Observable universe and overlapping spheres

[KobiashiBooBoo]

TL;DR Summary

A thought experiment outside our observable universe - laws of physics the same?

We observe a very distant galaxy, thanks to the JWST. That galaxy has the same laws of physics as we do. Now imagine yourself in that galaxy, in your observable sphere of the universe, using your own telescope, looking in a direction opposite our current one from earth. Would we be able to assume the laws of physics are the same? If so, what can we infer from "observable bubble hopping" as a thought experiment?

 

[phinds]

Our best current model of cosmology posits that all of space, however big it is, has the same laws of physics and every point in it has its own Observable Universe.

 

[jbriggs444]

If so, what can we infer from "observable bubble hopping" as a thought experiment?

The way a thought experiment works is that you postulate some set of principles or axioms. Then you see what you can deduce from that starting point.

If the universe is (on large scales) the same everywhere then yes, any inhabitants of a far away portion of the universe would see that the universe looks the same in every direction. Yes, they too could then postulate that the universe is the same everywhere.

So what?

Note that there is no way for communication with the far away inhabitants to leak information to us from outside our own cosmological horizon. Any data that they can observe (after speed of light delays) and relay to us (incurring further speed of light delays) could, in principle, have been directly observed by us (after speed of light delays).

 

[DaveC426913]

Setting aside our assumptions about the homogeneity of the larger universe, let's just look at your premises and their internal logic:

... imagine yourself in that galaxy, in your observable sphere of the universe, using your own telescope, looking in a direction opposite our current one from earth. Would we be able to assume the laws of physics are the same?

No. Why would we?

There's a missing step, implied but not stated, in the above logic chain. I will add it.

For clarity, I'll use

• observer A (you) in Milky Way galaxy A and observable sphere A,
• observer B (me) in Farpoint galaxy B and observable sphere B.
• galaxy C in OS C.

• I am in galaxy B "Farpoint" in Observable Sphere of universe B, using my own telescope,
• I am looking in a direction opposite you (who are in "Milky Way" galaxy A in Observable Sphere A)
• you, there in galaxy A assume that, just like your Observable Sphere A, my OS B will have the same laws in all directions
• Would you in galaxy A be able to assume the laws of physics are the same in galaxy C?

Here's the thing: if you can assume my galaxy is the same as your galaxy in what it observes, then you don't need to leapfrog at all; you are already assuming that any one galaxy anywhere sees their own OS the same as any other.

 

[KobiashiBooBoo]

Thanks, so in A we observe B, B observes C, etc. My assumption is that B is the same because we can see it. That would necessitate C etc being the same.

 

[DaveC426913]

Thanks, so in A we observe B, B observes C, etc. My assumption is that B is the same because we can see it. That would necessitate C etc being the same.

No.
We are in a Green Galaxy. We look "West" and see another Green Galaxy.
They too are in a Green Galaxy. They may look "East" and see a Green Galaxy, but when they look "West" and see a Blue Galaxy.

Just because we see neighboring galaxies that seem to be the same as ours does not mean that any other galaxy will see what we see. It's just an unsupported assumption.

 

[phinds]

Just because we see neighboring galaxies that seem to be the same as ours does not mean that any other galaxy will see what we see. It's just an unsupported assumption.

Yes, it IS an assumption but it is the heart of the current Cosmological Principle which is widely taken as a correct model.

 

[Hornbein]

There has been plenty of searching for different laws of physics in far away places. Nothing has been found.

 

[EDMM2]

I think what is interesting is when you extend the reasoning further. Each distinguishable location in spacetime will have its own unique "observable universe" and this will become quite important in the far future.

It appears that our universe is asymptotically approaching a de Sitter space, which means that ultimately, the cosmological horizon will become a Killing horizon. The recent results of the Dark Energy Survey appear to be consistent with this. This means that although these bounded regions may spatially "overlap" at one point in time, they ultimately end up as independent and separate causal patches. If the recently published paper, Killing horizons decohere quantum superpositions holds, then this could lead to some very interesting possibilities...

 

[PeterDonis]

although these bounded regions may spatially "overlap" at one point in time, they ultimately end up as independent and separate causal patches.

You might be confusing two separate things here.

It is true that for any given pair of comoving observers, their "observable universes" will eventually become causally disconnected.

However, the size of the observable universe for a given comoving observer does not decrease to zero. It asymptotically approaches a particular finite size that depends on the cosmological constant. For our universe, that is a very large size (billions of light years wide), and so there is still a lot of room for very large gravitationally bound systems (e.g., the supercluster in which our galaxy is currently bound) to be within our observable universe forever.

 

[phinds]

It is true that for any given pair of comoving observers, their "observable universes" will eventually become causally disconnected.

so there is still a lot of room for very large gravitationally bound systems (e.g., the supercluster in which our galaxy is currently bound) to be within our observable universe forever.

Peter, I'm confused. Those two statements seem to be contradictory. Take the second one first. OK, so in the long run our OU will be limited (similarly to what it is now) but WILL contain both the Earth and the Moon. An observer on the moon is comoving with an observer on the Earth, and they both have their own OUs but will be in causal contact with each other, thus in contradiction to your first statement, yes? no?

 

[Ibix]

An observer on the moon is comoving with an observer on the Earth

I assume Peter means co-moving in the cosmological sense of "at rest in FLRW coordinates". Neither the Earth nor Moon is one of those; not even the Milky Way is.

 

[phinds]

I assume Peter means co-moving in the cosmological sense of "at rest in FLRW coordinates". Neither the Earth nor Moon is one of those; not even the Milky Way is.

Ah. Good answer. Thanks. I confused myself with the Earth and moon being in similar realtionships to the CMB but I forgot that neither is comoving.

 

[PeterDonis]

Peter, I'm confused.

That's because you forgot what "comoving observers" means in cosmology. It means observers who follow comoving worldlines in the FRW spacetime geometry, and who therefore recede from each other at the appropriate recession speed based on the distance between them. It does not mean "moving together because they are gravitationally bound". So no, the Earth and Moon are not comoving. They are gravitationally bound.

Edit: I see @Ibix got there first.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

After moderator review, the thread will remain closed. Thanks to all who participated.