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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeJun 12th 2013

added to axion a pointer to this recent article:

• Joseph P. Conlon, M.C. David Marsh, Searching for a 0.1-1 keV Cosmic Axion Background (arXiv:1305.3603)

Primordial decays of string theory moduli at $z \sim 10^{12}$ naturally generate a dark radiation Cosmic Axion Background (CAB) with $0.1 - 1 keV$ energies. This CAB can be detected through axion-photon conversion in astrophysical magnetic fields to give quasi-thermal excesses in the extreme ultraviolet and soft X-ray bands. Substantial and observable luminosities may be generated even for axion-photon couplings $\ll 10^{-11} GeV^{-1}$. We propose that axion-photon conversion may explain the observed excess emission of soft X-rays from galaxy clusters, and may also contribute to the diffuse unresolved cosmic X-ray background. We list a number of correlated predictions of the scenario.

• CommentRowNumber2.
• CommentAuthorUrs
• CommentTimeMay 8th 2014

added also a pointer to F-theory axions and to axion inflation (a newly created stub), cross-linked with cosmic inflation

(no new content, just some more links and references for the moment)

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeJan 16th 2017
• (edited Jan 16th 2017)

I have added some content to axion, extracting and highlighting two arguments of Witten on the axion mechanism.

The first argument “As a solution to the strong CP problem” (due to Vafa-Witten 84) is the argument for why the axionic potential of the form $a tr(F \wedge F)$ really leads to $\theta \coloneqq \langle a\rangle$ vanishing. After mumbling “path integral” and “Wick rotation” this is a one-line argument (the whole article has only 1.5 pages of text) but somehow all later sources and reviews are shy about saying this. That’s why I felt inspired to go ahead and spell it out on the $n$Lab.

The second argument “As arising from string theory” (due to Svrcek-Witten 06) is almost as simple, too, but doesn’t seem to be widely advertised either. I tried to highlight the key ingredients that go into this.

There is quite a remarkable statement here, in conclusion: The strong CP problem (and maybe also the problem of dark matter, according to Witten et. al. 16) would be solved by the existence of a scalar field $a$ with the curious coupling term $a \, tr(F \wedge F)$. This raises the question whether such a coupling is not just as contrived as the original smallness of $\theta$. But then the observation is that such a coupling is precisely what becomes of the Green-Schwarz anomaly cancellation mechanism of heterotic string theory in 4d.

• CommentRowNumber4.
• CommentAuthorUrs
• CommentTimeFeb 5th 2017
• (edited Feb 5th 2017)

I have expanded further at axion:

1. added more of the historical references,

2. accomopanied the discussion of the axion from the perspective of the heterotic string with the paralell discussion from the perspective of the type IIA string.

• CommentRowNumber5.
• CommentAuthorUrs
• CommentTimeApr 1st 2018

Edit to: axion by Urs Schreiber at 2018-04-01 00:54:20 UTC.

hyperlinked pointer to textbook by Ibanez-Uranga

• CommentRowNumber6.
• CommentAuthorUrs
• CommentTimeMay 18th 2018

moved the previous pointers to Wiczek’s notes and to the ABRACADABRA experiment from “References – Experimental signature – In cosmology” to “In particle physics”

• CommentRowNumber7.
• CommentAuthorUrs
• CommentTimeJul 9th 2018

added pointer to today’s HJSSZ 18 (potential axion signatures in LIGO-type gravitational wave detection)

• CommentRowNumber8.
• CommentAuthorUrs
• CommentTimeMar 3rd 2020