Particle Horizon Question

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Post by Golden_Gonads » Mon, 29. Jan 18, 07:44

red assassin wrote: Where's the centre of the surface of a sphere?
The surface of a sphere is two dimensional though.

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Post by pjknibbs » Mon, 29. Jan 18, 08:28

Golden_Gonads wrote: The surface of a sphere is two dimensional though.
Obviously he's giving a simpler two-dimensional analogy to suggest how the universe works? There are theories suggesting the universe is like a three-dimensional analogy of the surface of the sphere, so if you kept flying in one direction for long enough you'd arrive back at where you started.

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Post by brucewarren » Mon, 29. Jan 18, 11:07

If I understood red assassin's explanation it's a three dimensional surface on the outside of a four dimensional hypersphere.

The true centre would the centre of the four dimensional object. If it existed at all it would be outside the visible universe in the same way that the centre of the Earth appears on no maps of the planet surface.

I'm getting old. Back in my youf I was told something about curved space but I must have forgotten. Of course it's also possible that my poor 3 dimensional brain can't visualise 4 dimensions and shut the memory out in self defence.

The fun part is that if a four dimensional balloon were inflated the surface could expand as much as it liked while remaining finite.

Of course it would make mrbadger's journey that much more difficult. Not only would he need to angle his engines at right angles to reality[1] but there might well be nothing there when he arrived.

[1] Yes that is a HHGTTG reference.

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Post by RegisterMe » Mon, 29. Jan 18, 12:09

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Post by red assassin » Mon, 29. Jan 18, 22:49

brucewarren wrote:As for the Earth being special I do have an argument of sorts.

According to the Weak Anthropic principle we are only able to hold this debate because the Earth is capable of supporting life. We are not random observers but privileged ones.

It doesn't prove that it was created for us of course but it does mean we just happen to live in the posh part of town.
Yeah, I'm actually a fan of the anthropic principle in general, and it's a decent explanation for why, for example, we should end up with a universe in which galaxies and stars and planets and bundles of organic molecules with delusions of grandeur can actually exist, when one could easily imagine universes in which none of this is possible.

However, I think it's a bit unhelpful for the question of whether or not the Earth is unique because it's jumping the gun a bit: in the case that life really is vanishingly unlikely to arise, the anthropic principle would neatly explain why we're here to ponder how unlikely it is anyway. But that's an entirely separate question which assumes the premise that the Earth *is* unique, it's not a reason to believe that a priori. If you want to argue the Earth is unique, you need some sort of reason for that to be true - something which is demonstrably vanishingly unlikely. And the awkward thing here is that the harder we look, the less sign of that we find. From animal intelligence to exoplanets to amino acids in space, the last few years have delivered a bunch of blows to Great Filter hypotheses. I'm being a little unfair here, of course - it's very difficult to prove the negative that we really are unique, in general. But if we'd looked out into the universe and not seen planets, for example, that might have been instructive.

Golden_Gonads wrote:The surface of a sphere is two dimensional though.
Yeah, as pjk and bruce have said, this wasn't so much an explanation of the shape of the universe as a general point that geometry is a lot more varied than Euclidean. There are proposed cosmological models that are simply an analogue of the sphere with an extra dimension (we occupy the three-dimensional surface of a four-dimensional hypersphere), and there are models with far more exotic topologies. This is perhaps a little moot at this point, as evidence at this point suggests the universe as a whole is pretty damn flat (which would imply that it's infinite), but local topology certainly isn't flat and it's important to keep in mind that that isn't a requirement.
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Re: Particle Horizon Question

Post by Morkonan » Tue, 30. Jan 18, 00:00

mrbadger wrote:....So, what if I got in a spaceship, found a handy wormhole and traveled to a galaxy right by the particle horizon?...
Keep in mind that the light cone moves relative to your position.
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.
It's relative... Our only reference is Earth.
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.
It's a relative boundary. With a catch. :)
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.
Yes/No

First. there are a number of things we have to accept, at least for now, as "True."

The physics of the Universe doesn't change from point to point and we are not a privileged observer.

The speed of light can not be exceeded by a mass-bearing object in normal space.

Space is "expanding" in all directions, possibly even accelerating in its expansion.

Gravity, and the other forces, are locally stronger than the forces of expansion.

Gravity, and the other forces, can not exceed the speed of light in their effects.

(PS- "Speed" is a term used loosely...)

<Incoming "Balloon Analogy>

It helps to picture the Universe as a balloon filling with air. On the surface of the balloon, someone has affixed little markers that represent galaxies. As the balloon expands, the galaxies move further apart from each other. In fact, every point on the surface of the balloon moves further apart from the other points. At any one point on the balloon, it appears as if all points are moving away from that point as the balloon expands.

Applying the analogy of the balloon to one of our Universe expanding isn't "exact", that's why it's an analogy. But, we'll take it a bit further.

Let's say that we are an ant one point on the balloon and that the balloon is expanding. But, it's also accelerating. We decide that we want to wal over to another point on the balloon. If it's close, we can make it. The relative rate of expansion between where we are and where we are going is comfortably within walking speed. But, if we wanted to go further, well that's another problem. See, by the time we got halfway, the relative rate of expansion between where we started, our galaxy, and where we wanted to go, a galaxy outside of our light cone, will be greater than the speed at which we can walk, which is lightspeed, which is as fast as our ant legs can carry us.

If we hopped onto the bumper of a car and were magically transported to a far away place on the surface of our balloon close to the limits of distance which we could walk, otherwise, due to the relative rates of expansion between the two points, then jumped out and looked around, we would find ourselves exactly in the same predicament. Further, if we wanted to return home, we couldn't, unless another magic car came by.

I'm not sure what to make of the "time" analogy other than that the definition of "time" is simply "something changes." The progress of time is certainly undeniable, but that it "exists" as "something" is arguable, at least according to some. Are we bound by it? Does it depend on our observation to exist? /shrug

The light cone (your "particle horizon") is relative. If you plotted two points with separate x and y locations on a graph, with the graph representing time in the z axis (Yay, 3d graphs..) and then moved the points in-sync along lines that diverged, in which all lines diverge in this model, then eventually those points would be, relative to each other, too far apart so that their light cones would eventually separate.

When two points separate away from each other at a combined acceleration faster than the speed of light, then nothing, not light, not "particles" not matter, can ever be exchanged between them. That doesn't mean either is moving faster than light can travel, it only means that the speed of the acceleration of distance between them is greater than the speed of light. That applies to every point in the Universe, ever.

Since we can't travel faster than light, we can never "see" beyond the light-cone relative to our little region of the Universe.

A few notes:

The period of rapid expansion during what is believed to be the first few moments of the "Big Bang" is responsible for the expansion of the Universe and the initial distribution of what would become "matter" once everything cooled down a bit. That's why a "Big Bang" by itself, like an explosion, didn't happen like a fire-cracker explosion analogy. It expanded more rapidly than any fire-cracker explosion could possibly expand, today, because... it was a baby Universe and didn't know any better... yet. :) This rapid expansion is responsible for the fact that stuff is spread all over the place, much more distant than we may have otherwise thought. It's also responsible in large part for matter being, more or less, evenly distributed. And, it's responsible for certain regions that have slightly concentrated ripples of matter, where the "wrinkles" were smoothed out, so to speak.

Gravity is the somewhat unknown variable, here. There's no Grand Unified Theory to tell us what it's really doing or what its effects are. Just remember that gravity, itself, can't effect distant objects faster than the speed of light. That means that the gravity from objects outside of our light-cone can't effect us, directly. BUT, we also know that gravity never "stops." There is no anti-gravity. Gravity's effect simply tapers off with distance, never reaching zero. Since it can't travel faster than light, though, then the gravity of star outside our light-cone can't effect us, but it can effect closer objects that are within its light-cone. And, those objects can effect us.

Something that might be worth considering - If the above is true, we, being our galaxy and the rest of our observable universe (light cone) will be less and less effected by the gravity of distant objects around us as the separation rate of expansion between us and them begins to exceed the speed of light. What will that mean for us? We're effected by gravity from "all sides." Granted, the effect is minuscule on the scale of interactions between truly distant cosmological objects. BUT, it's still "there."

Note: Objects like the Andromeda galaxy, which is due to combine with our own in the distant future, are accelerating towards us due to their relative velocity and the interactions of gravity, both of which are causing us to come together faster than the relative rate of expansion between us, since they are more significant and stronger than the force of the Universe's expansion. ie: Though we don't have a GUT, we "know" that gravity and the other forces are stronger than whatever is causing the expansion of the Universe. That means we won't be ripped apart... probably, at some future time. Or, so we think. :)


A note on the Anthropic principle and other semi-philosophical "scientific" points - Cosmology is not an experimental science. By definitions, which are required for the scientific plausibility of the Anthropic, and other, principles, it can not be proven. It's impossible to prove it. It's generally, or narrowly, accepted because of the convention of "Well, here we are, we gotta explain it in a sciencey sort of way, else we run the risk of trying to explain in in a non-sciencey sort of way." Also, there's the constant fight between Science and Religion that many consider to be somehow significant, which I think is a crock... Any time anyone discovers that "Cosmology" is about the "Universe" they're going to ask the cosmologist on the talk show "Where did we come from and why are we here." So, they came up with an answer.

IMO, as far as Science and Cosmology is concerned, that's not their question to answer. They don't study "whys", they study "hows." It's an entertaining diversion and helps some people sleep at night, but it's really just philosophizing with science. "Could" it be important or yield important results? It "could." But, no matter the results, without some new science, it can never be proven and likely can only be implied to a certain limit.

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Post by RegisterMe » Tue, 30. Jan 18, 00:34

"Chaining together" light (or graviton, or whatever) cones like that sounds distinctly sketchy to me.
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Re: Particle Horizon Question

Post by pjknibbs » Tue, 30. Jan 18, 08:31

Morkonan wrote: The speed of light can not be exceeded by a mass-bearing object in normal space.
The speed of light can't be exceeded by massless objects either. It can't be *reached* by an object with mass because it would take infinite energy to do so.

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Re: Particle Horizon Question

Post by mrbadger » Tue, 30. Jan 18, 09:07

Morkonan wrote: stuff that is too complicated without enough caffiene
I wake up, have one coffee and get hit with this...
Thanks...
But it makes some sense, I think.

Given that the big bang, which the consensus seems to agree was what happened, radiated out in all directions.

Would it be that anything on the 'other side' of the region of expansion from where we are would be always locked away from us from the start? The other side of the balloon as it were. So there would be something on the other side of the visible 'edge'.

Given that space is always expanding in all directions anyway we'd have enough trouble traversing or viewing the bit of the universe we can see.

I wish I'd studied modern physics in school. I'm probably missing something really basic.
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Post by Alan Phipps » Tue, 30. Jan 18, 14:53

"I wish I'd studied modern physics in school."

Nah, the relevant physics is old as the Universe. We just haven't pinned it down yet. :D
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Re: Particle Horizon Question

Post by Morkonan » Tue, 30. Jan 18, 22:36

pjknibbs wrote:
Morkonan wrote: The speed of light can not be exceeded by a mass-bearing object in normal space.
The speed of light can't be exceeded by massless objects either. It can't be *reached* by an object with mass because it would take infinite energy to do so.
I was being very specific and trying to avoid rebuttal comments in the other direction. :)
RegisterMe wrote:"Chaining together" light (or graviton, or whatever) cones like that sounds distinctly sketchy to me.
Why? It's pretty simple, really. Imagine any two points in space and time. Since nothing in the universe can exceed the speed of light, we have a boundary of "experience." Anything outside that boundary can't have any meaningful interactions. Suppose the two points separate, relative to one another. Now, imagine that the speed of separation, not their true velocity but the combined velocity, exceed the speed of light. In other words, let's say they are traveling in opposite directions and the space between them is increasing not only due to their own relative velocity but also due to the added factor of an expanding universe adding its numbers into the mix. The rate of increase of the separation distance between the two objects can exceed the speed of light in this case, even if the two objects do not. And, if that happens, then those objects can never interact. Light, gravity, and any other forces, so long as lightspeed is the speed limit, can never reach from one to another because it can't travel fast enough to overcome that separation rate.

That doesn't mean that events don't occur, however, but they may as well not have to observers at either point.
mrbadger wrote:...Given that the big bang, which the consensus seems to agree was what happened, radiated out in all directions. ...
It created "all directions." :) At least in how we experience them.
Would it be that anything on the 'other side' of the region of expansion from where we are would be always locked away from us from the start? The other side of the balloon as it were. So there would be something on the other side of the visible 'edge'.
That depends on what model one is looking at. We could be living in a universe filled with bubbles of other universes, each a product of their own "Big Bang." But, that there is any medium at all or even if "distance" exists between them is up for debate.

Personally, with only an enthusiast's knowledge, I think that we keep seeing evidence that what's "in" our Universe is largely the tip of the iceberg of "other things." IOW - The Cave of Socrates. What we experience are the shadows of forces lying "outside" our Universe. Our universe is where they interact/come together/aggregate effects, etc.. It's a view loosely based on the idea of an Ekpyrotic universe, a universe formed by what could be called a "collision" of other... stuffs. Imagine two bubbles coming together. They touch/intersect and, walla, there's a flat plane created. (The "flat plane" is just analogy for a universe, seemingly a third object, being created, not anything else. The formal ekpyrotic universe theory relies on string theory, though, which we have no evidence for.)

A couple of things regarding "particle horizon:"

I had assumed you meant our light horizon or a relative light horizon and described things in that way. A cosmological particle horizon is slightly different. ie: How ""far" did stuff go and what's it going to" vs what I was focusing on - "how distant does something have to be before we can no longer interact with it." They both have similar limits.

If your question is asking "what is our universe expanding into" then the answer varies. I think most would say it's not expanding into anything at all, but is simply getting bigger with increasing expansion. But, depending upon what model one is in love with, it could be expanding into anything from a shared medium in which there are many universes expanding, some that may have different laws, to "there is nothing else."

Bluntly, honestly, and with complete sincerity, there are many models and ideas revolving around "why this universe" that appear, IMO, to simply be rebuttals of religious doctrine and attempts to explain "creation" in ways that preclude any "Intelligent Design," specifically the "fine tuning" of the laws of this universe. So, angels and the heads of pins get invented where there is no need to invent them - Whatever the origin or the state of the existence of "everything else", we are here and the universe appears to exist. Science and Religion may go on about their way, confident they're not stepping on each other's egos.

But, if you're looking for an "outside" that could exist beyond the particle horizon of the Big Bang, then "Bubble Universe Theory" is where you should probably go. There has been "real science" trying to find evidence for this, by the way. Though, what the boundary between these bubble universes may "look like", if even that could apply, is up for grabs. To be sure, it's not like one would reach a point in our universe where we look in a specific direction "away" from some relative center to "see" the edges of other expanding universes.

Something else - You might reach a point where, if you could look across the entire universe towards where you came from with an pair of infinity googles, given enough time, you'd see your own backside. Or, well at least the point from which you started your journey. 1

In any event, our experiences are forcibly limited to our light-cones. We can not meaningfully interact with anything beyond those. If we somehow travel to this boundary, the goalposts are reset relative to our new position... It could be possible, though, to see things like gravitational interactions of extremely distant objects with objects that exist outside of our our relative light-cones. ie: We look at the most distant galaxy and somehow see that it is behaving unusually, hinting at the notion that there is "something else" beyond the light horizon which we can not detect or interact with. Is this "something else" just a cluster of galaxies beyond our horizon or evidence of gravity interactions with another universe? Dunno. :)

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Post by Chips » Wed, 31. Jan 18, 00:39

Erm, i don't follow. I was under the impression that ...

Light is point source, at any point if light is shining, it's travelling at speed of light. Move the light at 1,000,000 meters per second, it doesn't mean light relative to any instant is c - 1,000,000m/s - it is still c in every direction.

So even with expansion, it's possible A will see light from B even if they're both travelling at 0.8c in opposite directions.

Or is that wrong?

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Post by Observe » Wed, 31. Jan 18, 00:54

My understanding, is that while we measure photons traveling at the speed of light, for the photon, the travel time is instantaneous. From our point of view, photons emitted from the first stars, have taken 14 billion years to get to our eyes. However, from the photon point of view, it took no time and the distance was zero. Or, do I have things all wrong?

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Post by euclid » Wed, 31. Jan 18, 04:01

pjknibbs wrote:
euclid wrote: 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 actual edge of the observable universe is about 46 billion light-years away from us, though, not 14 billion, mainly because light from objects that used to be much closer has had time to reach us despite those objects now being much further away due to the expansion of the universe.
I never mentioned the distance of the "edge", only the time of the beginning of our universe. Just to be on the same page here: The limitation of the speed of information transport is in the space along a geodasic/geodatic. However, the Big Bang caused the space itself to expand which implies that speeds greater than the vacuum speed of light are possible (General Relativity) although no information is "transported". Thus, concluding from the time 14B years of the birth of your universe to its radius of 14B light years is not correct. The actual radius is somewhat debatable. It depends on the rate of expansion of our universe. The 46B light years are, again, a common assumption.
mrbadger wrote: .......
Given that the big bang, which the consensus seems to agree was what happened, radiated out in all directions.

Would it be that anything on the 'other side' of the region of expansion from where we are would be always locked away from us from the start? The other side of the balloon as it were. So there would be something on the other side of the visible 'edge'.
.......
That is an interesting question. In theory (see the classic "Gravitation", Misner, Thorne & Wheeler, 1970) there are two scenarios concerning the connectivity of a singualrity to the space in which it occurs: Either it is still connected like a black hole (observed and confirmed first in 1990: Cygus X-1) or it has separated to form an own space bubble. You can imagine that like a dripping tap in (very) slow motion freezing the "movie" at two instances: 1# The drop forms but still is connected via a thin bridge of water to the tap; 2# The drop just disconeccted from the tap to form a sphere.
If the Big Bang singuarity happened within the first scenario then it is possible that there is indeed an "outside", i.e. space beyond our universe expansion barrier. However, there seems to
be some evidence (at least in the sense of the consistency of the underlying theory) that our universe is a "closed" bubble (i.e. 2nd scenario) and hence there is no "outside".

And Chips, yes. That's the fundamental (and genious) idea of Einstein's Special Relativity.

Cheers Euclid
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Post by mrbadger » Wed, 31. Jan 18, 18:56

So at the moment at least some people think we can in fact see the entire universe?

I thought there were bits we couldn't see because the light from those places couldn't reach us due to expansion, but we knew they must be there. (sorry if I'm sounding dumb now, I'm a bit confused)
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Post by brucewarren » Wed, 31. Jan 18, 19:33

I'm going to have a stab at this. Feel free to laugh when I get it all wrong.

Since the speed of a light emitting object has no effect on speed of light, the light from the remotest object must get here eventually, but of course by then the image will be out of date. What we see in the telescope is not what the universe is but what the universe was. There will have been a time when the universe was still small enough for the light from the remotest bit, as it was then, to have got here by now.

So we can see the whole universe as it once was. We cannot see it as it now is.

Of course now we run into the philosophy of the tree falling in the woods. If the bear doesn't see or hear the tree as it falls on his head, does the tree exist?

To some folk if a thing cannot be measured it doesn't exist. To others it does.

So does the up to date universe we can't see, as opposed to out of date version we can, exist? I think it does but your mileage may vary.

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Post by RegisterMe » Wed, 31. Jan 18, 19:48

brucewarren wrote:the light from the remotest object must get here eventually
I think that's incorrect because the universe is expanding. Imagine two light bulbs plugged through a rubber sheet. They start ten feet away. Now stretch the sheet so that they are twenty feet away. The speed of light is the same but the distance it has to cover to be seen by the "other light bulb" has doubled.

This is why though the universe is "only" ~14 billion years old the radius of the observable universe is ~46.5 billion light years.
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Post by RegisterMe » Wed, 31. Jan 18, 20:23

Actually I think what I wrote above is wrong, and that Bruce is right. I'm now thinking that the "edge" of the observable universe is a time based horizon, and with the passage of time it gets further away from us.
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Post by Observe » Wed, 31. Jan 18, 20:46

I realize we are talking about 'space' expanding the distance between galaxies, but half in jest, I wonder if the universe is expanding, then the 'space' between the atoms in our body is expanding too? At some time in the future, will I be twice my size, but since the measuring rod will also be twice size, no evidence of expansion will be seen on this small scale? :D

[EDIT] If 'space' is expanding, isn't time also expanding. Otherwise, the speed of light would not remain constant?

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Post by RegisterMe » Wed, 31. Jan 18, 21:37

Observe wrote:[EDIT] If 'space' is expanding, isn't time also expanding. Otherwise, the speed of light would not remain constant?
Nope. Space is expanding, time remains constant, the speed of light remains constant, hence redshift.
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