Particle Horizon Question

[mrbadger]

We have the observable universe, and on the edge of that there is the particle horizon.

We can see things that are now beyond that particle horizon because the light from them crossed it a long time ago. The objects themselves are forever beyond our reach regardless of our level of technology. Anything else who's light hasn't had time to cross the particle horizon never will. It's cut off from us forever.

Apparently.

I have a question.

This particle horizon is a sphere, we sit in the centre of that sphere. This being that any point in the universe can serve as the centre from our frame of reference (if I'm describing that right).

So, what if I got in a spaceship, found a handy wormhole and traveled to a galaxy right by the particle horizon?

Would that then not give me access to the part of the universe otherwise inaccessible from Earth? It doesn't make sense to me that the particle horizon would still be centred on Earth. I should be at the centre of a new sphere.

I'm just thinking that the rest of the universe is only inaccessible to us because we can't get close to the particle horizon. Not because it's a definite boundary.

I heard it described that the edge of the universe is a point in time rather than a point in space. I'm not sure how that fits this.

[RegisterMe]

In short, yes. Last sentence of the first paragraph.

EDIT: just confused things.

[Alan Phipps]

"The particle horizon (also called the cosmological horizon, the comoving horizon (in Dodelson's text), or the cosmic light horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe. "

So the sphere radius is the maximum distance that particles could have travelled in the time of existence of the universe. The implication is that nothing relevant to the current universe existed before that time and/or outside the sphere. One assumes that the calculation of the sphere takes into account any movement/expansion of the universe applicable to the central focus location, or deems it a negligible factor for useful particle observational purposes. (The alternative name Comoving horizon would seem to support that.)

Instantly moving to the edge of an existing sphere just makes particle observation in most previously radially-outward directions completely pointless.

Think of it as you chasing the end of the rainbow.

[pjknibbs]

Yes, the particle horizon would move if you did what you suggested. The reason this isn't actually a problem is because what you suggested is impossible--you can't travel faster than light, after all.

[mrbadger]

But the universe is possibly infinite in size, therefore continues far beyond what we can see. That's what is confusing me. There should be, or could be infinitely more universe, but we can't see or reach it.

I can't see over the horizon, but if I move to the furthest point I can see, the horizon moves further away. I can't see everything. There's nowhere I could stand where I could see it all.

I find it hard to accept that our observable universe is all the universe there is. It would seem as naive as an ant thinking everything they could see was everything there was.

[red assassin]

By definition anything outside the observable universe can't affect us. We assume there's more, as there's nothing to indicate that we're at the centre of the universe. Depending on your exact cosmological model, the universe may or may not be infinite in extent.

[mrbadger]

Then how is it that we assume that the observable universe is a globe around us? At least in the representations I've seen.

That's my point, what if we moved elsewhere in that globe.

[Alan Phipps]

"But the universe is possibly infinite in size, .. "

Not before its creation it wasn't, and it almost certainly isn't now - not without resorting to exotic and extreme creation and expansion theories that are impossible as yet to confirm.

The most likely outcome is that any particle receivable today from any event that happened during the initial creation and expansion of the universe must, by definition, also have originated within the bounds of that sphere. The only alternative being that said expansion was somehow faster than particular travel; - and so we are back to pjk's relativity paradox.

" .. the observable universe is a globe around us?"

Yes, because we have designated the observable universe as a sphere of radius dictated by the universe's age and particle speed. The definition of the 'observable universe' as a sphere places no restrictions at all on the actual size and shape of the physical universe.

[UniTrader]

and yes, by (instantly) moving elsewhere inside (or outside for that matter) of that sphere you would be in a new "observable Universe" Globe.

Think of its as similiar to a Units Sight Range in Strategy Games. If you move the Unit it can see diffrent/new parts of the Map, but also lose sight of things it could originally see. And by definition this Sight Range is centered on the Unit (us)

[Observe]

I doubt there is any edge to the universe; because that would imply there is a line beyond which, nothing exists. I don't think "nothing" is possible. By the same token, "something" is not possible either. What we have, is the outcome of neither "something" nor "nothing". There never was creation and any discussion of it, is nonsensical - as proven by what I've just written.

[red assassin]

Then how is it that we assume that the observable universe is a globe around us? At least in the representations I've seen.

That's my point, what if we moved elsewhere in that globe.

I mean, the observable universe is a sphere around us because it's literally the section of the universe from which light has been able to reach us. The only assumptions involved in it being spherical is that there's no preferred travel direction for light or expansion direction for the universe, but we see no hint that either of those might be wrong. Other points in the universe have their own observable universes, which may or may not overlap with ours. Of course, the observable universe from a given point also changes with time.

Interestingly, due to the expansion of the universe, things that are currently at the edge of the observable universe are actually expanding away from us faster than the speed of light, so without breaking the speed of light, we can't ever move to see something we couldn't previously, but if the Hubble constant was different this wouldn't necessarily be true.

[mrbadger]

Just for example
If I were to identify a location right by the particle horizon and had a means to travel there instantly.
Then I went there and placed a beacon.

Then from the point, from my way of thinking there would be a new, equally distant particle horizon. So if I Traveled there and placed a second beacon that by some fantastical means could communicate with the first with no delay.

And I repeated this several times, as I have tried to demonstrate in this rather poor diagram (sorry, I don't have my drawing tablet downstairs)

[ external image ]

Could I not in effect communicate with an otherwise unreachable portion of the universe? In this case from region A, being ours, to region D, which is probably where Trump comes from.

[euclid]

Just to avoid confusion here: There are two inherently different notions of the size of the universe.

The observable universe, as stated in the OP, is indeed relative to the observer. It is just a consequence of Hubble's redshift equation v=H D
when setting v=c to obtain the radius D=c/H of the observable universe centered at the observer.

And, yes, as it has been said in several replies above, by changing (faster than the speed of light) the observation point the observable area is different.

However, even this theoretical play finds it's limit by the actual size of the universe. A common assumption is that the Big Bang took place about 14B years ago and is expanding ever since. The rate of expansion is under debate and, as it has been mentioned in previous replies, depends on the cosmological model.

Cheers Euclid

[Alan Phipps]

If you had the means to travel and communicate instantly over such distances then the whole concepts of 'observable universe' and 'particle horizon' would become redundant. These are just man-made concepts and logical limitations to investigational approaches and not any physical phenomena.

The real question is what you would expect to find if you could instantly travel to the physical expansion boundary of the actual universe (or beyond).

Anyway, how would you know where to travel to?

[mrbadger]

I assume you'd look for the oldest thing you could work out a location for and go there.

[Alan Phipps]

So ... that would be on the particle horizon maybe?

Or logically would that be the point of origin for the expanding universe and hence at the very centre of known creation?

This all assumes though that the associated particles received give an indication of point of origin and time in transit.

[red assassin]

Just for example
If I were to identify a location right by the particle horizon and had a means to travel there instantly.
Then I went there and placed a beacon.

Then from the point, from my way of thinking there would be a new, equally distant particle horizon. So if I Traveled there and placed a second beacon that by some fantastical means could communicate with the first with no delay.

And I repeated this several times, as I have tried to demonstrate in this rather poor diagram (sorry, I don't have my drawing tablet downstairs)

[ external image ]

Could I not in effect communicate with an otherwise unreachable portion of the universe? In this case from region A, being ours, to region D, which is probably where Trump comes from.

Q: If I could break the laws of physics, could I not use that to break the laws of physics?

A: Yes.

[brucewarren]

Wormholes? We of the Q laugh at your primitive technologies.

<snaps fingers>

In all seriousness I don't believe in wormholes myself. Yes I know they are supposed to be a solution of certain equations and all that. Thingy-wotsit bridges with cool looking computer graphics and everything but that's no guarantee that they exist, let alone big enough to fly a ship through.

[The Q]

Wormholes? We of the Q laugh at your primitive technologies.

<snaps fingers>

[mrbadger]

Q: If I could break the laws of physics, could I not use that to break the laws of physics?

A: Yes.

Would it break the laws of physics? Aside from the instant communication aspect. And we don't know that can't happen. We only know we can't do it.

We know Quantum entanglement exists. That can't be used to communicate information other than its own state, but it exists.

Since that does, it indicates at least the possibility that some other form of long distance instantaneous (or close to it) communication method may exist.