[Zer0 (OP)]

What is the difference between the Observable Universe & Thee Universe?

P.S. - Can We ever know the Answer for Certain?

[Kryptid]

The observable universe is as far as we can see (limited by how far light can travel in the current age of the universe), which is very probably not the universe as a whole. The entire universe might even be infinite in size, but we may never have any way of knowing for sure.

[Eternal Student]

Hi.

An alternative way of stating things:

   "The observable universe" is something we can see.   We're all fairly sure that it's a real thing.

   "The Universe" is just a word with various definitions, no-one has seen it.   It's generally understood to mean  "everything, absolutely everything".    This concept on its own is not a simple one.  Can we say it's a set of things, a really big set that has everything in it?   In Mathematics, most set theories are incompatible with the existence of such a set (see  Wikipedia's entry  if you're interested     https://en.wikipedia.org/wiki/Universal_set  ).   To say this more strongly, the  ZFC system of Mathematics - which is what you would have studied at school whether you knew it by the name ZFC or not -   this does not allow the existence of such a set.

Best Wishes.

[Halc]

Apologies to Zero for not keeping it entirely simple.

Hi.

Hi ES! Glad to see you up and around and posting now and then again.

The observable universe" is something we can see.

More than that I think.
I can see the sun of 9 minutes ago, but I can't see the sun now. I just presume from seeing the sun in the past that it is probably there still now, and thus part of the observable universe.

So what we see is for the most part confined to the edge of our past light cone, which at no point gets further away from 'here' than about 6 billion light years away. That's the limit of what we can see. But the visible universe is like the sun now, presumed to be there because we can see parts of its past. That's the usual definition of the visible universe, and it is about 48 billion light years in radius, about 8 times the maximum proper distance from Earth any light falling on it has had.
Similarly, I can't see a live T-rex, but they're part of the visible universe. Similarly, there are plenty of nearby burnt out stars that we can't see because their light has already completely passed us by. Those stars, like the T-rex, are part of the observable universe despite not being in any way observable.

Can We ever know the Answer for Certain?

The Universe" is just a word with various definitions

Exactly. Given that there are various definitions (and interpretations of empirical physics for that matter), there is no one correct answer to such a question. 'The universe' is just what you want it to be. It's just a word, or two of them actually.

It's generally understood to mean  "everything, absolutely everything"

Most often not I think, since then it would not make any sense to talk about a different universe, or the multiverse, or whatever, both of which are 'more of something', limiting 'the universe' to a subset of all that. I would say it generally is meant to refer classically to our particular chunk of 4D spacetime, but not other structures with the same or different physics/properties.

[Zer0 (OP)]

Thanks for keeping it Short & Simple Kryptid

Appreciate your Inputs Eternal

Pfft @ Hal

Fascinating to know that We cannot see the Whole Universe...
But a bit saddening too.

P.S. - So there Really might be a Flying Spaghetti Monster hidden from sight somewhere far far away receding at FTL speeds...hmm!
Maybe it's caught a Cold & keeps Sneezing, that would explain Dark Energy.
And maybe it's Spaghetti Tentacles are warped around in higher dimensions across Galaxies, that explains Dark Matter.
It Surely must have Farted long ago, hence Inflation...
YaaY!
(i finally know it all now)

[alancalverd]

"Observable" isn't restricted to visual, or even identifiable. There's all sorts of stuff out there that has at least a gravitational  influence on everything else, and as far as we know the observable universe therefore extends to the Schwarzchild radius of everything from here outwards.

It is entirely possible, indeed it would be vainly anthropocentric  and ludicrous to think otherwise, that there can be a whole lot of stuff outside, that we will never know about and will never affect us in any way.

[Zer0 (OP)]

Gravity perhaps cannot work FTL.

[Kryptid]

Gravity perhaps cannot work FTL.

Gravitational fields don't have a speed. Changes in a gravitational field, on the other hand, do travel at the speed of light.

[Eternal Student]

Hi.

There's all sorts of stuff out there that has at least a gravitational  influence on everything else, and as far as we know the observable universe therefore extends to the Schwarzchild radius of everything from here outwards.

1.     Could you check that you got the right words in this sentence?
The observable universe has a reasonably precise definition.   You can't increase the size of the observable universe by inferring the existence of something else, either light has been able to reach us from that place or it hasn't.    The universe is a different thing, you can try to make inferences about that.

2.   Inferring the existence of something in our universe based on gravitational effects is still limited.    An eternal black hole would be a good example here.   There's a gravitational pull from it.  In all respects it looks like there should be some mass at the centre of it.  However the interior of such a black hole is not in our observable universe (by definition, since light could never reach us from there).  For an eternal black hole, the interior never has been, is not now and never will be in our observable universe.   Applying what we believe about eternal black holes, the interior isn't even something you can describe with just space co-ordinates.   It's a place and a time and just a time right at the central singularity.  Anyway, hopefully you get my point.  Inferring that it is a place in our universe (the wider universe, not just our observable universe) just by the gravitational effects from it could lead you astray.   It is some gravitational effect with "cause unknown" or "no physical thing that can be located with just spatial co-ordinates".

3.    The only flexibility we have in defining the "observable universe" is whether we consider it to describe just some space (just pure space) or a region of spacetime (space and time co-ordinates).   If it describes some space then we must understand it to mean the space that exists now.  If it describes some  points  (x,t)  in spacetime then we have less of a problem.     Non eternal black holes are one example of something that causes these problems,  i.e.  black holes that weren't eternal but did have a formation time (so typically they form from the collapse of massive stars).    When a black hole forms,  a region of space is simply lost.   To see it more easily, we can't get light from that region any longer, so you could certainly say it isn't in our observable universe now.    If we allow the observable universe to be a set of spacetime coordinates (rather than just space co-ordinates), then   (x= location of the black hole, t)  is in the observable for universe for  t< time of formation      but   (x,  T) for  T >  time of formation is not.    Meanwhile, if we regard "the observable universe" as a set of space co-ordinates only then we must consider it to be a subset of the space that exists now   (e.g. take a time-slice through spacetime) and we see that  x=location of the black hole is not any sort of location that makes sense with t= now.   Any object with a worldine that passed through  (x, time of formation) simply terminated there and cannot be located at (x, t=now).
    Anyway, I'm not sure what "Schwarzschild radius" you were talking about.  There isn't just one boundary for the observable universe, there are multiple boundaries.   Every time we hit a black hole, we have a boundary.  So the observable universe is like Swiss cheese.   It's not even all that spherical at what you might describe as the particle horizon.  The universe is expanding at slightly different rates in different places,  so the observable universe is a hole-y and mis-shapen blob of space.   It is usually growing in size as time passes but not always, when a new black hole forms then some of it is lost and we have to consider the possibility that there could be other GR effects which could reduce the size of our observable universe sometimes.

Best Wishes.

[Zer0 (OP)]

Gravity perhaps cannot work FTL.

Gravitational fields don't have a speed. Changes in a gravitational field, on the other hand, do travel at the speed of light.

But Gravitational Fields might be having a Limit.

[Eternal Student]

Hi.

Gravity perhaps cannot work FTL.

    I don't know.    Nothing much works FTL (Faster than light).   All things should travel at light speed or less.

     However there is some theory about the Alcubierre drive.     See  https://en.wikipedia.org/wiki/Alcubierre_drive
 from which this quote is taken:
       The Alcubierre drive ([alˈkubie:re]) is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, ....

    It requires something like negative mass or a configurable energy field with less energy than the vaccum, i.e. something we haven't identified yet.

    It is very much based on the theory of gravity (General Relativity).  So you'd have some grounds for arguing that GR does hold FTL since it is precisely what makes this FTL system possible.

   I'm not going to say anymore about the Alcubierre drive since it remains speculative (plus - I just don't know much about it).   This is the main section of the science forum and not the "new theories" or "just chat" section, after all.

Best Wishes.

[paul cotter]

Hi Eternal Student, how is the grass now? You want to see mine( actually you don't want to see mine! ). Enough of the frivolities, I think I understand the highly speculative Alcubierre drive but apart from the technical obstacles I see some complexities: if travelling at greater than c as observed by static observers what becomes of the Lorentz factor? Similarly in the frame of the pilot of such a vehicle, does space and time become imaginary?       PS I hope this is not mumbo-jumbo, both my wife and myself have managed to acquire another virus, not covid but definitely making both of us wooly-headed.   

[Eternal Student]

Hi.

if travelling at greater than c as observed by static observers what becomes of the Lorentz factor?

    I've got to keep this short:
1.  It's a thread started by someone else, we can't hijack it.   
2.  It's not mainstream science.
3.  I'm not an expert on the Alcubierre drive.

      The Lorentz factor and things like Lorentz contraction describe what happens in flat space.   In curved space, an object may appear contracted (or enlarged) even when it is has zero velocity but just by virue of being in a different location to the observer and thus where the metric differs.   As such the short answer to your question is just,  Lorentz contraction, time dilation and all the effects we're familiar with from special relativity don't have such simple formulas in General Relativity.   The Lorentz factor  γ   ≠   1 / √(1 - v² / c² )  in curved space.


   Extract from Wikipedia:
  .... With regard to certain specific effects of special relativity, such as Lorentz contraction and time dilation, the Alcubierre metric has some apparently peculiar aspects.....
    Taken from the section under the sub-title  "Physics" in the Wikipedia entry.     https://en.wikipedia.org/wiki/Alcubierre_drive#Physics


Best Wishes.

[paul cotter]

Yes indeed, i'm guilty of diverting this thread, apologies to Zero. Thanks very much for your reply, Eternal Student, I will follow up the link you provided.

[Eternal Student]

Hi.

   I'm not sure that the Wiki article says all that much about Lorentz contraction.   I may have already picked out the most salient sentences.      However, it does have references and links of its own, if time is something you have plenty of.
    I'm also sorry to hear about your illness.  Rest up and get well soon.

Best Wishes.

[alancalverd]

However the interior of such a black hole is not in our observable universe (by definition, since light could never reach us from there).

I smell philosophy! Red rag!

The inside of a rock is not observable (obviously) but that doesn't exclude the rock from being inside our observable universe, and since the interior is contained within the exterior, the interior is also inside the OU.

Set theory trumps philosophy. And a lot of engineering is based on "black box" models:  we can know and exploit the transfer function  of a gizmo (like an automatic gearbox, for instance) without knowing what's inside it.

The Schwarzchild radius doesn't have to be isotropic or even continuous. You can define the SR of  Swiss cheese as the distance from where you are to the boundary between cheese and air.

[Zer0 (OP)]

Yes indeed, i'm guilty of diverting this thread, apologies to Zero. Thanks very much for your reply, Eternal Student, I will follow up the link you provided.

No Apologies warranted & None accepted.

Please don't view the OP as an Aeroplane, & do Not paint yourselves as Terrorists.

Rather, Please consider the OP as a plot of land demarcated for Gardening.

Sow your own Seeds & let them bud & Blossom.
🌹

Also, i have a Confession to make...
I'm a Trekkie..
& I Simply Adore Warp Drives!
🥰

P.S. - Hope & Wish for Mr & Mrs Cotter to Get Well Soon!
🖖

[Eternal Student]

LATE EDITING:  Not all of this is correct.  See the discussion in a later post  #23.  Sorry.

Hi.

I smell philosophy! Red rag!

   It's not philisophy.   It's just mathematics.

   The OU is understood to be limited by distance.    Specifically, the OU from here on earth is all those places in space where light could have travelled the distance from there to here since the big bang   (or since an early time like recombination when space first became transparent enough for light to travel).

   So my kitchen is in the OU even when I close the curtains and turn the lights off.   We don't care if the region actually did emit some light, only that light could have travelled the distance if it had been emitted.   Similarly, we don't care if something could have absorbed the light in some ordinary way along that journey  (the curtains in my example).
    Thus, the interior of your rock is in the OU because the distance from me to the interior of the rock is quite small and we don't care if some other bits of rock could have absorbed the light along the way or if the interior of the rock did emit any light to begin with.

    However, a black hole is something completely different.   The distance from the interior of a black hole to me is too large.

   Here's the Schwarzschild metric:
   dS²   =     (terms in dt)  +  (1-r_s/r)^-1 dr²   + (terms in dθ and dφ)
    Where we are using the Schwarzschild co-ordinates   (t, r, θ, φ) = (time, radial co-ord,  angle θ off the x-axis,  angle φ off the z axis).
   The bit in red  is what's important just because  when r ≈ r_s  = the Scwhrazschild radius  that term → ∞.
    I'm not going to bore everyone with the integration but it'll be enough to note what the metric is telling us.

LATE EDITING: I SHOULD have done the integration and not just take a short cut to thinking.  See https://physics.stackexchange.com/questions/85975/how-much-extra-distance-to-an-event-horizon

    It says, that when  r ≈ r_s    then  a small change in the radial co-ordinate  dr     is worth an infinite amount of distance,  ds.    To say that another way, even if we take a small step across the event horizon  (change r from  r_s-dr   to  r_s+dr ) that is an unbounded amount of physical distance.
   Anyway, that's too far (obviously).   Anything in the interior of a black hole is too far away from us,  it is outside the distance limits permitted by the definition of the OU.  Specifically light has not had enough time to travel from there to me here.   Since that distance between me and anything in the interior of a black hole is always infinite, then light will never have had enough time to reach me from there, so it will never be in my Observable Universe.
       To say this more succinctly, we can exclude the interior of every black hole from being in our OU just on the grounds of physical distance.

Best Wishes.


LATE EDITING:     Changed some phrases   with  "the Observable Universe"  to  "my Observable Universe".    The observable universe is different depending on where you are.   If I've left "the OU" in somewhere and there is some doubt about where we were located, then please assume I meant the OU from planet earth.

[Kryptid]

The inside of a rock is not observable (obviously) but that doesn't exclude the rock from being inside our observable universe

Technically, you could observe the inside of the rock by breaking it open, or via a less destructive method like sound or gamma rays. That's not really an option with black holes.

[alancalverd]

I see a semantic problem. We know that there is a black hole around 1600 lightyears away from us. That is well within the radius of the universe about which we can make meaningful statements (48,000,000,000 ly or thereabouts) so it is within the observable universe, even though its contents are not observable.

It is also probably true to say that we are not observable from inside a black hole, but there is no reason why a person inside a black hole couldn't hypothesise the existence of stuff (including people) outside his OU.

As I recall it, the term "Big Bang" was originally coined as an insult by Fred Hoyle, who propounded an alternative "Continuous Creation" theory for which there was, alas, no actual evidence. Now if we were inside a BH, we would be subject to an incoming rain of photons from spaghettified stuff crossing the event horizon. Pretty much like Hoyle's CC source: incoming energy with no information about its origin. And if a few bits coalesced into point masses, we would find stars and black holes within our SR - just as we do. Which rather suggests that the existence of anything can be considered consistent with there being stuff outside your OU, wherever you happen to be, in addition to unobservable entities wirthin the OU.