Keisuke Izumi

Orthogonal black di-ring solution

We construct a five dimensional exact solution of the orthogonal black di-ring which has two black rings whose

Massive gravity acausality redux

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Problems with Propagation and Time Evolution in f(T) Gravity

-rotating planes are orthogonal. This solution has four free parameters which represent radii of and speeds of

Cosmological perturbation in f(T) gravity revisited

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Stellar center is dynamical in Horava-Lifshitz gravity

-rotation of the black rings. We use the inverse scattering method. This method needs the seed metric. We also present a systematic method how to construct a seed metric. Using this method, we can probably construct other solutions having many black rings on the two orthogonal planes with or without a black hole at the center.

Unexorcized ghost in DGP brane world

Massive gravity (mGR) is a 5(=2s+1)5(=2s+1) degree of freedom, finite range extension of GR. However, amongst other problems, it is plagued by superluminal propagation, first uncovered via a second order shock analysis. First order mGR shock structures have also been studied, but the existence of superluminal propagation in that context was left open. We present here a concordance of these methods, by an explicit (first order) characteristic matrix computation, which confirms mGRʼs superluminal propagation as well as acausality.

Measurements of atmospheric muon spectra at mountain altitude

Teleparallel theories of gravity have a long history. They include a special case referred to as the Teleparallel Equivalent of General Relativity (TEGR, aka GR

Bubbles in the self-accelerating universe

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Hydro-elastic Complementarity in Black Branes at large D

). Recently this theory has been generalized to f(T) gravity. Tight constraints from observations suggest that f(T) gravity is not as robust as initially hoped. This might hint at hitherto undiscovered problems at the theoretical level. In this work, we point out that a generic f(T) theory can be expected to have certain problems including superluminal propagating modes, the presence of which can be revealed by using the characteristic equations that govern the dynamics in f(T) gravity and/or the Hamiltonian structure of the theory via Dirac constraint analysis. We use several examples from simpler gauge field theories to explain how such superluminal modes could arise. We also point out problems with the Cauchy development of a constant time hypersurface in FLRW spacetime in f(T) gravity. The time evolution from a FLRW (and as a special case, Minkowski spacetime) initial condition is not unique.

Superluminal Propagation and Acausality of Nonlinear Massive Gravity

We perform detailed investigation of cosmological perturbations in f(T) theory of gravity coupled with scalar field. Our work emphasizes on the way to gauge fix the theory and we examine all possible modes of perturbations up to second order. The analysis includes pseudoscalar and pseudovector modes in addition to the usual scalar, vector, and tensor modes. We find no gravitational propagating degree of freedom in the scalar, pseudoscalar, vector, as well as pseudovector modes. In addition, we find that the scalar and tensor perturbations have exactly the same form as their counterparts in usual general relativity with scalar field, except that the factor of reduced Planck mass squared Mpl2≡1/(8πG) that occurs in the latter has now been replaced by an effective time-dependent gravitational coupling −2(df/dT)|T = T0, with T0 being the background torsion scalar. The absence of extra degrees of freedom of f(T) gravity at second order linear perturbation indicates that f(T) gravity is highly nonlinear. Consequently one cannot conclusively analyze stability of the theory without performing nonlinear analysis that can reveal the propagation of the extra degrees of freedom.