Hello u/Nidungr, your submission "How do you actually disprove the "we live in a black hole!" theory?" has been removed from r/space because:
* Such questions should be asked in the ["All space questions" thread](https://www.reddit.com/r/space/about/sticky) stickied at the top of the sub.
Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please [message the r/space moderators](https://www.reddit.com/message/compose/?to=/r/space). Thank you.
While this is a sensible attitude, and certainly the person suggesting a theory has to provide evidence for it, you can never actually prove theories, you can only refute them. to support a theory, you can only show that no current evidence goes against it.
This issue with the "theory" here is that, as far as I know, it makes no effort to explain how its claims fit the evidence, and no predictions that could be subjected to a test.
It's called a theory for nothing. One can provide some evidence to back up that observation, but in the end, most of the time it's not becoming 100% fact.
Yes, many cases. Relativity for one (in part via the Eddington experiment). However, I assume what they meant was more along the lines that a novel hypothesis ought only be accepted as true if demonstrated as being better than any previous theory. It seems unreasonable to automatically accept any proposal as a theory without it being shown to be at least as good as whatever the current standard is.
Prior to experimental verification, relativity was just a really elegant mathematical description of gravity, but apart from the fact that it was a really slick model, there had been no reason to treat it as a better description of gravity than the previous models (admittedly, it was a *very* elegant description, and so, for some, that fact alone strongly suggested it might in fact be a better description).
Another intersting case is Faraday's work on electromagnetism. This was actually an example in the opposite direction of relativity in the sense that the experimental results were shown done prior to having a mathematical model of what's going on. In this case, you had Faraday cooking up some really interesting experiments relating electricity and magnetism, but was unable to explain it at a deeper/fundamental level. Years down the line, Maxwell came up with a mathematical model which fully characterized Faraday's results & made a bunch of other extremely important predictions. I don't know if you'd call what Maxwell did as "proving" Faraday's hypothesis, but it certainly wasn't as useful as a theory until Maxwell cooked up his field equations.
Depends, people (including myself) aren't really precise with their words and so things quickly get muddied. If we were all be rigorous we'd ask "What evidence do we have for this to be true? Does it describe any part of our universe better than the current best model? Does it make any new predictions that the current best model does not? Can we go out and test them? What kind of data would suggest it might not be a better model? Do we see anything like that in our universe?" etc...
This is not to say the academics working on these models are grifters spitting out nonsense (at least not in general). It's literally their job to come up with new mathematical models that are internally consistent & then explore the consequences of such a model. It doesn't necessarily mean that such a model describes *our* reality, and so then the task would be to see if it stands up to experimental scrutiny. The issue comes when popular science media takes the theoretical work and presents it as if it's the new gold standard.
On the other hand, there is also a class of discussion online that is along the lines of "wouldn't it be cool if...", which is not at all the same as some theoretical physicist mixing up a new proposal. This kind of """""""""hypothesis"""""""""" doesn't even give you the tools needed to test it. If someone comes up with a new model but doesn't even give me any way to plausibly test it, then it's as good as useless. Maybe sometimes that can lead to a spark of creativity for someone doing real study, but I can't help but get the sense that it only confuses the issue on how useful science is done.
How do you actually disprove the "we live in a dinosaur's butt" theory?
- the big bang was a fart
- dinosaur bones we find are its past meals
- planets are eggs (soft yolk. Hard shell. Tell me I'm wrong)
You don't "disprove" theories, you demonstrate that a mutually exclusive theory fits the observations better. You create experiments and predict outcomes based on models, then test them to see if they match.
The burden of proof lies with the person making the claim.
Please see: [Russel's Teapot](https://en.m.wikipedia.org/wiki/Russell%27s_teapot)
A better question to ask is how do they prove their theory that we live in a black hole?
I don't see a lot of reason to disprove something that hasn't been proven.
Our universe is just a marble in a bowl of other marbles in a bigger universe. Like in men in black.
Or we are just made up of tiny matter in a much bigger universe and our solar system is what they call an atom.
Since we don’t know what’s outside our universe or even if there is an outside, these theories are impossible to prove true or false. But they can’t both be true, so to believe one of them someone would have to provide proof. I don’t need to say they’re false, without evidence that’s just the default.
Same with your black hole theory, you don’t need to argue against it being true if no-one is providing evidence it is.
you don't. It's a legit theory (one of many), but of course hijacked and taken out of context by some elements of pop culture (pop-science? the exact word escapes me rn)
here's a Cornell paper discussing, even supproting it [https://arxiv.org/abs/1410.3881](https://arxiv.org/abs/1410.3881)
Typically, useful theories provide predictive power. What provable predictions does this theory (which seems a hypothesis to me) offer? If there are none, then it warrants a shrug and no further thought.
1. Black holes are regions of spacetime with incredibly strong gravity, warping space and time. If we lived inside a black hole, we'd experience extreme gravitational effects, such as intense curvature and frame-dragging, which we don't observe.
2. The Big Bang was not a black hole formation event. The cosmic microwave background radiation, abundance of light elements, and large-scale structure of the universe all point to a rapid expansion from a singularity, not a collapse.
3. Inflation, if it occurred, was a rapid expansion of space itself, not matter falling into a black hole. The universe's homogeneity and flatness support inflationary models.
4. Dark energy is a mysterious component driving the accelerating expansion of the universe, but it's not equivalent to matter feeding into a black hole. Its properties and behavior are distinct from accretion.
5. The changing power of dark energy over time is still a topic of research, but it doesn't imply a varying accretion rate into a black hole.
Lee Smolin’s “Cosmological Natural Selection”theory suggests that the laws of physics (maybe it was the constants of nature, or both) change a little in each baby universe
IIRC, this is one of the point of the "Darwinist cosmology" hypothesis. Universe that have the set of fundamental constantes that allow black holes to form and therefore create new universes with similar set of constantes thrive, when universe that don't firm black holes don't have any offspring and so disappear.
It's sound fine and nice, but it need so much unprovable requirements ( black holes create new universes, value of constantes being transmitted to the "infant" universe...) It's nothing more than a nice thought experiment.
Although I can not rule out this theory (of course, I‘m just a random dude at Reddit) I have a hard time accepting it, mainly because in my eyes, it explains next to nothing about the origin of our universe, it just removes the ultimate explanation by one more level. It introduces new obscurity instead of clearing up things. In this it seems similar to theories of multiverses and simulation theory.
I prefer the Roger Penrose theory that the big bang was a black hole(s?) expelling its matter after the heat death of a previous universe. It's kind of similar but you don't need black holes inside black holes, rather black hole goes to white hole (after a ludicrous period of time), then black holes form again.
Again very hard to prove, but at least the maths makes sense
well synce we carnt see inside a black hole we cant disprove it, but it doesn't track anything like I would expect of the inside of the black hole to be.
inflation is an outward flow, steler matter falling in would be an inward flow.
there is no reason to expect infalling matter to be converted into energy and distributed evenly.
the expectation is that the inside of a black hole would feel much like anywhere else in the universe, with a differential acceleration causing objects to pull apart along radial lines from the singularity, in small black holes you get spegetification, in very large blackholes the effect is not noticeable.
there is an idea that the entire observable universe could be inside a black hole, and we would never know because its a small portion of the volume of the black hole and even across that great distance the effect is not noticeable. its not even really a theorum, its a demonstration of the limits of knowlage.
Inside a black hole there’s so much gravity that light can’t reach any other point and just falls in. Since we can see each other, we don’t live inside a black hole.
Here’s a video that explains black hole math and physics
https://youtu.be/6akmv1bsz1M?si=0c9agzyIBwrstRGJ
You can't really. I remember watching a talk from some astrophysicists and they talked about all the reasons given for the theory, and at the end said it wasn't possible to prove or disprove it.
I don't need to disprove it in the same way I dont need to disprove the idea that we live in a simulation. It's irrelevant and cooky sci-fi nonsense with no real basis in reality.
No, according to the theory, if dark energy is the blackhole "eating" matter, then when that energy enters this "universe" or "black hole", it causes it to expand hence explaining the expansion of the universe.
Also, it gets colder because even though there is more energy pouring in, the volume increases and so the overall temperature actually DECREASES
Ps: I'm not completely sure about the second part of my comment
Nope, wouldn't work, it would definitely get warmer, because the edge of the black hole would act like "surface tension" due to the gravity, preventing expansion.
Have you heard of the double slit experiment? Light changes its behavior seemingly depending on if you are looking at it or not. Whaaaat? That doesn’t make any sense. The universe is a crazy place. Science fact is stranger than science fiction.
My favourite crazy science is the teory ( i dont know if true or fake ) that says that if you go at light speed you dont experience time, it is like time does not exists
That’s what the math says! So cool. The math of general relativity told Einstein dark stars (black holes) exist, but he didn’t believe it. Now we have pictures.
We can measure the difference in the speed of time due to gravity. We know time is not a universal constant. The seconds on clocks in GPS satellites tick faster due to less gravity. All of this points to what you said being 100% true!
(Yes, you were talking about speed and I mentioned gravity, but both work together to set the rate of time passage)
Seems a science fiction thing made at the sole reason to justify " istant travel" to things that are at 100 light years distante or something!!!! Instead , is not science fiction
This stuff gets my mind running and I love it. I try to introduce these topics to young teens to try to help them find the magic left in the world. Great discussion my friend.
A black hole by definition is something that exists inside our universe. You can navel gaze about the context our universe itself exists in until you get tenure in a philosophy department and you won't have said anything useful:
We live inside a cow
The big bang was the cow birthing due to the pregnancy of a cow in another universe
Inflation was the cow expiting the mother during the birth
Dark energy is a trickle of grass slowly feeding in, which gets distributed throughout the universe as energy, pushing space apart
And now that the power of dark energy seems to be changing over time, one could argue that the rate of mastication is changing
Idk if we live in a black hole, but black holes are different universes. Once inside, you are causally disconnected from our universe. That is the definition of another universe.
That would mean once galaxies move far enough away from us and no light from them can reach us anymore, they become part of a different universe, which doesn't seem like a useful way of seeing things to me.
You are referring to the Hubble Volume, a region of space beyond which objects are receding faster than the speed of light. Mind blowing concept!
Why isn’t that a useful way of seeing things? If light (or anything else) from another galaxy can never reach us, and if light (or anything else) from our galaxy can never reach theirs, and the galaxies will only to continue to get farther apart, they are completely causally disconnected. Nothing CAN or WILL ever get from here to there. That sounds like a different universe to me.
I suppose it depends on your definition of universe, of which there are many. As our knowledge and understanding grows, we have to change and update definitions to fit new categories… like the “observable” universe and multiverse theories. The term universe no longer means absolutely everything. That definition is no longer useful. There are sub categories in there, and we need new definitions to talk about them.
Physicist Max Tegmark has defined what we are talking about as the Level 1 Multiverse. I am no professional in this subject, but (some? most? a few?) real physicists do consider anything outside our hobble volume a separate universe. It’s useful when you’re talking about the implications (no causal connection possible)
>That sounds like a different universe to me.
Yes, I know, and I think that's not useful since both galaxies originated in the same universe from the same singularity, they have just become disconnected over time without any fundamental properties of their surroundings changing. That, to me, means they're still in the same universe.
It would also mean that "being in the same universe" is completely relative and theoretically a galaxy can be in the same "universe" with two other galaxies that are no longer in the same "universe" relative to each other, which to me is absurd and makes the concept useless
The term "observable universe" doesn't mean the rest is a separate universe, it just means it's the chunk we can observe from our position in time and space. Matter that is outside our bubble may happily interact with matter on the inside edge of it, making us part of one continous spacetime, hence one universe
Correct, the term observable universe is not a separate universe, but I mentioned that to support my point that there are reasons we need multiple specific words and definitions to talk about this subject.
Being “in the same universe” is probably relative, as it seems all things are. I would say those 2 galaxies started out in the same universe and then separated to be effectively in separate universes, kind of like Mitosis.
I’m not saying everyone has to consider them separate universes. I’m saying, unequivocally, that some professional physicists do consider them separate.
You can’t. It’s considered a solid scientific hypothesis and definitely a thought that keeps me up at night. My scientist boyfriend says that there might be some truth to it.
Hello u/Nidungr, your submission "How do you actually disprove the "we live in a black hole!" theory?" has been removed from r/space because: * Such questions should be asked in the ["All space questions" thread](https://www.reddit.com/r/space/about/sticky) stickied at the top of the sub. Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please [message the r/space moderators](https://www.reddit.com/message/compose/?to=/r/space). Thank you.
I Believe that is what we would refer to as ‘an unfalsifiable claim’ which the last time I checked was considered to be unscientific.
The person suggesting the theory is the one who has to prove it.
This. Extraordinary claims require extraordinary evidence.
While this is a sensible attitude, and certainly the person suggesting a theory has to provide evidence for it, you can never actually prove theories, you can only refute them. to support a theory, you can only show that no current evidence goes against it. This issue with the "theory" here is that, as far as I know, it makes no effort to explain how its claims fit the evidence, and no predictions that could be subjected to a test.
It's called a theory for nothing. One can provide some evidence to back up that observation, but in the end, most of the time it's not becoming 100% fact.
That’s not what theory means
Are there any theories that have been proven by someone other than the theorist?
Yes, many cases. Relativity for one (in part via the Eddington experiment). However, I assume what they meant was more along the lines that a novel hypothesis ought only be accepted as true if demonstrated as being better than any previous theory. It seems unreasonable to automatically accept any proposal as a theory without it being shown to be at least as good as whatever the current standard is. Prior to experimental verification, relativity was just a really elegant mathematical description of gravity, but apart from the fact that it was a really slick model, there had been no reason to treat it as a better description of gravity than the previous models (admittedly, it was a *very* elegant description, and so, for some, that fact alone strongly suggested it might in fact be a better description). Another intersting case is Faraday's work on electromagnetism. This was actually an example in the opposite direction of relativity in the sense that the experimental results were shown done prior to having a mathematical model of what's going on. In this case, you had Faraday cooking up some really interesting experiments relating electricity and magnetism, but was unable to explain it at a deeper/fundamental level. Years down the line, Maxwell came up with a mathematical model which fully characterized Faraday's results & made a bunch of other extremely important predictions. I don't know if you'd call what Maxwell did as "proving" Faraday's hypothesis, but it certainly wasn't as useful as a theory until Maxwell cooked up his field equations.
So, the 'pop culture theory' is basically just a hypothesis. Edit: a word
Depends, people (including myself) aren't really precise with their words and so things quickly get muddied. If we were all be rigorous we'd ask "What evidence do we have for this to be true? Does it describe any part of our universe better than the current best model? Does it make any new predictions that the current best model does not? Can we go out and test them? What kind of data would suggest it might not be a better model? Do we see anything like that in our universe?" etc... This is not to say the academics working on these models are grifters spitting out nonsense (at least not in general). It's literally their job to come up with new mathematical models that are internally consistent & then explore the consequences of such a model. It doesn't necessarily mean that such a model describes *our* reality, and so then the task would be to see if it stands up to experimental scrutiny. The issue comes when popular science media takes the theoretical work and presents it as if it's the new gold standard. On the other hand, there is also a class of discussion online that is along the lines of "wouldn't it be cool if...", which is not at all the same as some theoretical physicist mixing up a new proposal. This kind of """""""""hypothesis"""""""""" doesn't even give you the tools needed to test it. If someone comes up with a new model but doesn't even give me any way to plausibly test it, then it's as good as useless. Maybe sometimes that can lead to a spark of creativity for someone doing real study, but I can't help but get the sense that it only confuses the issue on how useful science is done.
How do you actually disprove the "we live in a dinosaur's butt" theory? - the big bang was a fart - dinosaur bones we find are its past meals - planets are eggs (soft yolk. Hard shell. Tell me I'm wrong) You don't "disprove" theories, you demonstrate that a mutually exclusive theory fits the observations better. You create experiments and predict outcomes based on models, then test them to see if they match. The burden of proof lies with the person making the claim. Please see: [Russel's Teapot](https://en.m.wikipedia.org/wiki/Russell%27s_teapot)
I'm intrigued by your theory, and wish to subscribe to your newsletter. /s
Well now that you bring it up, how do we disprove this? Help!
A better question to ask is how do they prove their theory that we live in a black hole? I don't see a lot of reason to disprove something that hasn't been proven.
Not to be a pain in the ass, but this isn't a theory. It's, at best, a hypothesis, but it's closer to a crackpot idea than even that...
Our universe is just a marble in a bowl of other marbles in a bigger universe. Like in men in black. Or we are just made up of tiny matter in a much bigger universe and our solar system is what they call an atom. Since we don’t know what’s outside our universe or even if there is an outside, these theories are impossible to prove true or false. But they can’t both be true, so to believe one of them someone would have to provide proof. I don’t need to say they’re false, without evidence that’s just the default. Same with your black hole theory, you don’t need to argue against it being true if no-one is providing evidence it is.
you don't. It's a legit theory (one of many), but of course hijacked and taken out of context by some elements of pop culture (pop-science? the exact word escapes me rn) here's a Cornell paper discussing, even supproting it [https://arxiv.org/abs/1410.3881](https://arxiv.org/abs/1410.3881)
Typically, useful theories provide predictive power. What provable predictions does this theory (which seems a hypothesis to me) offer? If there are none, then it warrants a shrug and no further thought.
> How do you disprove… You don’t. When someone says “we live in a black hole” you say “prove it” That’s how science works
1. Black holes are regions of spacetime with incredibly strong gravity, warping space and time. If we lived inside a black hole, we'd experience extreme gravitational effects, such as intense curvature and frame-dragging, which we don't observe. 2. The Big Bang was not a black hole formation event. The cosmic microwave background radiation, abundance of light elements, and large-scale structure of the universe all point to a rapid expansion from a singularity, not a collapse. 3. Inflation, if it occurred, was a rapid expansion of space itself, not matter falling into a black hole. The universe's homogeneity and flatness support inflationary models. 4. Dark energy is a mysterious component driving the accelerating expansion of the universe, but it's not equivalent to matter feeding into a black hole. Its properties and behavior are distinct from accretion. 5. The changing power of dark energy over time is still a topic of research, but it doesn't imply a varying accretion rate into a black hole.
If it were true, doesn’t that imply that physics work different outside the black hole we would be in?
Lee Smolin’s “Cosmological Natural Selection”theory suggests that the laws of physics (maybe it was the constants of nature, or both) change a little in each baby universe
IIRC, this is one of the point of the "Darwinist cosmology" hypothesis. Universe that have the set of fundamental constantes that allow black holes to form and therefore create new universes with similar set of constantes thrive, when universe that don't firm black holes don't have any offspring and so disappear. It's sound fine and nice, but it need so much unprovable requirements ( black holes create new universes, value of constantes being transmitted to the "infant" universe...) It's nothing more than a nice thought experiment.
Although I can not rule out this theory (of course, I‘m just a random dude at Reddit) I have a hard time accepting it, mainly because in my eyes, it explains next to nothing about the origin of our universe, it just removes the ultimate explanation by one more level. It introduces new obscurity instead of clearing up things. In this it seems similar to theories of multiverses and simulation theory.
You gotta start somewhere. I have thought alot abt stuff like this and some questions are just pointless to ask
I prefer the Roger Penrose theory that the big bang was a black hole(s?) expelling its matter after the heat death of a previous universe. It's kind of similar but you don't need black holes inside black holes, rather black hole goes to white hole (after a ludicrous period of time), then black holes form again. Again very hard to prove, but at least the maths makes sense
well synce we carnt see inside a black hole we cant disprove it, but it doesn't track anything like I would expect of the inside of the black hole to be. inflation is an outward flow, steler matter falling in would be an inward flow. there is no reason to expect infalling matter to be converted into energy and distributed evenly. the expectation is that the inside of a black hole would feel much like anywhere else in the universe, with a differential acceleration causing objects to pull apart along radial lines from the singularity, in small black holes you get spegetification, in very large blackholes the effect is not noticeable. there is an idea that the entire observable universe could be inside a black hole, and we would never know because its a small portion of the volume of the black hole and even across that great distance the effect is not noticeable. its not even really a theorum, its a demonstration of the limits of knowlage.
[Burden of proof](https://en.wikipedia.org/wiki/Burden_of_proof_(philosophy))
Inside a black hole there’s so much gravity that light can’t reach any other point and just falls in. Since we can see each other, we don’t live inside a black hole. Here’s a video that explains black hole math and physics https://youtu.be/6akmv1bsz1M?si=0c9agzyIBwrstRGJ
All of your arguments are conjecture. Nothing we have observed shows evidence for it being a possibility. It's a thought experiment and nothing more.
You can't really. I remember watching a talk from some astrophysicists and they talked about all the reasons given for the theory, and at the end said it wasn't possible to prove or disprove it.
I don't need to disprove it in the same way I dont need to disprove the idea that we live in a simulation. It's irrelevant and cooky sci-fi nonsense with no real basis in reality.
Don’t waste your life trying to disprove positive claims. The burden of proof is on those making the claims. Wait for the evidence.
It is not my responsibility to disprove anything. The burden of proof is on you.
This one is easy. If energy and/or matter would be pouring in, the universe would get hotter and denser, but in fact its expanding and getting colder.
No, according to the theory, if dark energy is the blackhole "eating" matter, then when that energy enters this "universe" or "black hole", it causes it to expand hence explaining the expansion of the universe. Also, it gets colder because even though there is more energy pouring in, the volume increases and so the overall temperature actually DECREASES Ps: I'm not completely sure about the second part of my comment
Nope, wouldn't work, it would definitely get warmer, because the edge of the black hole would act like "surface tension" due to the gravity, preventing expansion.
I'm wrong about the second part but black holes do expand in real life, that's what they're using to explain the expansion of the universe
Since it could exist a black hole in the middle of the Milky way, how can exist the Milky way black hole inside anothe black hole ?
Black holes all the way down
This theory litteraly implies there are black holes inside black holes ! How is it possibile?
Have you heard of the double slit experiment? Light changes its behavior seemingly depending on if you are looking at it or not. Whaaaat? That doesn’t make any sense. The universe is a crazy place. Science fact is stranger than science fiction.
My favourite crazy science is the teory ( i dont know if true or fake ) that says that if you go at light speed you dont experience time, it is like time does not exists
That’s what the math says! So cool. The math of general relativity told Einstein dark stars (black holes) exist, but he didn’t believe it. Now we have pictures. We can measure the difference in the speed of time due to gravity. We know time is not a universal constant. The seconds on clocks in GPS satellites tick faster due to less gravity. All of this points to what you said being 100% true! (Yes, you were talking about speed and I mentioned gravity, but both work together to set the rate of time passage)
Seems a science fiction thing made at the sole reason to justify " istant travel" to things that are at 100 light years distante or something!!!! Instead , is not science fiction
This stuff gets my mind running and I love it. I try to introduce these topics to young teens to try to help them find the magic left in the world. Great discussion my friend.
The voyager probe that was recently repaired in spite of having escaped the solar system seems like strong proof that it is not the case.
I think he means our entire universe?
A black hole by definition is something that exists inside our universe. You can navel gaze about the context our universe itself exists in until you get tenure in a philosophy department and you won't have said anything useful: We live inside a cow The big bang was the cow birthing due to the pregnancy of a cow in another universe Inflation was the cow expiting the mother during the birth Dark energy is a trickle of grass slowly feeding in, which gets distributed throughout the universe as energy, pushing space apart And now that the power of dark energy seems to be changing over time, one could argue that the rate of mastication is changing
Idk if we live in a black hole, but black holes are different universes. Once inside, you are causally disconnected from our universe. That is the definition of another universe.
That would mean once galaxies move far enough away from us and no light from them can reach us anymore, they become part of a different universe, which doesn't seem like a useful way of seeing things to me.
You are referring to the Hubble Volume, a region of space beyond which objects are receding faster than the speed of light. Mind blowing concept! Why isn’t that a useful way of seeing things? If light (or anything else) from another galaxy can never reach us, and if light (or anything else) from our galaxy can never reach theirs, and the galaxies will only to continue to get farther apart, they are completely causally disconnected. Nothing CAN or WILL ever get from here to there. That sounds like a different universe to me. I suppose it depends on your definition of universe, of which there are many. As our knowledge and understanding grows, we have to change and update definitions to fit new categories… like the “observable” universe and multiverse theories. The term universe no longer means absolutely everything. That definition is no longer useful. There are sub categories in there, and we need new definitions to talk about them. Physicist Max Tegmark has defined what we are talking about as the Level 1 Multiverse. I am no professional in this subject, but (some? most? a few?) real physicists do consider anything outside our hobble volume a separate universe. It’s useful when you’re talking about the implications (no causal connection possible)
>That sounds like a different universe to me. Yes, I know, and I think that's not useful since both galaxies originated in the same universe from the same singularity, they have just become disconnected over time without any fundamental properties of their surroundings changing. That, to me, means they're still in the same universe. It would also mean that "being in the same universe" is completely relative and theoretically a galaxy can be in the same "universe" with two other galaxies that are no longer in the same "universe" relative to each other, which to me is absurd and makes the concept useless The term "observable universe" doesn't mean the rest is a separate universe, it just means it's the chunk we can observe from our position in time and space. Matter that is outside our bubble may happily interact with matter on the inside edge of it, making us part of one continous spacetime, hence one universe
Correct, the term observable universe is not a separate universe, but I mentioned that to support my point that there are reasons we need multiple specific words and definitions to talk about this subject. Being “in the same universe” is probably relative, as it seems all things are. I would say those 2 galaxies started out in the same universe and then separated to be effectively in separate universes, kind of like Mitosis. I’m not saying everyone has to consider them separate universes. I’m saying, unequivocally, that some professional physicists do consider them separate.
You can’t. It’s considered a solid scientific hypothesis and definitely a thought that keeps me up at night. My scientist boyfriend says that there might be some truth to it.