Big Bang - No Single point of Expansion

[earamsey]

Cosmologist say that the universe is expanding but from no central point. It seems contradiction to me because the universe also started from a single point and EXPANDED outward. Further, they say if you reverse expansion everything would compress back to a singularity. Seems like expansion is occurred from a single point possibly occurring at one too if you can rewind to a single point!

[mgb_phys]

Before the universe there wasn't anything to expand into..
Don't think of the universe as exploding from a point out into black empty space - think of it as the space itself being created as it expands.

A common picture is to think of inflating a balloon. ANy two points on the surface of the balloon are expanding away form each other but there is no point on the surface that is the centre that everything is expanding from

[S.Vasojevic]

Space exploded into what?

[earamsey]

Intuitively it makes no sense that space is expanding into nothing. Is this something I am to accept and look the other way because no one understands it... like gravity?

[mgb_phys]

Intuitively it makes no sense that space is expanding into nothing.
Intuition is a sense of knowledge from your internal experience.
Do you have a lot of internal personal experience of the start of the universe?

In reply to somebody complaining about the confusing terminology of Quantum mechanics - to which one of the founders replied - "yes it's odd that a language invented to tell other apes where the ripe fruit is has difficulty with the notion of a particle as a wave"

[earamsey]

No, but I know what nothing is, I have lots of experience with nothing. Nothing is a common human concept and perhaps the problem is trying to explain non-classical events terms of the classical event driven human experience. I know that QM makes no sense until you start to view it in terms of hamilitonian space. Maybe the nothingness is something like that?

I have no problem with expansion but nothing is really abstract and there is actually no true instance of nothing except symbolic. For example, I did nothing to prevent shuttles take off in cold weather. In fact, here nothing means "took no action".

Also, I did not think that space exploded per se. After all "Big Bang" was met to be a derisive term and only stuck because of media like the term and I guess through constant usage made it acceptable. Explosion implies something blew up and BB came from nothing.

[russ_watters]

Intuitively it makes no sense that space is expanding into nothing. Is this something I am to accept and look the other way because no one understands it... like gravity? Intuitively, are you able to visualize a 3 dimensional curved surface (analogous to a two dimensional curved surface of a balloon)? If you can't, then your intuition is failing to help you understand the geometry of the universe and you must learn about a reality that is different from what your intuition tells you. Heck, if our intuition was always right, what would there ever be to learn?!

Trying to understand the baloon analogy would be a big help here.
No, but I know what nothing is, I have lots of experience with nothing. Ok....well think about how the concept of nothing could apply to the expansion of the universe. You're talking about a spherical universe with an edge and a center, expanding out into empty space surrounding it...... so where is this edge? Why don't we see it? Why don't the velocity vectors of galaxies have a preferential direction? I have no problem with expansion but nothing is really abstract and there is actually no true instance of nothing except symbolic. Actually, the concept of "nothing" really isn't abstract, but people often use imprecise wording that improperly frames the concept. For example, your quote: Intuitively it makes no sense that space is expanding into nothing. ...is an improper paraphrase of what people are telling you. They didn't say "space is expanding into nothing", they said space is not expanding into anything. These are two very different things because when people hear "nothing", they think "empty space", but the reality is that the universe is not expanding into empty space.....and empty space wouldn't be "nothing" anyway!

[justwondering]

Before the universe there wasn't anything to expand into..
Don't think of the universe as exploding from a point out into black empty space - think of it as the space itself being created as it expands.

A common picture is to think of inflating a balloon. ANy two points on the surface of the balloon are expanding away form each other but there is no point on the surface that is the centre that everything is expanding from

Is it best to think of this starting point as containing all of this (our universe) mass/energy/space at some instant, or as a point sized 'entry' location for things to follow? Could we ever know the difference?

[earamsey]

This is what I understand of BB. I will use outside observer to help illustrate. At BB this observer would notice some event, a flash of light and then an infinitely small region, a Point, would plop into existence. (I know photons would NOT be able to actually leave region but i say it anyway for illustration).

At this point, the infinitely small region, Point, is fixed in size to observer and will never change. However, internally, the region develops space, time and structure further this space starts to inflate.

So, now I space/time being mysteriously defined inside this region, which is still infinitely small externally. Space time attributes are unique and do not exist outside of region; observer has no concept of space or time only sees a dimensionless point. Now, since space is defined within this region and then the universe itself defines space, it can not expand into space but rather it's space is growing or expanding. At this point I guess I can say that Space/Time is localize to this region and expanding relative to every point within the region.

Now, if I run the process backwards, the space-time within this region would collapse into super dense stuff; but the outside observer not notice anything different until the space inside the point, or region to us, becomes so dense that it collapses and plops out of existence. At this point observer would think he was seeing things because it would have vanish as quickly as it appeared, if it did at all.

But I guess the stuff our universe, region or point, to observer, persists in is of no consequence since we or any law of classical or QM physics could exist there anyway. Our world and laws are inside this infinitely small but vast point. But still I wish I could somehow how see it anyway.

In second to last paragraph I said space-time collapses because, purely intuitively, I think that Time and Gravity were once unified but somehow broke apart.

[Nebozilla]

The Big Bang didn't come into existence since nothing existed. Also this observer which I assume is made of nothing but can still interpret things(...some how...i feel kind of bad saying that last sentence) wouldn't see the actual big bang unless he could see pure energy then Gamma rays, X-rays and ultraviolet then finally light which we can see.

[earamsey]

he is a pseudo hypothetical being I made up for illustrative purposes. Hmm, i would have met the region of space came into plopped into existence. But citing those few discrepencies I guess my understanding must be nearly correct since you had simple quibbles about post.

[Dmitry67]

notice some event, a flash of light and then an infinitely small region, a Point, would plop into existence. (I know photons would NOT be able to actually leave region but i say it anyway for illustration).


If (which is very likely) our universe is infinite now, then it ALWAYS was infinite, and the initial singularity was still INFINITELY BIG in size. Singulary is not (always) a point!

Singularity IS NOT a region. ALL places of the universe were in the same conditions. There were never "in" and "out" of the expanding region. All regions were the same. It is important.

[Cryptonic]

If (which is very likely) our universe is infinite now, then it ALWAYS was infinite, and the initial singularity was still INFINITELY BIG in size. Singulary is not (always) a point!

Singularity IS NOT a region. ALL places of the universe were in the same conditions. There were never "in" and "out" of the expanding region. All regions were the same. It is important.

I totally agree with this. Who says "WE" originated in a dimensionless point? A "dimensionless point" (ie singularity) is usually indicative of a flaw in our mathematics & logic.

Either we started with an infinite Big Bang (?), or we are "bouncing" back from a previous (finite) >4D-collapse...(is what I say).

Infinity (like singularities) does not make much cerebral sense. After all, we (our "minds") are part of the universe, and we don't see "infinities" nor "infinitesimal dimensionless points".

I think it's likely that the universe is "bouncing" (breathing) and is an organic entity. We are the neurons/synapses in this Universe's brain. We are the Universe's self-awareness.

The closest I come to this idea theoreticized is Smolin's theories.

Or maybe I'm stoned, who knows.

[Jimmy]

If you haven't already, check out the following thread:
Effort to get us all on the same page (balloon analogy anyone?) (http://www.physicsforums.com/showthread.php?t=261161)

[Nebozilla]

he is a pseudo hypothetical being I made up for illustrative purposes. Hmm, i would have met the region of space came into plopped into existence. But citing those few discrepencies I guess my understanding must be nearly correct since you had simple quibbles about post.

Those aren't just discrepancies, those are HUGE parts of understanding the Big Bang based on current theory.

[S.Vasojevic]

So there must be some kind of boundary between created space, and not yet created space?

[sylas]

So there must be some kind of boundary between created space, and not yet created space?

No. "created space" in this context just means that there's more space between things.

On sufficiently small scales (a couple of hundred million light years, perhaps) it is no different to things moving apart from each other in the way we are used to. On larger scales, you have to describe space and movements and expansion with general relativity, which doesn't always fit nicely with initial expectations or intuitions; but in general, it's still just everything moving apart from everything else, with no center, and no boundary apparent or implied.

Cheers -- sylas

[S.Vasojevic]

Before the universe there wasn't anything to expand into..
Don't think of the universe as exploding from a point out into black empty space - think of it as the space itself being created as it expands.

So it is expanding into its future self. There must be some propagation front that we can describe. Does GR strictly implies that it does not exist?

[sylas]

So it is expanding into its future self. There must be some propagation front that we can describe. Does GR strictly implies that it does not exist?

The term "propagation front" is not well defined. In the normal sense of the word, no there does not have to be a propagation front.

But I guess you might be able to think of "time" as a "propagation front". There is no location of a front in space; but all of space is expanding as time passes. This just means there is more space in between things; subject of course to the fact that objects are also moving with their own peculiar motions and gravitationally bound conglomerations like a galaxy don't expand.

I suspect you are still holding on to misleading metaphors and analogies with an explosion of material out from a point. You have to drop that idea; it's wrong. And you're probably best to forget about trying to shoehorn the idea of a propagation front into cosmological expansion. It's terminology that is bound to mislead.

Felicitations -- sylas

[S.Vasojevic]

Thanks sylas. If I understand this correctly galaxies are not moving apart because expanding space is exerting force on them which overcomes gravity, but simply because more space is being "stuffed" in between them?

[sylas]

Thanks sylas. If I understand this correctly galaxies are not moving apart because expanding space is exerting force on them which overcomes gravity, but simply because more space is being "stuffed" in between them?

There are better people here than me to answer this. Comment from Wallace would be welcome; and there are a number of others well placed for this also.

I shall try to go a bit further; and I will welcome informed instruction and criticism from experts here present.

Any answer in these terms is necessarily an analogy, or metaphor, for the underlying mathematical descriptions; and I am not sufficiently strong on those to be confident about all the pitfalls, or about identification of exactly how far an analogy works and where or how it breaks down.

My understanding is that things are moving apart because they are moving apart. That is, there's no force required; only an initial impulse. Additional pushes only slow it down or speed it up.

Expansion of the universe kicked off with a burst of inflation in the very very early universe, and since then (for the most part) everything has continued expanding, but gradually being pulled together again by gravity. Recently it has been discovered that the expansion of things is accelerating again; and this corresponds to a small additional kick from "dark energy", which pushes things apart rather than pulling them together as gravity would do.

The key point is that there is no indication of any boundary or edge, and no need for an edge. We can't see all the universe to be sure of what it looks like beyond the limits of telescopes. But the simplest and most straightforward models have all the universe pretty much like the observable universe, with everything expanding away from everything else, and with no limit or edge. If indeed everything is pretty much the same as the observable universe, then whether the universe is finite or infinite depends on the large scale "curvature" of the universe. A positively curved universe would be finite... a bit like the curved 2d surface of a planet is finite without having any edge. Except, of course, that space is 3d rather than a 2d surface.

The thing about relativity is that "Matter tells space how to curve, and space tells matter how to move." (Wheeler) There are small scale examples of how movement of matter leads to changes in spacetime which leads to movement of matter. For example: the "frame dragging" (http://en.wikipedia.org/wiki/Frame-dragging) effect means that a satellite in orbit around a rotating body (like the Earth) will experience a small "force" pulling it along with the rotation of the planet. I don't know how good a parallel that is; but with an expanding universe, I think that to say there is additional space between things another way of saying they are moving apart.

On the scale of the whole universe you have to deal with space in terms of general relativity; but it reduces on smaller scales to things just moving apart from one another, like a cloud of expanding gas becoming less dense -- although the cloud has no edge or boundary.

Cheers -- sylas

[jdz]

(this is a first post, I hope that it is on-topic; no thread-jack intended...)

I have a question about the curved 3-D space and "no edge and no boundry of the universe" idea.

Would it be possable to 'see' far enough to 'see' an edge or front of the universe, or does the curved 3-D space mean that any/every point in the universe is surrounded by the same (aprox the same) observable universe? That is, would the 'sky and stars' be the same, or look the same, from every where in the universe, whether the universe is open or closed?

thanks

[WhoWee]

Cosmologist say that the universe is expanding but from no central point. It seems contradiction to me because the universe also started from a single point and EXPANDED outward. Further, they say if you reverse expansion everything would compress back to a singularity. Seems like expansion is occurred from a single point possibly occurring at one too if you can rewind to a single point!

A central point would be possible if the universe first collapsed to a single point - then exploded in a Big Bang (once or cyclically).

[sylas]

A central point would be possible if the universe first collapsed to a single point - then exploded in a Big Bang (once or cyclically).

No; there is no distinguished central point in this case. The collapse to a point refers to all of space being reduced to a point; not collapse of all matter into a point in space.

There is no "central" point, in the sense of a point distinguished from other points being in the center. The collapse is when there is no space left between anything; all points are the same point; a singularity.

Felicitations -- sylas

[WhoWee]

No; there is no distinguished central point in this case. The collapse to a point refers to all of space being reduced to a point; not collapse of all matter into a point in space.

There is no "central" point, in the sense of a point distinguished from other points being in the center. The collapse is when there is no space left between anything; all points are the same point; a singularity.

Felicitations -- sylas

With no space left to fill after a rapid contraction - the only possible outcome (other than a static space-time) would be an outward expansion.

[S.Vasojevic]

My understanding is that things are moving apart because they are moving apart. That is, there's no force required; only an initial impulse. Additional pushes only slow it down or speed it up.




Shouldn't that imply that Universe is gaining gravitational potential energy, at expense of violation of first law of thermodynamics?

[sylas]

Shouldn't that imply that Universe is gaining gravitational potential energy, at expense of violation of first law of thermodynamics?

Energy in GR is a bit subtle, but the short answer is no.

Gravity slows the expansion down, as I indicated previously, and this is a bit like a gain in potential energy as things are further apart, and a loss in kinetic energy as the expansion slows. The problem is really that energy depends on the frame of an observer, and you run into difficulties defining energy for the universe. That get beyond what I can explain well.

But there is a correspondence with the normal exchange of potential and kinetic energy for objects moving in a gravitational field.

Cheers -- sylas

[sylas]

With no space left to fill after a rapid contraction - the only possible outcome (other than a static space-time) would be an outward expansion.

There's still no central point. The expansion of the universe is not really "outward", but an expansion of matter and energy that fills all of space, at any time slice you consider. No point in space can be distinguished as central. This applies before the hypothetical bounce while the universe contracts, or after as it expands with a hypothetical bounce.

[S.Vasojevic]

Gravity slows the expansion down, as I indicated previously, and this is a bit like a gain in potential energy as things are further apart, and a loss in kinetic energy as the expansion slows. The problem is really that energy depends on the frame of an observer, and you run into difficulties defining energy for the universe. That get beyond what I can explain well.


Yes, but I think that expansion is speeding up.

[WhoWee]

There's still no central point. The expansion of the universe is not really "outward", but an expansion of matter and energy that fills all of space, at any time slice you consider. No point in space can be distinguished as central. This applies before the hypothetical bounce while the universe contracts, or after as it expands with a hypothetical bounce.

There really is no way to continue the conversation without re-defining the word "point" to mean a place/area/location of an unspecified/unknown size/shape/dimension.

[sylas]

Yes, but I think that expansion is speeding up.

Quite right... and that is from an additional energy term, called "dark energy". But how this relates to the conservation of energy -- or rather, to the conservation results in general relativity which involve stress-energy-momentum pseudotensors -- I do not know. At this point the technical details are beyond me. Sorry.

[sylas]

There really is no way to continue the conversation without re-defining the word "point" to mean a place/area/location of an unspecified/unknown size/shape/dimension.

The word point refers to a specific location, without size or dimension or shape. No redefinition is required. It is a point like you are used to.

The only likely ambiguity is whether we mean a point in spacetime, or a location in space which persists through time. In either case there's no shape or size involved.

I explained previously what I mean by "no central point". Here it is again with a few additional words to see if it is more clear.

There is no "central" point, in the sense of a point that is distinguished from other points by being in the center of space. The collapse is when there is no space left between anything; all points are the same point; a singularity. This is not a "center", in the sense of a being in the middle of other points.

You can identify a distinguished point in spacetime, and you don't need to postulate a cyclic universe for that. It's called the singularity. But you can't give the location of the singularity in space... only in spacetime.

You can identify a point in space as a "timelike worldline". It means you can integrate proper time along the world line and give a unique event for each proper time value along the world line. This is how you trace the trajectory of a particle.

If you do this, you find that at the singularity, all world lines converge. There is no distinguished world line to give a spatial center to expansion. Only a singularity to give a temporal origin. At every instance after the singularity (or before, in a cyclic case) the universe is roughly homogeneous, with no distinguished center in space.

The reason for the thread is to explain this very thing.

Felicitations -- sylas

[Dmitry67]

No. A boundary is something between one region of space and another region of space.

There are no boundaries. Note that if Universe is finite it still has no boundaries.

[HowardS]

Hello. I can understand that there is no central point for the BB as it is the only point that exists. But it does not make sense that all of the universe's mass came into existence, from one point - whether that point IS the universe or is a point in eternal, empy space. You would have a continuous "sphere" of particles (mass), or equivalent energy, ejecting from this point, in all directions, for as long as it takes to make up the finite? amount of matter in the universe.

Howard

[Dmitry67]

OMG... I give up... Is there a short FAQ - "baloon analogy" thread is too long so people get frightened...

[HowardS]

Don't give up. Didn't your parents ever teach you not to be a quitter :)

I did not look at the balloon analogy thread yet, but I will - and you're right, 8 pages is alot to get in at a lunch break.

Since you seem very passionate about this analogy, can you give me 2 or 3 primary points of interest that I can keep in mind while reading it?

Thanks,
Howard

[sylas]

Hello. I can understand that there is no central point for the BB as it is the only point that exists. But it does not make sense that all of the universe's mass came into existence, from one point - whether that point IS the universe or is a point in eternal, empy space. You would have a continuous "sphere" of particles (mass), or equivalent energy, ejecting from this point, in all directions, for as long as it takes to make up the finite? amount of matter in the universe.


One thing to grasp is that existing physics breaks down at this singularity.

If you use good old general relativity, and extend it back as far as you can go, you get one great big "divide by zero" error, in which the total volume of the universe vanishes to zero and the density blows up to infinite.

It's usually just wrong to think of stuff expanding from a point, because this language conventionally means thinking of a point in space; and that's not what we mean.

Think of it like this. At a certain time, (t=1) all the universe was filled with a hot dense mix of stuff at enormous temperatures and density. And it was all expanding... just like galaxies are expanding now. That is, it was becoming less dense, as everything expands away from everything else. There's no center to this, mind. The whole universe, without any edge or boundary, is filled with this hot dense mix of stuff. As time passes the density drops, which is the same as saying there's more space between things than there was before. Except that the density was so incredibly high that the idea of space between things doesn't really work. Nevertheless, that's what expansion means. It means that density drops. Forget centers, forget boundaries, they have nothing to do with it. Just... density is falling.

But if you run the clock in reverse to look at the past, density is increasing, without any limit. It increases up to infinite density within a finite time in the past, at t=0. And physics breaks apart in such conditions.

Suppose that the universe is infinite. (Flat or negative curvature throughout.) If so, then it has ALWAYS been infinite. It's just that it used to be so phenomenally dense that everything we can currently see, out to billions of lights years distance, used to be all within a space of a basketball, or a pea, or an atom, or a proton, as you go back.

Just as there is presumably more of the universe beyond the horizon of visibility at present, so too there was back then more of the universe beyond the "basksetball/pea/atom" sized region that contained all the matter of the observable universe. The idea of the observable universe being the size of a pea in the past doesn't mean that there was an edge. It means only that everything we now can see used to be packed together into a small region within the whole infinite universe. And everything expanded.

I do mean everything. There's no center to this expansion; no central point from which it starts.

There is rather a singularity. A time in the past where classical physics would involve infinite density and zero size, for any region; and so classical physics breaks down. At any other time, we have a hot dense infinite universe filled with stuff, without a center and without a boundary... and it expands.

Felicitations -- sylas

PS. Never give up. Whether you are asking or explaining. This is not easy to grasp, and every new generation of students has the fun of finding it out. Ask away, don't be shy!

PPS. Dmitry67 gives a nice set of four points in the next post to try and keep in mind!

[Dmitry67]

Ok.

1. The theory of Big Bang DOES NOT include the Big Bang (surprise!). It is a theory about the expansion of the Universe, but it does not include the initial moment. So it does not answer questions "how all came into existence" and "why did it happen" or "what was before the BB"

2. "continuous "sphere" of particles (mass), or equivalent energy, ejecting from this point"
BB is NOT an explosion. There was no "sphere" of particles "ejecting from one point". At first, if Universe is infinite then it was always infinite, never a point. Particels were not "ejecting", in fact, if we ignore their ultra-high temperature, they were not moving at all (in the consmology coordinates)

3. Particles were always everywhere, so there was no "void" do be filled.

4. Finally (again surprise, surprise) energy and mass are NOT conserved in the Cosmology (this is a tricky subject and it deserved a separate thread, I provided a oversimplificated explanation)

[ViewsofMars]

I highly recommend people explore the following website from Lawrence Berkeley National Laboratory Physics Division, The Universe Adventure - The Eros of the Cosmos. :smile: I love it!

The Planck Epoch

In the beginning...
"...we have a viable theory of the universe back to about 10-30 seconds. At that time, the currently observable universe was smaller than the smallest dot on your TV screen, and less time had passed than it takes for light to cross that dot."
-George F. Smoot, Winner of the 2006 Nobel Prize in Physics

In the time before the first 10-44 seconds of the Universe, or the Planck Epoch, the laws of physics as we know them break down; the predictions of General Relativity become meaningless as distance scales approach the Planck length at which random quantum mechanical fluctuations dominate. Most particle physics models predict that during this epoch the four fundamental forces were combined into one unified force. Very little else is known about the early part of this era, and the mystery it poses is perhaps the central question in modern physics.
[snip]
Big Bang Theory predicts that sometime during the first second of Era 1, an unusual energy drove the Universe through a rapid, accelerating expansion. During this inflationary period, the Universe increased in size on the order of 1027.
http://www.universeadventure.org/eras/era1-plankepoch.htm
[Next]
Era 1: Inflation
Isotropy of the CMB
[snip]
What Happened Before 10-44 Seconds After the Big Bang?
We have no idea. Era 1 has provided cosmologists with several as-of-yet unanswered questions:
• What were the initial conditions for the Big Bang?
• How did the Big Bang start?
• What physical laws applied before the Big Bang?
• What is time?
Unfortunately, inflation appears to wipe out the clues that might help answer these questions. Inflation spreads out any initial conditions so that they are so diluted that the chance of finding anything from before inflation would be like finding a needle in a hay stack.
http://www.universeadventure.org/eras/era1-consequences.htm

[Sorry!]

This might be a bit off topic but it's a continuation question of things I've read here.. Just wondering:

Whats the "shape" of the universe? I've looked over the balloon analogy but I'm not sure if the analogy of the universe being spherical is correct (i think the analogy is just to show the expansion of all points). I understand that the universe would only be the 'rubber' of the balloon... but I'm still confused as to what the shape would be? Is it 2d?

[Dmitry67]

In General Relativity there is no observers "outside" of our space, so nobody can tell.

In superstrings it is possible that our universe is a part of a bigger structure: it can be a (3+1)D brane flying in a higher dimensional space. In that case it HAS shape.

Finally, curvature does not uniquely defines the topological pproperties of an object: for example, both sphere and half-sphere has the same curvature in every point, and half-sphere has 1/2 volume of a sphere.

[justwondering]

Should it be considered as a sign of mental scientific weakness if one can not envisage, or perhaps even construe, that our total Universe (mass,energy,space and time) was at one instant smaller than the volume of a Proton? I understand that all the 'evidence' points to this but, I just can't comprehend or truly believe it. Do I just need further indoctrination, or what? Woe is me.

[WhoWee]

Should it be considered as a sign of mental scientific weakness if one can not envisage, or perhaps even construe, that our total Universe (mass,energy,space and time) was at one instant smaller than the volume of a Proton? I understand that all the 'evidence' points to this but, I just can't comprehend or truly believe it. Do I just need further indoctrination, or what? Woe is me.

I feel your pain. Personally, I want to know what was going on 5 minutes before the BB.

[justwondering]

I feel your pain. Personally, I want to know what was going on 5 minutes before the BB.

For me that's easy, nothing!

[Dmitry67]

Should it be considered as a sign of mental scientific weakness if one can not envisage, or perhaps even construe, that our total Universe (mass,energy,space and time) was at one instant smaller than the volume of a Proton? I understand that all the 'evidence' points to this but, I just can't comprehend or truly believe it. Do I just need further indoctrination, or what? Woe is me.

Definitely there was time when our VISIBLE universe was smaller then Proton. Our VISIBLE Universe is always finite.

however, if(which is very likely) our Universe (as you say, "total", not just visible part) is infinite then it was ALWAYS infinite in the BB scenario.

[WhoWee]

Looking at the last 2 posts, time is finite and the Universe has always been infinite?

[Dmitry67]

Looking at the last 2 posts, time is finite and the Universe has always been infinite?

Without Dark Energy, there are 3 scanarios:
1. Closed universe: finite volume, finite time, after some time it collapses back to singularity. Singularity is a point.
2. Open universe: infinite volume, infinite time. It was born, but it will exist forever.
3. Flat unvierse. In general same as 2

With Dark Energy there are additional scenarios:
1a. Closed universe-II: finite volume, but because of DE expansion acccelerates, so time is infinite

If Dark Energy is a "Quintessence" then there are 2 additional scenarios:
2b - starts as open universe, then accelerates until the "Big Rip" in a finite time. So space is infinite, but time is finite, there are a beginning and final singularities
4b - closed universe also can end that way

We know for sure that Dark Energy exists, and expansion is accelerating, hence scenario 1 is impossible.

But we can not tell 2 from 3 from 1a, and we dont know what is DE, so 2b and 4b are not excluded.

[Sorry!]

Without Dark Energy, there are 3 scanarios:
1. Closed universe: finite volume, finite time, after some time it collapses back to singularity. Singularity is a point.
2. Open universe: infinite volume, infinite time. It was born, but it will exist forever.
3. Flat unvierse. In general same as 2

With Dark Energy there are additional scenarios:
1a. Closed universe-II: finite volume, but because of DE expansion acccelerates, so time is infinite

If Dark Energy is a "Quintessence" then there are 2 additional scenarios:
2b - starts as open universe, then accelerates until the "Big Rip" in a finite time. So space is infinite, but time is finite, there are a beginning and final singularities
4b - closed universe also can end that way

We know for sure that Dark Energy exists, and expansion is accelerating, hence scenario 1 is impossible.

But we can not tell 2 from 3 from 1a, and we dont know what is DE, so 2b and 4b are not excluded.

How do they know dark energy exists?

[George Jones]

How do they know dark energy exists?

Try

http://physicsworld.com/cws/article/print/31910.

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[Dmitry67]

Expansion is accelerating.

[Peter Watkins]

No it isn't. The increasing rate of galaxy separation is evidence of a slowing in the rate of expansion. Dark energy does not exist, nor does the need for it.

[Dmitry67]

ok, you're right, all astronomers in the world are wrong. I am listening to your Nobel-prize-winning theory :)

[Peter Watkins]

It is simplicity itself. As the universe expanded, density, and hence gravity, decreased faster in the outer regions. This shifted the centre of gravity inward which produced a disparity in expansion rates between inner and outer universe causing spreading. This produces the "faster with distance" view. Continued slowing increases the rate of separation. This should come as no surprise as it was always thought that gravity would slow expansion. And it is. The statement that increased separation is acceleration, is speculation. The same applies to the 1920's statement that the universe is expanding. What was observed is the fact that galaxies are moving apart at a rate that increases with distance. This could equally well be caused by collapse!

[Nebozilla]

How is there an outer universe or inner universe when there is no center to the universe? I don't understand what you mean that continued slowing accelerates seperation? Negative acceleration = positive acceleration???

No it isn't. The increasing rate of galaxy separation is evidence of a slowing in the rate of expansion. Dark energy does not exist, nor does the need for it.

Oh is this sarcasm? lol! Hopefully this is just a joke...

[Peter Watkins]

Why is it thought that there is no centre and no outer. That makes no sense and there can be no argument for it. The fact is that all observed phenomena can be explained by the four forces, and all large scale movement is the result of gravity acting upon matter. You should listen to what the universe is telling you. All the clues are there. For example, what sort of universe could you construct from the single observation that virtually all galaxies, in all directions, are exhibiting degrees of red-shift that increase with distance? Nothing else, and nobody elses interpretation.

[Nebozilla]

How CAN there be a center? The whole universe "exploded"...everywhere. The balloon or raisin bread analogy helps in understanding. If you make points all over the balloon and while its expanding, all points move away from each other the exact same amount so from any point it appears to be the center but that goes the same for the next point and so on. Its not normal so thats why its hard to grasp.

Well if i was Hubble when he first found the redshift and what it meant, it obviously means the universe is accelerating. The further out the "redder" they become.

[Peter Watkins]

The balloon analogy is pointless as it only describes the outer face of the universe. The raisin bread analogy is far more accurate in that it describes a three dimensional universe and demonstrates how the galaxies, (raisins), are moving apart. It also shows that as well as moving apart from each other, there is an overall movement in a single direction, that is, away from the baking tray, which represents the centre of the universe. The single loaf is, of course, only half a universe. The other half is beneath the tray.
The entire cosmological world accepts that the universe is expanding, ie., growing in volume, therefore, in times past it was smaller. The further back in time that we project, the smaller it was. There would have been a time when it was at it's smallest. Regardless of how small or large that was, that is the point from which it is expanding in all directions. This then is the region that could be said to be central. The outside edge needs no explaining.
Hubble did not discover the red-shift. Slipher did when studying distant nebulea. Hubble discovered that these are galaxies.

[Dmitry67]

Apparently, you dont understand the baloon analogy. Then here is another one.

Take an infinite line from -inf to inf
At a given time, t=1, put 'galaxies' at points marked with integer numbers.

Now, to look how that line looked at any time t, just multiple all distances by t.

For example, at t=1 galaxies are at -1,0,1,2,..
At t=10 they are at -10,0,10,20,
At t=0.001 they were at -0.001,0,0.001,0.002 etc

You can replace an infinite line with 2D or 3D surface

This analogy is not complete but it shows that:
1. There is no 'center' of expaction. The expansion looks the same from any point
2. If universe is infinite now, it was *ALWAYS* infinite, so BB is not a point.

[Dmitry67]

The entire cosmological world accepts that the universe is expanding, ie., growing in volume, therefore, in times past it was smaller
The further back in time that we project, the smaller it was. There would have been a time when it was at it's smallest. Regardless of how small or large that was, that is the point from which it is expanding in all directions. This then is the region that could be said to be central. The outside edge needs no explaining.
Hubble did not discover the red-shift. Slipher did when studying distant nebulea. Hubble discovered that these are galaxies.

This part is absolutely wrong.
The logic is very naive, like, "if I divide the infinity by sufficiently big number, I get 0" :)

Infinity multiplied by 0.000000000000000000000000000001 is not "smaller" then the original infinity

WHen we say that universe was "smaller" we always mean the "visible" universe, so, the finite part of it

[Chronos]

Peter, what other option is viable in the face of centuries of observational evidence? If the universe is not expanding, what is it doing? Einstein realized a static universe was not viable without ever looking through a telescope. Im not saying that is the last word on cosmology, but, it is incumbent on you to propose a viable alternative.

[Vanadium 50]

I am listening to your Nobel-prize-winning theory :)

Please don't encourage personal theories. As per PF-rules, they belong only in the IR section.

[Dmitry67]

It was a sarcasm.

[Peter Watkins]

Prior to 1912 and the discovery of red-shifted nebulea, the universe was thought to be fixed,immutable and perfect. The handiwork of God. We now know that it is, at best, a work in progress. Being thought perfect, it was stated by somebody who should have known better, that it would look the same no matter from where it was viewed. As this was in the nineteen teens, there could be no possible scientific basis for this statement. It was, at best, guesswork, and as such was foolhardy. I defy anybody to provide evidence that was known at that time, that could back up this claim.
With regard to expansion, I didn't say that the universe is not expanding, indeed the outer regions still are. What I said was that the "faster with distance" view could be produced by either an expanding or a collapsing universe, and that in 1929 they could not have known which it was. With the further information that we have today, they probably would have stated that collapse is more likely to be occurring than expansion.
With regard to the ballon analogy, this one dimensional surface is what the universe would resemble if escape velocity had been reached. There would, of course, be no clumps of matter, only the particles of the early, energy only universe.
So tell me, what is it that this forum has against the notion that gravity is exerting restraint from within that has caused the faster with distance view through slowing from within and that this produces the illusion of acceleration? It would seem entirely logical, obeys all physical laws and does not require the invention of a new force in order to explain things. We're not talking warp factors here, or time travel, merely interpreting information to produce the simpleist and easiest of answers.

[Nebozilla]

Oh Peter its not just this forum...Go to any astronomer and just ask what they believe is the current best theory behind the universe's future. Your assumption could be possible if there WAS a center but there isn't. Period. Also the balloon's surface is suppose to represent 3D. With the raisin bread, just imagine it suspending in nothing while expanding. No baking pan.

[Peter Watkins]

So why then does the einsteinonline website, which is heavily promoted by this forum, state categorically that the outward movement can be rewound to a single point? It also provides a pictorial representation which is essentially a "slice of pie" illustration whereby the universe is seen expanding from a single point. This type of picture is quite widely used and is to be praised for it's economical use of paper and ink. It is though, misleading. To understand fully what the universe is doing, simply fill in the rest of the pie and engage brain.

[Nebozilla]

So why then does the einsteinonline website, which is heavily promoted by this forum, state categorically that the outward movement can be rewound to a single point? It also provides a pictorial representation which is essentially a "slice of pie" illustration whereby the universe is seen expanding from a single point. This type of picture is quite widely used and is to be praised for it's economical use of paper and ink. It is though, misleading. To understand fully what the universe is doing, simply fill in the rest of the pie and engage brain.

Please do. Im done. I seriously thought this was a joke ...

[Peter Watkins]

So it's back to school for you then.

[Nebozilla]

Actually I am in school. I guess I will go back tomorrow then? Whats so hard to grasp about the Big Bang happening everywhere since there was no universe before it? Its hard to picture but it makes perfect sense. So maybe...So it's back to school for you then.