Proper definitions for "Universe", "Observable Universe" and "Visible Universe"?

This seems to be an area of some confusion and apparent mis-statements that I would like to see clarified. Too often, when I make a statement to clarify some issue in a discussion, I get a response that says something like "That isn't what the theory says," without any statement about what the theory does say. At best, such responses provide a link to read. But, after reading several, I see that the links do not fully agree with each other, either. A lot of the time, such reply posts seem to be intended to avoid clarity, rather than enhance it, perhaps to dodge problems with the theory itself?

So, here is my take on the 3 definitions. If there are criticisms or questions about these, please make them in a constructive and clarifying manner, so that we can resolve them into mutually accepted definitions for clear use in future discussions.

"Universe" means "everything" to me. If there is a theory of the universe, it includes everything we are thinking of it possibly including. Nothing can be outside the universe, even though we do not think we could ever detect it if it really exists. Not even if our theory doesn't address what it is, or where it is, or how it came to exist. If the theory says that the universe is finite, that means that there is absolutely nothing that can exist in the past, present or future other than what that theory says exists. Of course, theories that deal with what may be only a finite part of the universe that it can describe are OK, so long as they don't use the lack of explanation of anything beyond what they can describe as "proof" that nothing else exists. But, this leads to boundary and interface questions that should not be dodged with the assertion that they do not exist because the theory does not include anything about them.

"Observable Universe" seems to be a little conflicted. Sometimes, it seems to me to include what may be the whole universe or just a described part of the whole universe, with the description specifying that some aspect of an "observable" part must have been observable for at least some portion of that parts existence, even if it has since moved into a position from which whatever happens there now cannot ever be observed from here in the future. The fact that it was once in a position so that light it emitted could still reach us some day in the future, even with wavelength stretched to near infinite, apparently makes it theoretically "observable", as I see this term actually being used. But, I think that is misleading. I think a distinction needs to be made between what can eventually be observed here because it occurred in the past and the light will eventually get here, or maybe even did get here in the past, vs what is occurring now at such great distances that continuing expansion of the universe will preclude any light emitted now from ever getting here for us to observe it.

'Visible Universe" means to me the portion of the universe that we can actually detect. Things that are obscured by the scattering of photons before the combination of protons and electrons into neutral hydrogen atoms are rendered "invisible", at least with respect to any emissions of light from them. Perhaps gravitational wave detectors, neutrino detectors or other observation techniques that do not depend on photon transmission could render visible" some of those objects obscured by plasma before recombination. But, clearly, matter that, from our vantage point, has already fallen into black holes has now left our visible universe so far as any existing theories state.

So, some of the problems I am seeing involve quibbles about whether the BBT describes the whole "universe" or just the "observable universe" and, more specifically, what the radius of the "observable universe" is today. The current position of the BBT seems to be that it describes only the "observable universe" and does not address any boundary issues with anything else that might exist outside of what is described. That is OK with me, so long as word games are not being played to avoid the question about what might be outside the "observable universe" or what might happen at the finite radius where what is described might meet whatever is not described by the theory.

What really seems to get inconsistent opinions is whether things can move from inside the "observable universe" to outside of it. I see some supposedly authoritative web sites that claim that nothing can ever go from "observable universe" to beyond the observable universe, while another says that things currently more than 19 billion light years from Earth will eventually go beyond "observable", and neither of those statements make sense to me.

The way I envision the universe, as described by the BBT, is that it is everywhere the same at any specific point in time since its initial inception as a tiny spec. So, right now, every location in the universe described by the BBT is expanding at the same rate that we currently observe here from Earth and call the Hubble Constant. Extrapolating the Hubble expansion rate gets the speed of separation between here and points about 14 billion light years distant from here up to the speed of light now. When I do the math, it seems pretty clear that light emitted now by things that are now more than ~14 billion miles from us will never reach us here. I have seen some arm-waving "explanations" of how light would still eventually reach us from such distant locations, but I have never seen anybody actually show that with proper math. I do understand that when we see light that was emitted 13.something billion years ago we tend to think of those objects being 13.something billion light years away from us, but that is not how the theory actually works. According to the theory, the objects that we are seeing now emitted that light 13.something billion years ago, when they were much closer to us than that 13.something billion light years. What happened was that the space the light was traveling through to get to us was expanding as the light was traveling, and it took 13.something billion years for the light to get here - so that 13.something billion years is light travel time, not actual distance at time of emission. And, while that light traveling towards us was in-transit for 13.something billion years, the matter that emitted it was traveling away from us at an even higher apparent rate of speed, because, as the distance between us and the object increases, there is more expanding space between us as time goes on. The BBT calculates that things that emitted light that is just getting to us now after 13.something billion years of travel time have moved out to a distance of 30-something billion light years as of "now". (see https://astro.ucla.edu/~wright/CosmoCalc.html )

So, to me, the things that we are able to observe today are currently emitting light that will never reach us. At least not according to the current version of the BBT, which says that the observable universe is expanding and will continue to expand at rates that make the speed of separation between "here" and objects more than about 14 billion light years from here now exceed the speed of light.

If anybody thinks something that I have posted is wrong, please explain what and why you think that. Please do not simply refer me to another web site. If you really understand something, then you should be able to explain it clearly.
 
Last edited:

Catastrophe

"There never was a good war, or a bad peace."
Universe, to me, means "all there is" et cetera.

However, that assumes that we are equipped to understand or comprehend "all there is". Of course, we are not.

So we restrict "all there is" and "all we can observe", or other such limiting phrases. Thus, in essence, we could refer to "all we can observe" = "observable universe", OR "all we can comprehend" = "comprehendible universe". In this sense only, one can refer to "universes", since the "observable" applies to every individual, from their particular location in spacetime.

Cat :)
 
Cat, my goal with this thread is to try to get some conclusion about what people mean when they bandy about these words. And, it particular, when they say that something that we are trying to talk about is not in one of these terms.

I am trying to make sure that things are being clearly stated. And I am also trying to make clear when false logic is being used to argue against some idea. In particular, there used to be arguments that something cannot exist because it is not in the BBT. Such things as time before the "Bang" or anything outside the radius that the BBT estimates the "observable universe" to have reached at this point.

There are also some issues with boundaries on the BBT "universe", whatever it is said to be, if it is said to have a finite "radius". There are some folks making the argument that it has no boundaries, "like the surface of an inflating sphere", but that just pushes the argument to needing curved space and another dimension. And, most (not all) studies looking for curvature in intergalactic space come up with "flat" being the result. So, the "inflating sphere" argument does not have support. Same with the attempted analogy about "What was there before the Bang" being equated to being an inappropriate question like "What is north of the North Pole?" Obviously, "north" has one definition for beings that only understand a curved space with latitude and longitude as its dimensions, contrasted to beings that perceive a 3D space and see altitude (radius) , as well, and have no curvature in their perceived space. In other words, if the question doesn't fit the theory, it means you need to question the theory, not ignore the question.
 

Catastrophe

"There never was a good war, or a bad peace."
Proper definitions for "Universe", "Observable Universe" and "Visible Universe"?

However, that assumes that we are equipped to understand or comprehend "all there is". Of course, we are not.
I inadvertintly continued to refer only to individuals (humans?) at different locations. Of course, all sentient beings and their respective sensory mechanisms should be included. Thus, there are "observable universes" for every sentient species throughout the Universe as well as for every individual of every species. There may be necessary, a further extension of the definition of species (in this context) depending on any alternate definitions of "life" and "individual".

All of these questions must be applied to every possibility (and to every author of every possibility) throughout the Universe. Such eventualities, applying to the total Universe, are clearly unknowable.

Cat :)
 
I think there is great merit to being able to set limits on the Universe because of the strength of a fairly well-known value for expansion (~ 67 to 73), and the invariable speed of light. There is a point (fixed radius) where no light can ever reach an observer.

More distant beings will have the same radii but will encompass different edges due to their location. But, they could not communicate their unique views faster than c, so a defined limit should not be diminished by this nuance, IMO.
 

Catastrophe

"There never was a good war, or a bad peace."
Helio, I am not sure whether you agree with this, or not?

All of these questions must be applied to every possibility (and to every author of every possibility) throughout the Universe. Such eventualities, applying to the total Universe, are clearly unknowable.
Cat :)
 
I think the main issue is whether what we call the "observable universe" or "visible universe" is considered to be the total universe, or if theory supports that there are things beyond observation that can at least be inferred from the observations we are able to make. That seems to be true for the step between "visible" and "observable".

But, there seems to be a difference of opinion between "observable" and just plain "universe" with no modifier, thus the "total" universe.

For instance, does a theory dictate that there cannot be anything beyond what it calculates as the radius achieved since t=0 based on Inflation theory? Or does it just say that whatever, if anything, is beyond that radius, the theory is not able to address it?

There are logical questions about our universe having a limiting radius that the theorists seem to want to avoid. For instance, if the space we can perceive is flat and does not curve back into itself, then what is at the limiting radius? It seems we are once again back to those pre-Columbian sea charts that show the edge to a flat earth with the notation beyond the edge "Dragons be here", except some insist that there are no dragons because there is not "here" there.

And that is an entirely different question than asking whether observed or visible universes of 2 widely separated observers overlap in space or not.
 
Helio, I am not sure whether you agree with this, or not?
That might work in a Feynman interpretation, perhaps. But Einstein said that the “ most incomprehensible thing about the universe is that it’s comprehensible.” Thus implies there are objective facts and solid theories, some that bring us laws. Laws can be broken, but usually they are tweaked or broadened.

Given how much people like to claim metaphysics should be treated like physics, I’d prefer to hold that all considerations are worthy of scrutiny, but few will have sound merit.
 

ASK THE COMMUNITY