423 citations · 1.8k across the 30 of their papers we have counts for
30 papers
Gravitational Positivity for Phenomenologists: Dark Gauge Boson in the Swampland
Katsuki Aoki, Toshifumi Noumi, Ryo Saito +4
The gravitational positivity bound gives quantitative "swampland'' constraints on low-energy effective theories inside theories of quantum gravity. We give a comprehensive discussi…
Ostrogradsky mode in scalar-tensor theories with higher-order derivative couplings to matter
Atsushi Naruko, Ryo Saito, Norihiro Tanahashi +1
A metric transformation is a tool to find a new theory of gravity beyond general relativity. The gravity action is guaranteed to be free from a dangerous Ostrogradsky mode as long…
Antipodal Angular Correlations of Inflationary Stochastic Gravitational Wave Background
Zhen-Yuan Wu, Ryo Saito, Nobuyuki Sakai
The measurement of the inflationary stochastic gravitational-wave background (SGWB) is one of the main goals of future GW experiments. In direct GW experiments, an obstacle to achi…
Unified approach to secondary effects on the CMB B-mode polarization
Toshiya Namikawa, Atsushi Naruko, Ryo Saito +2
We develop a systematic and unified approach to estimate all possible secondary (i.e. non-primordial) nonlinear effects to the cosmic microwave background (CMB) polarization, named…
Dynamical Analysis of Screening in Scalar-Tensor Theory
Tomohiro Nakamura, Taishi Ikeda, Ryo Saito +2
We investigate linear and non-linear dynamics of spherically symmetric perturbations on a static configuration in scalar-tensor theories focusing on the chameleon screening mechani…
Local gravity test of unified models of inflation and dark energy in gravity
Mai Yashiki, Nobuyuki Sakai, Ryo Saito
We consider gravity theories which unify inflation and dark energy models. First, from the final Planck data of the cosmic microwave background, we obtain a condition,…
Is Trans-Planckian Censorship a Swampland Conjecture?
Ryo Saito, Satoshi Shirai, Masahito Yamazaki
During an accelerated expansion of the Universe, quantum fluctuations of sub-Planckian size can be stretched outside the horizon and be regarded effectively classical. Recently, it…
Phenomenological Consequences of the Refined Swampland Conjecture
Hajime Fukuda, Ryo Saito, Satoshi Shirai +1
We discuss phenomenological consequences of the recently-introduced refinements of the de Sitter swampland conjecture. The conjecture constraints the first and the second derivativ…
Heavy Particle Signatures in Cosmological Correlation Functions with Tensor Modes
Ryo Saito, Takahiro Kubota
We explore the possibility to make use of cosmological data to look for signatures of unknown heavy particles whose masses are on the order of the Hubble parameter during the time…
Chameleon Field in a Spherical Shell System
Tomohiro Nakamura, Taishi Ikeda, Ryo Saito +1
We investigate the validity of screening mechanism of the fifth force for chameleon field in highly inhomogeneous density profile. For simplicity, we consider a static and spherica…
Cosmological correlation functions including a massive scalar field and an arbitrary number of soft-gravitons
Ryo Saito
We study the imprint of a massive scalar particle on cosmological correlation functions, and suggest the way to determine the mass of the newly introduced particle, which is expect…
Scalar-tensor theories and modified gravity in the wake of GW170817
David Langlois, Ryo Saito, Daisuke Yamauchi +1
Theories of dark energy and modified gravity can be strongly constrained by astrophysical or cosmological observations, as illustrated by the recent observation of the gravitationa…
Towards Strong Field Tests of Beyond Horndeski Gravity Theories
Jeremy Sakstein, Eugeny Babichev, Kazuya Koyama +2
Theories of gravity in the beyond Horndeski class encompass a wide range of scalar-tensor theories that will be tested on cosmological scales over the coming decade. In this work,…
Relativistic Stars in Beyond Horndeski Theories
Eugeny Babichev, Kazuya Koyama, David Langlois +2
This work studies relativistic stars in beyond Horndeski scalar-tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter, focusing on a model based on the…
Beyond Fab Four
E. Babichev, C. Charmousis, D. Langlois +1
We show that the two additional Lagrangians that appear in theories beyond Horndeski can be reexpressed in terms of simple generalizations of the "John" and "Paul" terms of the Fab…
Modified gravity inside astrophysical bodies
Ryo Saito, Daisuke Yamauchi, Shuntaro Mizuno +2
Many theories of modified gravity, including the well studied Horndeski models, are characterized by a screening mechanism that ensures that standard gravity is recovered near astr…
Geodesic "curve"-of-sight formulae for the cosmic microwave background: a unified treatment of redshift, time delay, and lensing
Ryo Saito, Atsushi Naruko, Takashi Hiramatsu +1
In this paper, we introduce a new approach to a treatment of the gravitational effects (redshift, time delay and lensing) on the observed cosmic microwave background (CMB) anisotro…
Combined features in the primordial spectra induced by a sudden turn in two-field DBI inflation
Shuntaro Mizuno, Ryo Saito, David Langlois
We investigate the features generated by a sharp turn along the inflationary trajectory in a two-field model of Dirac-Born-Infeld inflation, where one of the fields is heavy. Disti…
Coleman-de Luccia instanton in dRGT massive gravity
Ying-li Zhang, Ryo Saito, Dong-han Yeom +1
We study the Coleman-de Luccia (CDL) instanton characterizing the tunneling from a false vacuum to the true vacuum in a semi-classical way in dRGT (deRham-Gabadadze-Tolley) massive…
Localized Features in Non-Gaussianity from Heavy Physics
Ryo Saito, Yu-ichi Takamizu
We discuss the possibility that we could obtain some hints of the heavy physics during inflation by analyzing local features of the primordial bispectrum. A heavy scalar field coul…
Testing general scalar-tensor gravity and massive gravity with cluster lensing
Tatsuya Narikawa, Tsutomu Kobayashi, Daisuke Yamauchi +1
We explore the possibility of testing modified gravity exhibiting the Vainshtein mechanism against observations of cluster lensing. We work in the most general scalar-tensor theory…
Hawking-Moss instanton in nonlinear massive gravity
Ying-li Zhang, Ryo Saito, Misao Sasaki
As a first step toward understanding a lanscape of vacua in a theory of non-linear massive gravity, we consider a landscape of a single scalar field and study tunneling between a p…
Femto-lensing due to a Cosmic String
Chul-Moon Yoo, Ryo Saito, Yuuiti Sendouda +2
We consider the femto-lensing due to a cosmic string. If a cosmic string with the deficit angle [femto-arcsec] [rad] exists around the line of sight to a…
Resonant Signatures of Heavy Scalar Fields in the Cosmic Microwave Background
Ryo Saito, Masahiro Nakashima, Yu-ichi Takamizu +1
We investigate the possibility that a heavy scalar field, whose mass exceeds the Hubble scale during inflation, could leave non-negligible signatures in the Cosmic Microwave Backgr…