So I’ve been remiss in my reading lately, but here are my picks from the past few weeks. We have Outstanding Problems in Galaxy Formation, Herschel on Dark Matter, Dark Matter Detection Discussed, “Symmetry Breaking” in Graphene?, Closed Timelike Curves and Postselection, Frame-Like Geometry of Double Field Theory, and Loop Lectures with Carlo Rovelli.

## Astrophysics and Gravitation:

### Outstanding Problems in Galaxy Formation

Joseph Silk (2011). Feedback in Galaxy Formation arXiv arXiv: 1102.0283v1

Abstract:

I review the outstanding problems in galaxy formation theory, and the role of feedback in resolving them. I address the efficiency of star formation, the galactic star formation rate, and the roles of supernovae and supermassive black holes.

Silk’s Figure 1. “The theoretical mass function of galaxies compared to the observed luminosity function.”

So, as most of us know, there are still quite a few puzzles when it comes to how galaxies form. Joseph Silk has put together a little discussion on some of these problems, and, perhaps more interestingly, ways in which they can be rectified (or already have been). For example, cold dark matter simulations alone predicted more *halo* dwarf galaxies than were observed (called the “missing satellites” problem; see Kravtsov and pdf slides). Through both a better understanding of observation (there were in fact more dwarfs out there than we though, they were just very faint) and more sophisticated models (taking into account baryonic physics too), this problem doesn’t seem so huge anymore (it’s not 100% resolved, mind you). There are many other much *less resolved* issues when it comes to gravitation and galaxy formation that are also deserving of some serious study.

### Herschel on Dark Matter

Alexandre Amblard, & et al. (2011). Sub-millimetre galaxies reside in dark matter halos with masses greater than 3×10^11 solar masses Nature arXiv: 1101.1080v1

From the press release:

ESA’s Herschel space observatory has discovered a population of dust-enshrouded galaxies that do not need as much dark matter as previously thought to collect gas and burst into star formation.

This is certainly good news for some galaxy formation theorists and another fun piece of the puzzle to think about. The latest analysis of Herschel observations suggest the existence of galaxies that are roughly 300 billion solar masses but with as many stars as expected from a galaxy of *5000* billion solar masses (ie. *that’s not got much dark matter in it*). This is quite fascinating, because most of the current theories dealing with galaxy formation require these huge amounts of dark matter to allow budding galaxies to stay together, but now there are observations that suggest otherwise. It looks like we’ll have to adjust our ideas of dark matter’s role in the galaxy (not that this should surprise anyone).

### For more, see Herschel finds less dark matter but more stars.

### Dark Matter Detection Discussed

Earlier this month, there was a really excellent guest post at Cosmic Variance about the state of dark matter detection experiments by Neal Weiner (to complete the discussion of dark matter in the galaxy) so you should give that a read.

## High Energy Physics and Particles:

### “Symmetry Breaking” in Graphene?

San-Jose, P., González, J., & Guinea, F. (2011). Electron-Induced Rippling in Graphene Physical Review Letters, 106 (4) DOI: 10.1103/PhysRevLett.106.045502

So this was a hot topic this month that I’m just getting around to: graphene as an analogy for the Higgs field. Now, as always with these analogy papers, I get a little nervous. When there isn’t an explicit (AdS/CFT-esque) correspondence, it’s really very difficult (and a somewhat philosophical matter) to say what we are actually able to learn from analogies. In this case, the analogy comes from the fact that the “energy landscape” of graphene moving in 2-dimensions is *similar *to that of the Higgs field in 3-dimensions, in that they are both described by a *similar *Mexican hat potential. Okay. There are other situations where we see Mexican hat potentials, like when rotating a bead on a circle, but that doesn’t mean that they would be at all useful in thinking about spontaneous symmetry breaking. Since I don’t really know anything relevant about graphene, quantum criticality, or… well, materials in general, I am completely unqualified to to judge this analogy, but it is still an analogy, not a correspondence. *Similar *and *the same *are, importantly, and fundamentally different.* *

For more, see Theorists turn to graphene for clues to Higgs.

## General Relativity, Quantum Gravity, et al.:

### Closed Timelike Curves and Postselection

Lloyd, S., Maccone, L., Garcia-Patron, R., Giovannetti, V., Shikano, Y., Pirandola, S., Rozema, L., Darabi, A., Soudagar, Y., Shalm, L., & Steinberg, A. (2011). Closed Timelike Curves via Postselection: Theory and Experimental Test of Consistency Physical Review Letters, 106 (4) DOI: 10.1103/PhysRevLett.106.040403

So this paper was very “win, lose, win” for me. “Closed timelike curves” (CTCs) mean general relativity, which means I’m happy. “Postselection” means quantum interpretations, which means I’m less happy. Now, this isn’t me saying that I don’t think interpretations are wonderful, in fact, they’re my favourite part of quantum mechanics, but it is a huge field, which takes a paper on time travel, away from general relativity, and thus outside of my wheelhouse.

As we know, CTCs are not forbidden by general relativity, although they are usually excluded because of the “logical” paradoxes they lead to (*if being able to kill your own grandfather really bothers you, that i*s). There are many people who are interested in universes that include CTCs however, not because of the *time travel* applications, but simply because they are not impossible, and thus might be able to be included in a self consistent model of our universe.

From the paper:

This self-consistency requirement gives rise to a theory of closed timelike curves via entanglement and postselection, P-CTCs. P-CTCs are based on the Horowitz-Maldacena ‘‘final state condition’’ for black hole evaporation, and on the suggestion of Bennett [pdf] and Schumacher that teleportation could be used to describe time travel.

This is pretty neat stuff, that has gone through a few iterations within the quantum information community, most famously led by David Deutsch in 1991 with his *Quantum Mechanics Near Closed Timelike Lines*. Given my naiveté when it comes to QI, I’m just going to have to take the authors’ word for it that their model doesn’t, in fact, agree with Deutsch’s (although it is consistent within their framework). What is really neat though is that, because of the nature of postselection, it’s possible to do (and they did) a little *grandfather paradox* experiment with their model using P-CTCs and photons. Their “grandfather paradox circuit” is worth a look, if you’re interested. The argument is technical, and I can’t personally say I followed it thoroughly, but I can appreciate their (wonderfully worded) conclusions:

[S]uicidal photons in a CTC obey the Novikov principle: they cannot kill their former selves.

Our P-CTCs always send pure states to pure states: they do not create entropy. Hence, P-CTCs provide a distinct resolution to Deutsch’s unproved theorem paradox, in which the time traveler reveals the proof of a theorem to a mathematician, who includes it in the same book from which the traveler has learned it (rather, will learn it). How did the proof come into existence? Deutsch adds an additional maximum entropy postulate to eliminate this paradox. By contrast, postselected CTCs automatically solve it through entanglement: the CTC creates a random mixture of all possible ‘‘proofs.’’

So, they have a resolution to the grandfather paradox within P-CTCs. While theoretically, their model is inequivalent to Deutsch’s, experimentally, one can not distinguish the two, unfortunately. They also state that they “cannot test whether a general relativistic CTC obeys [their] theory or not,” which confuses me a little by the terminology, because CTCs don’t make sense as a concept outside of general relativity, so what a “non-general relativistic CTC” is vs. as “general relativistic CTC” is, I can’t say (I’m assuming they just mean a CTC in a distinctly curved spacetime, which would be very hard to work into an experiment).

For more, see Time Travel Without Regrets.

### Frame-Like Geometry of Double Field Theory

Hohm, O., & Kwak, S. (2011). Frame-like geometry of double field theory Journal of Physics A: Mathematical and Theoretical, 44 (8) DOI: 10.1088/1751-8113/44/8/085404

The abstract:

We relate two formulations of the recently constructed double field theory to a frame-like geometrical formalism developed by Siegel. A self-contained presentation of this formalism is given, including a discussion of the constraints and its solutions, and of the resulting Riemann tensor, Ricci tensor and curvature scalar. This curvature scalar can be used to define an action, and it is shown that this action is equivalent to that of double field theory.

This is still in my “to read” list, but I thought I’d mention it as double field theory has a little bit of buzz right now, that makes it worth a look. I do find it curious though, that there seems to be several incredibly similar papers to the above on this topic, by the same authors, but I’m going to go with the most recent one.

### Loop Lectures

Carlo Rovelli (2011). Lectures on loop gravity arXiv arXiv: 1102.3660v2

The abstract:

This is a preliminary version of the introductory lectures on loop quantum gravity that I will give at the quantum gravity school in Zakopane. The theory is presented in self-contained form, without emphasis on its derivation from classical general relativity. Dynamics is given in the covariant form. The approximations needed to compute physical quantities are discussed. Some applications are described, including the recent derivation of de Sitter cosmology from full quantum gravity.

As if this needs explanation: Carlo Rovelli, one of the physicists who impresses me the most, has a great introduction to loop quantum gravity online (that is being updated currently). For anyone interested in the topic but wondering how to start, I imagine this is *the* recommendation now.

Speaking of time loops and quantum mechanics – I am always amazed that nobody knows about Mark Hadley’s attempt to derive quantum mechanics from CTCs in

classicalgeneral relativity, seen here and in most of his other arxiv papers.Hadley expresses his idea in terms of complementarity and nondistributive quantum logic, in a way which is a little opaque to me, so I’ll state the idea as I would express it, while warning that there may be a difference of detail. The idea, in my words: that the rules of quantum probability are due to the existence of microscopic space-time regions (bounded hypervolumes) containing CTCs. There will certainly be nonlocal consistency constraints on what can happen in such a space-time; the challenge is to determine whether these constraints, all by themselves, already give rise to quantum mechanics.

You could approach this by thinking of an initial hypersurface, a final hypersurface, and then postulating that CTCs occur in the intermediate hypervolume with a certain average density (n per unit hypervolume). Then we’d try to derive the initial-to-final transition probability. Note that there is not meant to be any quantum input: we have a purely

classicalprobability distribution on space-time histories, arising from the postulated frequency of CTCs, and we want to see what that does to the probability distribution on the future boundary, given the nonlocal consistency constraints affecting the existence of histories with CTCs.Frankly, as an explanation of quantum mechanics, this is simpler than I would want. But I find it curious that the idea has not been developed, that it’s not even talked about. There have been many variations on the theme of “quantum mechanics involves influences from the future as well as influences from the past”; and there is that striking similarity between quantum field theory in n space and 1 time dimensions, and statistical mechanics in (n+1) space dimensions; yet apparently no-one other than Hadley has suggested that general relativity can provide the microscopic explanation of how a causal chain can zigzag in time.

A sober, skeptical, orthodox assessment of the prospects for this idea would be that it will fall short somewhere, for a specific reason. The Wheeler-Feynman absorber theory is a historical precedent; it’s supposed to be unable to account for the Lamb shift. Instead, you need the full quantum approach, the sum over histories. Similarly, perhaps Hadley’s strictly classical approach can’t produce some necessary effect – maybe these histories dominate the quantum-gravity path integral, but don’t completely determine it – but still, one would like to see the details of this worked out.

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