NEQNET: Non-equilibrium Phenomena

Cosmology, turbulence, markets, non-equilibrium QFT and much more. No nonsense, just science

Lithium problem

Apart from being a very nice review of Big Bang Nucleosynthesis (BBN), recent paper by Karsten Jedamzik and Maxim Pospelov discusses an important open problem in the physics of BBN.

In short, the lithium problem in physics of Big Bang Nucleosynthesis is seeming underproduction of . What do we mean by that?

Well, the lithium abundance as predicted by the standard Big Bang Nucleosynthesis theory is

. (1)

On the other hand, the observed abundance seems to be about

(2),

i.e., (1) and (2) differ at least by , i.e., the deviation is statistically significant.

Before turning to possible scenarios explaining this anomaly, one has to explain in more details how exactly one estimates primordial abundance of lithium from observations.

As authors of the review explain, the value for primordial abundance of lithium is derived from absorption lines in atmospheres of low-metallicity galactic halo stars. If metallicity of such a star is sufficiently low, the abundance of turns out to be independent of the value of metallicity and temperature (in a certain range of both parameters – matallicity should remain small). This shows that the origin of lithium is cosmological: heavy elements that contribute to metallicity are themselves produced in stars.

One may think that the effect (anomalously low value of lithium abundance) might be related to erroneous atmospheric temperature determination, but this turns out to be ruled out. Two possibilities to explain the effect remain: a) lithium might be destroyed due to nuclear burning in the stellar interior (so, the underabundance of lithium is really an astrophysical problem) – this possibility is not very deeply analysed and b) underabundance of lithium means that standard BBN picture should be modified (so the problem is cosmological really). The first possibility seems to be more down-to-earth, while the second is of course more interesting physically. For example, in many models of dark matter where dark matter is made of supersymmetric partners of SM particles, underabundance of lithium-7 is naturally achieved (its presence is related to NLSP->LSP decays realized in the early Universe).

Rocky Kolb’s lecture on Dark Universe

… namely, about dark matter and dark energy as you may imagine. The lecture itself (2009 Buhl lecture at Carnegie Mellon U) is actually very clear and suitable for newcomers/non-scientists. So, if you want to know in some details (more or less technical) what the modern cosmology is all about, please check out this lecture.

Other interesting things in ArXiv (11 Jun 2009)

Basically, there were so many interesting and useful papers (or at least they were useful for me) – lecture notes, reviews – that it will give me hard time posting reviews of all of them here – since I am lazy, I’ll just try to list some of them.

Umut Gursoy et al. “Thermal Transport and Drag Force in Improved Holographic QCD“. Umut with collaborators have shown that bulk viscosity of strongly coupled quark-gluon plasma does not exceed shear viscosity, although grows in the vicinity of the phase transition. Also, if you want to know what exactly people mean by “improved holographic QCD”, a good minireview of it is contained in the beginning of the paper.

Gavin Salam, “Towards Jetography” – everything you need to know about jets in QCD.

Charlotte Gils et al., “Topology driven quantum phase transitions in time-reversal invariant anyonic quantum liquids“. A new type of phase transition in anyonic quantum liquids is discovered – it is driven by quantum fluctuations of topology. I think it is amazingly cool result and the paper definitely deserves to be called the best paper on cond-mat on the day of publication.

C. Germani et al., “Relativistic Quantum Gravity at a Lifshitz Point“. I was not able to go through the paper thoroughly yet, but it seems that they were able to relate Horava gravity to relativistic vector-tensor theory in some particular gauge – in other words, one can derive a kind of full Einstein-Hilbert action from the non-relativistic Horava gravity. If this is indeed so (I still have my doubts), then result is definitely a strong one. Did anybody check out this paper? If yes, what’s your opinion?

Notes on strongly coupled QCD in the continuum

By continuum here we mean using methods different from lattice QCD, which is currently our main instrument for quantitative understanding of QCD physics at strong coupling.

What can we actually do apart from lattice simulations to study properties of QCD in this regime? Not much really. As recent minireview paper by M. Pennington explains, one approach to the problem would be solving Schwinger-Dyson equations at strong coupling.

While coupling grows, we have more and more equations to deal with – which is a kind of apparent. Indeed, at very small coupling we have roughly three equations (not taking color/flavor index structure into account) – one for bosonic propagator (gluon), one for fermionic propagator (quark) and one for the boson-fermion vertex, see the fig. below.

Note that solid lines here mean exact propagators, so if we need to unleash the power of perturbation theory (pardon my French), we need to write diagrammatic expansion for them as well. What we will find is that Schwinger-Dyson equation relates 3-point function (vertex) to 2,3 and 4-point functions – therefore, we need additional equation for the 4-point function which in turn has terms
containing 2,3,4 and 5-point functions etc. etc. ad infinitum.

If coupling is small, we might expect that the chain of these equations might be effectively closed (by, say, neglecting at some point contribution of 10-point function into Schwinger-Dyson equation for 9-point function) – after that we are turning on numerics and start getting results like large effective mass of quarks nearly massless in zero approximation (from experiment we know that current u and d quarks have masses < 10 MeV, while constituent quarks, partons, have masses as large as several hundred MeV, essentially, ).

Will we get large parton mass this way? Not really – one can explicitly check that solutions of the corresponding Schwinger-Dyson (SD) equation give you as long as the (bare) coupling is small. On the other hand, if the bare coupling is larger than 1, there exist solutions of truncated SD equations that do feature parton mass , althogh bare mass is taken to be zero.

This shows that generation of large mass of constituent quark is a non-perturbative phenomenon. Is it the end of the story – i.e., is the theory absolutely untreatable in this regime? Yes and no. As it turns out, one can use renormalization group and rewrite Swinger-Dyson equations in terms of renormalized coupling, not the bare one. Second, gauge invariance (or Ward-Takahashi identities at the practical level) help to strongly reduce the number of Schwinger-Dyson equations one has to solve. On the other hand, taking properly gauge invariance into account also means further complications – for example, appearance of ghosts in diagrammatic expansions (using axial gauge you can get rid of ghosts but in a sense it reappears since gluon propagator now depends on two functions of momentum, not one).

Carnegie Mellon’s contribution to Star Trek universe

… apart from training Spock, I mean – the head of the Computer Science Department at Carnegie Mellon U. discusses several interesting projects being developed at the University which are worth putting into the universe of Star Trek.

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