Particle Horizon Question

[pjknibbs]

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.

[red assassin]

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.

Not being able to transfer any information faster than the speed of light is a core part of our understanding of physics. If that understanding is wrong, then sure, but it starts to become a little meaningless to ask questions at that point; the answers would just depend on our revised understanding of physics. Entanglement is not a communication mechanism and does not transfer information.

[Observe]

From what I understand, there is no center to the universe because the alleged Big Bang happened everywhere at once. Prior to the Big Bang, there was no time or space - so there was nowhere for the expansion to have originated. Anywhere you go in the universe, the expansion exists and the greatest distance you can 'see' is the same as from any other point. If you were somehow able to break laws of physics and see beyond the universe/expansion, you would find yourself looking back at yourself.

[brucewarren]

I've seen demonstrations on the Youtube that claim there is no centre. It's an impressive use of maths and modelling but I think it's garbage.

All finite spaces have a centre. This is basic maths. To not have a centre the universe would need to be infinitely large. If it's already infinitely large then how the heck does it manage to continue to expand?

For us not to be able to tell one part from another it would need mass everywhere. It would be infinitely heavy, but if it had infinite mass the initial explosion would have been impossible. It would have needed infinite energy to not immediately collapse.

Seems to me that there are people within the scientific community who don't want the universe to have a centre because it means having to confront ideas they don't like. If the Earth appeared to be close to the centre of the universe because it really was the that might imply the Earth was special and there are people who loathe the very idea.

For the same reason I don't trust SETI or the people who insist that life must be just about everywhere if only we look hard enough. Why? What evidence is there for such an assertion? So far we have no evidence either way but these people seem to desperately want it to be so because they don't want Earth to be special.



I know I'm ranting a bit, but seems to me that if the Earth really is unique, or at least sufficiently rare that there's no backup planet then we ought to be taking care of the place instead of pretending that we can all hot foot it to somewhere else when this one is uninhabitable.

[Observe]

All finite spaces have a centre. This is basic maths. To not have a centre the universe would need to be infinitely large.

Except prior to the 'beginning', there was no space. The Big Bang wasn't a normal explosion going outward from a starting point. There was nowhere for a point of origin to be - so the 'explosion' took place 'everywhere' at the same time.

I'm guessing there is no end to the size of the universe. Possibly there are clusters of universes as there are clusters of galaxies. The expansion we observe, may simply be our local universe "breathing" over a long period of time - rather the the totality of things expanding.

[red assassin]

I've seen demonstrations on the Youtube that claim there is no centre. It's an impressive use of maths and modelling but I think it's garbage.

All finite spaces have a centre. This is basic maths. To not have a centre the universe would need to be infinitely large. If it's already infinitely large then how the heck does it manage to continue to expand?

For us not to be able to tell one part from another it would need mass everywhere. It would be infinitely heavy, but if it had infinite mass the initial explosion would have been impossible. It would have needed infinite energy to not immediately collapse.

Seems to me that there are people within the scientific community who don't want the universe to have a centre because it means having to confront ideas they don't like. If the Earth appeared to be close to the centre of the universe because it really was the that might imply the Earth was special and there are people who loathe the very idea.

For the same reason I don't trust SETI or the people who insist that life must be just about everywhere if only we look hard enough. Why? What evidence is there for such an assertion? So far we have no evidence either way but these people seem to desperately want it to be so because they don't want Earth to be special.



I know I'm ranting a bit, but seems to me that if the Earth really is unique, or at least sufficiently rare that there's no backup planet then we ought to be taking care of the place instead of pretending that we can all hot foot it to somewhere else when this one is uninhabitable.

Where's the centre of the surface of a sphere? That's finite, but it doesn't have a centre, or an edge for that matter. It's completely possible to have a finite, unbounded space, it's just not a Euclidean geometry. But there's no particular reason to believe that the Universe's geometry *should* be Euclidean, and general relativity explicitly doesn't enforce that.

Imagine an infinite hotel, in which every room is occupied. An extra guest shows up, but the manager says "sorry, every room is full". However, the guest turns out to be really important, so the manager comes up with a plan: Every guest is ordered to move one room further along (that is, room number 1's occupant moves to room number 2, etc). Nobody isn't able to get a room, because the hotel is infinite. But you've just made space to add an extra person to it. Infinities can grow.

The point here is you *cannot* simply intuit things about cosmology. Your intuition has been exquisitely tuned by evolution and experience to deal with things roughly on the scale of a human being, but you can no more intuit the behaviour of the entire universe than you can intuit the behaviour of particles governed by quantum mechanics, and there's no reason to believe you should be able to. People love to suggest that all these jumped-up scientists are making things unnecessarily complicated for no reason and if they just applied a little good old-fashioned common sense it would all be clear, but that's absurd and frankly a bit insulting; of *course* the obvious explanations have been tried and others arise when they don't work - that's a literal description of hundreds of years of scientific progress!

The resistance to making our viewpoint special is just that: special cases suck and simpler explanations are better. If you want to special case things just for the bit of the universe we can see, you need a damn good reason emerging from the actual theory why that bit should behave differently. That's difficult to do and near impossible to test, which makes for bad science. Naturally, we have checked the observable universe anyway for signs that any of it behaves differently from our immediate vicinity and turned up nothing.

As for SETI, I don't think it has anything to do with any suggestion that we could find another planet - even if there were human-habitable worlds in every solar system, getting off planet and over there would be damn near impossible and we probably wouldn't be compatible with the native wildlife anyway. Again it's just because if you want Earth life to be unique you have to come up with a reason *why* that should be. Given that we haven't found life everywhere we've looked, there are a bunch of hypotheses, of varying degrees of comfort for our civilisation's future, but none of them are immediately compelling enough to suggest we should stop checking for other life.

[UniTrader]

As for SETI, I don't think it has anything to do with any suggestion that we could find another planet - even if there were human-habitable worlds in every solar system, getting off planet and over there would be damn near impossible and we probably wouldn't be compatible with the native wildlife anyway. Again it's just because if you want Earth life to be unique you have to come up with a reason *why* that should be. Given that we haven't found life everywhere we've looked, there are a bunch of hypotheses, of varying degrees of comfort for our civilisation's future, but none of them are immediately compelling enough to suggest we should stop checking for other life.

Also there are theories that Life didn't originate on Earth but was "imported" from Outer Space by Al... Asteroids. btw similiar but better-founded Case for Water, which under current models coundnt have formed in such quantities on Proto-Earth.



PS sorry for weasel-wording, but for me its not the right time now to look for sources...

[brucewarren]

@red assassin

I always hated Hilbert's Hotel but I have to concede that the maths works (even if it does make my head hurt )

If I came across as anti-science then I apologise. That was not my intent at all. If we abandon observation and experiment we might as well all go back to the caves and if I thought I had even a fraction of the science of actual scientists I would be an arrogant fool.

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.

@UniTrader

The cynic in means wonders whether attributing life on Earth to extra-terrestrial origins might be an attempt to kick the ball down the road if the task of working out what happened here was too hard. Not saying it couldn't have happened, just that I'm suspicious.

[UniTrader]

well, i see it as very plausible* but not proven yet.. we must find some microbes on Asteroids or other planets first which are similiar enough to life on our Planet.. but that proof could take a while... same for the counter-proof though. So far no Probe we sent out there is capable of either.


* plausible because life can Adapt to many environments

[mrbadger]

Ours is among the first second generation stars and our species evolved reasonably quickly on a geologic scale.

It is quite plausible that we are either the first intelligent species or among the first to evolve.

Not the first life perhaps, the first Sapient life maybe.

Someone has to be first.

[Golden_Gonads]

Where's the centre of the surface of a sphere?

The surface of a sphere is two dimensional though.

[pjknibbs]

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.

[brucewarren]

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.

[RegisterMe]

https://en.wikipedia.org/wiki/History_o ... e_Universe is worth a read.

[red assassin]

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.

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.

[Morkonan]

....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.

[RegisterMe]

"Chaining together" light (or graviton, or whatever) cones like that sounds distinctly sketchy to me.

[pjknibbs]

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.

[mrbadger]

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.

[Alan Phipps]

"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.