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Marcus, it's pretty clear that you have no idea what Witten's comment/question was all about! So, for you to belittle his comment is simply ridiculous. It was, in fact, a very relevant comment that pointed out at a well-known problem that exists in the scenario with low-scale SUSY and an axion, which Wilczek was advertising, so "Good luck with that!" was a way to dodge the issue. Again, this problem has nothing to do with bottom-up or top-down approach because it's simply a consequence of having supersymmetry: any axion must have a scalar associated with it and for the axion to remain ultra-light the mass of that scalar must be generated by susy breaking effects. This potentially leads to a cosmological moduli problem and that's what Ed was pointing to. It's a shame that Wilczek did not know what to say because there is, in fact, a good answer to Witten's comment/question. In short, having such light scalars (moduli fields) leads to a so-called non-thermal cosmological history of the universe with a long period of moduli domination, which gives novel cosmological signatures. Ironically, Wilczek's anthropic fine-tuning of the axion misalignment angle is reduced by several orders of magnitude in such a non-thermal scenario since the large entropy production from the moduli decays dilutes the axion relic density and almost eliminates the need of the fine-tuning. So, if Wilczek had attended, eg. this workshop:http://www.umich.edu/~mctp/SciPrgPgs/events/2011/NCHU/Prog.html", he would have learned how to answer Ed's commentmarcus said:There was only one "question" from the audience which turned out to be a long sermon seemingly to the effect that axions were "really" string theory. To which Wilczek had very little to say besides "Good Luck!" and to comment that he was coming at these things from a bottom-up approach. Witten's "question" or long comment took up so much time that there was no time for other questions from the audience.
I'm sure Ed is well aware of what I have said here because at least one of my collaborators has discussed this issue with him and since he is familiar with the relevant literature, unlike Wilczek, but do YOU have anything of substance to say?MTd2 said:Good thing smoit knows an answer that both Edward Witten and Frank Wilczek don't!
Yes, and who am I, right? Mind you, I'm not the only one who knows a possible answer to Witten's comment. You don't have to be a genius to follow the relevant literature if you work in the field. Here is the most recent paper, among many, that discusses the issue at hand: http://arxiv.org/abs/1104.4807" and it's a shame that "a specialist on the subject" has not paid attention to the recent developments.MTd2 said:I am just point out how funny this is. Wilczek, which is supposedly to be a specialist on the subject, and extremely reputed physicist, doesn't know about crucial issues of his own theories, while you do.
There was one graph I had to decode in order to make sense of the talk, the very densely illustrated graph with temperature, gas density, and virial velocity as coordinates, and with a yellow star marking a small parametric sweet spot which corresponds to our galaxy.marcus said:The second topic is Axions and it is covered in the 0.40 - 0.70 segment and for me was the most exciting. He argues persuasively for a line of speculation where the QCD THETA angle governs the RATIO OF DARK MATTER TO ORDINARY, in cosmology.
Then he reports on work with Max Tegmark looking at scenarios where the PQ (peccei-quinn) transition comes before inflation and we live in a patch of universe with a particular THETA and he looks at how favorable our theta would be to structure formation, like stars and galaxies. Some remarkable stuff comes up.
smoit said:it's a shame that "a specialist on the subject" has not paid attention to the recent developments.
mitchell porter said:... Then, in the "inflationary axion cosmology" where the Peccei-Quinn transition occurs after inflation, so you can have different regions in the multiverse (or even in the same inflationary region??) starting with different values of theta_0, the ratio of dark matter density to baryon density can also vary, and that's where Wilczek suggests that anthropic finetuning is at work, leading to his second, much simpler graph, showing a probability distribution for "dark matter per photon" densities that is peaked near the actual value.
I don't know that but I suspect that he's been thinking about the issue and I certainly know that he's aware of the non-thermal cosmological scenario. He said that this problem is worse in field theory compared to string theory but he did not elaborate on that point. The point is that in string compactifications one can find examples where the moduli are about O(10-100) heavier than the MSSM superpartners whereas in field theory one would generically expect them to be at the same scale. So the saxion with a mass, say 50 TeV, can decay before BBN and not screw up the abundances, while the superpartners are much lighter and can be reached by the LHC.MTd2 said:Why Witten didn't provide the answer right away?
atyy said:"0-brane perspective - that's "b" "r" "a" "n" "e"
I hadn't realized Americans spell "esthetics". That still leaves "amoeba" as an irregularity.
mitchell porter said:...the very densely illustrated graph with temperature, gas density, and virial velocity as coordinates, and with a yellow star marking a small parametric sweet spot which corresponds to our galaxy.
I believe the points on that graph correspond to populations of gas clouds in the early universe, before galactic formation, and the colored-in regions tell us about what happens to those gas clouds. Some just become black holes, some apparently become too crowded with stars for stable planetary systems to exist ("too close encounters"), etc. And then there were two thin regions of parameter space labeled "halo formation" and "halo destruction". Those would be dark matter halos in the gas clouds. I think dark matter is supposed to seed galactic formation. So the yellow star marks a point where the dark matter halo forms, and where the gas cloud also cools enough for planets to form.
Standard cosmology is characterized by about eleven parameters (dark matter density, total neutrino masses, etc). So this graph must be showing us the fate of those gas clouds with the cosmological parameters tuned to their observed values. Then, in the "inflationary axion cosmology" where the Peccei-Quinn transition occurs after inflation, so you can have different regions in the multiverse (or even in the same inflationary region??) starting with different values of theta_0, the ratio of dark matter density to baryon density can also vary, and that's where Wilczek suggests that anthropic finetuning is at work, leading to his second, much simpler graph, showing a probability distribution for "dark matter per photon" densities that is peaked near the actual value.
marcus said:I agree completely with the helpful summary in your first two paragraphs. Thanks! I don;t agree (or don't understand) your "occurs after", however. You may simply have accidentally misspoken. I think if you drag the time button to almost exactly 50% along the timebar you see him defining the "inflationary axion cosmology" oppositely to what you say here. He at first describes what has been "default" axion cosmology = if no inflation occurs after PQ.
Then he shifts gears and he has a slide that gives the new heading "Inflationary axion cosmology" and the following slide says:
"If inflation occurs after the PQ transition, things are very different... so we shouldn't average..."
Then by contrast you have a multiverse picture with different universe-sized regions each with a different (randomly determined--i.e. environmental) dark matter density.
By this time we are about 55% along the timebar.
The dark matter density is crucial (as you say) to how structure formation is going to play out. So some of these universe-sized regions have good structure formation and others do not. Might have too much or too little condensation.
That is where the overpacked graphs/figures come in, including the one you mentioned. The yellow star is, I guess where we are in that kind of "phase diagram". Max Tegmark leans towards "warm" graphics, it seems. Several times Wilczek seemed to be balking at the design style of his own graphics.
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