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astroscott
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Hi, Just a quick question.
Does anyone know the original reason for discarding the idea of the big bang as an explosion?
Does anyone know the original reason for discarding the idea of the big bang as an explosion?
The Hubble expansion.astroscott said:And how exactly are those things discounted in the BB theory?
Chronos said: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 said:For example, average density is the same everywhere. So pressure did not play any role.
Dmitry67 said:Even more, it slowed the expansion.
Dmitry67 said:Observational data proves that universe is the same in any direction. No center, no edges.
astroscott said: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.
astroscott said: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.
S.Vasojevic said: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.
Wallace said: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 said: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?
Wallace said: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.
Wallace said: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 said: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!
Wallace said: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.
Dmitry67 said: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.
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.astroscott said: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.
People provided several reasons - you just didn't like them!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.
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.russ_watters said: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.
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.russ_watters said: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.
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?russ_watters said:People provided several reasons - you just didn't like them!
S.Vasojevic said: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
S.Vasojevic said:1 Where would you put inflation on the blue curve?
2 t=1 is how much in seconds?
astroscott said: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?
S.Vasojevic said: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?
The whole Universe was in a hot dense state when 14 Billion years ago expansion started..
Dmitry67 said: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 said: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.