Big Bang - No Single point of Expansion

[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 a lot 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. 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 don't 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 don't 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.

You will have to register, but this free, and this is a great site.

 

[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 that's 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 don't 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. I am 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 balloon 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. I am 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.