I am not questioning cosmic istropy or gravities effect on how the universe evolves. I am asking: are disparate regions of the Universe separated/isolated from each gravitationally, due to the superluminal inflationary epoch, and b) if so, has that separation been properly accounted for or modeled when considering the expansionary behaviour of the universe - especially with regards to the apparent increase in acceleration? I'll try to illustrate:

For arguments sake, let's assume we have several 'arbitrary' spherical volumes of space within a larger arbitrary spherical volumen the universe aligend such that a cross-section of then align across their diameters, as illustrated below:


[ (---26.42 bly---) (---26.4 bly(---)26.4 bly---) (---26.4 bly---) ]
A B C D


Given a universe with a total diameter (not simply the visible horizons of any given volume) of at least 96bly, it's conceivable that:

1. Volume A would not be in any way gravitionally affected by Volumes B, C, D. The converse holds true for Volume D.

2. Volumes B and C intersect halfway through. This would make half the volume of A gravitationally affected by half the volume of the other (in other words, there's an equal volume representing the intersection of both (we're talking arbitrary boundaries here, so you can create 'arbitrary volumes' anywhere you please)). This would imply the the 'further' areas of either Volumes B or C would remain gravtiationally isolated (you can slice and dice volumes like this throughout space, if you so choose).

So...

1) Given an initial superluminal inflationary epoch, necessarily disparate areas of the universe would be separated BOTH electromagnetically AND gravitationally as a result; the inflation happens too quickly for both EM and Gravitaty to propagate to between areas, resulting the isolation between regions, and...

2) Given that expansion after inflation is subluminal and isotropic throughout the universe after the inflationary epoch,

it would seem that:

a) these regions would remain isolated, as while the named Volumes A, B, C, and D would expand over this time, so would the space between them expand, and...

b) this would have an effect on the overall expansionary evolution of the universe, and play some role in the current observed apparent increasing accelerating expansion.

Now, I am aware that during expansion, EM radiation is "stretched" (resulting in increased redshift and affecting real and apparent distances between observed objects) as a result of it needing to travel an increasing distance between points since it was emitted as a result of inflation increasing that during during it's travel. I have not seen this effect expanded with regards to gravitation. As this probably reflects more a lack of knowledge on my part, I'd like to know if this has been also considered, and if so, how (a link to a paper on the topic would help).

I have not seen this accounted for as a result of inflation (I think I recall some effect similar described for EM radiation, but I don't recall how it was treated, and I think it's still a shaky description, as I recall), and again, I don't recall gravitation being considered in this regard.

Is both EM and gravitation "stretched" during inflation? And even if so, how, after the inflationary epoch, could such widely separated regions remain gravitationally bound, given the distances involved and the limits placed on the propagation of EM and gravitation?

Please, no 'general comments' along the lines of 'you're misunderstanding the theory, and yadda yadda yadda' unless it includes a CLEAR explaination of HOW and WHY i am misunderstanding; I'm aware that it's likely this is the case, but I do not see exactly how. I am just as aware that it is possible at times for people to overlook something simple, or perhaps these ARE still unresolved questions. Please give the scenario some thought, too, before responding, unless you're already aware of and familiar with the answers in more than just a "I understand relativity and it says the universe is isotropic, so I believe that" way. I think the answers are not intuitively obvious...which is the purpose of my question.

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"...wait for the ricochet."

Despite your impression to the contrary, those questions are in fact the same as what I answered, and the answer is yes to all-- because of the cosmological principle and the way general relativity works.
There are some minor caveats owing to the difficulty light had moving around in the early universe when everything was highly ionized, it is basically true that if we can see two regions that have never sent any light back and forth, then those regions have also never had direct gravitational influence on each other. But inflation does not introduce that problem, it solves it-- we apparently cannot see any two such regions, because of inflation. If you want to conceptualize such regions beyond what we can observe, go to town-- but it's angels on the head of a pin. In a nutshell, it is inflation that gives us the cosmological principle, and as I said, that plus general relativity is all we need to understand the observations (providing we are willing to accept dark matter and dark energy, of course!).
No it wouldn't, by the way cause-and-effect works, in relativity, things can only affect what they can reach with light. So the causally disconnected regions you are talking about cannot have any effects at all on each other, and the nature of the expansion that gave rise to that disconnect is all completely included in the equations of cosmology with inflation included. That's if we are including inflation correctly, of course-- we still don't know about that.

That is because you are now talking about gravity waves, not the effects of gravity on spacetime (which as I said, is included). That you have not seen the "redshift" of gravity waves is simply because gravity waves have not yet been detected. If and when they are, we expect them to be redshifted just like light waves, and if that isn't true, it'll be quite a dramatic discovery (gravity waves themselves would be a dramatic discovery!).

You have never "seen" the redshift of EM radiation by inflation either-- we'll never observe it, it happened long before light "broke free" of matter. But the theories do describe how both EM radiation and gravity waves would redshift with inflation. The entire process happened in conditions of thermodynamic equilibrium, and expansion of space preserves that, so we expect inflation to do nothing but change the temperature of everything. Gravity waves might not have been in thermodynamic equilibrium at that point, so there might be some additional information to garner from observing them, but forget about ever "seeing" gravity waves from before inflation! We'd be happy to see gravity waves from a nearby black hole merger.
Gravity waves are stretched. but we would probably not apply that term to gravitation itself (at least not until there is a theory that links gravitation and particle physics)-- gravitation obeys Einstein's equations and reacts to the local density and to the spacetime curvature that fits the appropriate boundary conditions. Inflation is all in there.They don't need to "remain gravitationally bound". For one thing, it does not appear that any of the mass of the universe is "bound", except on smaller scales like gravity clusters that cropped up long after inflation. But even if it did, the "binding" would not work quite like what you are imagining. The way gravity works in GR is that local mass concentrations alter the local spacetime curvature, and the global spacetime curvature is caused by adding up all those local contributions in connection with fitting them to some global boundary condition. There is no literal "action at a distance" in GR-- that is one of its crowning successes (indeed Newton himself was always troubled by the concept of "action at a distance", and I suspect would have vastly preferred GR as a result). Thus "gravitationally bound", when applied to the whole universe, would be the statement that comoving temporal frames include a spatial description that closes on itself like an expanding balloon. Inflation only changes the radius of curvature of such a "balloon", it does not break the balloon. Also, there is no evidence that we can see any such curvature-- space appears to be quite flat. Then again, inflation makes the "balloon" so huge that we might expect it to look flat-- in effect it may have expanded so far that we'll never know what its shape is-- it might have been a bust of Caeser for all we'll ever know.

Either I am not stating the problem clearly enough, you are not reading it carefully enough, or I'm just above moron level. I am not sure which...

You are speaking in generalities, ones which I am already familiar with, Ken. I understand about causality and transfer of information limitations (you misread the contextual use of the word 'cause' in the section you mentioned this). The issue IS that regions gravitationally separated would have no effect on each other, thereby the dynamics cosmic expansion would (or could) change, as I see it.

[also note: the idea of 'action at a distance' is not relative to this discussion, and so far as I can see, nothing I am saying is implying it or dependent upon it (even indirectly). ]

I previously operated under the assumption that ALL matter in the universe was gravitationally bound - hence, I thought that, given sufficient mass the universe would slow, and would eventually contract; insufficient mass, it would be open, expanding forever at a slower and slower rate until eventually it died heat death, with black holes and even eventually protons, etc. evaporating after about 10^100 years or so. The discovery of dark matter would have seemed to seal the fate for a closed universe, however it's apparently behaving in the opposite way, and some form of 'dark energy' is being posulated as being a repulsive force.

As I understood it previously, one of the determining factors in whether or not the universe was 'open' or 'closed' in this respect the total amount of mass in the universe; too little matter, it's open. Too much, it's closed. Earlier observations seemed to indicate that there wasn't enough matter in the universe to close it, but later obervations and more refined calculations showed that there MUST be more matter than we can see.

Given the likely that much of the universe is gravitationally isolated from other parts, this could concievably have the effect on how the universe behaves in this respect, including whether or not the universe behaving as if it were open, if there were otherwise sufficient mass in the universe for it to be considered closed.

Anyway, as the case for dark energy seems to rest almost entirely on the recent observation that cosmic expansion is appears to be accelerating (and tied to other theories from other problems tied to ideas like 'vacuum energy'), I seek other possible reasons for this apparent behavior, to include the possibility of misinterpretation of our observations or some fundamental misunderstanding of other things (even (especially) on my part).

Dark energy, while an ingenious and strong idea for many reasons, also seems to be a possible way of inject unnecessary complexity into the picture. It could also be that a combination of some weaker "dark energy" (such as a very weak force associated with dark matter only) in concert with other, more mundane 'causes' to explain the observations we have. One such mundane cause could be mis-considering the effect of early inflation and the gravitational separation matter. Understand - I am only thinking out loud here, tossing out an idea, and looking for it to be authoritatively shot down, or...authoritively allowed a "Hmm....let's think about that.". If wrong, I would like it explained WHY, not just given a litany of general 'things we understand', with some of it out of context (at least that's how I see the response above, and I believe that's due, in at least part, to me not being able to clearly articulate my question).

It would probably help ME in taking your criticism more seriously if I knew more about your relative knowledge/understanding of the subject: are you a physicist? Or are you, like me, someone very interested in science who follows MANY fields, to varying degrees, and who believes themselves to have a fairly good basic grip of at least most of what's involved, but, being largely self-taught, lacks some of the deeper understanding a physics background affords? That would be helpful to know. and could allow the discussions to continue in a constructive manner, given a better understanding of where each one's coming from and the relative levels of understanding we each likely have, and could wind up even being more enjoyable!
.

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"...wait for the ricochet."

Ken, as an aside, perhaps - if what you said is true in relation to 'gravitational red-shift', there would be indirect means to detect that effect, possibly leading to indirect evidence of gravity waves (or excluding them, if not found)? Or perhaps it could also impact on our interpretations OF EM relation observations...

I can't think of any...this is just an afterthought.

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"...wait for the ricochet."

I think I understand your question. I suspect, however, that not too many people have analyzed the problem. Hasn't the theory that we have a 13+bly visible light horizon only recently gained acceptance? If so, (and if gravity does propigate at c) then if there is a massive galaxy cluster out there beyond the light horizon that we can't see, it makes sense that any gravity from all that mass wouldn't have reached here either (but as I understand it, we're so far away that any gravitational attraction that did reach us would be miniscule, nicht wahr?)

That's more or less it. There's actually 'most' of the universe out there beyond our visible, though, as I understand it, anyway. Now, there are a few considerations at play here, too; for example, galaxies we 'see' as 13.2 bly away from us are actually 'now' much further away. All in all, it's a bit of a mess to sort thought with all the adjustments/corrections that need to be made in make estimates of the size of the universe, but I think basically you hit it in what you said.

It seems an inescapable conclusion that ANY superluminal inflation would have the effect of separating large parts of the universe from others, and I'm just not sure how to deal with it properly. Given what we know and despite what KenG says (one or both of us is oversimplifying or overcomplicating things), it's not so obvious from GR or SR how would really play out, especially with so many poorly understood variables in the mix now that need keeping track of. It's also possible this is an obvious one that's not been (though I tend to doubt it).

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"...wait for the ricochet."

But why do you "see it" that way? I've explained why you should not see it that way, why no theory or observation suggests that this is a valid problem. Nothing you've said is not entirely part of present GR and present cosmology, there is no "change" that needs to be incorporated based on your thinking. It's a complete non-issue-- unless it's wrong. You need to re-acquaint yourself with modern cosmology. Dark matter in no way "seals" the universe's fate in terms of closure-- it still looks flat. Indeed, if dark energy does not exist, the universe is open even with dark matter, and if dark energy does exist, the curvature is flattened but the expansion will all the more never turn around into a contraction.

But not nearly enough to close it. All that positive vs negative curvature stuff is outmoded thinking, I personally don't think it should even be taught any more, it's entirely academic. Current observations indicate that comoving space is either flat or close enough.
I've told you why this is incorrect. Gravity is completely accounted for in the current model, unless we have gravity wrong. You are not adding anything that is not already part and parcel of Big Bang plus inflation, it's all in there, because of, yes, the cosmological principle. You can claim this is not the answer to your question, but it is.
As I said, what you are talking about is already in there, including it still requires dark energy. Also, if there is no dark energy, the universe will just expand even faster. There is zero evidence that anything on that scale is "bound". I'm repeating myself.


Then you will also need a new theory of gravity, because the current one includes all this, so you would require that the current theory is wrong. Thus you need to move to the ATM section.
Guessing the right context is not so easy, so I've tried all that you might have been thinking about. But why are you not hearing: the effect you are talking about is already in there, it's in the equations of gravity with an inflationary term coupled with the cosmological principle that is engendered by that inflation. Now, exactly what is "out of context" there?
I steadfastly refuse to stand on authority, but I am an acting physicist. Still, I would rather think my argument stands on its own merits: What aspect of general relativity do you think is inconsistent with the concept of causality?

Present GR and cosmology, from what I see, assume that all matter in the universe is currently bound, eg. all matter existing is gravitationally attracted by all other matter. If this is a mistaken assumption, please point me to where this is explained otherwise.

This is also the assumption I've been working with until recently myself. If you do not see the question I am asking, you are just not allowing yourself to see it. It IS a valid question. It may BE based on a an incorrect assumption made FOR the purposes of the question ONLY. Please don't get all condescending with me here; try to SEE what the question IS before jumping all over it.

Hmm. I think this is, generally, what I said? Why did you feel a need to restate in a manner that implied I was misunderstanding something? I was stating a generalization that is true: sufficient matter/closed. insufficient, open/flat. Dark energy would serve to 'prevent' closure. Where was I wrong in this VERY simple generalization?

Umm, no, it doesn't deal with the question as I am asking it. but then, that's because I feel you are NOT seeing the real question. I am not saying our understanding of gravity is wrong or it's being dealt wrongly or trying to add anything - please stop implying that. But ok. enough. I really wish someone with a different style of engagement would answer it in a way that explained, not simply asserted it goes against the grain as currently understood by everyone except by people living in the remotest regions of the amazon basin and me.

One last try: given the superluminal inflationary epoch and associated vast relative increase in spatial volume of the universe during a very brief time, WHY would NOT some areas of the universe be gravitationally isolated from others? It's been determined already, I believe, that most regions of the universe are visibly isolated from other regions. If gravity and EM both propagate at v=c, then why would this not be the case for gravity, too? I know your response is because GR doesnt allow for that (or something similarly general). I am seeking an explanatory answer that takes the situation and explains WHY it doesn't evolve that way.

As a side note, it's also clear (and generally so) that should expansion continue to accelerate, the acceleration WOULD eventually (at least appear to) exceed c, resulting in an EM and graviational unbinding of regions. There is NO reason why this should not occur during ANY superluminal inflation/expansion that I can obviously see.

Also keep in mind, I am NOT insisting that the case with regards to the initial inflationary epoch DOES result in what I am suggesting. I just do not see how it could NOT - what I am wondering is if it COULD BE the case, but if not, WHY not...Please don't read more into what I am asking than I am, and please don't make too many assumptions about what I do and don't understand because I am asking this question.

Yes, you are. Me too. (ARGGH!) Do you know what I mean by 'bound' or 'unbound' in this case? It's simple - any objects within their mutual gravitational fields, however severely attenuated, are 'bound' If two objects are located at a distance sufficiently far apart such that their gravitational fields have not propagated sufficient far to affect each other, they are considered unbound. This situation should arise during any superluminal inflationary event of sufficient parametric scope so as to separate objects from each other far enough that their EM radiation or gravitational fields are unable to propagate far enough to affect/reach the (some) others. Parameters that I imagine would affect this would be the rate/amount of inflation in terms of volume/time and the initial separations between any two given objects.

Again wrong. I am not claiming any theory is wrong, and NO new theory is needed to deal with it, so far as I can see. At the very most, I am suggesting the possibility of a misrepresentation of the dynamics of a specific event could be faulty, and I am VERY much open to that misinterpretation being mine - INSOFAR as this question goes. That you seem to think this is what I am claiming demonstrates to me a strong likelihood you do NOT get the basis of my question.


Fine with me. I am not trying to present a new theory, however, nor suggesting anything is wrong with what we have. I see a logical problem that I cannot resolve. I am asking only if this problem is real, but as yet unconsidered, or is not a problem and - specifically - why it's not.


[/quote]Guessing the right context is not so easy, so I've tried all that you might have been thinking about. But why are you not hearing: the effect you are talking about is already in there, it's in the equations of gravity with an inflationary term coupled with the cosmological principle that is engendered by that inflation. Now, exactly what is "out of context" there?[/quote]

I didn't think there was a lot of guesing involved. But anyway - explain it then, please. Don't just state it. Take the model based (or similar to but 'more correct' if needed) on I describe and explain WHY there is not the effect I described w/regards to gravitation, yet there IS the same effect with regards to EM?


Ken, I don't really know how or why you do this - I have NOT EVER stated GR is in ANY WAY inconsistent with causality? You inferred this from a faulty interpretation of a statement I made that was UNRELATED to the issue of causality. Partly my fault in not being precise enough with my usage of the word 'cause', but then I never expected it could be so construed in the context in which it was used.

Further, am not asking you to "stand on authority". I ask so that I can put your responses in better context and perspective based on how authoritative they may or may not be. You have presented no real arguments that I can use here, and have mainly made assertions that are either obvious, not aimed at what I am asking, or both, and done it in a tone that's more than a little bit condescending/patronizing.

Really, realize that I am looking for a simple explaination to a relatively simple question, but I think it's not one based on total ignorace.

Ken, you post a lot - evidently you read a lot of posts. Do you read them and ponder them, or do you skim them and react to trigger words you come across?

Really, I am not trying to be difficult or make a big problem out of nothing here. I have a simple question. I am seeking a consise and simple explanation, not just assertions. If you are a physicist, then perhaps you can explain better. And try to keep in mind I am not trying to make ANY claims here; I am just trying to understand a simple point, and you seem to think I am saying/asking things that I am not saying or asking. Let's keep it respectful and civil (I sense it's going away from those conditions, and I prefer it doesn't). I really hate this mode of debate and hope that it can shift to a better mode - or barring that, perhaps the thread can just be closed or deleted, having served no useful purpose.

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"...wait for the ricochet."

Thothicabob,

Whooa -- let me try to help shed a little heat that's building up here fast.

First you are using "bound" in a way different from Ken, and a non-standard way too, and that's part of the problem. A gravitationally bound (or electrically bound, etc, etc) is a system that doesn't have enough energy to come apart. For example, the earth and planets are bound to the sun. The solar system is a gravitationally bound system. Fire a rocket off the earth and give it escape velocity, and it is no longer bound to the earth. The earth's gravity still affects it, but it can escape. That's the meaning of bound and unbound.

Now, the Big Bang metric, a very general, large scale description of the space-time of the universe, depends on the total mass-energy (and cosmological constant like stuff) contained in it. Whether local pieces of that total mass are gravitationally bound are not is an independent question.

Local systems are just little "dimples" in a larger space-time. The larger space-time is determined by the whole (and the initial conditions) and does what it does, expands.

This is sort of a similar thing as "how does a black hole's own gravity escape if light can't escape". The short answer is, it doesn't have to, because the space-time, gravity, was made as it went down. And that's sort of how to see the question you're asking.

All the mass-energy-lamba made a space-time to begin with, and that space-time is expanding. The effects of all that are already here locally, as they started out in the beginning. What happens to local matter past any horizon doesn't matter.

For example, if we drop something into a black hole, it leaves a little wake of its own gravity behind as it falls. Once it crosses the horizon, in its own proper time, any changes -- say the object splits in two or something -- never make it out. The external observer never sees that gravitional "wiggle". But it doesn't matter. He sees the gravity it left behind.

Another example, the Rindler horizon. If you accelerate at a constant rate of 1g (in your own frame), your notion of space-time has an event horizon about 1 light-year behind you. To an observer watching you accelerate away, that horizon is just a big plane moving at light speed chasing you. Any events that happen after that horizon passes are causally disconnected from you.

So, say you acceleratate away at 1g from the earth (assuming you started out a little faster to escape the gravity well first). Anything that happens on earth after about 1 year's time will not affect you. It doesn't happen. Suppose there was a gravitational change to the earth. Say it blew up. There would be change in space-time that would propagate out at light speed spherically. But you would never see it as it would never catch you.

You would only see the space-time due to the earth (which would be very small at 1 light year away) before it exploded. The change would never get to you. The existing gravity of the earth would be still be acting on you all the way. You'd never see the change unless you stopped accelerating.

-Richard

Okay, thanks Richard!

I get all you said, and it more or less fits with how I have understood things generally, too.

By ' gravitationally bound' I think I had in mind a similar definition; eg. I previously assumed that no matter (or region) in the universe had enough energy to escape the TOTAL system (without exceeding the assumed escape velocity) c, in any case); in essence, all matter in the universe was essentially 'gravitationallly bound' in that sense.

Now, when you speak about what happens to local matter past any horizon not mattering, I understand that also, but...well, in a sense, this is where some questions begin, too.

First, am I right in assuming that such separated/localized regions are a direct result of the inflationary epoch and subsequent expansion? Assume I don't mean fixed 'discrete' regions, but generally, any region defined by an arbitrary location bounded by it's EM/Grav (causal) horizon in relation to another arbitrary region contained within its own horizon not intersecting the other.

I am also okay with such regions being causally unbound (separated) and thus not 'worth considering' in a real sense. It's a bit of a complex picture, but also pretty obvious, and one I can deal with.

The question I think I have is that, when considering the question of closure with regards to cosmic expansion, is it the TOTAL volume of the universe (and associated mass) - to include regions causally distant/separated from us - being considered, or just that region bounded by our own local horizon? (Also, if assuming cosmic isotropy for any given region in the universe, it's of course obvious this same would hold true for any arbitrary region).

One reason I ask is that I'm just trying to properly put in context discussions involving: a) the total amount of mass as determined by observations 'in the universe' vs. b) calculations as they apply to expected amounts generated in the Big Bang vs. c) the total estimated 'volume' of the entire cosmos (including regions outisde our horizon, which evidently constitute a much bigger volume relative to our own region). I am not always sure (at least recently) that this sort of discussion is specific to our region (within our causal horizon) or to that of the entire universe, including those disparate unbound regions outside our horizon.

Side issue: There are also the apparent coincidental ratios of: a) the observed mass in the universe being somewhere around 6 percent of that what was calculated to have been created in the Big Bang and the amount needed/expected for closure (in terms of expansion), and; b) the ratio of the total volume (and mass)of our 'universe' (within our causal horizon) and that of the 'entire' cosmos (including aforementioned "nether regions".

I ask myself:

Is this coincidental? Is this an accurate and real estimate/relationship, or am I comparing apples/oranges somehow? A bit of math here would help, but I've no idea where to really really take it, given various corrections/adjustments needed to account for relativity/expansion/etc. - or even if it's something worth doing to begin with.

Anyway - this is the crux of my questions; from your reply I understand that for any 'real' purposes we can and do ONLY consider what is going on in/related to our own causal horizon; that we can assume that the same holds true for any region outside of our horizon, however - in any case - it doesn't really 'matter' for us what, if anything, is going on outside it.

This I understand, and generally HAS been my understanding until I had cause to look at things a bit differently. Perhaps I'm just taking "what if...?" too far when I ask what I'm asking.

But...what if I'm not?

Anyway, thanks - your response was the sort I'm looking for, and is helpful; my questions are aimed at refining MY understanding and clearing up questions, not trying to redefine the universe as we know it.

Thanks.

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"...wait for the ricochet."

False. This sounds like a Newtonian interpretation-- you are aware of general relativity, yes? General relativity simply does not talk about the gravitational influence of one body on another, that's just an approximate (Newtonian) way to frame the situation that breaks down in GR and in cosmology. See any elementary text on cosmology and general relativity. I do see the question you are asking, and I'm telling you, everything you are talking about is already in general relativity. Don't believe me, find someone else to tell you the same thing.
It's wrong, that's where. Without dark energy, the universe would be open. Dark energy renders it flat. It has nothing at all to do with preventing closure. Your understanding is wrong, would you have me not tell you?Why do you keep hearing that? I never said what you are saying about causally disconnected regions "goes against" any grain-- I said it is already a part of the theory. Do you understand? I cited the cosmological principle, which means that everywhere is doing the same things. That's why you don't need to specifically mention what other regions are doing, the equations of GR applied to cosmology appear to be completely local equations when you have a cosmological principle. It's because invoking the cosmological principle allows you to solve everything just from the equations, the initial conditions, and consideration of what is happening locally, because it's the same everywhere! All nonlocal causal disconnects are hardwired right in there, they are included completely automatically-- causality is built into GR. There's simply nothing to do that relates to what you are asking-- it's already done, automatically. I can't think of a clearer way to say this, you'd better try someone else or just go on wondering.
I already told you, it would! I said it quite a few times, in fact. It would-- and it's irrelevant because the cosmological principle allows you to apply GR entirely locally, where all nonlocal influences are encoded into the cosmological principle instead of direct causal terms. If it's happening everywhere else just like it is here, why do you need to say explicitly if you are causally connected to everywhere else? GR has no action at a distance, it is a local theory combined with a prescription for spatially connecting to boundary conditions, but in cosmology the need for spatial boundary conditions is replaced by the cosmological principle. This answers your question, yes it does-- causal connections are hardwired and require no changes to the theory.
A thousand times no. Where do you get that? What I said over and over is that GR not only allows for it, it already includes it in the Big Bang plus inflation model.
And no one ever said it wouldn't, did they? That has never been in contention in this discussion, anywhere.So you think that the "gravitational field" is "owned" by particular objects? That's a big part of the problem right there, you are indeed using a Newtonian model. In GR, masses don't "own" their own gravitational fields, such that you can talk about "mutual attraction"-- that's why I mentioned action at a distance, you are using action at a distance concepts here. In GR, there is just one gravitational field at each point, really a curvature of spacetime, and it is due to everything that can influence it-- which all comes from the application of Einstein's equations to the chosen boundary conditions. The cosmological principle makes all the nonlocal effects appear local, so that there is only one spacetime curvature everywhere, changing only with age. That's the whole purpose of that principle-- and it does not require the manual introduction of any causality issues, it is just a symmetry that automatically includes them. Your question stems from a misconception about how GR works, which I'm attempting to clear up, but you think you understand everything and I'm just missing what you are asking. I fear you may just not know enough about how GR works to understand why I am answering your question, so that's why I'm telling you this stuff about GR, which you are complaining about being told.
Exactly-- then you have answered your own question without realizing it.

I hope you agree by now that this is just what I have answered. It's not a problem-- not because it's wrong, but because it is already in there. In the action of GR under the cosmological principle, all such causality issues are hardwired in, and inescapably included.
Yes you did, but you apparently didn't realize it. You see, if you understood that GR automatically includes causality issues, cannot possibly ever produce a result that is blind to any and all causal disconnects, then you could not ask the question you are asking-- the answer would have been obvious: it's in there. Perhaps this is all I needed to say from the start, but I couldn't tell where your disconnect was coming from, so I decided to also clear up some of your misconceptions. Sorry if you found that "patronizing", I only care about what is the truth.

I always present arguments. I never say anything on my own authority, though I will at times refer to the authority of a theory. In other words, I will sometimes cite the result of a theory without deriving it from first principles, that is hardly the same thing as not presenting an argument.
I completely understood your question, right from the start. It is you who have still not understood my answer, though it is still the correct answer. There has been an unfortunate disconnect in communication, but when you understand GR in cosmology better, I predict you can go back to my very first answer and understand what I was really saying, but I can understand why it did not come across. But I did give you the entire answer in post #8 when I said: "No it wouldn't, by the way cause-and-effect works, in relativity, things can only affect what they can reach with light. So the causally disconnected regions you are talking about cannot have any effects at all on each other, and the nature of the expansion that gave rise to that disconnect is all completely included in the equations of cosmology with inflation included. " How could it not be, do you doubt that GR automatically encodes causality issues? Is that the part I needed to explain? Relativity invokes all that as a fundamental postulate, I thought that was obvious.
I understand, I do. And even if there can be testiness in an exchange that involves a lot of potential for miscommunication, ask yourself this: why would I be willing to spend so much time on this? The answer is, for two reasons-- one, it is an interesting issue to think about, and two, I believe that you will come to understand why I am telling you the answer, I really am. And I am hardly "asserting" them-- I'm explaining the backdrop of GR, cosmology, and the cosmological principle. It just wasn't obvious at first how much I would need to say by way of explanation.

Enough Ken. I'm not here to be mocked; I'm just looking to ask some questions and discuss things. I am not suggesting anything radical is wrong with current understanding - I'm looking for some clarification on a specific issue. Thanks anyway.

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"...wait for the ricochet."

And that is precisely what I gave you, with no mocking. But you didn't like the answer, so I had to tell you what you are wrong about-- that is "mocking"? If you perceive it that way, I apologize-- I assumed you wanted to know the truth.

Thoth,

I don't really understand things well enough to comment much further. If you're interested the best thing would be to read up on the basic FLRW metric and the Friedmann equations (what I call the Big Bang metric) which.

What matters is the *density* (and always at a given point in time compared to certain critical values that come from the equations). Add a cosmological constant, which can be interpreted as "negative pressure" type of something -- ie "dark energy", and the density of that comes in as well. The dark stuff could be something that acts a bit different, but that all gets involved.

Density is different from the total energy of the stuff. By the cosmological principle, we assume/assert the universe is the same everywhere, so the density we can see is that of the whole thing.

So you can have any different "total volume and mass" for a given density, and it acts the same.

I suppose the total universe could be much bigger than we can see by far, but the evolution of what we see is determined by only what is causally connected here.

-Richard

Thanks. I'm doing some checking based on things you and ken noted.

I am sort of curious about one aspect - some (roughly half) of what 'we' are causally connected to is likely also causally related to things we're not; I'm not sure how that affects the 'Big Picture' - I understand, so far as we're concerned it likely doesn't in any real terms, but still, something interesting to consider.

This is perhaps a part of what I was thinking of before. Understand, that I realize since information travels only a v=c, even if something within in our causal vicinity IS affected by something outside it, it's far enough away from us than any such effect would not be able to have any effect on us nor could we observe such an effect on it, in reality or in principle - if I understand it right.

Still, what I am curious about is how this would affect the larger system (entire cosmos) overall if, in anyway, it could. I know what the standard answer must be, but it IS a bit hard hard to accept intuitively, even if 'logic' dictates otherwise. Even if the hipbone is not connected to the ankle bone, they're both connected to the leg bone.

Yeah, yeah...bad analogy and one where GR has no card to play...but still, irrestible, nonetheless.

Anyway, I'll ponder this one on my own time - I won't bother people with such pointless musing. Let's say 'knowing' the answer isn't always the same as 'understanding' or 'feeling' it, and this IS the sort of problem that - if you devote much more bandwidth to it than you should, can be the cause of much gnashing of teeth and rending of robes...

Thanks for the help!

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"...wait for the ricochet."

I think I see the problem now, you are asking under what circumstances would causality disconnects alter the picture of how the universe should evolve under GR, and I'm saying that if one invokes the cosmological principle (that the universe is the same everywhere so changes with age but not with location) then all causality disconnects (due to inflation or anything else) are automatically included. So the answer to your question is-- you'd have to relax the cosmological principle in order for causality disconnects to alter the Big Bang model with inflation. If you relax that assumption, you could perhaps get all kinds of things, maybe an accelerating universe, so perhaps you could alleviate the need for dark energy. But the cost would be very high indeed-- that principle is supported by all observations, and it vastly simplifies our understanding, so I think most people would rather keep it and allow for dark energy than get rid of it. That doesn't make it true, but it does make it good science, at least until there is a better reason to let it go. In other words, the "solution" to dark energy that most people would like to see is either a cosmological constant or something equally simple, not something a lot more complex like relaxing the cosmological principle. I think that is the best answer to your question, perhaps I should have just said it that way from the start!

Thanks Ken...it certainly helps, a lot, anyway! Between you and Richard, I think some of the issues I was wrestling with are much clearer. Thanks!

The remaining obstacle I have is dealing with causality (or rather causal isolation) when considering that there's a continuum involved; even with realizing there are "causal regions" separated from each other, parts of such separated regions CAN be causally connected. While I do understand that due to GR this doesn't matter for any two given locations (as opposed to regions) - information simply cannot propagate from one point to another outside it's causal (EM/graviational) horizon - I do still have some issues when considering this in a global context, eg. the entire extent of the cosmos; there's a continuum of causally (partially) intersecting regions, even if some of them themselves are completely disconnected.

I'm pretty sure I know what the answer "must be" - the issue is likely my own grokking of it. Perhaps the problem comes, in part, from thinking in terms of regions instead of points/locations; due to relativity, each region itself is a continuum as well, with no instanteous communication within it, either.

I know, it's naiive, or at least apparrently so; and everyone knows that turtles just can't breath in a vacuum.

Thanks for the help, in any case!

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"...wait for the ricochet."

Yes, I think that's the problem with the region concept-- events occur at points, and so points can affect entire regions, but regions don't affect regions: any effect due to a region would really be a sum of effects due to all its constituent points, so some might contribute and some might not in any given situation. Put differently, a "region" is an arbitrary designation-- reality does not recognize what we choose to lump together that way.

Well, I agree, but with qualifications. Causal regions are not necessary 'arbitrarily' defined except in terms of selecting the center of a causal region; the volume it ecompasses is defined specifically as that region within which events at some other point can affect the central point, largely. The issue of continuity (in terms of a 'causal contiuum') arises when looking at the picture globally.

Now, I'm not trying to be stubborn here and insisting on some untenable point. I'm simply having an issue dealing with a "super-region" that constists of a continuum of such locations with limited but intersecting causal horizons and trying to properly understand how such a super-region would evolve over time, given a set of initial conditions and some basic assumptions.

We assume, for example, the immediately prior to the inflationary epoch that all matter was homogenous and isotropic, and that inflation would not change this. I'm good so far.

Can we also assume that, prior to inflation, that all matter was also causally connected, and it is inflation that CHANGED this? I think one can argue we MUST make this assumption, however one can also argue that DUE TO the homogeneity and isotropy of the initial conditions, there's no need for such an assumption to be true. One could also, perhaps with some risk, take the position that in those those conditions, causality has no meaning/relevance. Of one accepts this stance, then does the implication necessarily follow that it was inflation that "brought" causality into existence in the first place (hmm. did I just imply that something "caused causality"? ).

If we take the former argument as our assumption (causality is meaningless with regard to initial conditions), then we conclude that inflation little or no impact on the GLOBAL h/i characteristics of the universe (with local variation being due to other characteristics of matter that we need not get into now), but did induce a causal contiuum, which results in a separation between regions (defined as above).

One could easily let go at this point, accepting that regardless of the existance or lack of causality between regions, the entire system would evolve homogenously; all share the same initial conditions and experienced the same (or similar) histories, and there's no reason to believe that any region, however causally distant, should experience anything 'different' than any other. Assuming my reasoning is correct to this point, I'm still fine with this.

Hmm. I think I'll stop here - I have some more thoughts on this theme (and springing from it) but I find I need to think some more, and would like to still post this bit for comment in the meantime...

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"...wait for the ricochet."

Right, but that's a relationship between a region and a point, not a region and a region. Causality has to be referenced to an event (which is either the cause or the effect), but one can piece together "regions" of events using any prescription you like. But if you want to connect regions to regions by a shared causality, then you are doing an intersection of the causal relationships of the events therein, not a union. Perhaps that is the key point you are wondering about. If B is in the set of all things affected by A, and B is also in the set of all things affected by C, it says nothing about the relationship between A and C but it says a lot about what is happening to B.

I think this is the key issue-- the super-region does not evolve per se, what evolves is all those little points within it, but the points "B" were affected by A and C, but A and C did not affect each other even though according to B they are in the same super-region. If you don't have a cosmological principle, you have to query every point individually to see what is affecting it and what it will do, but if you do have a cosmological principle you just look at B and say you're done-- the rest will do the same things.
We would only expect all matter to be causally connected if the universe had an infinite age, unless we introduce some new physics along the lines of "causality of initial conditions". At present, there is no physics of initial conditions at all, they are entirely ad hoc additions that are outside physics at all levels. Inflation is part of the theory that the universe had a finite age and with "normal" causality, indeed the age was very short, so we would not expect all matter to be causally connected at that time. In this context, what I mean by "causally connected" is "could have been affected by that material at any time in the history of the universe up until then". Thus inflation does not change whether or not we were ever causally connected to something, that is an unchangeable fact, it only changes if we are causally connected to it after inflation. So inflation does two things, one is to make us not currently causally connected to anything that happened to a huge bunch of stuff after the epoch of inflation, and the other is to make us causally connected to the stuff that we would otherwise be just seeing now "for the first time" (and this second piece is its raison d'etre). It means that (very) early astronomers could have seen that stuff, then inflation happened and they couldn't see it any more, but just now we are beginning to see it again. That's why it appears to be causally connected to us (at the same temperature, basically)-- because it was at some time in the past, before inflation. There should have been other stuff that was never causally connected to us, but we're a very long way from ever seeing it, because of inflation.

Yes, if we are willing to assert h/i conditions in the initial condition, we don't need inflation for that. But some people aren't happy with h/i initial conditions, it seems like "too much to ask", and they like the way inflation imposes h/i conditions now-- because we are not seeing material "for the first time", we are seeing our long-lost brothers and sisters in deep space that were causally connected to us long ago, and that's why they exhibit h/i conditions. It's a way to save causality as the "cause" of the h/i conditions, rather than ask causality to be an emergent property of the universe later on down the line, coupled with initial conditions that somehow simulated it or had different rules about it. Which is "true" I have no idea, it's very much the metaphysics of what good science should do.