Why was the big bang not an explosion?

[astroscott]

Hi, Just a quick question.

Does anyone know the original reason for discarding the idea of the big bang as an explosion?

 

[Dmitry67]

Big Bang - is a solution to Einstein equations, and it does not have any properties of an explosion. Nobody never was thinking about the 'explosion'. The image of 'something exploding into empty space' comes from very bad popularisations, created later.

So originally there was a correct idea, then came a crowd of journalists, asking stupid questions like 'where was the center of Big Bang'?

 

[astroscott]

Just so we're on the same page here. What would you say are the properties of an explosion?

 

[Dmitry67]

1. Pre-existing space/time (before the exposion).
2. High pressure at the center and empty space around
3. Pressure accelerated matter outside.
All that is not correct for the BB.

 

[astroscott]

And how exactly are those things discounted in the BB theory?

 

[Chronos]

An explosion is a pop sci description of the origin of the universe. Would you characterize the unfolding of a blanket as an 'explosion'? The initial state of the universe was not unlike the unfolding of many blankets. An explosion is descriptive of a blast wave on Earth - a pressure wave propogated by the atmosphere. Space has no atmosphere, so the analogy is invalid.

 

[Dmitry67]

For example, average density is the same everywhere. So pressure did not play any role. Even more, it slowed the expansion. Observational data proves that universe is the same in any direction. No center, no edges.

But if we look at General Relativity, it is even easier than that. There are no solutions where there is empty space for some time, then something 'explodes' in that space, creating matter.

 

[D H]

And how exactly are those things discounted in the BB theory?

The Hubble expansion.

If you must insist on thinking of the big bang as an explosion, it is far better to think of it as an explosion of space rather than an explosion in space.

 

[astroscott]

An explosion is descriptive of a blast wave on Earth - a pressure wave propogated by the atmosphere. Space has no atmosphere, so the analogy is invalid.

And yet you can have an explosion in space, say nuclear, as long as it doesn't require a component of our atmosphere in the reaction.

 

[astroscott]

For example, average density is the same everywhere. So pressure did not play any role.

This just proves that the pressure didn't come from a central distinguishable source within whatever exploded. However there still could have been pressure from every part of the 'mass' acting on every other part.

Even more, it slowed the expansion.

I thought gravity slowed the expansion.

Observational data proves that universe is the same in any direction. No center, no edges.

Hate to disagree but observational data only proves that the universe is the same for 12 billion ly or so in all direction. Any speculation on centers or edges is just that, speculation.

 

[lazypast]

what provided the driving force for this expansion? surely the force of gravity for a universe, say a second old, would be overwhelming.

 

[S.Vasojevic]

Astroscott, nobody can't tell you exactly what BB was, except that it was the beginning of time as we know it, and that things were much more dense than today.
It was an event 13.7 By ago, which marked the transition from unknown state, not describable in our language or physics, to universe that we can study and talk about.

But I guess that scientific community is to blame for common misconceptions, not pop science. If you call something "Big Bang" then you should expect questions about explosion. "Big Birth" would be more appropriate, at least acronym could stay.

 

[Dmitry67]

1 I thought gravity slowed the expansion.

2 Hate to disagree but observational data only proves that the universe is the same for 12 billion ly or so in all direction. Any speculation on centers or edges is just that, speculation.

1 Pressure increases gravity, and gravity slows the expansion.

2 If you agree that there is no center, then what you "simple question" is about? Do you have any doubts, and if yes, what exactly?

 

[Wallace]

Hate to disagree but observational data only proves that the universe is the same for 12 billion ly or so in all direction. Any speculation on centers or edges is just that, speculation.

The point is that we have no evidence that any point in the observable Universe is special, to be specific that any point is the centre of some original explosion. It is possible (though hard to explain theoretically) that the Universe suddenly becomes very different just beyond the reaches of the observable Universe in such a way as to imply some kind of special point somewhere.

The point of the 'BB was not an explosion' catch-cry is to counter to common misconceptions that modern cosmology has evidence that the Universe started with an explosion at some initial point. This is a misunderstanding of the theory we have based on the evidence we have. The evidence is consistent with there not being a special point that is the origin of all material. It may not completely rule out the possibility, but it does not in any way suggest it, which is the what is commonly misinferred.

I tell students ( and anyone else that will listen!) that it is okay to think of the BB as an explosion, as long as your mental picture has the original exploding material having infinite extent, or at least bigger than the observable universe, and uniform. For instance if your mental picture of the BB imagine a nuclear bomb in space blowing up and throwing material into the void, then this is incorrect. However if you imagine that the bomb itself is infinite in size, or at least bigger than the observable universe, then if you think through what happens when it goes off then you'll get a reasonable picture of the BB.

Note that on a technical level this explanation is not correct, but the reasons for that are somewhat subtle and if you need to resort to mental pictures rather than understanding GR then you won't need to worry about them.

 

[Wallace]

Astroscott, nobody can't tell you exactly what BB was, except that it was the beginning of time as we know it, and that things were much more dense than today.
It was an event 13.7 By ago, which marked the transition from unknown state, not describable in our language or physics, to universe that we can study and talk about.

But I guess that scientific community is to blame for common misconceptions, not pop science. If you call something "Big Bang" then you should expect questions about explosion. "Big Birth" would be more appropriate, at least acronym could stay.

The reason that we have such a bad name for the Big Bang is that it was actually Fred Hoyle, a BB critic, who first coined the phrase. He intended it to be a silly name for what he thought was a bad theory, but somehow that was the one that stuck!

The one other major misconception I'd like to counter is this idea that 'the Big Bang was the beginning of time'. This is a bit more subtle and not so bad, but still a misconception. We have a set of theories (GR, QM etc) that tell us with a good degree of certainty what happened going back around 13-14 Billion years from now. At some point, which corresponds to when we think the Universe was above a certain average energy density, our theories stop being meaningful. That doesn't mean that the Universe stopped being meaningful, just that we can't say what happened before this time. Suggesting that 'time began' at this point is a possible interpretation, but its not the most reasonable or probable.

The FRW model of the universe has an important function a(t) (loosely speaking the size of the Universe in a comparative sense) in which a->0 when t->0, but that's the result of extrapolating the maths beyond the point where the physics makes sense. We all want to know what happened before the furthest point we can currently discuss with any certainty, and progress is being made on it, but for now we don't really know. Suggesting that 'time began' is only one possibility, with no supporting evidence.

 

[S.Vasojevic]

The one other major misconception I'd like to counter is this idea that 'the Big Bang was the beginning of time'. This is a bit more subtle and not so bad, but still a misconception. We have a set of theories (GR, QM etc) that tell us with a good degree of certainty what happened going back around 13-14 Billion years from now. At some point, which corresponds to when we think the Universe was above a certain average energy density, our theories stop being meaningful. That doesn't mean that the Universe stopped being meaningful, just that we can't say what happened before this time. Suggesting that 'time began' at this point is a possible interpretation, but its not the most reasonable or probable.

The FRW model of the universe has an important function a(t) (loosely speaking the size of the Universe in a comparative sense) in which a->0 when t->0, but that's the result of extrapolating the maths beyond the point where the physics makes sense. We all want to know what happened before the furthest point we can currently discuss with any certainty, and progress is being made on it, but for now we don't really know. Suggesting that 'time began' is only one possibility, with no supporting evidence.

Ok, but if you are suggesting that time in any way existed before t=0, than BB was not birth of the universe, but (loosely speaking) some kind of 'phase transition'. If universe began at BB then time also did so.
I agree that 'time began at BB' is my preferred way, but only because I can't see it any other way.
What are your thoughts on imaginary time? Is that concept of any use?

 

[Dmitry67]

Ok, but if you are suggesting that time in any way existed before t=0, than BB was not birth of the universe, but (loosely speaking) some kind of 'phase transition'. If universe began at BB then time also did so.
I agree that 'time began at BB' is my preferred way, but only because I can't see it any other way.
What are your thoughts on imaginary time? Is that concept of any use?

It does not suggest that something existed at t<=0 either.
Regarding "I can't see it any other way" I can give you 2 examples if you want.

 

[S.Vasojevic]

Of course Dmitry.

 

[Dmitry67]

1. So t>0. What if t (what we observe) is an operator. The underlying, more basic concept, is another time, let's call it Q

Say Q = t - 1/t

While t->0 Q-> -inf
t->+inf Q-> +inf

So we just mapped ]0,+inf[ to ]-inf,+inf[

So Universe had infinite history and existed forever, but the time operator 'underestimates' time intervals close to BB.

2. Big Bounce with 'even' solution:

StateofTheUniverse(t) = StateOfTheUniverse(-t)

like cos(x)

So negative times are just equivalent to positive ones.

 

[astroscott]

The point of the 'BB was not an explosion' catch-cry is to counter to common misconceptions that modern cosmology has evidence that the Universe started with an explosion at some initial point. This is a misunderstanding of the theory we have based on the evidence we have. The evidence is consistent with there not being a special point that is the origin of all material. It may not completely rule out the possibility, but it does not in any way suggest it, which is the what is commonly misinferred.

I guess my original question is based on what I like to think of as the common sense way of looking at things. To me it seems that if you can follow events backward and find a time when everything was in a compressed state it makes sense to assume it came from some sort of explosion. Now I understand that you can't always rely on common sense, but to completely discount it should require a sound reason. I am yet to find that reason and was hoping someone here could provide it.

Note that on a technical level this explanation is not correct, but the reasons for that are somewhat subtle and if you need to resort to mental pictures rather than understanding GR then you won't need to worry about them.

Are you talking here about the idea that if it were an explosion then there would be no way to account for Inflation due to the speed limit that matter from the explosion could travel at?

The reason that we have such a bad name for the Big Bang is that it was actually Fred Hoyle, a BB critic, who first coined the phrase. He intended it to be a silly name for what he thought was a bad theory, but somehow that was the one that stuck!

I read somewhere recently that Hoyle has said that he didn't mean it like that, though I'm not sure whether that's true or he was just trying to backtrack.

The one other major misconception I'd like to counter is this idea that 'the Big Bang was the beginning of time'. This is a bit more subtle and not so bad, but still a misconception. We have a set of theories (GR, QM etc) that tell us with a good degree of certainty what happened going back around 13-14 Billion years from now. At some point, which corresponds to when we think the Universe was above a certain average energy density, our theories stop being meaningful. That doesn't mean that the Universe stopped being meaningful, just that we can't say what happened before this time. Suggesting that 'time began' at this point is a possible interpretation, but its not the most reasonable or probable.

I can only agree with this. While I can assume that we probably do not have the same view of the way it all happened, It's refreshing to read this as it's so different from what I'm used to reading.
I'll have to leave it there for now as I have to go to work now.

 

[jambaugh]

You can model the dynamic universe by mathematically embedding it as a hyper-surface in a higher dimensional space-time. It is in that sense that you can properly think of the big bang as "an explosion" of a point into an expanding spatial "balloon". In that sense the "center of the explosion" is the abstract mathematical center of this "balloon" universe. It is not a point in physical space but a point in the abstract extended space of the model. There is nothing physically real about this extended space and we should treat it as an analogy which helps us visualize the physical cosmology.

When we speak of the changing radius of the universe we mean circumferential radius defined as the circumference over 2pi. There is no way to draw this radius as a geometric line without stepping outside physical space by invoking those extra dimensions. We are bugs living in the peel of an apple which is all peel and no interior. There is no "where" outside the peel yet it can stretch and grow over time.

(Now you can speculate all day about whether these extra dimensions really exist but that's leaving the realm of science and entering the realm of philosophical speculation about the unobservable a.k.a. mysticism.)

 

[S.Vasojevic]

1. So t>0. What if t (what we observe) is an operator. The underlying, more basic concept, is another time, let's call it Q

Say Q = t - 1/t

While t->0 Q-> -inf
t->+inf Q-> +inf

So we just mapped ]0,+inf[ to ]-inf,+inf[

So Universe had infinite history and existed forever, but the time operator 'underestimates' time intervals close to BB.

2. Big Bounce with 'even' solution:

StateofTheUniverse(t) = StateOfTheUniverse(-t)

like cos(x)

So negative times are just equivalent to positive ones.

1. Are we living in Q time or t time? If first is true than you need +inf time interval, just to get to any real number. If we live in the t time, then t time had beginning at t=0.

2. No objections here.

 

[S.Vasojevic]

Red line is t, blue line is Q. It would be hard to interpret how your function grew from -inf, considering that it would need +inf time to pass.

Where would you put inflation on the blue curve? t=1 is how much in seconds?

 

[russ_watters]

I guess my original question is based on what I like to think of as the common sense way of looking at things. To me it seems that if you can follow events backward and find a time when everything was in a compressed state it makes sense to assume it came from some sort of explosion.

Not really. As others suggested, that idea came later and was a misunderstanding of the issue - it has never been seriously considered by scientists and hadn't even been considered by ill-informed laypeople until the BBT came around.

If I remember the history correctly, prior to the BBT, most scientists and even religious crackpots believed the universe to be static and infinite. But it was shown mathematically that such a situation would be unstable. Thus the idea of an expanding or contracting universe was born.

Now I understand that you can't always rely on common sense, but to completely discount it should require a sound reason. I am yet to find that reason and was hoping someone here could provide it.

People provided several reasons - you just didn't like them!

 

[astroscott]

Not really. As others suggested, that idea came later and was a misunderstanding of the issue - it has never been seriously considered by scientists and hadn't even been considered by ill-informed laypeople until the BBT came around.

I would guess that was because until the BBT came around most people hadn't considered the possibility of an expanding universe so would have had no reason to believe it had been the result of an explosion.

If I remember the history correctly, prior to the BBT, most scientists and even religious crackpots believed the universe to be static and infinite. But it was shown mathematically that such a situation would be unstable. Thus the idea of an expanding or contracting universe was born.

Though this wasn't taken seriously until Hubble's discovery of the Galactic Redshift. And even then wasn't actually accepted until Penzias and Wilson found the CMBR.

People provided several reasons - you just didn't like them!

I have just reread all the responses and can't find anywhere where someone actually gives any solid reasons. Could you perhaps let me know where they were or give a reason of your own?

 

[Dmitry67]

Are we living in Q time or t time? If first is true than you need +inf time interval, just to get to any real number. If we live in the t time, then t time had beginning at t=0

In that particular model, we live in BOTH
We interpret it as t while mathematically is Q

 

[Dmitry67]

1 Where would you put inflation on the blue curve?
2 t=1 is how much in seconds?

1
There is 1 to 1 mapping between t and Q
So take inflation era in t and map it to Q

2
This is a model only
I wrote Q=t-1/t but it can be
Q=A*t-B/t with some unknown A and B
Or another function at all.
I just wanted to show that you can map FINITE interval after the BB until NOW to an INFINITE interval.

 

[Wallace]

I have just reread all the responses and can't find anywhere where someone actually gives any solid reasons. Could you perhaps let me know where they were or give a reason of your own?

I think the miscommunication here is that I suspect when you say "the big bang was like an explosion", what you mean by 'explosion' and what myself and others are assuming you mean might not be the same thing.

Therefore, to clarify the issue it might help to give some more detail about how you are picturing the expanding universe, then we can see more clearly if you are on the right track. It's possible we are using the same words to mean different things; it is a common problem in these sorts of discussions!

In the interum, here are some ways in which the BB is and isn't like an explosion:

NOT like an explosion:

* An explosion occurs due to a pressure differential. The ignition of the bomb raised the pressure at the site, so material moves from that region outwards into the lower pressure surroundings. This is not like the early universe, because the early universe was homogenous so all regions had equal pressure.

* If an explosion sends material outwards, then the expanding ball of material has a centre, an origin. Due to the pressure differential involved, the outflow is not homogenous. If you were riding on some particle in this outflow you could work out where the centre was even if you couldn't see that point by tracking the paths of the particles around you and tracing them back. This is not like the expanding Universe, because the expansion is homogenous.

IS like an explosion:

* If something explodes and sends material outwards, the initial 'kick' gets everything moving, and afterwards the material keep moving due to the conservation of momentum. This is one way in which the BB is somewhat like an explosion, the initial 'kick' from inflation (or possibly some other initial accelerating mechanism) started the expansion, however this quickly switches off and the expansion of the Universe proceeds in an analougous way, it is simply conservation of momentum. The key is that this 'kick' occurs everywhere, equally and isotropically (the same push in every direction).

Note that there are effects at play that alter the expansion after the intial kick is provided. Gravity continually acts to slow it down, and we also are pretty sure that something (dark energy, cosmological constant, something else...) is also now acting to speed up the expansion. It's best not to try and understand these parts in terms of an explosion.

 

[Wallace]

Ok, but if you are suggesting that time in any way existed before t=0, than BB was not birth of the universe, but (loosely speaking) some kind of 'phase transition'. If universe began at BB then time also did so.
I agree that 'time began at BB' is my preferred way, but only because I can't see it any other way.
What are your thoughts on imaginary time? Is that concept of any use?

I realize the thread has moved on somewhat, but I wanted to respond to this (my response is relevant to the later discussions). I've made a very rough graph to help explain things.

This shows the scale factor, a(t), as a function of time. This first thing to note is that t=0 corresponds to today, as is the norm for doing cosmological calculations. From our position at t=0, we look back in time at the Universe and from these observations and known physics we construct a model for a(t). This is shown by the black line. At some point, a(t) becomes sufficiently small that known physics doesn't give a sensible answer. This is shown by the point around the "?". A lot of guff has been written based on the false assumption that you can simply extend the mathematical model we have for a(t) right down to a=0, this extrapolation is shown in red. There is no reason for doing this, we have no physics that tells us this is likely to be true, our model is simply not applicable in this regime.

Right now, we have no idea what happened before this time. I've put a bunch of aqua lines there to show some possibilities, but the point is we don't know (yet). There are a lot of people working on this problem, and some progress has been made, but nothing is solid enough to have become an established theory as of yet. Hopefully one day this graph can be completed in black, but not yet.

So you can say that 'time began' at the point where a(t)=0, but that is an assertion that is as unsupported by evidence as any other (at present) so I don't know why you would do so.

It is also, I feel, of little use to employ physicsless trickery like negative or imaginary time. As you can see, all time in the past is 'negative' and if the Universe is much older than 13 Billion years (very possible) we don't need to manufacture some kind of crazy new way of thinking about time in order to picture this or theorise about it.

The problem comes back to the name "the Big Bang" which implies some kind of moment, instant of origin. In fact modern cosmology theory tells us nothing about some instant of creation, instead it tells us a great deal about the history of the Universe from today going back to about 13-14 Billion years ago. At the point at which currrent theory 'gives up' the Universe is very hot and dense, but importantly it is not infintely dense, or infinitely small in size. Those concepts are derived from inappropriate extrapolations of current theory.

A far more descriptive name would be the "expansion from a hot dense state" theory, but that's not quite as catchy as "the Big Bang". Incidently though, the TV show of the same name has a theme song that starts with

The whole Universe was in a hot dense state when 14 Billion years ago expansion started..

That's as good a catch-cry as I've heard for it!

 

[S.Vasojevic]

1
There is 1 to 1 mapping between t and Q
So take inflation era in t and map it to Q

2
This is a model only
I wrote Q=t-1/t but it can be
Q=A*t-B/t with some unknown A and B
Or another function at all.
I just wanted to show that you can map FINITE interval after the BB until NOW to an INFINITE interval.

You are introducing new quantity as a function of time, and then calling that quantity time? And on top of that, you are implying that it would be useful to stretch it to infinity, which is very thing that should be avoided.