Publications (13)
General theory of cosmological perturbations in open and closed universes from the Horndeski action
Shingo Akama, Tsutomu Kobayashi
Our Universe is nearly spatially flat, but this does not mean that it is exactly spatially flat. In this paper we derive general quadratic actions for cosmological perturbations in…
Primordial non-Gaussianities of scalar and tensor perturbations in general bounce cosmology: Evading the no-go theorem
Shingo Akama, Shin'ichi Hirano, Tsutomu Kobayashi
It has been pointed out that matter bounce cosmology driven by a k-essence field cannot satisfy simultaneously the observational bounds on the tensor-to-scalar ratio and non-Gaussi…
Scalar parity-odd trispectrum from gravitational Chern-Simons interaction vertices
Giorgio Orlando, Shingo Akama, Chunshan Lin
In this paper, we explore parity violation in a scalar trispectrum from a dynamical Chern-Simons gravity theory. So far, a graviton-mediated diagram with two vertexes being of gene…
Imprints of primordial gravitational waves with non-Bunch-Davies initial states on CMB bispectra
Shingo Akama, Hiroaki W. H. Tahara
It has been shown that both scalar and tensor modes with non-Bunch-Davies initial states can enhance the amplitudes of the primordial bispectra compared to those with the Bunch-Dav…
Generalized multi-Galileons, covariantized new terms, and the no-go theorem for non-singular cosmologies
Shingo Akama, Tsutomu Kobayashi
It has been pointed out that non-singular cosmological solutions in second-order scalar-tensor theories generically suffer from gradient instabilities. We extend this no-go result…
Stochastic gravitational wave background anisotropies from inflation with non-Bunch-Davies states
Shingo Akama, Shin'ichi Hirano, Shuichiro Yokoyama
It is known that stochastic gravitational wave backgrounds (SGWBs) have anisotropies generated by squeezed-type tensor non-Gaussianities originating from scalar-tensor-tensor (STT)…
Primordial tensor non-Gaussianities from general single-field inflation with non-Bunch-Davies initial states
Shingo Akama, Shin'ichi Hirano, Tsutomu Kobayashi
It has been found that the primordial non-Gaussianity of the curvature perturbation in the case of non-Bunch-Davies initial states can be enhanced compared with those in the case o…
Primordial full bispectra from the general bounce cosmology
Shingo Akama
Primordial non-Gaussianities are key quantities to test early universe scenarios. In this paper, we compute full bispectra of scalar and tensor perturbations generated during a con…
Testing gravitational wave polarizations with LISA
Shingo Akama, Maxence Corman, Paola C. M. Delgado +16
In this paper we quantify the ability of the Laser Interferometer Space Antenna (LISA) to test the presence of non-tensorial polarizations as well as modifications to the tensor on…
Primordial non-Gaussianity from Galilean Genesis without strong coupling problem
Shingo Akama, Shin'ichi Hirano
Galilean Genesis is generically plagued with a strong coupling problem, but this can be avoided depending on the hierarchy between a classical energy scale of genesis and a strong…
Bypassing the Lyth Bound with Entangled Gravitons: Primordial Signatures and Late-Time Noise
Shingo Akama, Chunshan Lin
We demonstrate that quantum entanglement between primordial gravitons in dynamically decoupled gravitational sectors can parametrically enhance the tensor power spectrum during inf…
Towards testing the general bounce cosmology with the CMB B-mode auto-bispectrum
Shingo Akama, Giorgio Orlando, Paola C. M. Delgado
It has been shown that a three-point correlation function of tensor perturbations from a bounce model in general relativity with a minimally-coupled scalar field is highly suppress…
Parity violation in primordial tensor non-Gaussianities from matter bounce cosmology
Shingo Akama, Mian Zhu
It has been shown that primordial tensor non-Gaussianities from a cubic Weyl action with a non-dynamical coupling are suppressed by the so-called slow-roll parameter in a conventio…