what is the big bang and how do i understand it?

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

No, you sound quite on track 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.

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

Exacty the same as when you throw a ball into the air... "gravity" decreases as height is gained, but it doesn't necessarily allow the the ball to escape to infinity. As mentioned in my reply to DS, the "sphere" is an output of the model. This is not an output of the model, but just conjecture. It also doesn't make a huge amount of sense given the nature of the singularity. It is possible that quantum gravity could describe what you are imagining, but now there would be no south/north pole, but a thin "neck" connecting each "sphere"... but I find it difficult imagining how this would work. I would have thought we would require a major deceleration of the collapse long before we actually enter the planck scale. But it could be done with quintessence. True, the spherical model is a depiction of the classical closed big-bang scenario. Because it is an excellent model for understanding how to imagine 4d space-time... and up until a few years ago was a primary model.[Edit to correct an infinite number of typos]This message has been edited by cavediver, 08-12-2005 03:45 AM

Well, in a closed big-bang, it's topologically a sphere (minus two points) and that's good enough for me

It's not, unless quintessence evolves in a very strange way... much like what may be required for your oscillating universe. Why? This does not follow...

Not really... string theory (and supergravity before it) suggests several mechanisms for a cosmological constant. The question was always "why don't we see a cosmological constant?". We know think that we do. So it makes total sense to look at some of the implications for our universe, especially as we are looking at a dynamic field providing the "cosmological constant". That field could evolve in a number of ways, and a very simple evolution would be some form of power law. I'm not sure how this follows. As mentioned, the question has always been "why is dark energy zero?" (not that we called it dark energy back then)

Agghhh, I don't have time to do this justice... oh well, i can try. Sorry about the piecemeal approach; I'd prefer to reply in kind, but it's quicker this way, if somewhat lacking in eloquence. This "problem" is far more to do with our lack of understanding of time, than any problem with GR or QM. GR leaps way beyond our preconceptions of time, where-as QM is naturally embedded within them. It is not a matter of tweaking GR until it "dovetails with QM". I can easily imagine a world where GR would fit with QM, in the sense that a we had a renormalisable theory of a spin 2 massless gauge field. We would declare that GR fits with QM, but we would not have solved the real issue. But everyone outside of QG would think that everything was fine.But I agree wholeheartedly with your point on "elegant" solutions. The problem is usually the lack of elegance in our understanding... Just as slight quibbles, string theory was not developed to address the GR/QM problem. GR fell out by accident. But even string theory does not in any way even begin to solve the problem of time. It just provides a "solution" in the sense I mentioned above.And string theory does not call upon extra dimensions to solve problems as such. String theory only works in certain dimensions. It is a requirement of the theory, and thus a strength of the theory. GR does not specify a dimension. You can have it in as many dimensions as you like. You're right in that these extra dimensions are very useful in solving various issues. Ok, this ties in with a major point I want to make that covers your entire post: observation... it is very easy to take observation for granted. But our geocentric viewpoint is rather restricted when it comes to the universe as a whole. We are at the limits of observability of predicted grav waves. We are many orders of magnitude away from being able to detect a graviton interaction. This is not exactly a problem.When it comes to dark matter, there seems to be more matter out there than is actively emitting in a remarkably narrow range of the EM spectrum. There even seems to be matter out there that is not active electromagnetically at all (that we can detect). So what? Is this surprising? We are conditioned to observe and think electromagnetically because1) our universe contains a massless abelian gauage field (photon) - if the photon were not abelian, there would be no "sight" possible in the universe2) we have an immense local source of said photons (Sun)And dark energy... I have already explained in my previous post that dark energy is part of GR. It is part of the theory. The question has always been why does it appear to be zero? It is not a new invention or addition to the theory. So the Big Rip study is entirely valid. It would have made sense to relativists any time in the last 70 yrs, but has more interest now after the recent discovery of potential accelerating expansion. It does not minimise the need for extra dimensions. It is routed in string/m-theory so we are still looking at 10/11 target-space dimensions. And it comes from exactly the same maths as the dark energy calculations.More importantly, it is not a new "theory". It is still string theory. Just as with GR, the theory can predict an infinite number of different universe, depending on the parameters set. It is just another output from the theory, with some interesting behaviour. Not sure about this. Apollo was all Newtonian, and I would be very surprised if there has been any need for GR for any probe trajectory calculations, but I may be wrong. I've not seen this. Any references? GR is not empiracally based... in fact, Einstein was beaten to the Einstein Equation by a few days using mathematical reasoning.Anyway, sorry again for this being so slapdash. There are a number of points here we could possibly break off into seperate threads...

Hmmm, I owe RAZD a big reply, but I'm sure he won't mind if I fit this one in first Yes (I don't know is the less obtuse answer) It appears that it might, contrary to what we believed just a few years ago. True and true True again (I would say rate-of-expansion just to be clear). I thought you were reasoning against collapse even without the recently discovered acceleration factor... however, if the acceleration is dynamic, it might reduce or switch off, in which case we could still have a (delayed) collapse. I think you misunderstand me. Despite what most think (including "science" correspondants), our models of the universe are not observation based but mathematics based (GR), with observations providing the parameters for the model. All of our predictions come from the mathematics. The big bang is simply not a result of running the expansion backwards... it is the output of the mathematics of General Relativity. My picture of the globe representing space-time is not just an analogy but a representation of the actual mathematics. Your picture of the globes on top of each other is a (good) analogy of the oscillating universe conjecture, but it is no longer an accurate representation of any of the mathematics, and actually can create confusion as to the nature of time. Why should there only be one globe touching our original globe at the pole? Why couldn't several globes touch here? The "neck" concept saves this problem. Also, the idea of two globes touching at a point, and expecting some form of sensible propagation across this point (the singularity) is hopeful at best... Unfortunately, [conventional] human reasoning is very counter-productive in this field. What can and can't be done is down to the mathematics. Believe me this is not a restriction. For every idea you have that is shown to be implausible, the mathematics will offer up ten ideas that we could never hope to dream up...AbEThis message has been edited by cavediver, 08-15-2005 08:47 PM

Ok, I see your reasoning. But a big crunch does not imply your "stack of earths" is correct...

I've always found finite more aesthetic mathematically. It makes for much more interesting global topology. But I have no problem with infinite. Not at all But it is a difficult field in which to make sensible conjectures without a thorough grounding in the subject (i.e. be actively working in the field) And by sensible, I mean consistent with the known mathematics. The analogies of relativity and cosmology have created more confusion than clarity, and they must not be used a starting points for extrapolation. As an example, the book "Has Hawking Erred?" was based upon the fact that us relativists had never explained what the thickness of the rubber represented in the balloon model of cosmology, nor had we explained what was inside or outside of the balloon... That's all the maths you need for relativity. If you're happy (well, aquainted with will do) diff equations and you've a good grounding in vector calculus, then you could go straight to a GR textbook. My favourite for someone in your position is D'Inverno (An Introduction to General Relativity). It is an excellent mix of maths and phsyics, and is also quite readable. But be prepared to put some work in... otherwise, it's probably back to Greene's book. I've never read it, but many here seem to like it. This sounds so reasonable and believable, that it is so hard to convey the fact that it just doesn't make sense You cannot progress through the singularity to "earlier in time". If you do bolt on an extra globe, you have no guarentee that its time is before our time, is after our time, or is totally seperate from out time. The "neck" radically changes the geometry (and topology) to ensure that progress backwards before t=0 is well defined.

This question is totally reasonable, but it is just so ill-defined. I don't mean this anyway as a criticism. It is just a freaky application of conventional thinking (how long) to a highly non-conventional concept (singularity). I could say that it was no time at all... but that would be wrong, as time itself has no definition there. If we smooth off the singularity with the no-boundary proposal, then this region involves imaginary time. Now what does this mean? Passage of time is a reasonable concept well away from the singularity, but it is not globally defined.

Yeah, yeah, alright... I'm trying to run a business here Absolutely, and time is the biggie... to GR time is just a dimension with an odd signature in the metric. It's not something you travel along or through... there is no dynamics in GR (GR is what I take as God's eye view on reality) But we know that our place in time is much more exotic than this. We travel through time, whatever that means. By incorporating time as just a dimension, relativity has brought far more confusion to the nature of reality than it has clarity! No, this isn't true. In fact it was precisely the opposite that gave the hint that Maxwell's EM was not all it seemed... take EM:div B = r
div E = 0
curl E = -dB/dt
curl B = dE/dt + Jwhere E, B and J are 3-vectors.Now form the Faraday 4-tensor: and just use (where J^0 is r) and You are hard pressed to distinguish time and space here. There most certainly is. The best demonstration I can think of is Wheeler showing the curvature of space-time around the earth (taken from Gravitation, MTW, page 33)Throw a ball through the air, then fire a bullet, both from ground level such that both hit the ground 10m away. The ball has horizontal velocity 5m/s and takes 2s, rising 5m in height. The bullet has horizontal velocity 500m/s and takes 0.02s, rising 0.0005m in height. They both follow curved paths, but their radii of curvature are very different. Why should this be, given that the space-time curvature is the same? Becuase we're only looking at the spatial distance and not the temporal. The ball has travelled not 10m, but sqrt(10^2 + (2 x 3e8)^2) = 6e8m when time is factored in. The bullet has travelled sqrt(10^2 + (.02 x 3e8)^2) = 6e6m. The respective heights over the adjusted base-line distances now give identical radii of curvature... essentially the radius of curvature of space-time at the surface of the earth.So gravity and distance in time is inextricably linked...More to come This message has been edited by cavediver, 08-20-2005 11:25 AM

We formulate a string theory in N dimensions. We then try to make the mathematics correpsond to reality. We find that unless N is some specific value, we get unreal physics. By reality, I mean things like unitarity, or conservation of probability; the things that make science possible. So N is fixed right at the beginning. In bosonic theory, N is 26. In superstring theory, N is 10. We have known for 80 yrs or so that extra dimensions are good, as they enable us to unify the various forces. Well, we certainly have indirect evidence of gravitational waves. The spin-up of a neutron-star pair has been measured with exceptional accuracy, and agrees with the GR predicted rate based on the system's energy loss due to grav wave emission. I'll have to dig out references.And we have the rest of our confirmation of GR. Grav waves are just one prediction. Should we worry that we haven't directly detected them, when our instrumentation is only scratching at the sensitivity required? Whatever lies behind GR as some more fundemental theory, I will bet BIG money that it will not contradict GR's grav wave predictions. It will make modifications of course at QG limits... It's usually the halo that is thought to be the residing place of the dark matter. I'm a long time out of this, but I would have thought that dynamics and observations of the globular clusters would tell us something. And the luminous matter is thought to be ~10% of the total mass of the Galaxy. The 4% figure comes from considering the entire universe. I'd have to see references to be convinced. Random noise from solar radiation and wind would surely wamp any GR corrction.That Pioneer Anomaly stuff was great. Thanks for the references. But consider the level of effect they are looking at, and the level of precision of the calculations. They are using post-Newtonian approximation n-body work to determine if an 8e−8 cm/s2 acceleration is real! That is a measure of the accuracy of GR in the solar system... we can account for all but 8e−8 cm/s2 with the theory. I think we could go many orders of magnitude greater than this and still not lose a Mars probe due to bad physics! Easy, it's 1 What it is to be a theorist...Seriously though, the value of G is one of the two free parameters in GR. GR cannot really be decribed as empirical just becasue we use observation to fix a couple of free parameters! I think this is a valid point, but I will have to go back to study "ekpyrosis" a little more before making a serious comment. At the same time, ekpyrosis is just a string based cosmological model, albeit novel, of which we have literally thousands! This is precisely the view of SR, GR, QFT, etc... But I'm not sure what you are saying here... can you expand a bit?

Don't worry My point was just that space and time seem to have different roles when EM is looked at traditionally, but the simple dimensionality of time becomes apparent when using the language of tensors.Did you look over the later points in the post? They weren't quite so gory...