Universe versus Known Universe

[ObjectivelyRational]

Is there empirical evidence demonstrating that the entire universe and the known (directly) observable universe are one and the same? Is there definitive evidence that what we observe (directly) is simply not a small patch of something much larger? That instead of our observations characterizing an entirety of existence (and its rate of expansion etc.) they are applicable only to some local patch of the universe and thus only characterize what is going on in our area?

EDIT: Clarification, by observable I meant "directly" observable. I leave open the possibility that a future observation of (hypothetically) a large scale structure or asymmetry (of a particular nature and magnitude) which might constitute an indirect observation of the larger "patch" of the universe outside of the range of direct observation.

 

[phyzguy]

The part of the universe outside of the observable universe is, by definition, unobservable. Given that, how could there be empirical evidence of whether it exists or not?

 

[nikkkom]

Is there empirical evidence demonstrating that the entire universe and the known observable universe are one and the same?

No.
And unless Universe is unexpectedly found to be finite and rather small (about the size of observable patch), there will never be such evidence. The unobservable far-away parts may be very different, but we will never know for certain. (We can become "reasonably sure" based on our theories how they look, but we will have no experimental verification of it).

 

[Orodruin]

By definition you cannot make any observations regarding what is outside the observable universe. Of course you can make predictions based upon current theories, but there is no way that you could determine whether they are correct or not.

 

[ObjectivelyRational]

By definition you cannot make any observations regarding what is outside the observable universe. Of course you can make predictions based upon current theories, but there is no way that you could determine whether they are correct or not.

Clarified the question to distinguish between directly observable versus indirectly observable.

 

[ObjectivelyRational]

If we cannot know the actual extent of the universe how could we ever know whether it is open or closed? Would the implications of our observations be only that the "patch we observe" is will likely be "open" or "closed" for a "really long time"... I mean could we really have anything scientific to say about the "universe" in the limit of time approaching infinity?

 

[nikkkom]

I mean could we really have anything scientific to say about the "universe" in the limit of time approaching infinity?

You move goalposts and ramble in your questions. You were asking about "empirical evidence" at first, but now you shifted to "anything scientific". Those are not same things.

Try asking specific questions.

 

[ObjectivelyRational]

You move goalposts and ramble in your questions. You were asking about "empirical evidence" at first, but now you shifted to "anything scientific". Those are not same things.

Try asking specific questions.

I did. The answer to the first question has implications for the answer to the second question. Also, the second question explicates the importance of the answer to the first question.Since I did not initiate against you or your post any of the aggression which you have chosen to direct toward me, specifically, by characterizing my posts as rambling, I have no reservations to respond to you in full measure. In fact had you chosen the course of polite rational discourse, you may have learned with time and discussion that I very much agree with and appreciate your first post, but alas that is not what has transpired.Do not conflate logic with rambling they are not the same, and such confusion is an error.

The context of the question(s) regarding "evidence" is aimed at the nature of the evidence we collect, is it about the entire universe or a patch of it, what can we determine from that evidence. The importance of understanding the extent and context of the evidence is illustrated by the second question (which I am also interested in), namely what kinds of things can we validly attempt to say about the universe either as a whole, or in the limit of time, which could rightly labeled as "scientific" (rather than sheer speculation or pseudoscience). Although "evidence" and "science" are related these concepts are not to be conflated, one of which has implications for the second. As such, a little reflection would suffice to realize that although the questions are related they are not by any means "shifting" or as you imply lacking in specificity.If you prefer not to engage in a rational discussion with me by all means please refrain from having any discussion with me. There are plenty other things you could do with your time which you would find more valuable or enjoyable to you. That said, I also am certain others who are interested and willing to objectively discuss the questions with me will respond in due course.

 

[Drakkith]

If we cannot know the actual extent of the universe how could we ever know whether it is open or closed?

We cannot know with absolute certainty. All we can do is create a model that best explains our observations using known laws, which may or may apply to the rest of the universe that we cannot observe.

 

[kimbyd]

The edge of the observable universe is the edge of our observations, full stop.

The only way to gain some confidence regarding what lies beyond the observable is through the use of a model. For example, the standard model of cosmology assumes that the universe is homogeneous and isotropic. This suggests that the overall properties of the universe remain mostly unchanged quite far beyond our cosmological horizon. But the model itself doesn't say how far (for a flat or open universe, the model is infinite, but we don't know how far beyond our horizon the model holds, and have no way of testing that experimentally).

 

[Chronos]

The term observable universe is not related to what we actually observe, it is related to what is possible to observe. The customary definition for that is the celestial sphere surrounding Earth whose boundary is defined by the distance a photon emitted at the instant of the big bang could have traveled.

 

[timmdeeg]

The customary definition for that is the celestial sphere surrounding Earth whose boundary is defined by the distance a photon emitted at the instant of the big bang could have traveled.

Do you mean the proper distance now between us and this photon? Just to make it clear saying "the photon has traveled" you don't mean the distance between the locations where the photon was emitted then and observed later, right?

 

[ObjectivelyRational]

The edge of the observable universe is the edge of our observations, full stop.

The only way to gain some confidence regarding what lies beyond the observable is through the use of a model. For example, the standard model of cosmology assumes that the universe is homogeneous and isotropic. This suggests that the overall properties of the universe remain mostly unchanged quite far beyond our cosmological horizon. But the model itself doesn't say how far (for a flat or open universe, the model is infinite, but we don't know how far beyond our horizon the model holds, and have no way of testing that experimentally).

Does the observed large scale structure of the directly observable (to us) universe fit well with the models which assumes the universe is homogeneous and isotropic? What can be said about "observers" at the "edge" of what is observable to us (assuming what is observable to us is just a patch). Do models assume they would see a different patch (some overlapping with ours) which essentially looks similar in its large scale pattern to what we are observing?

 

[ObjectivelyRational]

The term observable universe is not related to what we actually observe, it is related to what is possible to observe. The customary definition for that is the celestial sphere surrounding Earth whose boundary is defined by the distance a photon emitted at the instant of the big bang could have traveled.

Are there theories which relate the speed of light at any epoch of the universe, with either the size ("amount" of space?) or rate of expansion of the universe? or does all the evidence show that the speed of light has indeed remained the same.

 

[Bandersnatch]

Do you mean the proper distance now between us and this photon? Just to make it clear saying "the photon has traveled" you don't mean the distance between the locations where the photon was emitted then and observed later, right?

These two distances are the same. The situation is symmetrical, only in one case you have a photon traveling towards us, and in the other away from us.
You can see this e.g. on light cone graphs such as this one (from Lineweaver & Davis' paper):

Where it is obvious that the past light cone and the particle horizon have the same extent in terms of comoving distance, so they're also the same in terms of current proper distance.
bapowell elaborates upon this some more in his Insights article on horizons (see discussion around fig.6):
https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

 

[timmdeeg]

These two distances are the same.

I'm not sure if I understand you correctly. Do you say that these two distances defined by a spacelike and a lightlike geodesic resp. are the same? And how do we define a distance in expanding spacetime between the events emission then and absorption now?

 

[Chronos]

Does the observed large scale structure of the directly observable (to us) universe fit well with the models which assumes the universe is homogeneous and isotropic? What can be said about "observers" at the "edge" of what is observable to us (assuming what is observable to us is just a patch). Do models assume they would see a different patch (some overlapping with ours) which essentially looks similar in its large scale pattern to what we are observing?

The 'edge' of the universe that we observe [e.g., CMB] is an illussion. In fact, everything observable is an outdated image of what existed in the past - due to the finite speed of light. It's no different than looking at your picture in your high school yearbook. How does that compare to what you see in the mirror? Let's just say you and another observer at the edge of the universe shared identical watches, then; everything you both can agree has ever occurred is at, or less than half the distance between the two of you - and everything beyond that range, has not yet been seen by both of you. The only events you both can agree happened at the same time occurred exactly half way between between both locations at that very instant. In effect, you two share a unique and specific definition of simultaneity. The only objects really suitable to serve as watches are variable quasars.

 

[Bandersnatch]

I'm not sure if I understand you correctly. Do you say that these two distances defined by a spacelike and a lightlike geodesic resp. are the same?

But these are both lightlike - they're just light cones, only one is the future light cone of the emission event, and the other is the past lightcone of the reception event.

And how do we define a distance in expanding spacetime between the events emission then and absorption now?

Take comoving distance of the emission event and multiply it by the scale factor at reception.

 

[ObjectivelyRational]

The 'edge' of the universe that we observe [e.g., CMB] is an illussion. In fact, everything observable is an outdated image of what existed in the past - due to the finite speed of light. It's no different than looking at your picture in your high school yearbook. How does that compare to what you see in the mirror? Let's just say you and another observer at the edge of the universe shared identical watches, then; everything you both can agree has ever occurred is at, or less than half the distance between the two of you - and everything beyond that range, has not yet been seen by both of you. The only events you both can agree happened at the same time occurred exactly half way between between both locations at that very instant. In effect, you two share a unique and specific definition of simultaneity. The only objects really suitable to serve as watches are variable quasars.

Ah yes. So if we take the "edge" to be the farthest parts of the universe which are at least potentially causally correlated with us, they can be affected by the same event at some time in the past, and that event lies "halfway" between us in space, would they also see events "on the other side of them from us" which occurred at that same time in the past but which would be potentially shared with others on the other side of them.. another full distance (equal to the "farthest distance") in space? (I am not building into this a curved space such that we are on the other side of them... also...)

 

[timmdeeg]

But these are both lightlike - they're just light cones, only one is the future light cone of the emission event, and the other is the past lightcone of the reception event.Take comoving distance of the emission event and multiply it by the scale factor at reception.

I see, thanks for your response.

 

[Bandersnatch]

would they also see events "on the other side of them from us" which occurred at that same time in the past but which would be potentially shared with others on the other side of them.. another full distance (equal to the "farthest distance") in space?

Yes, providing the cosmological principle holds. A snapshot of their observable universes at the current epoch would look something like this:

I.e. in the overlapping section there are events currently observable by both A and B, even though neither A nor B can see each other. You can draw such overlapping observable universes for any number of observers, as long as there's more homogeneous and isotropic universe left for them to occupy. Which might very well be infinite.

 

[Chronos]

Yes, and the only events that are simultaneous for Adam and Barbara bisect the overlap region. Everything on Barbara's side of that line is seen by Barbara before Adam can see it, and vice versa.

 

[JMz]

Does the observed large scale structure of the directly observable (to us) universe fit well with the models which assumes the universe is homogeneous and isotropic? What can be said about "observers" at the "edge" of what is observable to us (assuming what is observable to us is just a patch). Do models assume they would see a different patch (some overlapping with ours) which essentially looks similar in its large scale pattern to what we are observing?

Yes, they do. And as far as we can observe, those models are accurate.

 

[ObjectivelyRational]

Thank you to everyone who replied. Very interesting stuff.

 

[gary350]

How can the universe be so big it never ends?

If the universe ends what is on the other side?

 

[phinds]

How can the universe be so big it never ends?

So you think if it never ends it should be small ?
EDIT: I mis-read the question. We don't know HOW it can be infinite, we don't even know that it IS infinite. But it could be.

If the universe ends what is on the other side?

Whether it is infinite or finite, there is nothing "on the other side". There IS NO "other side"

 

[JMz]

At best(?), there could be regions with no matter or energy, just quantum background. Describing that as either part, or not part, of the universe doesn't seem like a good use of language. In any case, from our position in the universe, there is no evidence that parts of the universe like ours ever stop: If there's an edge or a spatial "end", we are nowhere near it.

 

[phinds]

At best(?), there could be regions with no matter or energy, just quantum background. Describing that as either part, or not part, of the universe doesn't seem like a good use of language. In any case, from our position in the universe, there is no evidence that parts of the universe like ours ever stop: If there's an edge or a spatial "end", we are nowhere near it.

No, it is a PERFECT use of language. It falls in the very definition of "universe" that EVERYTHING is a part of the universe.

 

[rootone]

Got to agree with Phinds on this one although we don't always agree.
Anything that exists is part of the Universe.
Stuff which doesn't exist doesn'l matter.

 

[JMz]

Got to agree with Phinds on this one although we don't always agree.
Anything that exists is part of the Universe.
Stuff which doesn't exist doesn'l matter.

What does that sentence even mean?

FWIW, I merely meant that people do not always mean the same by universe that you do.

 

[weirdoguy]

Then they should understand that on PHYSICSforums "Universe" has a precise meaning and they should use this word with that meaning.

 

[JMz]

I don't think even all cosmologists would agree with you. In particular, for those working with either multiverses or quantum-foam bubbles (followed by inflation), they seem to find it helpful to distinguish "everything" from "everything that shares our BB", with the latter being the "universe" and the former being "other universes" that do not, or have not so far, interacted with ours.

Not my usage, but I understand why they might find it useful.

 

[Stavros Kiri]

I don't think even all cosmologists would agree with you. In particular, for those working with either multiverses or quantum-foam bubbles (followed by inflation), they seem to find it helpful to distinguish "everything" from "everything that shares our BB", with the latter being the "universe" and the former being "other universes" that do not, or have not so far, interacted with ours.

Not my usage, but I understand why they might find it useful.

Thus we may re-ask the initial question(s) within the Standard Model of Cosmology ...
Some answers will still hold, some will not ...