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Nereid
2010-Jun-20, 12:13 AM
Rather than take an existing Q&A thread OT, I've started this one.

If by observable universe we mean everything which, in principle, we could detect a signal from - photons, neutrinos, gravitational wave radiation, ... - is there anything in the observable universe which is causally disconnected (from our point of view)? I think not, by definition (caveats apply, wrt 'time').

Are there parts of the universe which are not within the observable universe but which nevertheless are causally connected (with us)?

I'm interested in this both from a general perspective - a universe in which GR rules, but which does not have to be DE/CDM/radiation/other dominated - and the universe we seem to live in (flat, DE dominated, etc).

Jeff Root
2010-Jun-20, 12:26 AM
I'm copying this from the other topic where I just now posted it:

Whether widely-separated parts of the Universe are causally connected
or not is something I started to argue with Ken and DrRocket about, but
I never posted the last posts I wrote. It had been my understanding
that the mainstream view was that different parts of the Universe were
causally disconnected at the very beginning, and that that disconnection
was the primary reason for hypothesizing inflation (in order to explain
the observed uniformity of the CMBR). On that basis, I tried to argue
for causal connection from the beginning. However, DrRocket then told
me that the mainstream view is that everything was causally connected
at the start of the Big Bang. If that is true (I have not yet confirmed it),
I do not understand what the original motivation was for Inflation.

If the expansion is accelerating, distant parts of the Universe should be
causally disconnecting from us. Everything we can see was, of course,
once causally connected with us, but it isn't necessarily connected with
us any more. Some things that we can see can never be affected by us,
now or in the future. No beam of light or spaceship from here, leaving
now, could ever reach such distant places. But they are now seeing
light which left the Milky Way billions of years ago, just as we are seeing
them as they were billions of years ago.

-- Jeff, in Minneapolis

Jeff Root
2010-Jun-20, 12:42 AM
Part of my argument with Ken and DrRocket was over the meaning of
"causally connected". I argued that everything which was involved in
the Big Bang, and is now participating in the cosmic expansion, had to
be causally connected, whether it is now observable by us or not.
They apparently took me to be asserting that we are *now* causally
connected, so that what we do now will affect distant parts of the
Universe, and what happens in those distant places now will affect us.
That of course is not what I meant. I meant only that the matter and
energy which would eventually become us and stuff in distant parts
were causally connected at the very beginning, in the Big Bang.

So there is a terrible ambiguity, and I don't know whether there is a
standard way of resolving it. If there *is*, no doubt Ken and DrRocket
were using the standard way of using the term, and I was not. But
what I intended is such a useful idea that it should have its own term.
Lacking that, we need to specify the direction and/or the time of any
assertion of causal connection. For example: What they did then is
affecting us now, but what we do now can never affect them.

-- Jeff, in Minneapolis

Ken G
2010-Jun-20, 01:39 AM
There's no doubt that the first step is a meaningful definition of "causally connected", starting with what the connection is between. For example, is it a connection between two events, such that one event can affect another event, or is it a connection between two world lines, such that some event on one world line is causally connected to some event on another world line? And are we referring strictly to causal connections with ourselves, at this moment that we are observing, or are we allowed to stipulate causal connections between two world lines on which we are now observing very distant events? Certainly, everything we observe, as an event, is causally connected to ourselves making the observation, as an event, because the event is having an effect on us (perhaps we are saying "wow" or some such thing). But when people talk about "the need for inflation", they usually mean that we are seeing two events on different world lines, and each event is not causally connected to any events on the other world line, without inflation.

So the OP question does not appear to be referring to the need for inflation, it seems to be asking whether or not it is the case that we are causally connected by everything that we could (in principle) observe. We have that we are causally connected in practice to everything we observe in practice, so the question seems to be are we causally connected in principle to everything we can observe in principle. That would also seem to follow, though it's less clear what is meant by a connection "in principle." Also, in regard to the second question, anything that could be causally connected to us would seem (by definition) to be in the "observable" (in principle) universe. However, if we make certain assumptions that extend our observations, such as that the laws of physics apply to regions outside our observable universe, then there are regions that are causally connected to regions that are causally connected to us, and we can make inferences about those regions. Call that extended universe the "inferrable universe" if you will, in contrast with the "observable universe."

speedfreek
2010-Jun-20, 10:29 AM
The way I understand it, before inflation was hypothesized we had a "Horizon Problem". We could not explain how regions on the opposite sides of our observable universe could be in such similar states, as, if they had to have been separating faster than light for the whole history of the universe, they would had been causally disconnected all along, and the conditions in these regions would never have had time to find any equilibrium. The universe was expanding too fast for everything to have mixed together properly!

With inflation, the universe is given time for everything to mix together properly before the superluminal expansion occurs.

So, regions on opposite sides of our observable universe have been causally disconnected since the beginning of the inflationary epoch, but not since the beginning of the whole enchilada.

speedfreek
2010-Jun-20, 01:06 PM
Are there parts of the universe which are not within the observable universe but which nevertheless are causally connected (with us)?

The "edge" of the observable universe is usually defined as the particle horizon or surface of last scattering. The "inferable" universe, as Ken put it, extends beyond the particle horizon if we assume the laws of physics apply there.

Inflationary theory posits that a large proportion of (what was, or would have been) our observable universe was "inflated" beyond our cosmological horizon, such that the universe increased in size by many magnitudes in a fraction of a second. So this seems to suggest that the inflationary volume (that comprises of what was our observable universe before inflation) was causally connected before inflation.

But the real question is, was that inflationary volume, which took what was our observable universe and inflated it, the whole thing or not? How large was the whole pre-inflationary universe? Atom sized, larger, or perhaps infinite?

Jeff Root
2010-Jun-20, 03:31 PM
speedfreek,

What you described was my understanding of the mainstream description.
The cosmic expansion supposedly prevented different parts of the new
Universe from having any contact with other parts, so Inflation was added.
My problem with that is very obviously that everything *was* in contact
with everything else at the beginning. It had to be, or it wouldn't all be
doing the same thing. So Inflation isn't needed. Instead what is needed
is a description of how the Universe began, which we don't really have
much of a clue about.

I like the term "inferrable Universe, although there would be a range
of degrees of inferrability. Some things might be inferred with high
confidence, others only with low confidence. Some things might be
inferrable out to great distances, others not so far.

-- Jeff, in Minneapolis

Nereid
2010-Jun-20, 05:26 PM
I implicitly excluded any inflationary period; let me explicitly exclude it, from the scope of this thread.

I'll say a few words about world lines in a bit; for now, consider this scenario: a nice, compact relatively cool cloud, close to our horizon. From that cloud's point of view, a really, really bright GRB happens, a very long way away, way beyond our event horizon, with the jet lined up directly at the cloud*. The cloud lights up. We observe the fireworks (assume we have visual/NIR waveband telescopes whose effective diameters are, say, 100,000 km).

Is the GRB in our observable universe? If not, is there a causal connection between the GRB event and the detection of the cloud's 'lighting up', here near Earth? By assuming the 'laws of physics' are the same, as here, in the universe beyond our event horizon, can we infer the existence of events beyond it (as in my GRB example)?

* or the jet from a pair of colliding SMBH, for more firepower

Ken G
2010-Jun-20, 05:47 PM
From that cloud's point of view, a really, really bright GRB happens, a very long way away, way beyond our event horizon, with the jet lined up directly at the cloud*. The cloud lights up. We observe the fireworks (assume we have visual/NIR waveband telescopes whose effective diameters are, say, 100,000 km).

Is the GRB in our observable universe?Yes, because if a signal could get from the GRB to the cloud, and then to us, then it must also be true that a signal could get from the GRB directly to us-- at least in the usual topology of cosmology. In cases where a "scattered" signal could get to us, but no direct path is possible, then it's still causal connection, albeit a somewhat more subtle form.

speedfreek
2010-Jun-20, 06:56 PM
for now, consider this scenario: a nice, compact relatively cool cloud, close to our horizon. From that cloud's point of view, a really, really bright GRB happens, a very long way away, way beyond our event horizon, with the jet lined up directly at the cloud*. The cloud lights up. We observe the fireworks (assume we have visual/NIR waveband telescopes whose effective diameters are, say, 100,000 km).
But, by definition, if the GRB happened way beyond our event horizon, then we cannot observe the event. Nor can we see the light from that event by proxy - if there is time for the light from that event to light up a cloud and for the light from that illuminated cloud to reach us, then the GRB was within our event horizon!

Nereid
2010-Jun-20, 07:54 PM
Now would be a good time to introduce the question - sub-question to the OP really - of the difference between world lines and light cones. Doing this will, I hope, help to provide a framework to discuss the difference between event horizons and causal connections.

What are world lines?

What are light cones?

How are these related to our (cosmological) event horizon?

Is it possible - in standard GR-based cosmological models - for an event to be on our world line, but outside our light cone?

Is it possible - in standard GR-based cosmological models - for an event to be within our light cone, but not on our world line?

Ken G
2010-Jun-20, 10:34 PM
I can't give you authoritative answers in general relativity, but my understanding is:


What are world lines?
A series of events experienced by an observer, or a clock. Either can be hypothetical, but the motion must be physically possible.


What are light cones?
The set of all events that a given event can affect (future light cone) or be affected by (past light cone), even in principle.


How are these related to our (cosmological) event horizon?
I would say that any event that could not get a signal to us (here and now) lies outside our cosmological event horizon. So it is a limit to our past light cone that exists expressly because the universe has a finite age.

Is it possible - in standard GR-based cosmological models - for an event to be on our world line, but outside our light cone?There really is no such thing as "our world line" unless you mean our own lifetime, but we could talk about the world line of the comoving material that defines our local comoving frame. There might be a way to trace a kind of "line" through the center of that comoving material, and call it "our world line", that seems OK to me though I wouldn't try to make it rigorous (recall that prior thread). Anyway, if such a concept works, I would say that the past on our world line lies entirely within our past light cone, and the future on our world line lies entirely in our future light cone.


Is it possible - in standard GR-based cosmological models - for an event to be within our light cone, but not on our world line?
Certainly, that's even possible in special relativity. Any event that affects us now but that we did not locally experience, and any event that we will affect but will not locally experience, provides such an example.

speedfreek
2010-Jun-20, 10:35 PM
What are world lines?
World lines are the paths that objects take through spacetime.


What are light cones?
A light cone can show the region of spacetime that a flash of light emitted at a certain event will be able to reach (future light cone of that event), or it can show the region of spacetime from which emitted light can reach a certain event (past light cone for that event).


How are these related to our (cosmological) event horizon?
Our cosmological event horizon represents the extent of our future light cone. The light from events that happen now on Earth can approach our event horizon but never quite reach it, and conversely the light from events happening "now" on our event horizon can never quite reach us.

But the world lines of objects on our event horizon were all inside our light cone(s) at some time in the past, meaning that we have seen, or will see, events from earlier on the worldlines of those objects.


Is it possible - in standard GR-based cosmological models - for an event to be on our world line, but outside our light cone?
Yes, all past events on our worldline are outside of our future light cone. And all future events on our worldline are outside our past light cone! I'm not sure that is what you meant though!


Is it possible - in standard GR-based cosmological models - for an event to be within our light cone, but not on our world line?
Yes, this applies to all the events in the universe we will see in the future that don't happen right here! But again I think I must be missing your point.

Nereid
2010-Jun-21, 12:38 PM
I implicitly excluded any inflationary period; let me explicitly exclude it, from the scope of this thread.

I'll say a few words about world lines in a bit; for now, consider this scenario: a nice, compact relatively cool cloud, close to our horizon. From that cloud's point of view, a really, really bright GRB happens, a very long way away, way beyond our event horizon, with the jet lined up directly at the cloud*. The cloud lights up. We observe the fireworks (assume we have visual/NIR waveband telescopes whose effective diameters are, say, 100,000 km).

Is the GRB in our observable universe? If not, is there a causal connection between the GRB event and the detection of the cloud's 'lighting up', here near Earth? By assuming the 'laws of physics' are the same, as here, in the universe beyond our event horizon, can we infer the existence of events beyond it (as in my GRB example)?

* or the jet from a pair of colliding SMBH, for more firepower
I want to explore this a bit more.

Suppose instead of a distant, compact, cloud there was a super-duper neutrino detector, which broadcasts its measurements in our direction. Per Ken G and speedfreak, the colliding SMBH - whose jet contains vast numbers of ultra-high energy neutrinos - would be 'closer' than our event horizon, because if the we can see the neutrino detector, we can also see the neutrinos.

Right?

Suppose, instead of a super-duper neutrino detector, there is a super-duper gravitational wave radiation (GWR) detector (not a gravity wave detector, that would be a very different thing!). Again, the colliding/merging SMBH would emit vast amounts of GWR, which we could also detect (in principle); we don't need the intermediary detector.

Right?

So is it possible to show, using various GWR thought experiments, that mass-energy beyond our event horizon cannot have any detectable effect - in principle - on anything that is observable (to us)?

Nereid
2010-Jun-21, 12:45 PM
Re light cones: I am interested only in past light cones, for now - we can see only our past.

World lines: the paths through space-time (or spacetime) of comoving (or co-moving) observers, OK?

And in the standard GR homogeneous and isotropic universe, which was once radiation dominated, co-moving means at rest wrt the CMB - yes? no? something else??

Somewhat OT question: is it possible to be at rest wrt the CMB, but not at rest wrt the cosmic neutrino background? If so, how?

Ken G
2010-Jun-21, 03:09 PM
So is it possible to show, using various GWR thought experiments, that mass-energy beyond our event horizon cannot have any detectable effect - in principle - on anything that is observable (to us)?What you said up until this is correct, but here we get into the difference between an event and a bunch of mass-energy. A coherent blob of mass-energy (like a galaxy, or SMBH, or some such thing) is not a single event, it has its own world line. So we can say that to know anything about that coherent blob of mass-energy, some event on its world line must be within our past light cone, but the entire world line of the object need not be in our past light cone. So you could have some mass-energy that is an identifiable object, that spends most of its time within our "inferable universe", but only a part of its time within our past light cone. This is true, for example, with many of the quasars we now see. It is also how we use fluctuations in the CMB to extrapolate to the "smoothness" of the inferable universe, far beyond what we can observe at age 13.7 billion LY.

Ken G
2010-Jun-21, 03:11 PM
World lines: the paths through space-time (or spacetime) of comoving (or co-moving) observers, OK?
OK

And in the standard GR homogeneous and isotropic universe, which was once radiation dominated, co-moving means at rest wrt the CMB - yes? no? something else??
Sounds fine to me.

Somewhat OT question: is it possible to be at rest wrt the CMB, but not at rest wrt the cosmic neutrino background? If so, how?Anything's possible, but if we observed that to be the case, we'd have to dump the cosmological principle, because the difference in frames would have a velocity associated with it, and hence a preferred direction.

George
2010-Jun-21, 03:38 PM
And in the standard GR homogeneous and isotropic universe, which was once radiation dominated, co-moving means at rest wrt the CMB - yes? no? something else?? If you want to be a bit picky about this, there is the CMBR dipole, which simply reveals a slight Doppler shift of the CMB due to our motion within co-moving space (ie Hubble Flow).

Also, there is that curious slight anisotropy, which is the only possible explanation as to why Ken and others are smarter than I am. :)

Nereid
2010-Jun-21, 03:59 PM
So is it possible to show, using various GWR thought experiments, that mass-energy beyond our event horizon cannot have any detectable effect - in principle - on anything that is observable (to us)?What you said up until this is correct, but here we get into the difference between an event and a bunch of mass-energy. A coherent blob of mass-energy (like a galaxy, or SMBH, or some such thing) is not a single event, it has its own world line. So we can say that to know anything about that coherent blob of mass-energy, some event on its world line must be within our past light cone, but the entire world line of the object need not be in our past light cone. So you could have some mass-energy that is an identifiable object, that spends most of its time within our "inferable universe", but only a part of its time within our past light cone. This is true, for example, with many of the quasars we now see. It is also how we use fluctuations in the CMB to extrapolate to the "smoothness" of the inferable universe, far beyond what we can observe at age 13.7 billion LY.
Here's where it starts to get interesting, I think.

Let's take some of these quasars, during the Era of Reionisation (EoR)*. Collectively, they ionised the hydrogen in the then intergalactic medium (IGM). Are there any which did that that are no longer in our (past) light cone? Is any part of the IGM, ionised by some of these quasars, within our light cone (i.e. observable)? From what was said earlier, I got the impression that if we can observe a patch of ionised IGM, waaaay back when, then we can still see all the quasars which did the ionising (well, in principle).

Similarly with blobs of mass-energy.

I can create a thought experiment whereby an intense burst of GWR is emitted by (part of) any galaxy- or cluster-sized blob, and by my distant GWR detector (see previous post) detects this, a looong way from where it was emitted, and a looong way from here.

It follows, then, that all the blogs of mass-energy must themselves be observable, and within our light cone.

* for readers who don't know this is: at the surface of last scattering - when the universe became transparent to the photons that we now see as the CMB - the universe was composed of neutral H and He (more or less). Some time later, the universe became transparent to UV; the hydrogen (and He?) was (mostly) ionised and has existed ever since as a (mostly) fully ionised plasma. When this occurred is called the EoR, and is thought to have happened over a quite extended period of time.

Nereid
2010-Jun-21, 04:02 PM
If you want to be a bit picky about this, there is the CMBR dipole, which simply reveals a slight Doppler shift of the CMB due to our motion within co-moving space (ie Hubble Flow).
Yes, I want to explicitly remove this dipole, by considering observers who are at rest wrt the CMB (i.e. they see no dipole).



Also, there is that curious slight anisotropy, which is the only possible explanation as to why Ken and others are smarter than I am. :)
There is? What is it?

Grey
2010-Jun-21, 06:51 PM
From what was said earlier, I got the impression that if we can observe a patch of ionised IGM, waaaay back when, then we can still see all the quasars which did the ionising (well, in principle).

...

It follows, then, that all the blogs of mass-energy must themselves be observable, and within our light cone.Close, but not quite. From the fact that we can observe things affected by some quasar or other object, we can deduce that we must at some point in the past have been able to observe that object directly, at least in principle. However, it does not follow that it is still observable. As Jeff notes, given our current models for the expansion of the universe, very distant things which were once able to affect us may have moved beyond the horizon. So there can, in principle, be a "blob of mass-energy" which is beyond the observable horizon, but whose existence we might be able to infer from some effect it had previously, when it was still within that horizon.

Ken G
2010-Jun-21, 07:34 PM
It follows, then, that all the blogs of mass-energy must themselves be observable, and within our light cone.
Yes, any blob of mass-energy that ever had any influence on anything we can observe is something that we could in principle observe now, but in practice we generally only get the opportunity to see a single event on the world line of any such blob. So we can create a combination of direct observations, and indirect inferences relying heavily on the cosmological principle, to reconstruct that world line. When there is acceleration in the expansion, that reconstruction can apply to world lines that leave our past light cone, and when there is deceleration, it can apply to world lines that enter out light cone from outside it. But I think the point you are making is that there is no such thing as a world line of a blob of mass-energy that is always outside our past light cone that can have any effect on anything that is within our past light cone, and that would seem to be true. However, the use of the cosmological principle extends the theory to beyond anything we can check by observation or even inference-- at that point, it becomes a purely philosophical stance.

George
2010-Jun-21, 07:57 PM
There is? What is it? The CMBR anisotropy. :)

It seems to me that the acceleration of the expansion rate is key to the demise (observational loss) of outer outerspace (O2S). [This would apply to light and the GWR.]

Nereid
2010-Jun-21, 09:33 PM
From what was said earlier, I got the impression that if we can observe a patch of ionised IGM, waaaay back when, then we can still see all the quasars which did the ionising (well, in principle).

...

It follows, then, that all the blogs of mass-energy must themselves be observable, and within our light cone.Close, but not quite. From the fact that we can observe things affected by some quasar or other object, we can deduce that we must at some point in the past have been able to observe that object directly, at least in principle. However, it does not follow that it is still observable. As Jeff notes, given our current models for the expansion of the universe, very distant things which were once able to affect us may have moved beyond the horizon. So there can, in principle, be a "blob of mass-energy" which is beyond the observable horizon, but whose existence we might be able to infer from some effect it had previously, when it was still within that horizon.
In post #8, I briefly described a situation in which something like this happens (poor choice of word, perhaps).

In their responses, both Ken G and speedfreak said that if we can observe the effect, on something 'closer', then we can observe the cause ... which seems to contradict what you just wrote (that I'm quoting).

One of the things I'm trying to explore, in this thread, is just what the relationship between 'causally connected' and 'observable' is. And I'm doing this through quite specific, narrowly defined thought experiments.

Nereid
2010-Jun-21, 09:37 PM
It follows, then, that all the blogs of mass-energy must themselves be observable, and within our light cone.Yes, any blob of mass-energy that ever had any influence on anything we can observe is something that we could in principle observe now, but in practice we generally only get the opportunity to see a single event on the world line of any such blob. So we can create a combination of direct observations, and indirect inferences relying heavily on the cosmological principle, to reconstruct that world line. When there is acceleration in the expansion, that reconstruction can apply to world lines that leave our past light cone, and when there is deceleration, it can apply to world lines that enter out light cone from outside it. But I think the point you are making is that there is no such thing as a world line of a blob of mass-energy that is always outside our past light cone that can have any effect on anything that is within our past light cone, and that would seem to be true. However, the use of the cosmological principle extends the theory to beyond anything we can check by observation or even inference-- at that point, it becomes a purely philosophical stance.
As I just said, in responding to Grey's post, I'm starting small, with quite detailed, narrow thought experiments.

And as I noted, there is disagreement ... perhaps my GRB, or colliding/merging SMBHs, are no longer in our (past) light cone? Perhaps they are no longer observable (in principle, of course)?

Ken G
2010-Jun-21, 10:50 PM
And as I noted, there is disagreement ... perhaps my GRB, or colliding/merging SMBHs, are no longer in our (past) light cone? Perhaps they are no longer observable (in principle, of course)?Actually, I think Grey is answering a different question. Any event that can affect anything we can observe is in our past light cone. The world line that connects to such an event is another story, and what it can do is harder and I don't think we've quite figured that out yet in this thread, we have some more work to do if we want to understand that part.

forrest noble
2010-Jun-22, 12:26 AM
Nereid,


If by observable universe we mean everything which, in principle, we could detect a signal from - photons, neutrinos, gravitational wave radiation, ... - is there anything in the observable universe which is causally disconnected (from our point of view)? I think not, by definition (caveats apply, wrt 'time').


From my interpretation of the standard cosmology model and nearly all other models which I think are worthy of consideration, everything at least causally is related, whether by its initial cause or by a relationship of causes. Maybe some of the multi-verse ideas may claim unrelated causes but I haven't heard of any. Infinite-universe models in time however are generally non-causal.


Are there parts of the universe which are not within the observable universe but which nevertheless are causally connected (with us)?


The BB model and all other causal models that I am aware of assert a commonality of cause.


I'm interested in this both from a general perspective - a universe in which GR rules, but which does not have to be DE/CDM/radiation/other dominated - and the universe we seem to live in (flat, DE dominated, etc).

Although many theorists have strongly adopted the DM and DE ideas many still have not. Many still adhere to the cosmological constant concerning expansion and are not willing to give it up as yet. Many simply believe the DM, DE, the dark flow, etc. are just the current place-holders until something better comes along.

According to all versions of the BB model that I am aware of, however, there is a causal connection between all entities within the universe.

forrest noble
2010-Jun-22, 01:47 AM
Nereid,


So is it possible to show, using various GWR thought experiments, that mass-energy beyond our event horizon cannot have any detectable effect - in principle - on anything that is observable (to us)?

Although mass-energy could presently be beyond our observable horizon it could still be within the observable and gravitational horizons of distant galaxies. They might just be a hop, skip, and jump away from the most distant of these galaxies or quasars, lets say a few hundred million light years away. Such entities could gravitationally or otherwise effect or be affected by such entities within their own horizons. Such a present-day theoretical entity is the mass (galaxy clusters) that is supposedly causing "the dark flow."

Nereid
2010-Jun-22, 12:00 PM
Actually, I think Grey is answering a different question. Any event that can affect anything we can observe is in our past light cone. The world line that connects to such an event is another story, and what it can do is harder and I don't think we've quite figured that out yet in this thread, we have some more work to do if we want to understand that part.
Yes, it seems so.

Once we have light cones and their associated causal connections nailed down it'll be time to move onto world lines.

Nereid
2010-Jun-22, 12:04 PM
Nereid,

If by observable universe we mean everything which, in principle, we could detect a signal from - photons, neutrinos, gravitational wave radiation, ... - is there anything in the observable universe which is causally disconnected (from our point of view)? I think not, by definition (caveats apply, wrt 'time').

From my interpretation of the standard cosmology model and nearly all other models which I think are worthy of consideration, everything at least causally is related, whether by its initial cause or by a relationship of causes. Maybe some of the multi-verse ideas may claim unrelated causes but I haven't heard of any. Infinite-universe models in time however are generally non-causal.
I think I said that, in this thread, I'm limiting the scope to just universes where GR rules, and excluding inflation. In fact I did ... and you even quoted me!

For simplicity, I think isotropic and homogeneous universes are what we need too, at least to start.



Are there parts of the universe which are not within the observable universe but which nevertheless are causally connected (with us)?
The BB model and all other causal models that I am aware of assert a commonality of cause.
That may be so, but it's beyond the scope of this thread.



I'm interested in this both from a general perspective - a universe in which GR rules, but which does not have to be DE/CDM/radiation/other dominated - and the universe we seem to live in (flat, DE dominated, etc).

Although many theorists have strongly adopted the DM and DE ideas many still have not. Many still adhere to the cosmological constant concerning expansion and are not willing to give it up as yet. Many simply believe the DM, DE, the dark flow, etc. are just the current place-holders until something better comes along.

According to all versions of the BB model that I am aware of, however, there is a causal connection between all entities within the universe.
Again, this is OT.

Nereid
2010-Jun-22, 01:22 PM
Nereid,

So is it possible to show, using various GWR thought experiments, that mass-energy beyond our event horizon cannot have any detectable effect - in principle - on anything that is observable (to us)?
Although mass-energy could presently be beyond our observable horizon it could still be within the observable and gravitational horizons of distant galaxies. They might just be a hop, skip, and jump away from the most distant of these galaxies or quasars, lets say a few hundred million light years away. Such entities could gravitationally or otherwise effect or be affected by such entities within their own horizons. Such a present-day theoretical entity is the mass (galaxy clusters) that is supposedly causing "the dark flow."
This is just what my thought experiment seeks to examine.

Some super-distant event affects a galaxy (or something within a galaxy) which is, itself, within our light cone.

Per Ken G and speedfreak, earlier in this thread, if the (closer) galaxy is within our light cone, then the super-distant one must be too.

The point is that "gravitationally affect" (or similar) statements can be examined by my thought experiment, involving GWR. In particular, any such gravitational effect will, per Ken G and speedfreak, will be within our light cone. Conversely, there can be no gravitational effect produced by (caused by) mass-energy beyond our light cone.

At least, that's where I think we're up to now ...

speedfreek
2010-Jun-22, 06:21 PM
If I may quote a paragraph from the wiki on the dark flow (my bold added from emphasis):


The authors (A. Kashlinsky, F. Atrio-Barandela, D. Kocevski, and H. Ebeling) suggest that the motion may be a remnant of the influence of no-longer-visible regions of the universe prior to inflation. Telescopes cannot see events earlier than about 380,000 years after the big bang, when the universe became transparent (the Cosmic Microwave Background); this corresponds to the particle horizon at a distance of about 46 billion (4.61010) light years. Since the matter causing the net motion in this proposal is outside this range, it would in a certain sense be outside our visible universe; however, it would still be in our past light cone.

Our past light cone extends beyond the particle horizon and even beyond the inflationary epoch (which we weren't supposed to be talking about, but which is relevant to question of the dark flow). Perhaps we need some diagrams!

Grey
2010-Jun-22, 06:27 PM
Actually, I think Grey is answering a different question. Any event that can affect anything we can observe is in our past light cone. The world line that connects to such an event is another story, and what it can do is harder and I don't think we've quite figured that out yet in this thread, we have some more work to do if we want to understand that part.


Yes, it seems so.

Once we have light cones and their associated causal connections nailed down it'll be time to move onto world lines.Yes. It's a subtle distinction. Any event that can effect us, even indirectly, could necessarily also affect us directly, and is therefore in our past light cone. We would therefore be able, in principle, to have observed (or now be observing) that event. However, you were also asking about a blob of matter, and suggesting that any blob of matter associated with such an event is still observable in principle. That's not necessarily the case. Some object could have been associated with an event in our past light cone, but that object may have later moved out of our past light cone, so that although it was once observable, it no longer is. I think everyone here has agreed about that; I just wanted to make certain that you didn't get an incorrect impression.

forrest noble
2010-Jun-22, 07:22 PM
Nereid,

It would seem that in GR there are no bounds concerning gravitational event horizons even though accordingly gravity travels no faster than the speed of light. This is seemly a contradiction but in this way I see a parallel between Mach's writings and Einstein's theories relating to GR.

Mach believed that the entire universe and all of its parts has influence on all other parts (although like Newton he instead believed such influences were instantaneous). In Einstein's cosmological equations based upon GR, he introduced the forth dimensional warp characteristic of gravity that accordingly closed that bounds of the universe. The exact extent of this "warping of spacetime" would seemingly be dependent on each and every part/ physical characteristic and all the relationships between mass-energy and spacetime on an ongoing basis.

It would seem that in an expanding universe this warping characteristic and forth dimensional closure proposed by Einstein due to the overall density of matter in spacetime, accordingly would not seem to hold up in the long run based upon the validity of expanding spacetime -- at least the world-line concerning gravitational influences. Without a forth dimensional warp (or something like it) the universe would seemingly have true boundaries and edges.

Hope this is relevant to your queries and related conjecture.

Nereid
2010-Jun-23, 11:49 AM
Nereid,

It would seem that in GR there are no bounds concerning gravitational event horizons even though accordingly gravity travels no faster than the speed of light.(bold added)

That's interesting, but not at all what even simple explanations of a GR universe (isotropic and homogeneous).

The distinctions that have already been made, in this thread - but not yet clarified - make this clear.


This is seemly a contradiction but in this way I see a parallel between Mach's writings and Einstein's theories relating to GR.
As I said, the scope of this thread is limited; speculations about how GR might be related to Mach's writing are well and truly OT.


[...]

Hope this is relevant to your queries and related conjecture.
Thanks for your attempt; unfortunately, as I said, it is neither.

speedfreek
2010-Jun-23, 07:25 PM
Perhaps we need to discuss some definitions? I think we definitely have a clash of semantics going on here. :)

Observable - (As per OP definition) Events we can detect right now with the best hypothetical equipment.

Our past light cone - A history of the distance we can theoretically detect radiation from, expressed over time, usually in the form of a space-time diagram.

World line - The comoving path through space-time that an event occurred on (in this case a receding galaxy, quasar, or lump of something).

Cosmological Event horizon - Usually, the future evolution of our past light cone. But if taken to mean our cosmological event horizon right here and now, then simply the edge of our past light cone.

Particle horizon - Comoving distance to the most distant world line with an observed event in our past light cone.
_______

Our past light cone theoretically extends all the way back to the Big Bang, and all the events in our past light cone have comoving world lines that meet at the Big Bang.

A photon could trace a path all the way along the edge of our light cone, from here all the way back to recombination, and in an idealised situation all the way back to the Big-Bang.

Nereid
2010-Jun-25, 05:09 PM
Perhaps we need to discuss some definitions? I think we definitely have a clash of semantics going on here. :)
That seems to be so.



Observable - (As per OP definition) Events we can detect right now with the best hypothetical equipment.
Yep.



Our past light cone - A history of the distance we can theoretically detect radiation from, expressed over time, usually in the form of a space-time diagram.
Lots packed in here.

"the distance we can theoretically detect radiation from" - this needs refining; for example, "distance" means what, exactly?

Also, I'm interested in any distinction between a past light cone and a past QWR cone.

FWIW, "theoretical" has a precise meaning, in my question: equipment that can detect the light (or GWR) could be constructed, hypothetically (as a thought experiment).



World line - The comoving path through space-time that an event occurred on (in this case a receding galaxy, quasar, or lump of something).
Yep ... with the further restrictions that the scope is limited to a GR homogeneous and isotropic universe, excluding inflation.



Cosmological Event horizon - Usually, the future evolution of our past light cone. But if taken to mean our cosmological event horizon right here and now, then simply the edge of our past light cone.
The latter, extended to include GWR.



Particle horizon - Comoving distance to the most distant world line with an observed event in our past light cone.
Needs further refinement (e.g. "distance")



_______

Our past light cone theoretically extends all the way back to the Big Bang, and all the events in our past light cone have comoving world lines that meet at the Big Bang.
This is going too fast (see above); also, since we know GR and QM are incompatible at the Planck scale, we cannot extend as far as the Big Bang itself.



A photon could trace a path all the way along the edge of our light cone, from here all the way back to recombination, and in an idealised situation all the way back to the Big-Bang.
Not sure about this ...

speedfreek
2010-Jun-25, 06:01 PM
"the distance we can theoretically detect radiation from" - this needs refining; for example, "distance" means what, exactly?
Distance along the spatial axis. This can be a proper distance or a comoving distance, depending on how you plot the space-time diagram. I was hoping that by saying that the history of the distance is expressed over time, that it would obviously mean a spatial distance.


Also, I'm interested in any distinction between a past light cone and a past QWR cone.
So am I. Does GWR travel at c? If so, then it may as well be a photon for the purposes of this discussion about light cones.


FWIW, "theoretical" has a precise meaning, in my question: equipment that can detect the light (or GWR) could be constructed, hypothetically (as a thought experiment).
Yup, that was my meaning too.


Yep ... with the further restrictions that the scope is limited to a GR homogeneous and isotropic universe, excluding inflation.
Can I use a universe like ours in other respects? I.e decelerating then accelerating? It is still a GR homogeneous and isotropic universe, it just includes a cosmological constant. After all, you did say in the OP that "I'm interested in this both from a general perspective - a universe in which GR rules, but which does not have to be DE/CDM/radiation/other dominated - and the universe we seem to live in (flat, DE dominated, etc)" and you have only precluded inflation since.


Needs further refinement(e.g. "distance")
It is a spatial distance, expressed over time. The particle horizon represents the earliest event in the universe that we can currently detect - recombination. As this event happened everywhere, it will have always have been detectable from our world line and with increasing time we would have been able to detect particles from this event that were originally released from increasing distances. So, as we today detect particles released at recombination from regions of the universe with a comoving distance of 46 billion light-years, we assume we will in the future be able to detect particles from regions currently more distant than 46 billion light-years. In the future we will be seeing the recombination event from a world line that was more distant in space at the time of the event, and is more distant in space now.


Not sure about this ...
Heh, it doesn't help when I say it backwards like that! If you don't want to idealize the situation all the way back to the Big-Bang, then a hypothetical photon could trace a path along the edge of our light cone from the earliest event we can observe all the way to here and now, passing every event on that edge during its journey. I.e. a CMBR photon passes a z=10 quasar then passes a z=6 galaxy then a z=2 supernova and then enters our Hubble sphere as it passes a galaxy at z=1.4 (a galaxy that was, at that time, apparently receding from here at c) and then eventually reaches us 13.7 billion years after it was emitted, at the same time as we detect all the other events.

EDIT: I just noticed what you meant about the description of the particle horizon and have edited my reply above.