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19 June, 2016

posted 19 Jun 2016, 04:57 by Scott Melville   [ updated 3 Jul 2016, 02:07 ]
A note on viability of nonminimally coupled f(R) theory
(4 pages)

An original paper (1308.3401) studied an f(R) theory with non-minimal coupling to a massless scalar field, and used the absence of classical instabilities and superluminal propagation to constrain the gravitational sector f and the matter sector coupling. A response (1406.6422) suggested that these constraints may be too strong – arguing that (i) superluminal propagation could be interpreted as a result of considering unphysical matter content, and so it is possible to evade constraints by restricting attention to only certain matter fields, and (ii) even in the case of a massless scalar field, the derivation had neglected potentially important fluctuations. This brief note (1606.04060) attempts to justify the original claimed constraints, addressing (ii) by redoing the derivation with a different formalism, keeping track of fluctuations explicitly and showing that they have no effect (and briefly addressing (i) by arguing qualitatively that more complicated fields suffer similar pathologies to massless scalars, so more general matter fields give rise to similar constraints).

Writing down the most general Lagrangian theory one can construct using the Ricci scalar curvature, one arrives at nonminimally coupled f(R) theory. Employing classical consistency conditions, as in these papers, is an effective way to reduce the theory space (the formal constraints can immediately rule out many naively possible theories). This leaves (ideally) only a small number of theories, which can then be studied phenomenologically and compared to experiment.

Revisiting Conserved Charges in Higher Curvature Gravitational Theories
(20 pages)

The ‘solution phase space method’ is a covariant formulation of gravitational phase spaces which can be used to calculate conserved charges for black hole solutions (and potentially other solutions). Here it is applied to a theory of the form

  f(R) + Rm n Rm n + Rm n r s Rm n r s ,

nonminimally coupled to gauge and scalar fields, yielding conserved charges and the first law of thermodynamics for certain black hole solutions (BTZ and 3-dimensional z=3 Lifshitz black holes).

In addition to the formal merits of a powerful calculation technique for conserved charges and simple proof of the first law in higher curvature theories, these explicit calculations of the mass, angular momentum, and entropy of black holes provide useful phenomenological comparators for black hole solutions in GR and in a modified theory of gravity.   

The Cosmological Memory Effect
(22 pages)

The Christodoulou (gravitational) memory effect is the change in the metric induced by the flux of gravitational radiation at null infinity. In general, there is no null infinity in an FRLW spacetime.
By specializing to only point-particle sources, the authors can distinguish and separate the non-radiative gravitational effects without taking a null limit, thus allowing them to define a memory effect on FRLW.

This is of cosmological importance: this memory effect in FRLW can be enhanced compared to the to the effect in Minkowski. Gravitational memories may be detectable in the near future with Advanced LIGO and the upcoming eLISA mission.