I hope others will watch and comment. I think it is an important talk. Or maybe two important talks packaged together.
The first 40% of the timebar is about quantitative unification. with SO(10) and assuming some low energy Susy.
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.
This 0.4 - 0.7 part of Wilczek's talk is definitely something I want to watch over again, maybe several times. And I want to find a writeup on the arxiv for it.
It draws some new connections between cosmology and particle physics for me.
Then around 0.7 on the timebar he talks about ways to TEST his axion+cosmology ideas, including with experimental particle physics. That is a short bit. Maybe 0.7 - 0.8
The last 20% I didn't get much out of. 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 found this paper by Wilczek and others which appears to have some overlap with the talk:
Axion Cosmology and the Energy Scale of Inflation
Mark P Hertzberg, Max Tegmark, Frank Wilczek (MIT)
(Submitted on 11 Jul 2008 (v1), last revised 3 Apr 2010 (this version, v3))
We survey observational constraints on the parameter space of inflation and axions and map out two allowed windows: the classic window and the inflationary anthropic window. The cosmology of the latter is particularly interesting; inflationary axion cosmology predicts the existence of isocurvature fluctuations in the CMB, with an amplitude that grows with both the energy scale of inflation and the fraction of dark matter in axions. Statistical arguments favor a substantial value for the latter, and so current bounds on isocurvature fluctuations imply tight constraints on inflation. For example, an axion Peccei-Quinn scale of 10^16 GeV excludes any inflation model with energy scale > 3.8*10^14 GeV (r > 2*10^(-9)) at 95% confidence, and so implies negligible gravitational waves from inflation, but suggests appreciable isocurvature fluctuations.
Comments: 10 PRD pages, 4 figs
The link to Wilczek video no longer works:
I for one would like very much to watch it again. Does anyone have any ideas?
Om du klickade på en länk på Medfarms mediabibliotek och fick upp den här sidan, skriv till firstname.lastname@example.org. Ange särskilt vilken länk du klickade på.
The error message says to mail email@example.com and report which link is broken. Unless they removed it for a reason, I assume they'll fix it.
Thanks, and your assumption was right! I just tried the link and it has been fixed.
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 comment
Good thing smoit knows an answer that both Edward Witten and Frank Wilczek don't!
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?
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.
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" [Broken] and it's a shame that "a specialist on the subject" has not paid attention to the recent developments.
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.
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.
Why Witten didn't provide the answer right away?
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.
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.
the video is ~300mb so you might want to download it if repeat viewing is desired (something like vlc player will play it)
direct download link (may need to right-click and choose 'save link as..' or similar)
Wilzcek claims the lightest supersymmetric particle candidate for dark matter 'gets lost' and cold dark matter is rather axions. Shame he didn't get onto portals.
"0-brane perspective - that's "b" "r" "a" "n" "e" :rofl:
I hadn't realized Americans spell "esthetics". That still leaves "amoeba" as an irregularity.
The prevailing usage in Usa is aesthetics.
But in surgery you get anesthetized. The simplified spelling prevails although anaesthetic is acceptable. So it isn't consistent.
Anyway he used the LESS common variant spelling, I think wisely. It comes across as less high-fallutin literary, more regular-guy, middlebrow.
Exceptions to the American simplification rule include aesthetics and archaeology, which usually prevail over esthetics and archeology, respectively, as well as the stronger case of palaestra, in which the simplified form palestra is a variant described by Merriam-Webster as "chiefly Brit[ish]."
In the interesting section from 40% to 70% across the timebar, about Inflationary Axion Cosmology, there was one point of definition where I thought I understood something different from Mitchell. In general Mitchell's summary was excellent and helped me see what was going on in one of the graphs:
Mitchell the way you describe Infl. Axion Cosm. (IAC) inflation occurs first, and then PQ. Maybe the distinction does not matter but I thought he said that corresponded to an earlier view of Axion Cosmo. and that what he meant by IAC was the case where PQ transition comes first (fixing a dark matter ratio in a certain region) and then inflation expands that region so that abundance prevails throughout the whole apparent universe.
My take on this is that it may be just a verbal slip analogous to forgetting the minus sign in an equation. But I want to be sure because I may have misunderstood something in what either you or Wilczek said.
Obviously he has a paper in the works with Max Tegmark, and it will (I would guess) be coming out quite soon.
The interesting thing is that it PROVIDES A TESTABLE ALTERNATIVE TO THE LIGHTEST SUPERPARTNER hypothesis about the composition of dark matter.
He points to ways to test it.
I think that a scientific alternative hypothesis deserves respect, if it is testable, whether it eventually proves right or wrong. I don't want to bet on stuff like this, but want to honor it as extremely interesting. And he describes approaches to testing starting around 75% along the timebar.
You're right Marcus, I was confused (and still am) about how it works, but evidently Wilczek requires that some inflation occurs after the PQ transition. But note that inflation can be occurring before the transition as well! In that case, the PQ transition occurs during inflation, rather than before it. The important part is that regions with the various nonzero values of theta_0 which arise after the PQ transition get blown up to astronomical scales.
In terms of part 2 of http://arxiv.org/abs/astro-ph/0610440" [Broken], I think Wilczek must be talking about Case 1, "inflation occurs with reheat temperature smaller than T_PQ [temperature at which PQ transition occurs], and the axion field is homogenized over enormous distances". But it's all a little complicated - there are thermal axions and cold axions; the axion gets its mass at the later transition, the QCD transition; the axion field even mixes with QCD meson fields! And then there are the extra twists that smoit mentions.
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