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.
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.
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