令和元年度
京都大学大学院理学研究科

Ｄ３発表会アブストラクト（令和元年７月３０日）

物理

学

第二分

野

New viable theories of modified gravity:
Minimal Theories and Quasidilaton
Yukawa Institute for Theoretical Physics

OLIOSI Michele

Abstract In light of recent developments in the study of cosmology, it is interesting to revisit
large-scale modifications of gravity. I will present two new types of models, based on the idea ofminimalism in terms of number of degrees of freedom. These have an interesting phenomenology
and respect current constraints.
© 2019 Department of Physics, Kyoto University

In the last few decades, several alternative models of gravity have been developed in the hopeto produce large-scale deviations from general relativity (GR). The principal aim of this programis to uncover what physics could be at works behind the present-day acceleration of the Universe,
other than a fine-tuned cosmological constant. However, satisfying the constraints from current
observations while guaranteeing full theoretical consistency is still a challenging task forseveral such constructions. Two notable examples are the instabilities that plague some of thecosmological realizations of massive gravity theories and its extensions, or the recent
restrictive bound on the speed of tensor modes from the multi-messenger observation of a neutronstar binary merger. Finally, in light of the large number of future observational efforts toconstrain the cosmological dynamics as well as the behavior of gravity on these scales,model-building efforts come by as crucial tools to be able to interpret the future data mostefficiently.
We show that there exist new classes of alternative theories of gravity that are observationallyand theoretically viable, and produce interesting phenomenology. In particular, we focus firston minimally modified gravity (MMG) theories, which propagate only the two tensor modes by meansof violations of the Lorentz symmetry in the gravitational sector. In this context, we present
a class of theories [1] constructed on the basis of the existence of a frame in which thegravitational Lagrangian is equivalent to GR. As observational constraints, we consider the boundon the speed of tensor modes, as well as on the variation of the gravitational constant. We findthat there subsists a wide class of interesting possibilities to modify GR.
As a second example of a new alternative theory of gravity, we construct and study the minimal
theory of quasidilaton massive gravity (MQD) [2]. This theory is motivated by some difficulties
to find viable homogeneous and isotropic cosmologies in the context of quasidilaton massive
gravity theories, but can also be effectively understood as an extension of a specific MMG theory,the minimal theory of massive gravity, by rendering dynamical part of the fiducial metricstructure. We show that MQD is viable for a wide region of its parameter space, that it will
be efficiently constrained by future cosmological surveys, and can sustain interesting
phenomenology, in particular produce weak gravity while propagating the same number of degrees
of freedom as usual scalar-tensor theories.
References[1] K. Aoki, A. De Felice, C. Lin, S. Mukohyama and M. Oliosi, “Phenomenology in type-I minimally modifiedgravity”, JCAP 01 (2019) 017.
[2] A. De Felice, S. Mukohyama and M. Oliosi, “Phenomenology of minimal theory of quasidilaton massivegravity”, Phys. Rev. D 99, 044055 (2019).