|
|
Post by Progenitor A on Sept 22, 2010 12:46:11 GMT 1
First, you are mixing up two different kinds of singularities (which in maths terms just means some physical quantities become infinite). So a black hole singularity is a singular point within a pre-existing spacetime, where the curvature becomes infinite. Whereas in the big bang singularity, density was infinite. I have already pointed out this difficulty but you appear to ignore it. Let me try to make myself clear You say that the big bang singularity density was infinite -OK Some force must have been compressing that matter together to make the density infinite. It was not gravity because you say that gravity is the curvature of space-time and you say there was no space time outside the infinite mass. What force caused the infinte density mass? |
|
|
|
Post by speakertoanimals on Sept 22, 2010 13:19:09 GMT 1
You're basing your reasoning on false analogies again.
Okay, if we want to compresss some gas in a box, then we have to apply a large force to compress it and keep it compressed. But that is only because there is an outside at a lower pressure!
For the big bang, if everything was at such a high density, then no force needed, because there is nothing else outside at a lower pressure into which the matter might try to escape.
For a particular bit of matter, at high density, what stops it thinning out? The fact that all the matter around it is a similarly high density.
In fact, pressure does enter into the cosmological solutions, but the pressure exerted by ordinary baryonic matter is not important on cosmological scale.
Moral -- you can't take what you know about gas in boxes, and try and apply it to the entire universe, where there is no box and no outside either!
|
|
|
|
Post by speakertoanimals on Sept 22, 2010 13:33:00 GMT 1
Pressure is confusing in general relativity, because our intuiotive noiton is that something at high pressure needs to be held in, and will do work as it expands (steam engines anyone?). But that intuition is just wrong for the universe, when the whole universe is full of stuff at the same pressure, and there is no outside holding it in. It is NOT like a balloon, where pressure inside tries to expand it.
Why? Because to do work, high-pressure gas needs to expand out into some region of lower density. but if there is no such region, then no work is done.
There is another twist, which is that pressure (which in some sense means energy) occurs in the Einstein equations, and just as energy (that is mass) produces gravity, and just as momentum produces gravity, so does pressure. So, being energy, pressure actually strengthens gravity, rather than being the expansive sort of force we see for high-pressure regions in a lower pressure environment.
As I keep saying, you have to ditch many of your everyday assumptions about what high density means, about what high pressure means, in order to get any handle on cosmology.
|
|
|
|
Post by Progenitor A on Sept 22, 2010 13:33:45 GMT 1
You're basing your reasoning on false analogies again. Okay, if we want to compresss some gas in a box, then we have to apply a large force to compress it and keep it compressed. But that is only because there is an outside at a lower pressure! Actually I never did use the gas analogy For the big bang, if everything was at such a high density, then no force needed, because there is nothing else outside at a lower pressure into which the matter might try to escape. This sheer speculation admit it! Lumps of matter compressed into infinite density because they have nowhere else to go! You are joking matey. |
|
|
|
Post by speakertoanimals on Sept 22, 2010 13:49:25 GMT 1
If you think that is joking, then you have not grasped even the first thing about big bang cosmology!
As I keep saying, the big bang was NOT an explosion. Instead, it was everywhere, and everywhere was at much higher density then. not because all the matter was being held in one place, waiting to expand into some nice empty space, but because space itself was just 'smaller' then (I put smaller in inverted commas to allow for the fact that we could have an infinite universe, with an infinite density initial singularity, that still expands and remains infinite!).
Not pure speculation, it is what the observations tells us. We can see the expansion now, we can even see as far back as a few hundred thousand years after the event, in terms of the CMB. The fact that the universe was once hotter and denser, and uniform, is there for all to see.
It's not LUMPS of matter, it's ALL matter, and if you think matter was in lumps at the pressures and temperatures involved, you are loopy. Solid matter was much further down the line. We are talking about hot plasma and radiation, and only when electrons started binding to nuclei, did the universe become transparent, which is the CMB we see today. Before that, we had even hotter and denser, so that protons could not hold together (quark-gluon plasma), with various weird and wonderful at even higher densities and temperatures.
Matter in this context just means matter particles (electrons, protons, quarks), as opposed to radiation (photons). It doesn't mean lumps of solid matter.
You were using it without realising it, by the very question as to what holds the matter at such high densities.
Stop messing me about, if you knew anything at all about cosmology, you wouldn't think this was a joke.
|
|
|
|
Post by Progenitor A on Sept 22, 2010 14:33:19 GMT 1
If you think that is joking, then you have not grasped even the first thing about big bang cosmology! Quite frankly you are the first physcist in my experience (I am widely read) that professes to know what was going on in the singularity before th ebig bang. [Not pure speculation, it is what the observations tells us. We can see the expansion now, we can even see as far back as a few hundred thousand years after the event, in terms of the CMB. The fact that the universe was once hotter and denser, and uniform, is there for all to see. You mean physicists have observed the singularity before the big bang? No they have not of course and theories about what went on before the big bang are pure speculation. The fact that you profess to know makes me ponder somewhat [It's not LUMPS of matter, it's ALL matter, and if you think matter was in lumps at the pressures and temperatures involved, you are loopy. Did I say I thought that? Matter in this context just means matter particles (electrons, protons, quarks), as opposed to radiation (photons). It doesn't mean lumps of solid matter. You don't say! [Stop messing me about, if you knew anything at all about cosmology, you wouldn't think this was a joke. Why so arrogant I wonder? You consider that becaus I ask questions that you cannot answer properly that I am messing you about You are messing yourself about my friend I am interested in physics, and, for one who professes to be a physicst/cosmologist, none of your explanations are sufficient; they are opaque and even dogmatic - professing to know what went on before the Big Band indeed! |
|
|
|
Post by speakertoanimals on Sept 22, 2010 15:50:04 GMT 1
Who said anything about before? Yiou are thinking of the singularity, it seem to me as something that hung about for a while before going bang, which is wrong.
Even if the density doesn't go all the way back to singular, the points about pressure etc still stand.
No, just as no force is needed to maintain the density when the whole universe was very very dense (just not infinite). There can be no force, because no outside, and no force needed, because everywhere was equally dense.
I never said ANYTHING about before the big bang, you only think I did because you have some incorrect idea of some infinitely-dense cosmic egg that sat around and then somehow exploded when the pressure was released.
All I was talking about was the conventional running the film backwards to the point of the big bang singularity, which according to current theories was infinite. But that was the start of time, not some state that hung around, and I certainly never mentioned any before!
No, you are asking questions that I answer properly, but you don't know enough physics to understand the answers, and don't appear willing to learn. you don't appear to know much english either, if you keep claiming I was talking about BEFORE the big bang................
Perhaps if you were more interested in learning, rather than your mistaken assumption that I (and the physics community at large) don't know what they are talking about when it comes to the simplest basic cosmological models.
I never said I was a cosmologist, and all that you are getting here is the intro page on cosmology for undergraduates, the most basic stuff, so we are really talking kiddy level here, and you can't get your head around even that! And then you assume the rest of the book that you haven't got to yet is nonsense, based on your inability to understand the first few paragraphs.
Arrogant? Why is that the constant complaint from those who are being told they have got it wrong, they don't know enough physics, and don't seem interested in learning. More often the sort of complaint you hear from various bridge-dwelling denizens..................
|
|
|
|
Post by Progenitor A on Sept 22, 2010 17:27:47 GMT 1
I guess STA, that you are not getting through to me and I am inexplicble to you
Lets leave it at that eh?
|
|
|
|
Post by speakertoanimals on Sept 22, 2010 18:24:33 GMT 1
The facts are that theories about the big bang aren't that wild really. The prediction of expansion and a very dense (possibly infinite) initial state of the universe follow on very naturally from our current best theory of gravity (general relativity). Indeed, Einstein made what he described as his greatest blunder when he fudged up the cosmological constant in order to try and get a static universe, because at that time people thought that was what was needed.
The evidence for the expansion is the Hubble redshift. The direct evidence for the hot dense beginning is the cosmic microwave background, which is surprisingly uniform on opposite sides of the sky, and also gives the black body spectrum, and in fact is the most precisely measured black body spectrum in nature.
Then we also have the fact that it isn't totally uniform -- and without some irregularities, we wouldn't have any gravitational clumping of matter, hence the lumpy structures like clusters of galaxies that we do see.
An indirect confirmation comes from nuclear physics. If things were really that hot and dense, then we would expect various nuclear physics processes to go on, and in particular, why there wasn't just hydrogen in the early universe. The conventional models predict the primordial abundances of the light elements (helium, deuterium, lithium), and the fact that the measured values fit the model is an enormous plus on the side of the big bang. And this is taking us back way before the CMB (3000K and 400,000 to 500,000 years after the bang), since the nucleosynthesis took place in the first few minutes.
Given all this is just irks me when people suggest that cosmologists (and every other physicist that ever did an undergraduate course on cosmology) was in some way too stupid to ask things such as -- what was the force that made it so dense then? As I keep saying, no force was needed, and hot and very dense is just what seems to have been the initial conditions for our universe. From a hundreth of a second up til now, we have a pretty good idea as to what went on. Before that, we are in the realm of more speculative physics, we need to know a bit more about particle physics and quantum gravity to fill that bit of the story in.
So, for your contribution:
No lumps, as I said before. And no force needed, because what was being compressed (running the film backwards as it were) was not the matter, but ALL of space. And since matter was spread everywhere, it had no choice but to increase its density, there was no where else to go!
This isn't just speculation, it is perfectly good science, based on theories we can test here and now, and measurements we can take here and now. Indeed, if we could see microwaves, the bang would be written large all over the sky, and would have been one of the first puzzles that science tried to answer. Instead, we kind of came upon it in another way, by wondering why the sky was dark in the visible (olbers paradox).
Inexplicable? Yes, why aren't you willing to admit that you might be wrong, that the current knowledge you have of physics just isn't sufficient to deal with matters like the entire universe. Instead, you seem to prefer to assume that all us scientists are a bit dim, and haven't answered the obvious questions, even if we can't explain the answers to you
|
|
|
|
Post by abacus9900 on Sept 22, 2010 18:44:08 GMT 1
STA, I recall that on a previous occasion you stated that it is impossible to keep regressing in terms of the constituent parts of sub-atomic particles, yet here you seem to be saying that the initial universe was (possibly) infinity. The two positions just don't square, do they STA?
Or, perhaps, it is because I don't know enough physics. Which is it, STA?
|
|
|
|
Post by speakertoanimals on Sept 23, 2010 12:07:12 GMT 1
STA, I recall that on a previous occasion you stated that it is impossible to keep regressing in terms of the constituent parts of sub-atomic particles, yet here you seem to be saying that the initial universe was (possibly) infinity. The two positions just don't square, do they STA? Or, perhaps, it is because I don't know enough physics. Which is it, STA? Make your mind up, do you mean infinite in spatial extent, or infinite density? The point about the infinity is that it is the consequence of a classical theory of gravity. When we can do quantum gravity, then perhaps we will find that space is no longer discrete on the smallest scales, so that the infinity goes away. It is characteristic of the untutored to get all hung up on the infinity that appears in general relativity, either for black holes, or in the big bang. All that really is saying -- new physics comes in here, but apart from that, the rest of the story is probably pretty good. But we knew that anyway, since we were just using a classical theory of gravity. Still doesn't change the fact that the arguments about there must be a force to hold it all that dense are totally wrong. And the fact still rmeains that this standard cosmology isn't all wild speculation -- some cosmology is more speculative, but that is usually when people are trying what sort of effects new physics might being into it, and HOW we might see the effects of that new physics. And that's the whole point really -- asking if so and so were true, what would we see NOW, what can we measure to falsify that hypothesis. Which is science. |
|
|
|
Post by abacus9900 on Sept 23, 2010 12:34:45 GMT 1
Well, you did mention 'possibly infinite' in your post so....
|
|
|
|
Post by Progenitor A on Nov 7, 2010 9:22:36 GMT 1
Now here's a puzzle
Dr Tong of The Cambridge Cosmology Dept tells me that the Cosmic Background Radiation that we can all see (if we have the equipment) occurred 400,000 years after t=0 of the Big Bang.
Other Cosmologists tell me that the Big Bang occurred 14 billion years (or so)
I am also told that because space is expanding so rapidly , the actual diameter of the Universe is 43 billion light years or so.
Now all this means that the Cosmic Background radiation occurred - on the scale of the Universe- very close to the birth of the Universe.
As that is so then Cosmic Background radiation is at the extremity of the Universe, that is about 20 billion light years away from us the radius of the universe
Now what does this all mean?
Well the Cosmic Background Radiation that we see has travelled a distance of 20 billion light year in just 7 billion years to interfere with our satellite communications
In other words the electromagnetic radiation of Cosmic Background Radiation is travelling at 3c!
Other people tell me that the observable Universe is 14 billion light years in diameter - anything beyond that is moving away so fast it cannot be seen.
But we can see Cosmic Background Radiation!
We should not be able to of course.
If the Universe really is expanding at 3c then any em energy from the extremity should not be visible should it?
But it is
What on earth is going on?
|
|
|
|
Post by abacus9900 on Nov 7, 2010 10:50:50 GMT 1
Doesn't the CBR just become more diluted the more the universe expands?
|
|
|
|
Post by Progenitor A on Nov 7, 2010 11:09:36 GMT 1
Doesn't the CBR just become more diluted the more the universe expands? Ih yes, definitely, but we shouldn't even be seeing it at all. Should we? |
|
