Open AccessPosted Content
Distinguishing modified gravity with just two tensorial degrees of freedom from general relativity: Black holes, cosmology, and matter coupling
Aya Iyonaga,Tsutomu Kobayashi +1 more
TL;DR: In this article, the authors consider spatially covariant modified gravity with two tensorial degrees of freedom and show that the effects of modified gravity appear only through the modification of the background evolution.
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Abstract: We consider spatially covariant modified gravity in which the would-be scalar degree of freedom is made non-dynamical and hence there are just two tensorial degrees of freedom, i.e., the same number of dynamical degrees of freedom as in general relativity. Focusing on a class of such modified gravity theories characterized by three functions of time, we discuss how modified gravity with two tensorial degrees of freedom can be distinguished observationally or phenomenologically from general relativity. It is checked that the theory gives the same predictions as general relativity for weak gravitational fields and the propagation speed of gravitational waves. We also find that there is no modification to asymptotically flat black hole solutions. Due to a large degree of freedom to choose the time-dependent functions in the theory, the homogeneous and isotropic cosmological dynamics can be made close to or even identical to that of the $\Lambda$CDM model. We investigate the behavior of cosmological perturbations in the long and short wavelength limits and show that in both limits the effects of modified gravity appear only through the modification of the background evolution. Finally, it is remarked that in the presence of a galileon field in the matter sector, the scalar degree of freedom is revived, ruining the essential feature of the theory.
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Citations
Cosmological gravity probes: connecting recent theoretical developments to forthcoming observations
Shun Arai,Katsuki Aoki,Yuji Chinone,Rampei Kimura,Tsutomu Kobayashi,Hironao Miyatake,Daisuke Yamauchi,Shuichiro Yokoyama,Kazuyuki Akitsu,Takashi Hiramatsu,Shin'ichi Hirano,Ryotaro Kase,Taishi Katsuragawa,Yosuke Kobayashi,Toshiya Namikawa,Takahiro Nishimichi,Teppei Okumura,Maresuke Shiraishi,Masato Shirasaki,Tomomi Sunayama,Kazufumi Takahashi,Atsushi Taruya,Jun Tokuda +22 more
TL;DR: In this article , the authors present an overview of a variety of modified theories of gravity and the cosmological observables in the cosmic microwave background and large-scale structure, supplemented with a summary of predictions for cosmology observables derived from Cosmological perturbations and sophisticated numerical studies.
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Gravitational wave constraints on spatial covariant gravities
21 Feb 2023
TL;DR: In this article , the authors consider the observational effects of spatial covariant gravities on the propagation of GWs in the cosmological background and obtain the observational constraints on coupling coefficients in the action of spatiotemporal gravities from LIGO/Virgo/KAGRA observations.
Testing gravity with the cosmic microwave background: constraints on modified gravity with two tensorial degrees of freedom
TL;DR: In this paper , a cosmological test of modified gravity with two tensorial degrees of freedom and no extra propagating scalar mode is provided, where the Boltzmann code is modified to incorporate the present model.
Static, spherically symmetric objects in type-II minimally modified gravity
09 May 2022
TL;DR: In this paper , spherically symmetric solutions representing stars made of barotropic perfect fluid are studied in the context of two theories of type-II minimally modified gravity, VCDM and VCCDM, which share the property that no additional degree of freedom is introduced in the gravity sector, and propagate only two gravitational waves besides matter fields, as in General Relativity.
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Minimally modified gravity with auxiliary constraints formalism
25 May 2023
TL;DR: In this paper , the authors investigated the possibility of reducing the number of degrees of freedom (d.o.) starting from generic metric theories of gravity by introducing multiple auxiliary constraints (ACs), under the restriction of retaining spatial covariance as a gauge symmetry.
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References
•Posted Content
Spatially covariant gravity with two degrees of freedom: perturbative analysis
Yu-Min Hu,Xian Gao +1 more
TL;DR: In this article, the authors revisited the problem of building the Lagrangian of a large class of metric theories that respect spatial covariance, which propagate at most two degrees of freedom and in particular no scalar mode.
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Black holes in a type-II minimally modified gravity
TL;DR: In this paper, the authors studied spherically symmetric vacuum solutions to explore the strong gravity regime and showed that the vacuum solutions are locally different from the Schwarzschild or Schwarzschild-(A)dS metric in general and thus the Birkhoff theorem does not hold.
Spatially covariant gravity theories with two tensorial degrees of freedom: the formalism
Xian Gao,Zhi-Bang Yao +1 more
TL;DR: In this paper, the conditions for having only the two tensorial degrees of freedom were studied, and the condition that the dimension of the phase space at each spacetime point is even, so that the scalar type degree of freedom is eliminated completely.
Effective scalar-tensor description of regularized Lovelock gravity in four dimensions
TL;DR: In this paper, the authors reformulated the regularized version of Lovelock gravity in four dimensions as a scalar-tensor theory and showed that the warp factor of the internal space can be expressed as a metric of scalar degree of freedom.