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Author Topic:   what is the big bang and how do i understand it?
mick
Member (Idle past 3275 days)
Posts: 913
Joined: 02-17-2005


Message 1 of 122 (229483)
08-03-2005 9:08 PM


Hi,

This is a message from a physics-virgin.

I'm a biologically-oriented member of the forum and I would be grateful if people here more knowledgeable about physics could help me to understand "big bang theory".

I have in the past tried to read a few pop-science books on the subject, and don't really feel that I understand the basic idea.

I should say that I'm happy to look at maths but I doubt I'm up to the standard required for a proper understanding of the theory, so would like a verbal/conceptual description (if such a thing is possible). However I'm not a stupid person. If it is possible for a lay-person to understand the big bang, then it is probably possible for me to understand it.

On a side-note, given that i doubt I will ever be able to understand the maths of modern physics, am I doomed never to fully appreciate the theory of the big bang?

Thanks for any help you can give; pointers to articles, books, websites or whatever would be greatly appreciated.

Mick

ps. i should say that my current knowledge of physics is wholly newtonian... I can understand cannon balls firing in neat trajectories, and rotating balls on string, pendulums, etc., but that's about it.

This message has been edited by mick, 08-03-2005 09:10 PM


Replies to this message:
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AdminJar
Inactive Member


Message 2 of 122 (229487)
08-03-2005 9:12 PM


Thread moved here from the Coffee House forum.

This is not a Coffee House thread. I'll overlook bypassing the PNT but I'm moving it to BB&C

This message has been edited by AdminJar, 08-03-2005 08:18 PM


  
Chiroptera
Member
Posts: 6804
From: Oklahoma
Joined: 09-28-2003
Member Rating: 6.6


Message 3 of 122 (229494)
08-03-2005 9:45 PM
Reply to: Message 1 by mick
08-03-2005 9:08 PM


Actually, mick, I have a master's degree in physics, but I don't have any experience in astrophysics, cosmology, or even high energy particle physics, so, believe it or not, I'm no better off than you!

However, here is my "cartoon" (read: over simplified explanation) of Big Bang.

It is observed that the spectra of distance galaxied pretty systematically exhibit "red-shifts". That is, the spectral lines that identify particular elements are all shifted toward longer wavelengths ("redder" colors). This usually indicates movement away from the earth at a particular velocity. The further the galaxy is, the greater the red shift. The obvious interpretation is that the galaxies are moving away from the earth, and the further a galaxy is, the faster it is moving away.

Actually, a more satisfactory interpretation is that space is expanding, and carrying the galaxies along with it. Think of (classic analogy) raisins in a loaf of bread that is rising -- the raisins are moving away from one another, and the further apart two raisins are, the faster the distance between them is increasing.

Now we think of the implications. If the universe is expanding, then if we "run the clock backwards", all of the galaxies must have been closer together -- go back far enough, and everything must have been packed together -- physics tells us that the universe must have been hotter, too, so the temperature was higher. If we run the clock backwards, the universe must have been infinitely hot and dense -- what is called the singularity.

We can use the laws of physics to tell us what the conditions were like in the past when things were hotter and denser. Often there are several possible models, and we use the observations of the current universe to decide which models are correct -- different models will predict different distributions of galaxies and such.

Also, at some point the universe gets too hot and dense for our current laws of physics to work -- for the time between the singularity and about 10^-40 sec later, our laws of physics are completely inadequate to describe what the universe was completely like.

This is what Big Bang is -- the observation that the universe was much denser in the past, and the use of the laws of physics to determine what the universe was like at these earlier epochs.

What the Big Bang is not is a theory of the origin of the universe. Big Bang is a model (or several competing models) of the early universe after its origin. In fact, since the singularity is when "time began", it makes no sense to speak of "before" the singularity, and so speaking of "causes" poses, in my opinion, something of a philosophical problem.

There are ideas of the origin of the universe, the origin of the singularity, but as far as I know none of these ideas are as yet even close to being observationally verified even in principle.

If this doesn't answer your query, then maybe it's enough to start some questions.


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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 4 of 122 (230049)
08-05-2005 7:51 AM
Reply to: Message 1 by mick
08-03-2005 9:08 PM


Hi Mick! Chiroptera has already given you an excellent overview of the astronomical/cosmological picture of the big bang. However, I will give you a more fundemental mathematical picture, which predates any of the observations...

Just after completing General Relativity, Einstein used it to model the universe. He discovered to his dismay that GR predicted that the uinverse should be expanding or contracting. Einstein "fudged" his equations to remove this expansion, as he thought it completely unphysical. It was only when evidence of the expansion of the universe was discovered that he unfudged his equations.

An important point that is a little obscured in Chiroptera's post is that the observation that the universe is expanding is not what leads us to the big bang, it is just evidence for the big bang. The nature of the big bang comes to us from the mathematics of General Relativity. It is only in the mathematics that you realise that questions about "before the big bang" have no meaning, that the universe did not "explode", but merely expanded, that there is no "outside the universe" or "beyond the universe". The big bang is as obscure and exotic as a black hole, and trying to apply any level of reasoning outside the framework of General Relativity (and its descendants) is doomed to failure. This is why it is such a difficult concept for someone outside the field to make any sensible comment.

The first step in understanding the big bang properly is to start thinking not of space, but of space-time: the universe appears 4-dimensional. Points in space-time are "events": something happening at a particluar place and at a particlar time; like me clapping my hands at 1200 GMT in my office at home. Your life is a line stretching though space-time, wit the end of the line marked by your birth event and your death event.

Ok, picture the globe with lines of longitude and latitude. Longitude stretch from north pole to south pole and latitudes encircle the earth... the equator being the longest line of latitude.

Now, this is the space-time of the universe... the lines of latitude are what we call space at different times. The lines of longitude are lines of time, with time 0 at the north pole: the north pole is the big bang and the south pole is the big crunch. The lines of latitude just south of the big bang are very small but as we head south they expand quickly. This is the universe expanding. Picture a galaxy every 30 degrees around a line of latitude, so 12 galaxies in our universe. Just after the big bang they are very close together, but as we head south, the galaxies naturally move away from each other, following their lines of longitude. At the equator, they are at a max distance from each other, and then they collapse back together towards the big crunch at the south pole.

What happens before the big bang? Well, that is just asking what is north of the north pole... oh, there is no north of the north pole; all directions head south! Similarly at the big crunch.

Get your head around this picture and come back with your questions. If you can appreciate this picture, you are far along the road to understanding :)


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Sylas
Member (Idle past 3549 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 5 of 122 (230095)
08-05-2005 10:01 AM
Reply to: Message 4 by cavediver
08-05-2005 7:51 AM


What cavediver said is spot on.

The driving model for the big bang is general relativity. This is one of the most stringently tested models in science, and so far it has passed all tests will flying colors. A consequence of this model is an expanding spacetime; and this too is confirmed by observations. No other model makes sense of what we observe; and no other model is as thoroughly tested.

Ironically, however, we know that general relativity fails at quantum scales. It is a "classical" theory; and a unified physics will need to deal with gravity on quantum scales. It follows that as you approach the singularity there is a region where existing physics fails. But what happens in that region will not be a confirmation of population intuitions. There is a great unknown near the singularity; but if you think of it as not knowing what happened "before" then you underestimate its strangeness. Time and space are no longer anything like what we expect, and the popular associated of a word like "before" will not have their familiar meanings.


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New Cat's Eye
Inactive Member


Message 6 of 122 (230316)
08-05-2005 6:37 PM
Reply to: Message 4 by cavediver
08-05-2005 7:51 AM


tall stack of earths
At the equator, they are at a max distance from each other, and then they collapse back together towards the big crunch at the south pole.

How does the expansion stop and the contraction begin? Why, instead of a sphere, wouldn't the model be a cone/hemisphere?

Also, as we reached the equator the expansion rate would slow, so is there anyway for us to estimate which latitude we are at? Perhaps compare the expansion rate over a large enough time scale to determine what point of the globe we are at. Since the expansion rate is still increasing, we’d have to be north of 45 degrees latitude.

What happens before the big bang?

It seems that after the big crunch there would be another big bang. I think the singularity existed for a split second after contraction and immediately began expanding again, it just happens over and over again forever. With this view, you can ask what happened before the big bang. In your earth model, north of north pole would be the south pole of another earth sitting on top of it, and another earth would be south of the south pole too, a whole stack of them, a tall one.

Or am I completely wrong?


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Replies to this message:
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DominionSeraph
Member (Idle past 3043 days)
Posts: 365
From: on High
Joined: 01-26-2005


Message 7 of 122 (230770)
08-07-2005 7:24 PM
Reply to: Message 6 by New Cat's Eye
08-05-2005 6:37 PM


Re: tall stack of earths
Catholic Scientist writes:

How does the expansion stop and the contraction begin?

Gravity would do that if the universe was dense enough.

Catholic Scientist writes:

Why, instead of a sphere, wouldn't the model be a cone/hemisphere?

Different models are used.

Catholic Scientist writes:

Also, as we reached the equator the expansion rate would slow, so is there anyway for us to estimate which latitude we are at? Perhaps compare the expansion rate over a large enough time scale to determine what point of the globe we are at. Since the expansion rate is still increasing, we’d have to be north of 45 degrees latitude.

No. Using that model, the rate of expansion would be steadily decreasing, reach 0 at the equator, and then switch over to a steady increase in contraction. However, as the rate of expansion is increasing, the 3D representation would be more of a horn shape. (I think)

Catholic Scientist writes:

It seems that after the big crunch there would be another big bang. I think the singularity existed for a split second after contraction and immediately began expanding again, it just happens over and over again forever. With this view, you can ask what happened before the big bang. In your earth model, north of north pole would be the south pole of another earth sitting on top of it, and another earth would be south of the south pole too, a whole stack of them, a tall one.

Or am I completely wrong?

Not if you're talking about 'before' in imaginary time. The sphere represents space-time, so the totality our time is on it. Within time, once you get to the North Pole, all directions are 'later'/'after'. 'Up' is a direction in imaginary time.

Of course, it's been awhile since I studied this stuff, so I may be off a bit.


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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 8 of 122 (230774)
08-07-2005 7:32 PM
Reply to: Message 7 by DominionSeraph
08-07-2005 7:24 PM


Re: tall stack of earths
Of course, it's been awhile since I studied this stuff, so I may be off a bit.

No, you sound quite on track :)

Catholic Scientist writes:
Why, instead of a sphere, wouldn't the model be a cone/hemisphere?

Different models are used.

Careful, the shape of space isn't the model but an output of the model. The model is GR and an isotropic, homogeneous space. The shape depends upon how much mass there is in the universe, and whether you include a cosmological constant. If we try to mimic the observed acceleration, then yes, the output is no longer closed up into a sphere and is indeed more of a horn shape.


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Replies to this message:
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Omnivorous
Member (Idle past 1256 days)
Posts: 3808
From: Adirondackia
Joined: 07-21-2005


Message 9 of 122 (230788)
08-07-2005 8:28 PM
Reply to: Message 8 by cavediver
08-07-2005 7:32 PM


Re: tall stack of earths
cavediver:

quote:
If we try to mimic the observed acceleration, then yes, the output is no longer closed up into a sphere and is indeed more of a horn shape.

If the sphere can be read as a closed universe, Big Bang/Big Crunch, does the horn suggest infinite expansion?

My understanding (woefully limited as it is) is that a very small percentage of our universe is "ordinary" matter and energy, considerably more of it "dark" matter, and an even greater portion dark energy, the latter accounting for the accelerating expansion.

Is there still a defensible model for a closed or cyclic universe?

This message has been edited by Omnivorous, 08-07-2005 08:30 PM


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Chiroptera
Member
Posts: 6804
From: Oklahoma
Joined: 09-28-2003
Member Rating: 6.6


Message 10 of 122 (230794)
08-07-2005 8:37 PM
Reply to: Message 9 by Omnivorous
08-07-2005 8:28 PM


Re: tall stack of earths
Google "big rip".

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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 11 of 122 (230874)
08-08-2005 5:52 AM
Reply to: Message 9 by Omnivorous
08-07-2005 8:28 PM


Re: tall stack of earths
If the sphere can be read as a closed universe, Big Bang/Big Crunch, does the horn suggest infinite expansion?

Yes, though of course the rate of expansion can be slowing, steady or increasing.

Is there still a defensible model for a closed or cyclic universe?

Closed, yes. It is only the standard big-bang model where closed equates to collapse and open equates to continued expansion. When you throw in a cosmological constant, you can have a closed expanding universe.

Cyclic? This model is a bit of red-herring, and really comes from a less than ideal understanding of time. I'm so busy at the mo I can't really do justice to posts, but DominionSeraph started to touch on this issue. That's not to say that it can't be cyclical, but it's not an output of the model... it's just a fanciful idea.


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Omnivorous
Member (Idle past 1256 days)
Posts: 3808
From: Adirondackia
Joined: 07-21-2005


Message 12 of 122 (230912)
08-08-2005 8:49 AM
Reply to: Message 11 by cavediver
08-08-2005 5:52 AM


Re: tall stack of earths
cavediver writes:

quote:
Cyclic? This model is a bit of red-herring, and really comes from a less than ideal understanding of time. I'm so busy at the mo I can't really do justice to posts, but DominionSeraph started to touch on this issue. That's not to say that it can't be cyclical, but it's not an output of the model... it's just a fanciful idea.

:)
Fanciful ideas are what I'm here to rid myself of...I'll keep reading.

Also, thanks, Chiroptera, for the google "big rip" tip. Fascinating.


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New Cat's Eye
Inactive Member


Message 13 of 122 (232434)
08-11-2005 6:50 PM
Reply to: Message 7 by DominionSeraph
08-07-2005 7:24 PM


Re: tall stack of earths
Gravity would do that if the universe was dense enough.

But the density is decreasing with the expansion, so graity would have less and less of an affect.

Different models are used.

duh...I was asking why that model was a sphere.

However, as the rate of expansion is increasing, the 3D representation would be more of a horn shape.

ahhhh, horn shape...thats what I was getting at with the cone/hemisphere description.

Within time, once you get to the North Pole, all directions are 'later'/'after'. 'Up' is a direction in imaginary time.

But at time=0, you exist on both earths simultaneously, the south pole of the upper earth is the same point as the north pole of the other, so you could go north, not up, but just on a different sphere.

Basically, we're saying the Big Crunch and the spherical space-time model don't work with the increasingly expanding universe we observe from the red-shifts.

So, why did cavediver bring it up in the first place?


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Son Goku
Member
Posts: 1152
From: Ireland
Joined: 07-16-2005


Message 14 of 122 (232437)
08-11-2005 7:55 PM
Reply to: Message 13 by New Cat's Eye
08-11-2005 6:50 PM


Re: tall stack of earths
He was using a sphere as an analogy for demonstrating the fact that time is just a coordinate.

The actual shape of spacetime isn't actually anything like a sphere.


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New Cat's Eye
Inactive Member


Message 15 of 122 (232438)
08-11-2005 8:02 PM
Reply to: Message 14 by Son Goku
08-11-2005 7:55 PM


Re: tall stack of earths
But he also brought up the Big Crunch. I don't see how the Big Crunch is possible with the horn-shape model. Conversly, if we do consider the Big Crunch, then before the Big Bang is possible.

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