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    I really like this quote from Colin Powell: What is the greatest threat facing us now? People will say terrorism. But are there any terrorists in the world who can change the American way of life or our political system? Only we can change ourselves.


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  1. Simon, welcome. Sisyphus and Mr.Skeptic are explaining things in words but you might also need to get some mental imagery. So I suggest you google "wright balloon analogy" and watch it for 3 or 4 minutes as the two 2D universe expands Then type a number 2 into the URL and get the second version "Balloon2", that has light in it. Little wigglers representing photons, that move across the 2D surface while it is expanding. Also you mentioned the special relativity (1905) speed limit. That does not govern the rate that distances can expand in the improved (1915) theory called general relativity. They do you a real disservice in school or in popular science books when they give you the impression that everything (including rates of largescale distance increase) is governed by the 1905 speed limit. General Rel says you can't TRAVEL faster than light. You can't catch up to and pass a photon. You can't approach some destination faster than the standard rate c. But it does not say that distances between galaxies cannot increase faster than c. Even when the galaxies are essentially standing still in their surrounding space. You can even see that happening in the small movies I mentioned, if you are observant. when you type 2 into the URL you will get http://www.astro.ucla.edu/~wright/Balloon2.html Watch it. The galaxies are all standing still (constant longitude latitude in the 2D spherical surface which is their universe) and the wigglers are all traveling at constant speed across the surface I would say about 3 millimeters per second, based on the scale of my computer screen. But there are photons heading in the direction of some galaxy which never get there, in fact get dragged back farther and farther away from their aimed-for destination, by the expansion of distance. And you may see photons whose distance from their point of departure is increasing much faster than the 3 mm/second rate they crawl through their local neighborhood. Special Rel (1905) applies to NON-EXPANDING SPACE. What it tells you is only approximately right because our local small-scale space is approximately not expanding. Only thing to do is watch the movie and get some new mental imagery. Words by themselves often do not work for people.
  2. Sometimes a user will refuse to admit they made an error and continue arguing. Not listen to what they are told. Misinterpret what is being said, take issue with strawmen, and so on. It gets to seem like they are arguing just for argument's sake. We need a place for posts that might arise this way, so we can move them here and take some time to think about it---see if the post might have merit, or arises from honest misunderstanding, or is just arguing for its own sake. So here's a thread for that kind of thing, so we can move posts here and check them out. And in some cases move them back into the original thread! Hopefully, if on reflection there seems to be no intent to argue for its own sake and whatever confusion has been resolved. Anyway that's what I plan to try here, and other staff (mods and others) who work in Astro/Cosmo are welcome to move stuff here, in and out of this thread.
  3. These links are old. The thread is dated. I will move it to the speculations trash and think what new resource links for QC and QG to post.
  4. As DH said, stop putting words in our mouths. Nothing wrong with isotopes being formed in iron core supernova. Didn't say there was. You seem more interested in arguing than in learning. DH already called troll. I have to reluctantly go along.
  5. ===quote DH== ... In fact, they produce quite a bit of iron. The iron is produced by the supernova event in the case of a pair instability supernova or a Type 1a supernova. ... ==endquote== I have been reading the first of those two references in post #25---the one about SN 2007bi. An important PISN (pair instab. SN) signature was the large amount of Nickel-56. Predicted by the PISN model. 56 is divisible by 4. 14 helium nuclei come together. Energetically favored. Ni-56 is made at the time of the explosion and decays over a few days to iron. I gather the PISN model has been around, been studied, for 30 years or more. It's clearly mainstream. Simply that PISN events are rare and the first clear instance was 2007bi. They knew what they were looking for (big release of Ni-56 etc etc.) Also it's good you pointed out the similarity with the (much smaller) Type 1a! That is another one where it is not iron-core-collapse. There is a core of potentially good fusion fuel, it just needs something to set it off and you get a runaway thermonuclear explosion. The Chandrasekhar trigger. That should also have a Nickel-56 signature, I suppose, but it would be different from PISN in other ways.
  6. Steevey, you aren't listening. Please read post #17 again. When a high enough temperature is reached the gamma radiation is no longer efficient at supporting outer layers. And the hotter it gets the less effective fusion becomes at supporting outer layers. That is why your reasoning just now is bad. Your reasoning in this post is bad. A typical "non-metallic" core in this case might be primarily oxygen and around 100 solar masses. (star more massive, core say 100 solar). All that oxygen is potential fusion fuel. But the fusion energy is now being bled off into the pair creation reaction and no longer serves to support outer layers. And the hotter the core gets the less effective a given amount of fusion energy becomes, because more gets trapped into pair creation. So the star cannot "return to normal" by increasing rate of fusion in core. This stabilizing feedback is what fails to work when pair-instability conditions are reached. Please let us know if you have contributed anything to Wikipedia or to Wiki-answers, and give us links to articles you have edited. There is a serious possibility that your work is unreliable and the relevant admins should be warned. Thanks. If you show consistent refusal to listen to physical reasoning we will have to think of some way to get your attention.
  7. I should have spoken more precisely. As I understand it they found that amount of patterning can occur randomly. As one paper said, to the extent that Penrose found concentric circles you could just as well find triangles or squares. There will always be randomly occurring patterns but it does not mean anything. So the papers contested the claim that the findings were significant. ==================================================================== I may be mistaken. PENROSE AND GURZADYAN posted a second paper today, answering their critics and maitaining that the observed patterning is significant. http://arxiv.org/abs/1012.1486 More on the low variance circles in CMB sky V.G.Gurzadyan, R.Penrose 2 pages (Submitted on 7 Dec 2010) "Two groups [3,4] have confirmed the results of our paper concerning the actual existence of low variance circles in the cosmic microwave background (CMB) sky. They also point out that the effect does not contradict the LCDM model - a matter which is not in dispute. We point out two discrepancies between their treatment and ours, however, one technical, the other having to do with the very understanding of what constitutes a Gaussian random signal. Both groups simulate maps using the CMB power spectrum for LCDM, while we simulate a pure Gaussian sky plus the WMAP's noise, which points out the contradiction with a common statement [3] that 'CMB signal is random noise of Gaussian nature'. For as it was shown in [5], the random component is a minor one in the CMB signal, namely, about 0.2. Accordingly, the circles we saw are a real structure of the CMB sky and they are not of a random Gaussian nature. Although the structures studied certainly cannot contradict the power spectrum, which is well fitted by LCDM model, we particularly emphasize that the low variance circles occur in concentric families, and this key fact cannot be explained as a purely random effect. It is, however a clear prediction of conformal cyclic cosmology."
  8. http://arxiv.org/abs/1012.1268 A search for concentric circles in the 7-year WMAP temperature sky maps I. K. Wehus, H. K. Eriksen http://arxiv.org/abs/1012.1305 No evidence for anomalously low variance circles on the sky Adam Moss, Douglas Scott, James P. Zibin Sean Carroll pointed these out. Looks like Penrose circles not confirmed. http://blogs.discovermagazine.com/cosmicvariance/2010/12/07/penroses-cyclic-cosmology/
  9. Thanks for the correction. Should have said wavelength. Since neutrinos are massive particles I'll just think of them as slowing down as they lose momentum, in contrast to light.
  10. Short answer is YES neutrinos lose momentum--from the standpoint of observers at rest relative to primordial light (CMB). Since neutrinos have mass, losing momentum means losing speed. An article I read a couple of years ago said that STEVEN WEINBERG in his textbook COSMOLOGY goes through the math for a general particle with mass, traveling over long period of time in expanding space and shows how it loses momentum (so speed) due to expansion. The author provided what he said was a more concise proof. It is not too surprising. People use this fact when they run simulations of structure formation in the universe. Most of the matter is dark matter. so how does DM condense into clouds and cobwebby filaments? Good question. If DM is falling into an overdense region, how can it dump kinetic energy? How can it disipate energy so that it can STAY in the cloud? The answer involves some gravitational interaction so that some DM is trapped and some other DM gets extra and escapes. then the escaped DM gradually due to expansion loses kinetic energy and is able to participate in condensation somewhere else. That is more than you asked about, so ignore it if you want. the main answer is yes expansion does have the effect of draining momentum from stuff (seen from CMB rest) and when it is light we call it REDSHIFT but when it is a particle like neutrino or DM particle then you can think of it as simple SLOWING DOWN. Or I guess you can think of it as redshifting the quantum wavefunction of the particle but that seems a bit fancy when all it is is slowing down.
  11. In part your questions show me my understanding is inadequate. So I looked around and didn't find much. I did find this: http://www.scholarpedia.org/article/Accretion_discs I may find more later. You sound like you have it pictured in mind at least as well as I do, or did until reading the Scholarpedia on Accretion discs. My intuitive picture is based on the ideas of conservation of energy and of angular momentum. I know that around a BH there is a minimum circular orbit size, where stuff is circling at near the speed of light. I haven't done a back-of-envelope calculation to estimate the amount of gravitational potential energy something has when it is, say, 5 million miles out from solar mass hole, compared with what it has when it is 5 miles out. It seems to me that if you could lower something on a pulley from 5 million miles down to 5 miles you could extract a lot of work. If something is spiraling in, then much of that energy has to be dissipated. A lot of energy (and angular momentum) has to be somehow blown off. I think you were hinting at that in what you just said, and offering images of how the energy might be ejected. so as to let the thing spiral down in closer. I have to go do something. Can't satisfactorily get this clear right now. Maybe someone else will help.
  12. Be careful about dismissing people if you don't always know what you are talking about, Steevey. Nobody knows it all (not me anyway.) Arch gave a fairly clear (if very brief) mention of the pair-instability hypernova mechanism. Indeed it is believed to proceed by a kind of chain reaction which in effect traps increasing numbers of gamma photons once they reach a certain threshhold energy---so they no longer help to support the outer layers by radiation pressure. The gamma photon reacts with an atomic nucleus to produce an electron positron pair. The particles and antiparticles in the core then annihilate to produce more gamma photons with the required energy to repeat the interaction. This can happen in a massive (>150 solar) star long before she has run out of fuel. The core full of good stuff ready for a runaway thermonuclear explosion if the temperature gets high enough. All that needs to happen is the temperature reaches the threshhold where photons are diverted by the pair-production reaction from their job of supporting the outer layers. Then there is an abrupt drop in pressure. The outer layers come crashing. Temperature rises. More gamma photons are absorbed by the pair-production. More outer layer crashes down. And the remaining fuel in core experiences runaway detonation. In a pair-instability hypernova you don't even necessarily get a neutron-star or black hole remnant, at least according to the theoretical model. It's cool. I think that was what Arch was trying to say in a few words.
  13. You know an asteroid falling towards some other body when it hits can release enough heat to melt rock. It can melt part of the asteroid and part of the crater---even vaporize. That is just a small conversion of gravitational energy. How much energy depends (among other things) on the square of the escape velocity at the surface of the target body. The grav. energy in this context is what does the heating. It isn't just a "fancy talk" equivalence---it is a meaningful straight-talk equivalence. Nuclear fusion only gets you on the order of 1% of the mass converted to energy (gamma radiation, kinetic or heat energy) Gravity lets you make a much more efficient conversion. As long as the grav. field that the thing is falling thru is strong enough. The creation and annihilation of matter/antimatter would not play an essential role or have a net effect. The essential thing must be the conversion of gravitational potential energy into kinetic/heat energy (by compression and "friction" in the accelerating/inspiraling disk of matter).
  14. Nice! Did you do the labeling on that diagram? Ned Wright's cosmology tutorial had some basic diagrams like that, but last time I looked it did not have the labeling with words like "distant galaxy" and "Milkyway". Ned Wright's diagrams had the teardrop shape lightcones, and the worldlines with little local forward lightcones along them. A light ray worldline passing one of those has to be running parallel to one side or the other of the small local triangle. The plot is with proper time and proper distance. It's a really good tutorial and the diagrams are a great help. But I haven't looked at Wright's UCLA website for quite a while, so I don't know if he has new material. What I think is that you made something responsive and useful by correctly labeling one of his. In any case thanks for a valuable contribution!
  15. I defer to Spyman and only comment in support of his explanation, but yes and you can figure for yourself that since the redshift z = 8.6 the surrounding galaxies were about 10 times closer than today on average. The relevant ratio is z+1 = 9.6 or about 10. You are talking about the universe at redshift 8.6 when it was already about 600 million years into its expansion history (correct me if you mean something else.) So the CMB radiation would have a temperature 10 times hotter. Today it is about 2.75 Kelvin. So back then the microwave soup around you would be 27.5 Kelvin. Still pretty cold :-D And objects would be abouty 10 times closer. And the average density of matter in space would be about 1000 times denser. Galaxies would still be in the process of forming. People talk about "proto-galaxies". Smaller clumps that would be growing, and merging to form larger clumps. Structure would be in the process of assembling itself. So there could have been more little pieces and fewer big pieces. But if you allow for that qualitative adjustment in your imagination it would be OK to say, I think, that galaxies were 10 times closer. There is one caveat. The Hubble law does not refer to gravitationally bound assemblages of matter, like clusters of galaxies. Our Local Group of galaxies does not participate in the expansion process and so it would not appear to contract as you go back in time. Likewise the Virgo Cluster of galaxies is gravitationally bound. So those distances are stable at least in some average sense. The pattern of expansion does not apply to IMMEDIATE neighbors, like Andromeda a member of our Local Group. It only applies to more distant galaxies which are not bound to us. But that is almost all (the Local Group is only a couple of dozen out of millions, so we can forget about it). We can say that almost all galaxies were 10 times closer back then, than they are today.
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