A note on
viability of nonminimally coupled f(R) theory
(4 pages)

An original paper (1308.3401) studied an f(R) theory with
nonminimal 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)
+ R_{m n} R^{m n}
+ R_{m n r s} R^{m 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 3dimensional 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 pointparticle sources, the authors can distinguish and separate
the nonradiative 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.
