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Good News for Big Bang theory
Phillip Helbig---remove CLOTHES to reply wrote:
In article , " writes: So we have agreement on several important issues, but some remaining differences, as discussed below. There is an ongoing debate over whether the very large-scale distribution of galaxies is fractal, or goes over into a homogeneous distribution. It is ongoing, but not in serious circles. There IS a scale above which there is large-scale homogeneity. "Serious" cosmologists can say that as many times as they like, but it is still a matter of faith, and I use that term literally. What scientific proof do you offer for your claim of homogeneity? The microwave background used to be the best supporting evidence, but many strange anomalies (recently the possibility of a quadrupole anisotropy) and correlations with the Solar System's ecliptic, etc. have clearly shown that we probably have much more to learn about the large-scale structure of the observable universe? As scientists, we need be sure that the evidence is unambiguous before we declare an issue closed. Ok, but quite simply: is there some prediction or test by which the new, and heavily reworked, modified, adjusted, Big Bang paradigm can be falsified, or does the present incarnation of the Big Bang paradigm not meet Popper's criterion for science? Certainly. But you have to define exactly what you mean first. Second, it is a widespread misconception that disproving one aspect of a theory also disproves the foundations. That is not necessarily the case. Please show me at least one definitive prediction, by which we might put the Big Bang paradigm to a rigorous, quantitative scientific test. |
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#23
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Good News for Big Bang theory
Thus spake "
Phillip Helbig---remove CLOTHES to reply wrote: In article , " writes: . What scientific proof do you offer for your claim of homogeneity? The microwave background used to be the best supporting evidence, but many strange anomalies (recently the possibility of a quadrupole anisotropy) and correlations with the Solar System's ecliptic, etc. have clearly shown that we probably have much more to learn about the large-scale structure of the observable universe? As scientists, we need be sure that the evidence is unambiguous before we declare an issue closed. The large scale distribution of galaxy clusters is also observed to be extremely homogeneous. But actually, what would be really difficult is justifying the formulation of a theory which did not obey the cosmological principle. Ok, but quite simply: is there some prediction or test by which the new, and heavily reworked, modified, adjusted, Big Bang paradigm can be falsified, or does the present incarnation of the Big Bang paradigm not meet Popper's criterion for science? Certainly. But you have to define exactly what you mean first. Second, it is a widespread misconception that disproving one aspect of a theory also disproves the foundations. That is not necessarily the case. Please show me at least one definitive prediction, by which we might put the Big Bang paradigm to a rigorous, quantitative scientific test. I think, notwithstanding possible anomalies, the microwave background is such a test. It's overall character is well established and in accordance with a big bang. Only some of the detail may need attention. Another is the proton-neutron balance. This can be precisely calculated as a product of big bang nuclear synthesis, and depends quite critically on Hubble's constant. Observation and prediction fit extremely well. But there are simpler and more obvious ones. Without a big bang we would be caught up in Olber's paradox. A big bang is the most natural solution of Einstein's field equation (the same is actually true in Newtonian gravity). The fact that scientists, including Einstein himself, did not tend to adopt a big bang model had to do with theoretical prejudice, and had nothing to do with whether cosmological expansion was a prediction of the model. It was only when a prediction of the model was observed by Hubble, that the model started to be adopted. Einstein's theory of general relativity has also been put to a number of rigorous experimental tests, and has passed all of them while alternative models have failed. One must accept that any model must form a self consistent mathematical whole. One cannot accept parts of it and then accept other things which are mathematically inconsistent. It seems to me that any scientific model must satisfy this criterion even before one applies observational tests. In that regard, a big bang obeying general relativity has no serious competitors. One can hardly say this is not the result of rigorous scientific quantititive testing. Regards -- Charles Francis substitute charles for NotI to email |
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Good News for Big Bang theory
"John (Liberty) Bell" wrote:
[...] wrote: wrote: Phillip Helbig---remove CLOTHES to reply wrote: Prediction, prior observation, prediction, assumption later verified by observation. By my accounting this list should read: 1. Background Radiation: prediction, but initially off by about 400% and had to be adjusted to come into agreement with observations. Your 400% seems way off -- Alpher and Herman's 1948 prediction was "about 5K." The figure I seem to remember was a factor of 3 difference, but I certainly couldn't quote references after all this time. I would guess from the above that various people had a stab at this calculation, with differing results. If so, cherry picking the best, after the fact, would seem to be no more logical than cherry picking the worst. Alpher and Herman's 1948 Nature paper was, I believe, the first to make a prediction. There was some variation after that, but it came from lack of a good observational value for the present density of the universe (and probably also from the fact that people were making order-of-magnitude estimates of a quantity they didn't expect to be observable). Furthermore, you're not quite asking the right question. CMBR temperature varies with time, so to predict the value "now" you need to know when "now" is, I don't think anyone could argue with that. relative to, say, primordial nucleosynthesis. Clarification here would be appreciated. I was under the impression that CMBR is black body radiation at the transition from plasma physics to the point where protons and electrons combine to form atomic hydrogen, thus indicating the temperature at which this transition is believed to take place. Right, but not relevant to the present issue. To compare a past value to a present one, we need to be able to compare two independent quantities, one to relate the times and the other to allow a prediction of temperatures. At recombination, we know the temperature (from the ionization energy of hydrogen), but don't know anything obvious that would tell us the time between then and now. At primordial nucleosynthesis, though, we know both the temperature and the baryon density. So we can start then, predict the evolution of both temperature and density until recombination (which then gives us a density at recombination), and then compare the density at recombination to density now to calibrate the time. Remember also that the prediction was not just a temperature, but a spectrum. Black body spectra are hard to make (since temperatures of different sources get different red shifts); Hmm. Black body radiation is black body radiation, isn't it? Black body radiation at a given temperature is all the same. But if you combine two black body spectra at different temperatures, the result is not a black body spectrum at some intermediate temperature -- it's not black body at all. Observation of a uniform black body spectrum means either that the universe was once a black body at a uniform temperature, or that some huge series of implausible coincidences somehow combined a bunch of different spectra to mimic a black body. Steve Carlip |
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Good News for Big Bang theory
I find myself largely agreeing with Oh No here, except for the
following paragraph. Oh No wrote: A big bang is the most natural solution of Einstein's field equation (the same is actually true in Newtonian gravity). I think you must mean gtr. EFE (as first published) contained a cosmological constant for the explicit purpose of preventing any such global geometrical dynamism. The fact that scientists, including Einstein himself, did not tend to adopt a big bang model had to do with theoretical prejudice, That is unfair. Every generation is a product of its educational background. In Einstein's case that was in the 19th century, when the thermodynamics of steam engines was still the 'state-of-the-art' in the technological application pure physical principles. Einstein supressed global geometrical dynamism within his mathematical apparatus, in order to produce a general theory of relativity which was consistent with the then known facts about the universe. If that is prejudice, then we are certainly still guilty of it now. Perhaps we would all be a little wiser if we learned from the lessons of history, by recognising that nothing ever really changes, in that respect. John Bell (Change John to Liberty to bypass anti-spam email filter) |
#28
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Good News for Big Bang theory
Thus spake "John (Liberty) Bell"
-- red shift dependence of CMBR temperature (for observations, well after the predictions, see Battistelli et al., astro-ph/0208027; Srianand et al., astro-ph/0012222; Molaro et al., astro-ph/0111589) Yes, that certainly confirms global expansion (which I don't think anybody doubts). However, I dont see how this necessarily nails things down any more precisely. Take, for example, Oh No's theory (which I am definitely not claiming to support). In this you should still get such changing T with changing z despite the timescales being radically different. In fact my proposal only affects quantum phenomena for which, by definition, continuous observation is not possible, and for which, incidentally, there is not an alternative existing theory. It gives classical general relativity in the classical correspondence. The cosmic microwave background is continuously observable, so it must be treated classically and obeys the same redshift relationship as found in the standard model. Regards -- Charles Francis substitute charles for NotI to email |
#29
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#30
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Good News for Big Bang theory
Phillip Helbig---remove CLOTHES to reply wrote:
Indeed. All the research on CMB inhomogeneities (and there is a lot of research to do) has obscured two Very Important Facts in the popular mind: the black-body spectrum, as Steve mentioned, and the fact that the signal is very homogeneous. The latter doesn't just mean that the inhomogeneities are small, but is in itself very important. I agree that within the local Hubble Bubble the CMB is approximately "homogeneous", but then there are the dipole, quadrupole and octopole anisotropies. The difference between the Newtonian and GR predictions for the precession of the perihelion of Mercury was "small" too. The devil is sometimes in those "small" details, no? Also, does the distribution of matter look homogeneous, with all those very much unpredicted sheets, filaments and voids? I might go along with a claim of statistical homogeneity, but claims of a more idealized, literal homogeneity seem to me to be in conflict with accepted observations. Rob |
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