arXiv subjects

Shuntaro Mizuno

Publications and source records attributed to Shuntaro Mizuno.

At least 19 recordsLinked to original sources

arXiv2021

Inflationary gravitational waves in consistent $D\to 4$ Einstein-Gauss-Bonnet gravity

We study the slow-roll single field inflation in the context of the consistent $D\to4$ Einstein-Gauss-Bonnet gravity that was recently proposed in \cite{Aoki:2020lig}. In addition to the standard attractor regime, we find a new attractor regime which we call the Gauss-Bonnet attractor as the dominant contribution comes from the Gauss-Bonnet term. Around this attractor solution, we find power spectra and spectral tilts for the curvature perturbations and gravitational waves (GWs) and also a model-independent consistency relation among observable quantities. The Gauss-Bonnet term provides a nonlinear $k^4$ term to the GWs dispersion relation which has the same order as the standard linear $k^2$ term at the time of horizon crossing around the Gauss-Bonnet attractor. The Gauss-Bonnet attractor regime thus provides a new scenario for the primordial GWs which can be tested by observations. Finally, we study non-Gaussianity of GWs in this model and estimate the nonlinear parameters $f^{s_1s_2s_3}_{\rm NL,\;sq}$ and $f^{s_1s_2s_3}_{\rm NL,\;eq}$ by fitting the computed GWs bispectra with the local-type and equilateral-type templates respectively at the squeezed limit and at the equilateral shape. For helicities $(+++)$ and $( -- )$, $f^{s_1s_2s_3}_{\rm NL,\;sq}$ is larger while $f^{s_1s_2s_3}_{\rm NL,\;eq}$ is larger for helicities $(++-)$ and $(--+)$.

gr-qc↗

arXiv2020

Universal Upper Bound on the Inflationary Energy Scale from the Trans-Planckian Censorship Conjecture

We study the constraint on the inflationary energy scale from the recently proposed Trans-Planckian Censorship Conjecture (TCC). We find a universal upper bound on the inflationary Hubble expansion rate $H_\text{inf}$ which is solely determined by the reheating temperature $T_\text{rh}$: $H_\text{inf}/M_\text{Pl}\lesssim T_0/T_\text{rh}$, where $T_0$ is the photon temperature today. The upper limit can be saturated by a post-inflationary oscillatory stage with the critical equation-of-state parameter $w\approx-1/3$, or by an inflation model with multiple stages. In the lowest reheating temperature required for big bang nucleosynthesis, the upper bound on the tensor-to-scalar ratio $r$ at the CMB scales is $r\lesssim10^{-8}$, which can be realized in many string-inspired inflation models.

astro-ph.CO↗

arXiv2019

Hyperbolic field space and swampland conjecture for DBI scalar

We study a model of two scalar fields with a hyperbolic field space and show that it reduces to a single-field Dirac-Born-Infeld (DBI) model in the limit where the field space becomes infinitely curved. We apply the de Sitter swampland conjecture to the two-field model and take the same limit. It is shown that in the limit, all quantities appearing in the swampland conjecture remain well-defined within the single-field DBI model. Based on a consistency argument, we then speculate that the condition derived in this way can be considered as the de Sitter swampland conjecture for a DBI scalar field by its own. The condition differs from those proposed in the literature and only the one in the present paper passes the consistency argument. As a byproduct, we also point out that one of the inequalities in the swampland conjecture for a multi-field model with linear kinetic terms should involve the lowest mass squared for scalar perturbations and that this quantity can be significantly different from the lowest eigenvalue of the Hessian of the potential in the local orthonormal frame if the field space is highly curved. Finally, we propose an extension of the de Sitter swampland conjecture to a more general scalar field with the Lagrangian of the form $P(X,φ)$, where $X=-(\partialφ)^2/2$.

hep-th↗

arXiv2019

Primordial Tensor Non-Gaussianity from Massive Gravity

We calculate tensor bispectrum in a theory of massive tensor gravitons which predicts blue-tilted and largely amplified primordial gravitational waves. We find that a new 3-point interaction can produce a larger tensor bispectrum than the conventional one from general relativity. The bispectrum peaks at the squeezed configuration and its slope towards the squeezed limit is determined by the graviton mass. This squeezed tensor bispectrum may be observed as the quadrupolar modulation of the tensor power spectrum by interferometers.

astro-ph.CO↗

arXiv2019

$α$-attractor-type Double Inflation

We study the background dynamics and primordial perturbation in $α$-attractor-type double inflation. The model is composed of two minimally coupled scalar fields, where each of the fields has a potential of the $α$-attractor type. We find that the background trajectory in this double inflation model is given by two almost straight lines connected with a turn in the field space. With this simple background, we can classify the property of the perturbations generated in this double inflation model just depending on whether the turn occurs before the horizon exit for the mode corresponding to the scale of interest or not. In the former case, the resultant primordial curvature perturbation coincides with the one obtained by the single-field $α$-attractor. On the other hand, in the latter case, it is affected by the multifield effects, like the mixing with the isocurvature perturbation and the change of the Hubble expansion rate at the horizon exit. We obtain the approximated analytic solutions for the background, with which we can calculate the primordial curvature perturbation by the $δN$ formalism analytically, even when the multifield effects are significant. We also impose observational constraints on the model parameters and initial values of the fields in this double inflation model based on the Planck result.

hep-th↗

arXiv2018

Blue-tilted Primordial Gravitational Waves from Massive Gravity

We study a theory of massive tensor gravitons which predicts blue-tilted and largely amplified primordial gravitational waves. After inflation, while their mass is significant until it diminishes to a small value, gravitons are diluted as non-relativistic matter and hence their amplitude can be substantially amplified compared to the massless gravitons which decay as radiation. We show that such gravitational waves can be detected by interferometer experiments, even if their signal is not observed on the CMB scales.

gr-qc↗

arXiv2018

Quantum Entanglement in Multi-field Inflation

We study the emergence of quantum entanglement in multi-field inflation. In this scenario, the perturbations of one field contribute to the observable curvature perturbation, while multi-field dynamics with the other fields affect the curvature perturbation through particle production and entanglement. We develop a general formalism which defines the quantum entanglement between the perturbations of the multiple fields both in the Heisenberg and Schrödinger pictures, and show that entanglement between different fields can arise dynamically in the context of multi-field inflationary scenarios. We also present a simple model in which a sudden change in the kinetic matrix of the scalar fields generates entanglement and an oscillatory feature appears in the power spectrum of the inflaton perturbation.

astro-ph.CO↗

arXiv2017

Primordial perturbations from inflation with a hyperbolic field-space

We study primordial perturbations from hyperinflation, proposed recently and based on a hyperbolic field-space. In the previous work, it was shown that the field-space angular momentum supported by the negative curvature modifies the background dynamics and enhances fluctuations of the scalar fields qualitatively, assuming that the inflationary background is almost de Sitter. In this work, we confirm and extend the analysis based on the standard approach of cosmological perturbation in multi-field inflation. At the background level, to quantify the deviation from de Sitter, we introduce the slow-varying parameters and show that steep potentials, which usually can not drive inflation, can drive inflation. At the linear perturbation level, we obtain the power spectrum of primordial curvature perturbation and express the spectral tilt and running in terms of the slow-varying parameters. We show that hyperinflation with power-law type potentials has already been excluded by the recent Planck observations, while exponential-type potential with the exponent of order unity can be made consistent with observations as far as the power spectrum is concerned. We also argue that, in the context of a simple $D$-brane inflation, the hyperinflation requires exponentially large hyperbolic extra dimensions but that masses of Kaluza-Klein gravitons can be kept relatively heavy.

hep-th↗

arXiv2017

Cosmological Dynamics of D-BIonic and DBI Scalar Field and Coincidence Problem of Dark Energy

We study the cosmological dynamics of D-BIonic and DBI scalar field, which is coupled to matter fluid. For the exponential potential and the exponential couplings, we find a new analytic scaling solution yielding the accelerated expansion of the Universe. Since it is shown to be an attractor for some range of the coupling parameters, the density parameter of matter fluid can be the observed value, as in the coupled quintessence with a canonical scalar field. Contrary to the usual coupled quintessence, where the value of matter couple giving observed density parameter is too large to satisfy observational constraint from CMB, we show that the D-BIonic theory can give similar solution with much smaller value of matter coupling. As a result, together with the fact that the D-BIonic theory has a screening mechanism, the D-BIonic theory can solve the so-called coincidence problem as well as the dark energy problem.

hep-th↗

arXiv2016

Vainshtein mechanism in massive gravity nonlinear sigma models

We study the stability of the Vainshtein screening solution of the massive/bi-gravity based on the massive nonlinear sigma model as the effective action inside the Vainshtein radius. The effective action is obtained by taking the $Λ_2$ decoupling limit around a curved spacetime. First we derive a general consequence that any Ricci flat Vainshtein screening solution is unstable when we take into account the excitation of the scalar graviton only. This instability suggests that the nonlinear excitation of the scalar graviton is not sufficient to obtain a successful Vainshtein screening in massive/bi-gravity. Then to see the role of the excitation of the vector graviton, we study perturbations around the static and spherically symmetric solution obtained in bigravity explicitly. As a result, we find that linear excitations of the vector graviton cannot be helpful and the solution still suffers from a ghost and/or a gradient instability for any parameters of the theory for this background.

hep-th↗

arXiv2016

Constraining equilateral-type primordial non-Gaussianities from imaging surveys

We investigate expected constraints on equilateral-type primordial non-Gaussianities from future/ongoing imaging surveys, making use of the fact that they enhance the halo/galaxy bispectrum on large scales. As model parameters to be constrained, in addition to $f_{\rm NL}^{\rm equil}$, which is related to the primordial bispectrum, we consider $g_{\rm NL}^{(\partial σ)^4}$, which is related to the primordial trispectrum that appeared in the effective field theory of inflation. After calculating the angular bispectra of the halo/galaxy clustering and weak gravitational lensing based on the integrated perturbation theory, we perform Fisher matrix analysis for three representative surveys. We find that among the three surveys, the tightest constraints come from Large Synoptic Survey Telescope ; its expected $1σ$ errors on $f_{\rm NL}^{\rm equil}$ and $g_{\rm NL}^{(\partial σ)^4}$ are respectively given by $7.0 \times 10^2$ and $4.9 \times 10^7$. Although this constraint is somewhat looser than the one from the current cosmic microwave background observation, since we obtain it independently, we can use this constraint as a cross check. We also evaluate the uncertainty with our results caused by using several approximations and discuss the possibility to obtain tighter constraint on $f_{\rm NL}^{\rm equil}$ and $g_{\rm NL}^{(\partial σ)^4}$.

astro-ph.CO↗

arXiv2016

Gravitational Baryogenesis after Anisotropic Inflation

The gravitational baryogensis may not generate a sufficient baryon asymmetry in the standard thermal history of the Universe when we take into account the gravitino problem. Hence it has been suggested that anisotropy of the Universe can enhance the generation of the baryon asymmetry through the increase of the time change of the Ricci scalar curvature. We study the gravitational baryogenesis in the presence of anisotropy, which is produced at the end of an anisotropic inflation. Although we confirm that the generated baryon asymmetry is enhanced compared with the original isotropic cosmological model, taking into account the constraint on the anisotropy by the recent CMB observations, we find that it is still difficult to obtain the observed baryon asymmetry only through the gravitational baryogenesis without suffering from the gravitino problem.

hep-ph↗

arXiv2015

Halo/Galaxy Bispectrum with Equilateral-type Primordial Trispectrum

We investigate the effect of equilateral-type primordial trispectrum on the halo/galaxy bispectrum. We consider three types of equilateral primordial trispectra which are generated by quartic operators naturally appeared in the effective field theory of inflation and can be characterized by three non-linearity parameters, $g_{\rm NL} ^{\dotσ^4}$, $g_{\rm NL} ^{\dotσ^2 (\partial σ)^2}$, and $g_{\rm NL} ^{(\partial σ)^4}$. Recently, constraints on these parameters have been investigated from CMB observations by using WMAP9 data. In order to consider the halo/galaxy bispectrum with the equilateral-type primordial trispectra, we adopt the integrated Perturbation Theory (iPT) in which the effects of primordial non-Gaussianity are wholly encapsulated in the linear primordial polyspectrum for the evaluation of the biased polyspectrum. We show the shapes of the halo/galaxy bispectrum with the equilateral-type primordial trispectra, and find that the primordial trispectrum characterized by $g_{\rm NL} ^{\dotσ^4}$ provides the same scale-dependence as the gravity-induced halo/galaxy bispectrum. Hence, it would be difficult to obtain the constraint on $g_{\rm NL} ^{\dotσ^4}$ from the observations of the halo/galaxy bispectrum. On the other hand, the primordial trispectra characterized by $g_{\rm NL} ^{\dotσ^2 (\partial σ)^2}$ and $g_{\rm NL} ^{(\partial σ)^4}$ provide the common scale-dependence which is different from that of the gravity-induced halo/galaxy bispectrum on large scales. Hence future observations of halo/galaxy bispectrum would give constraints on the non-linearity parameters, $g_{\rm NL} ^{\dotσ^2 (\partial σ)^2}$ and $g_{\rm NL} ^{(\partial σ)^4}$ independently from CMB observations and it is expected that these constraints can be comparable to ones obtained by CMB.

astro-ph.CO↗

arXiv2015

Modified gravity inside astrophysical bodies

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 astrophysical bodies. In a recently introduced class of gravitational theories that goes beyond Horndeski, it has been found that new derivative interactions lead to a partial breaking of the Vainshtein screening mechanism inside any gravitational source, although not outside. We study the impact of this new type of deviation from standard gravity on the density profile of a spherically symmetric matter distribution, in the nonrelativistic limit. For simplicity, we consider a polytropic equation of state and derive the modifications to the standard Lane-Emden equations. We also show the existence of a universal upper bound on the amplitude of this type of modified gravity, independently of the details of the equation of state.

gr-qc↗

arXiv2014

Orthogonal non-Gaussianity in DBI Galileon: prospect for Planck polarisation and post-Planck experiments

In this note, we study cosmic microwave background (CMB) constraints on primordial non-Gaussianity in DBI galileon models in which an induced gravity term is added to the Dirac-Born-Infeld (DBI) action. In this model, the non-Gaussianity of orthogonal shape can be generated. We provide a relation between theoretical parameters and orthogonal/equilateral non-linear parameters using the Fisher matrix approach for the CMB bispectrum. In doing so, we include the effect of the CMB transfer functions and experimental noise properties by employing the recently developed \textsf{SONG} code. The relation is also shown in the language of effective theory so that it can be applied to general single-field models. Using the bispectrum Fisher matrix and the central values for equilateral and orthogonal non-Gaussianities found by the Planck temperature survey, we provide forecasts on the theoretical parameters of the DBI galileon model. We consider the upcoming Planck polarisation data and the proposed post-Planck experiments COrE and PRISM. We find that Planck polarisation measurements may provide a hint for a non-canonical sound speed at the 68% confidence level. COrE and PRISM will not only confirm a non-canonical sound speed but also exclude the conventional DBI inflation model at more than the 95% and 99% confidence level respectively, assuming that the central values will not change. This indicates that improving constraints on non-Gaussianity further by future CMB experiments is invaluable to constrain the physics of the early universe.

astro-ph.CO↗

arXiv2014

Covariant second-order perturbations in generalized two-field inflation

We examine the covariant properties of generalized models of two-field inflation, with non-canonical kinetic terms and a possibly non-trivial field metric. We demonstrate that kinetic-term derivatives and covariant field derivatives do commute in a proper covariant framework, which was not realized before in the literature. We also define a set of generalized slow-roll parameters, using a unified notation. Within this framework, we study the most general class of models that allows for well-defined adiabatic and entropic sound speeds, which we identify as the models with parallel momentum and field velocity vectors. For these models we write the exact cubic action in terms of the adiabatic and isocurvature perturbations. We thus provide the tool to calculate the exact non-Gaussianity beyond slow-roll and at any scale for these generalized models. We illustrate our general results by considering their long-wavelength limit, as well as with the example of two-field DBI inflation.

astro-ph.CO↗

arXiv2014

Combined features in the primordial spectra induced by a sudden turn in two-field DBI inflation

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. Distinct features are generated by two different effects: the mixing of the light and heavy modes during the turn, on the one hand, and the resonance between the oscillations along the heavy direction after the turn, on the other hand. Contrary to models with standard kinetic terms, the resonance effect is not strongly suppressed because the action contains derivative interactions. Working in the potential basis, we study the oscillations after the turn and compute the amplitude of the mixing and resonance features in the power spectrum, as well as in the bispectrum for the latter effect. We find that the amplitudes and positions of these combined features obey specific consistency relations, which could be confronted with cosmological data.

hep-th↗

arXiv2014

Trispectrum estimation in various models of equilateral type non-Gaussianity

We calculate the shape correlations between trispectra in various equilateral non-Gaussian models, including DBI inflation, ghost inflation and Lifshitz scalars, using the full trispectrum as well as the reduced trispectum. We find that most theoretical models are distinguishable from the shapes of primordial trispectra except for several exceptions where it is difficult to discriminate between the models, such as single field DBI inflation and a Lifshitz scalar model. We introduce an estimator for the amplitude of the trispectrum, $g_{\rm NL} ^{equil}$ and relate it to model parameters in various models. Using constraints on $g_{\rm NL} ^{equil}$ from WMAP5, we give constraints on the model parameters.

astro-ph.CO↗