Question About the Universe

You would never see it. Your CH (Cosmological Horizon) basically represents the point where space is expanding so much that even something travelling at the speed of light will never reach you.
The only way you would ever see it, is if he could "chuck it" faster than the speed of light. Since the speed of light is the speed limit of the universe, this is impossible.Not only would you never see it, it would also never get to you. In fact, it would only get further and further away from you I think it's best to describe in terms of units.Speed of light has the units: m/s (meters per second)
Expansion of the universe has the units: (meters per second per meter)... or the "speed of the expansion" per "distance of space that is expanding"Here's a (really long) example that kind of shows the math behind it, if that helps: Example: Think of an ant walking along a rubber band that is being stretched out.

Let's say the ant walks at 1 inch/s

Let's say that each inch of rubber expands at 0.1 inch/s

Let's mark two points on the rubber band that are 3 inches apart.

Say the ant starts walking from one point to the other...

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After 1 second:
-The 3 inches from the ant to the other mark has expanded by 0.3 inches
-Therefore, the ant had to walk 3.3 inches to the other mark
-The ant has walked 1 inch
-Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 2.3 inches.

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After 2 seconds:
-The 2.3 inches from the ant to the other mark has expanded by 0.23 inches
-Therefore, the ant had to walk 2.53 inches to the other mark
-The ant has walked 1 inch
-Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 1.53 inches.

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After 3 seconds:
-The 1.53 inches from the ant to the other mark has expanded by 0.153 inches
-Therefore, the ant had to walk 1.683 inches to the other mark
-The ant has walked 1 inch
-Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 0.683 inches.

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After 4 seconds:
-The 0.683 inches from the ant to the other mark has expanded by 0.0683 inches
-Therefore, the ant had to walk 0.7513 inches to the other mark
-The ant has walked 1 inch
-Therefore, the ant has reached the other mark, and actually walked passed it by 0.2487 inches.

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Now we're going to do the same thing, but instead of marking two points 3 inches apart, we're going to mark two points 30 inches apart:

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After 1 second:
-The 30 inches from the ant to the other mark has expanded by 3 inches
-Therefore, the ant had to walk 33 inches to the other mark
-The ant has walked 1 inch
-Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 32 inches!

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After 2 seconds:
-The 32 inches from the ant to the other mark has expanded by 3.2 inches
-Therefore, the ant had to walk 35.2 inches to the other mark
-The ant has walked 1 inch
-Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 34.2 inches!

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...and it doesn't matter how much longer the ant is walking towards the other mark, it's getting further and further away, he'll never get there, it's impossible.

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It's not so much that there's "too much expanding universe pushing back" just that there's simply "too much expanding universe to overcome at the speed of light."

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The Cosmological Horizon depends on 2 things:
1. The speed of travel (speed of light vs walking speed of the ant)
2. The speed of the expansion of the universe (currently about 74.2 km/sec/Mpc for our universe vs 0.1 inches/sec/inch for the ant)

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Our CH is about 46 billion light years (vs 10 inches for the poor ant...).

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The "area of the universe" between you and your keyboard is actually expanding at the exact same rate. It's just that there's so little "space" there that you can easily overcome it's expansion with the movement of your arms (or to even notice). For something billions of light years away... there's just a lot more "space" in between us that's all expanding to overcome.

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Some Dude's Blog on the Speed of the Expansion of the Universe
Wikipedia - Cosmological Horizon

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Oh, and if you're wondering... the expansion of our universe (74.2 km/sec/Mpc) is about the same as:
2.4x10^-18 inches/sec/inch, or again written as:
0.000 000 000 000 000 002 4 inches/sec/inch

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Edited by Stile, : Converting expansion of our universe into inches/sec/inch bitches!!

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Edited by Stile, : Corrected title 'cause it was wrong

Lacking the requisite math, I rely on analogies to reach some small degree of understanding about the Big Bang and other cosmological marvels.When the pieces come together (determined mostly by being able to rephrase the analogy in a way an expert finds acceptable), it is exhilarating: I often star gaze, and sometimes mull over what I understand about what I see.So, to my question and current hope for enlarged understanding: Is the universe more or less spherical? Does the expansion of space following the Big Bang occur in spherical symmetry?Or are we lumpish? The explanatory diagrams I've seen show the various cosmic epochs as unfolding in a sort of expanding trumpet shape--is this merely a matter of graphical convenience?Does the universe have a center away from which everything is accelerating?I realized I had not the slightest idea about the answers to these questions the other night after hiking to a nearby mountain top to watching for taurid meteors. They were few, but grand.

Thanks Stile So that sounds reasonable. CH is a bit like earth’s.it’s all about where i stand.the horizon is real, but in a way its an artifact.But then the satellite won’t ever catch up! & Ned’s comment: Seems all^ makes the CH very real and physical. Like you’ve just defined the edge. There is no shift of position that will ever enable me to see over it (?!)Every star in a whole galaxy’s worth of galaxies could go supernova just over my CH, and it would make zero difference to me, ever.no matter what I did (?!)Hey? Are we closed then? But its a horizon!!! If we’re in a pocket, then isn’t every other point in the universe in overlapping pockets? Don’t we shift from one pocket to another just by moving through our own pocket? Oh no! Lost again.Say then you and I got together in the early days and each took one end of that rubber band and we glued the poor old ant to the middle..space has expanded since and we’re now over each other’s horizon.Is the band now infinitely stretched? What do you experience if I let go? Where is the ant?Sorry if I’m being dense. It fascinates me, but cosmology is like a cupboard of horrors sometimes. You know, open the draw, shout with horror, slam the door shut again. Not the best way to learn, but to be fair you guys throw out some pretty full on things. A horizon that is coming to get me!Anyways. Thanks for this too Stile, i WAS wondering: .Take care,Lurkey....EDIT: since posting, found this which helped a little:http://www.mathpages.com/home/kmath666/kmath666.htmOne benefit of presenting horizons in a spacetime diagram, rather than in a purely spatial diagram, is that it makes clear the transitive nature of the partial ordering of events. In other words, if event b is inside the event horizon of event a, and if event c is inside the event horizon of b, then c is inside the event horizon of a. Lack of clarity on this point sometimes leads to confusion over the fact that galaxy C can be outside the horizon of galaxy A while it is inside the horizon of galaxy B, which is still inside the horizon of A. If horizons were purely spatial, this would seem to imply that C could send a signal to B, which could then relay the signal on to A, and hence C can send a signal to A, contradicting the fact that C is outside the event horizon of A. Needless to say, this reasoning is invalid, because it overlooks the fact that if C is outside the event horizon of A, then, even if B is presently inside the event horizon of A, by the time a signal from C reaches B, the latter must have passed outside of the event horizon of A.Edited by Lurkey, : No reason given.

No, not really. Imagine an ant jogging as fast as it can on an expanding rubber band. Now, if we know the speed of the ant and the rate of the expansion, then there is a certain point on the band that we could identify and say "the ant will eventually traverse every point on the band up until there, and will never traverse any point on the band beyond there". And yet there is no edge. There's nothing special about that point of the rubber band, because it only exists relative to the point that that particular ant started from. Another ant that started an inch to the right on the rubber band would run right over that point and not even notice that there was anything special about it. Because there isn't.And it's the same with us. Physics tells us that there are limits to how much of the universe we can see and explore, and that even if we could travel at the speed of light we could only reach such-and-such a galaxy and no galaxy beyond it. And yet there is no edge. A race of aliens living a billion lightyears away from us could see stars beyond our "edge" and (with sufficiently advanced technology) could cross that "edge", and they would never notice that they'd done anything special, or "crossed an edge", because the edge is only an edge for someone starting from Earth. It is otherwise completely unremarkable.As with the ants, the "edge" only marks the point that we can't get to if we start from here. There is nothing special or remarkable about it, it's only the "edge" with respect to the place where we are, not with respect to the Universe.Edited by Dr Adequate, : No reason given.

I wonder if that means that we can meet halfway the creationist point of view that the Earth is at the centre of the expanding universe. It's at the centre of the limit of our perception of the expanding universe.

No, it doesn't mean that.If a looney thought that he was at the center of a flat Earth, would it support his point to explain to him that (in a flat landscape) he must always perceive himself to be at the center of his view of the Earth as limited by his horizon?

Catholic Scientist's analogy is useful here:
EvC Forum: Size of the universe
(In fact it's not even a full analogy, it's just making the properties of a four dimensional space/shape, the universe, more obvious by using a two-dimensional space/shape that shares some of those properties)So the Big Bang is the North pole and every point in time after that is a circle of constant latitude. A circle of latitude being all of space at one time. As you the circles get larger the further south you go, so the spatial dimensions of the universe increase the further along in time you go.Since the circles represent the whole universe at a given time, a point in space at a given time is like a point on a given circle.So phrase your question in terms of this analogy:
The centre of the universe from which everything is accelerating, would be the equivalent of some special point on one of the latitude circles. However there is no special point on these circles that the circles are expanding way from, they are expanding "away" from the North Pole
Simply as a consequence of being circles on a sphere.Similarly the universe is not expanding away from some point where the Big Bang happened. It's just that space is getting larger as you move forward in time (down the sphere).Again, if you reverse time the circle shrinks and all points on the circle converge on the North Pole, so the North Pole "happens" at all points on the circle. Similarly the Big Bang happened everywhere.

Oh, I wasn't intending to support the looney's point - I was wondering if we could re-frame the looney's terms of reference, so that the looney's opinion more closely resembled reality.(Still, if I pursue this, it'll be off-topic, so I'll leave it for the time being).

Thank you. That is quite helpful.

Yes No - each "pocket" as you call it is defined by a point in space *and* time. You are not free to move to any other pocket, only those that can be reached by you travelling at less than the speed of light.Someone in a neighbouring pocket has areas of the Universe within his horizon that are not within yours. He can potentially reach these regions - you cannot. But for him to do so, he needs to set off *now*, otherwise those regions will be lost to him. By the time you manage to get to where he is now, those regions will be long gone. Make sense?Oh, and just to correct some common confusion - we're not talking about the Cosmological Horizon (or Particle Horizon) here but the (Cosmological) Event Horizon.

Thanks everyone yeah CD, just by themselves all the names are confusing enough! No information flow. No casual contact. This just sounds so much like an edge!But i know, i know... that's^ my problem. I think i'm making some headway but its taking me ages...one thing leads to another, you know?Hopefully i'll get there one day.By the way, I like the idea that the event horizon is our universe's ultimate light cone.Also the whacky one where we're living in an inside out black hole Hey and i'll cheer any extremely brave and patient person who cares to try and dumb this down for me:
http://ned.ipac.caltech.edu/...ineweaver/Figures/figure1.jpg

The expansion of space is detectable by the red shifting of light. This is expansion along the vector that the light is travelling. Can we see the expansion of space orthogonal to that vector?To put it another way, would 2 streams of photons that are parallel at their sources remain parallel across big space?

At long last, "You can't get there from here" is no joke.

My Guess: No. Yes.Movement is relative.
Light is only moving in one direction. Therefore, if the two rays/streams are parallel, there is no movement "toward" or "away" from each other.Therefore, we can consider them as if they are stationary with respect to each other (as far as their "parallel-ness" is concerned).Therefore, they will "grow apart" as the space in between them expands. However, they will remain parallel.
They will never overlap. Or cross....or, that's my guess, anyway.Maybe if there was somehow a constant stream of two light rays running parallel (not sure how this could happen naturally, but let's just say it did).
We could measure the distance between them near the source and get some value.
And measure the distance between them at some great distance away from their source and get a larger value?...or, perhaps, this is all impossible because of a light ray's (or lazer's) natural tendency to diffuse out like a flashlight beam (get wider the further from the source...) anyway. Therefore, it might not be possible to measure the distance between two beams since they wouldn't stay as a straight, solid "beam" of light. Therefore, the two parallel streams would overlap at some point. Like two conical sources of light... like a car's headlights.So maybe I won't answer your questions at all, and just talk about some stuff for a bit.Chew on that!

Only in flat space... In non Euclidean space parallel lines may intesect (for example lines of longitude on the surface of a sphere). Or parallel lines may diverge in a space with a different curvature.