Question about "our observed universe"

[ronald_dai]

TL;DR Summary

Our unobserved universe might still influence us since they might be observable to others

I feel a bit troubled by the use of "our observed universe" as a concept of reason concerning the nature of universe. For example, the following is part of the answer of Halc to my previous question:
[
Rotation around an attractor only works given the causal reach of the mass, so nothing outside our visible universe can have an influence on our motion, and thus we cannot rotate around it, however slowly.
]

The universe beyond our observed part could be the observed universe for those at the far end of our observed universe. Therefore, if there is a big attractor beyond our observed universe, but in their observed universe, their movement could be influenced by that attractor ...and if they are big enough, they would influence some other part in our observed universe, and if those again are big enough, they would influence us as well...SO it seems not strictly proper to use "our observed universe" as the reason to deny the influence from the far away...

Thanks

 

[PeroK]

Summary:: Our unobserved universe might still influence us since they might be observable to others

I feel a bit troubled by the use of "our observed universe" as a concept of reason concerning the nature of universe. For example, the following is part of the answer of Halc to my previous question:
[
Rotation around an attractor only works given the causal reach of the mass, so nothing outside our visible universe can have an influence on our motion, and thus we cannot rotate around it, however slowly.
]

The universe beyond our observed part could be the observed universe for those at the far end of our observed universe. Therefore, if there is a big attractor beyond our observed universe, but in their observed universe, their movement could be influenced by that attractor ...and if they are big enough, they would influence some other part in our observed universe, and if those again are big enough, they would influence us as well...SO it seems not strictly proper to use "our observed universe" as the reason to deny the influence from the far away...

Thanks

It all takes time. Galaxies currently outside our observable universe may influence us in the future. Currently, neither their light nor their gravitational influence has had time to affect us.

 

[ronald_dai]

[ It all takes time. Galaxies currently outside our observable universe may influence us in the future. Currently, neither their light nor their gravitational influence has had time to affect us. ]

But this does not exclude the possibility that our galaxy is rotating around something beyond "our observed universe"...our unobserved universe is even not necessary to be the unobserved universe for the center of our galaxy...if the center of our galaxy is influenced by something beyond our observed universe, and we are rotating around the center of our galaxy, how could we claim that we have nothing to do with that unobserved thing?

 

[FactChecker]

The universe beyond our observed part could be the observed universe for those at the far end of our observed universe. Therefore, if there is a big attractor beyond our observed universe, but in their observed universe, their movement could be influenced by that attractor ...and if they are big enough, they would influence some other part in our observed universe, and if those again are big enough, they would influence us as well...SO it seems not strictly proper to use "our observed universe" as the reason to deny the influence from the far away...

If an unobservable object has influenced an observable object, that influence can not be observable to us yet. In other words, the influence of an unobservable object, or evidence of it, can not get to us faster by going through intermediary objects.

 

[PeroK]

[ It all takes time. Galaxies currently outside our observable universe may influence us in the future. Currently, neither their light nor their gravitational influence has had time to affect us. ]

But this does not exclude the possibility that our galaxy is rotating around something beyond "our observed universe"...our unobserved universe is even not necessary to be the unobserved universe for the center of our galaxy...if the center of our galaxy is influenced by something beyond our observed universe, and we are rotating around the center of our galaxy, how could we claim that we have nothing to do with that unobserved thing?

If something influences the centre of our galaxy then it will influence us directly or indirectly about 30,000 years later (at most). It's not the case that something can influence one part of the galaxy and not another for longer than the number of light years between the two.

If the question is whether our galaxy can be in a gravitationally bound orbit around some huge mass outside the observable universe, then the answer is clearly no.

If you're talking about the universe as a whole rotating, then that was answered by @PeterDonis in the previous thread: there is no evidence for a Goedel rotating universe.

 

[ronald_dai]

If something influences the centre of our galaxy then it will influence us directly or indirectly about 30,000 years later (at most). It's not the case that something can influence one part of the galaxy and not another for longer than the number of light years between the two.

If the question is whether our galaxy can be in a gravitationally bound orbit around some huge mass outside the observable universe, then the answer is clearly no.

If you're talking about the universe as a whole rotating, then that was answered by @PeterDonis in the previous thread: there is no evidence for a Goedel rotating universe.

Thanks for your explanation...But I think the key in the issue here is that the speed of light is a constant, but the expansion rate of the universe is not...so what lie beyond our observed universe now might not be beyond our observed universe long time ago...and once the momentum of the rotation started, it needs some other force to stop it...so based on our current observation capacity, it does not seem to be logically strict to me to draw the conclusion that as long as the thing is not in our current observable universe, it has never influenced us and it is absolutely impossible for us to move around it...

 

[ronald_dai]

If an unobservable object has influenced an observable object, that influence can not be observable to us yet. In other words, the influence of an unobservable object, or evidence of it, can not get to us faster by going through intermediary objects.

The key issue here is that the speed of light which we use to gauge the size of the observed universe is a constant, but the expansion of the universe is accelerating, which mean what lie beyond our current observed universe could be within our observed universe long time ago...but once its mass exerted some influence on our planet, we need some counter force to remove that influence...otherwise, it will remain there...and if the influence led to a rotation with a peculiar velocity perpendicular to the expansion, then the expansion itself would not eliminate that peculiar velocity...

 

[PeroK]

Thanks for your explanation...But I think the key in the issue here is that the speed of light is a constant, but the expansion rate of the universe is not...so what lie beyond our observed universe now might not be beyond our observed universe long time ago...

The horizon of the observable universe has been moving outwards since the CMB radiation was emitted. There hasn't been a period of inflation since then. The CMB pattern is as far back as we can see. Anything that is influencing our galaxy's dynamics today must be in that picture somewhere.

But, to be honest, the gravitational influence of objects that far away is hardly relevant. You can do the maths. The Earth's orbit round the Sun is almost entirely due to the Sun. Not to the other stars in our galaxies, nor neighbouring galaxies, let alone galaxies at the edge of the observable universe, and certainly not galaxies outside the observable universe.

The role of distant galaxies in terms of cosmology is not in terms of local galaxy dynamics, but in terms of large scale isotropy, the FLRW model and the expansion of the universe.

Your attempts to introduce distant galaxies into local galaxy dynamics are counter-productive in terms of your learning cosmology, IMO.

 

[ronald_dai]

The horizon of the observable universe has been moving outwards since the CMB radiation was emitted. There hasn't been a period of inflation since then. The CMB pattern is as far back as we can see. Anything that is influencing our galaxy's dynamics today must be in that picture somewhere.

But, to be honest, the gravitational influence of objects that far away is hardly relevant. You can do the maths. The Earth's orbit round the Sun is almost entirely due to the Sun. Not to the other stars in our galaxies, nor neighbouring galaxies, let alone galaxies at the edge of the observable universe, and certainly not galaxies outside the observable universe.

The role of distant galaxies in terms of cosmology is not in terms of local galaxy dynamics, but in terms of large scale isotropy, the FLRW model and the expansion of the universe.

Your attempts to introduce distant galaxies into local galaxy dynamics are counter-productive in terms of your learning cosmology, IMO.

Well, I guess I probably did not phrase it extremely clearly...my concern in this issue is not [in terms of local galaxy dynamics]...I am referring to the logic of using "our observed universe" as a logical concept in general...If CMB could tell us the whole history of the universe since the Big Bang and we can base our knowledge on that, then we won't have the limit of the "our observed universe"...as I said, if the peculiar velocity was created towards some system long time ago, then it will remain there unless we have some other counter force to eliminate it...

I believe my way is the best for me to know the nature of things based on my personal background, no matter in which field...I am not taking a course in cosmology, I am more interested in the grand view of the universe as a philosopher...again, what bothers me is not just the argument here, but the use of "our observed universe" as an absolute base for reasoning in general!

 

[phinds]

I believe my way is the best for me to know the nature of things based on my personal background, no matter in which field...I am not taking a course in cosmology, I am more interested in the grand view of the universe as a philosopher...again, what bothers me is not just the argument here, but the use of "our observed universe" as an absolute base for reasoning in general!

Then you are not discussing science but philosophy (apparently based misunderstandings of science) and you'd likely get discussions more of interest to you by posting this on a philosophy forum (which PF does not have).

 

[phinds]

I believe my way is the best for me to know the nature of things based on my personal background, no matter in which field

That is a truly awful way to approach science. We humans evolved over a VERY limited range of phenomena and a lot of things in the very large (cosmology) and very small (quantum mechanics) are TOTALLY non-intuitive and trying to use our "common sense" or "intuition" or experience to explain them is usually a dead end.

 

[ronald_dai]

Then you are not discussing science but philosophy (apparently based misunderstandings of science) and you'd likely get discussions more of interest to you by posting this on a philosophy forum (which PF does not have).

Your quote of what I said is my response to the moral education made to me by a senior member here...and your comment is another moral education while you could not respond to my argument in scientific term...I am really surprised how proud people here with "physics" aura would argue using moral education when they could not respond to the issue in scientific logical terms...Please, argue with scientific term, not moral education here!

 

[PeroK]

I am referring to the logic of using "our observed universe" as a logical concept in general...

I am more interested in the grand view of the universe as a philosopher...

I don't know what either of these terms means, so they don't form any basis on which a discussion can take place. Cosmology is about the Friedmann equation and cosmological models. "Logical concepts" and "grand views" don't come into it. Only models and observations.

 

[Ibix]

Please, argue with scientific term, not moral education here!

We can't. You aren't talking about any well-defined model based on observations and rigorous reasoning and consistent with experiment, but about something much vaguer that you have invented. Thus there is nothing here that can be discussed in scientific terms. The pointlessness of such debates is why they're out of bounds in this forum.

We can, if you are interested, tell you about the actual models and observations and suggest ways for you to learn about them. For example, the causal structure of spacetime is a well-founded concept, based on the observed existence of a finite invariant speed (which turns out to be that of light in vacuum). The existence of an observable universe, and the fact that the current behaviour of everything we can see can only depend on other things we can (at least in principle) see follows directly from that. The chain of reasoning in your first post implies that you don't understand that. Given that, I would suggest that in order to reason about cosmology, you first need to start with learning about special relativity before advancing to general relativity and cosmology. There are a great many resources (many of them free) that we can suggest to help with this.

 

[PeroK]

... starting with this excellent Insight:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

 

[jedishrfu]

Since this thread is waxing philosophic, i think its time to close. Thank you all for contributing.