Monday, April 23, 2012

Paper of the Day (gr-qc)

http://arxiv.org/abs/1204.4524


During the weekend I though it could have been a good experiment having a small "journal club" on papers submitted daily on the arXiv (i'll write something on arXiv one of these days). I cannot promise this blog-review will be very regular, but i'll do my best for the gr-qc and hep-th sections. Other contributors are in charge for other sections.

Our idea is to pick selected papers and describe them very briefly and informally. This brings me to the next problem: selecting the papers.

ArXivwise, Monday is usually quite a boring day. Papers appearing on Mondays are those submitted during the weekend and this explains why there are (on average) less papers. Well, not today!

Today the choice was quite hard (and not only because Frank Wilczek's "A Long View of Particle Physics"). Anyway my choice goes to Kent Yagi's paper above, which (or probably also because) is closely related to some projects I'm recently involved to.

Yagi is interested in constraining "alternative theories of gravity", i.e. theories that modify General Relativity (GR) in some regime. Typically, alternative theories differ from GR in  the description of the dynamics of black holes, neutron stars and other compact objects, but they are conceived in such a way that they pass Solar System tests. I'll postpone the motivation to investigate these theories and my personal interest for them to a future post, for the time being let me just say that it is very hard to modify GR in a way that is compatible with current and past experiment.

In brief, the paper above discuss: (i) a way to put very stringent constraints (in fact, much more stringent than those coming from Solar System experiments) to a particular class of theories that modify GR in the strong-curvature regime and (ii) that, despite these strong constraints, these theories "could" explain an unexpected astrophysical result: apparently, the orbital separation of some binary system (i.e. the distance between two compact objects orbiting each other) decreases in time faster than what is predicted in GR. That the distance decreases is well-known: it is due to the emission of gravitational waves (GWs) from the system. The fact that the this decrease is faster than expected, points to some poor understanding of the astrophysical processes involved in the process or to a more copious emission of GWs than what expected in GR. A quite common feature of modified gravities is that they predict a more efficient GW emission but (iii) the results of this paper would be very difficult to obtain by considering a alternative theory other than what Yagi considers. This should come as a surprise, given the ridiculously large number of proposed alternatives but, on the other hand, it makes the results more interesting. 

As the author stresses all over the paper, the cause of this discrepancy is most probably of astrophysical origin. However, it's intriguing to observe that current observations do not rule possible deviations from Einstein's theory at astrophysical level and, most importantly, that these corrections may eventually play a role in explaining the dynamics of compact stars and black holes [i'm sure the astro-contributors won't agree with me here :)]

[Note added in proof: next posts will be shorter!]
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