102 research outputs found
Hairy black holes in theories with massive gravitons
This is a brief survey of the known black hole solutions in the theories of
ghost-free bigravity and massive gravity. Various black holes exist in these
theories, in particular those supporting a massive graviton hair. However, it
seems that solutions which could be astrophysically relevant are the same as in
General Relativity, or very close to them. Therefore, the no-hair conjecture
essentially applies, and so it would be hard to detect the graviton mass by
observing black holes.Comment: References added. 20 pages, 3 figures, based on the talk given at the
7-th Aegean Summer School "Beyond Einstein's theory of gravity", September
201
On Consistent Theories of Massive Spin-2 Fields Coupled to Gravity
We consider the issues that arise out of interpreting the ghost-free bimetric
theory as a theory of a spin-2 field coupled to gravity. This requires
identifying a gravitational metric and parameterizing deviations of the
resulting theory from general relativity. To this end, we first consider the
most general bimetric backgrounds for which a massless and a massive spin-2
fluctuation with Fierz-Pauli mass exist. These backgrounds coincide with
solutions in general relativity. Based on this, we obtain nonlinear extensions
of the massive and massless spin-2 fields. The background value of the
nonlinear massive field parameterizes generic deviations of the bimetric theory
from GR. It is also shown that the nonlinear massless field does not have
standard ghost-free matter couplings, and hence cannot represent the
gravitational metric. However, an appropriate gravitational metric can still be
identified in the weak gravity limit. Hence in the presence of other neutral
spin-2 fields, the weak gravity limit is crucial for compatibility with general
relativity. We also write down the action in terms of the nonlinear massive
spin-2 field and obtain its ghost-free couplings to matter. The discussion is
then generalized to multimetric theories.Comment: Latex, 31 page
The hemispherical asymmetry from a scale-dependent inflationary bispectrum
If the primordial bispectrum is sufficiently large then the CMB hemispherical asymmetry may be explained by a large-scale mode of exceptional amplitude which perturbs the zeta two-point function. We extend previous calculations, which were restricted to one- or two-source scenarios, by providing a method to compute the response of the two-point function in any model yielding a 'local-like' bispectrum. In general, this shows that it is not the reduced bispectrum fNL which sources the amplitude and scale-dependence of the mode coupling but rather a combination of 'response functions'. We discuss why it is difficult to construct successful scenarios and enumerate the fine-tunings which seem to be required. Finally, we exhibit a concrete model which can be contrived to match the observational constraints and show that to a Planck-like experiment it would appear to have |fNL-local| ~ |fNL-equi| ~ |fNL-ortho| ~ 1. Therefore, contrary to previous analyses, we conclude that it is possible to generate the asymmetry while respecting observational constraints on the bispectrum and low-ell multipoles even without tuning our location on the long-wavelength mode
The separate universe approach to soft limits
We develop a formalism for calculating soft limits of -point inflationary
correlation functions using separate universe techniques. Our method naturally
allows for multiple fields and leads to an elegant diagrammatic approach. As an
application we focus on the trispectrum produced by inflation with multiple
light fields, giving explicit formulae for all possible single- and double-soft
limits. We also investigate consistency relations and present an infinite tower
of inequalities between soft correlation functions which generalise the
Suyama-Yamaguchi inequality.Comment: 28 pages, 7 figures. This is an author-created, un-copyedited version
of an article published in JCAP. IOP Publishing Ltd is not responsible for
any errors or omissions in this version of the manuscript or any version
derived from it. The Version of Record is available online at the DOI below.
v3: Updated to match version published in JCA
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