arXiv subjects

Jun'ichi Yokoyama

Publications and source records attributed to Jun'ichi Yokoyama.

At least 109 records · Page 6Linked to original sources

arXiv2011

f(R) Gravity and its Cosmological Implications

We have investigated the evolution of a homogeneous isotropic background of the Universe and inhomogeneous subhorizon matter density perturbations in viable $f(R)$ models of present dark energy and cosmic acceleration analytically and numerically. It is found that viable $f(R)$ models generically exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$. Furthermore, it is shown that the growth index of perturbations depends both on time and wavenumber. This anomalous growth may explain properties of the observational matter power spectrum from the SDSS data and can also partially counteract the spectrum suppression by massive neutrinos making larger values of the total sum of neutrino rest masses possible.

astro-ph.CO↗

arXiv2010

Higgs G-inflation

A new class of inflation models within the context of G-inflation is proposed, in which the standard model Higgs boson can act as an inflaton thanks to Galileon-like non-linear derivative interaction. The generated primordial density perturbation is shown to be consistent with the present observational data. We also make a general discussion on potential-driven G-inflation models, and find a new consistency relation between the tensor-to-scalar ratio $r$ and the tensor spectral index $n_T$, $r = -32 \sqrt{6}n_T / 9$, which is crucial in discriminating the present models from standard inflation with a canonical kinetic term.

astro-ph.CO↗

arXiv2010

G-inflation: inflation driven by the Galileon field

We propose a new class of inflation model, G-inflation, which has a Galileon-like nonlinear derivative interaction of the form $G(ϕ, (\nablaϕ)^2)\Boxϕ$ in the Lagrangian with the resultant equations of motion being of second order. It is shown that (almost) scale-invariant curvature fluctuations can be generated even in the exactly de Sitter background and that the tensor-to-scalar ratio can take a significantly larger value than in the standard inflation models, violating the standard consistency relation. Furthermore, violation of the null energy condition can occur without any instabilities. As a result, the spectral index of tensor modes can be blue, which makes it easier to observe quantum gravitational waves from inflation by the planned gravitational-wave experiments such as LISA and DECIGO as well as by the upcoming CMB experiments such as Planck and CMBpol.

hep-th↗

arXiv2010

Full nonlinear growing and decaying modes of superhorizon curvature perturbations

We clarify the behavior of curvature perturbations in a nonlinear theory in case the inflaton temporarily stops during inflation. We focus on the evolution of curvature perturbation on superhorizon scales by adopting the spatial gradient expansion and show that the nonlinear theory, called the {\it beyond} $δN$-formalism for a general single scalar field as the next-leading order in the expansion. Both the leading-order in the expansion ($δN$-formalism) and our nonlinear theory include the solutions of full-nonlinear orders in the standard perturbative expansion. Additionally, in our formalism, we can deal with the time evolution in contrast to $δN$-formalism, where curvature perturbations remain just constant, and show decaying modes do not couple with growing modes as similar to the case with linear theory. We can conclude that although the decaying mode diverges when $\dotϕ$ vanishes, there appears no trouble for both the linear and nonlinear theory since these modes will vanish at late times.

astro-ph.CO↗

arXiv2010

Matter power spectrum in f(R) gravity with massive neutrinos

The effect of massive neutrinos on matter power spectrum is discussed in the context of $f(R)$ gravity. It is shown that the anomalous growth of density fluctuations on small scales due to the scalaron force can be compensated by free streaming of neutrinos. As a result, models which predict observable deviation of the equation-of-state parameter $w_\DE$ from $w_\DE=-1$ can be reconciled with observations of matter clustering if the total neutrino mass is $O(0.5 \eV)$.

astro-ph.CO↗

arXiv2010

New cosmological constraints on primordial black holes

We update the constraints on the fraction of the Universe going into primordial black holes in the mass range 10^9--10^17 g associated with the effects of their evaporations on big bang nucleosynthesis and the extragalactic photon background. We include for the first time all the effects of quark and gluon emission by black holes on these constraints and account for the latest observational developments. We then discuss the other constraints in this mass range and show that these are weaker than the nucleosynthesis and photon background limits, apart from a small range 10^13--10^14 g, where the damping of cosmic microwave background anisotropies dominates. Finally we review the gravitational and astrophysical effects of nonevaporating primordial black holes, updating constraints over the broader mass range 1--10^50 g.

astro-ph.CO↗

arXiv2010

Geometrically Consistent Approach to Stochastic DBI Inflation

Stochastic effects during inflation can be addressed by averaging the quantum inflaton field over Hubble-patch sized domains. The averaged field then obeys a Langevin-type equation into which short-scale fluctuations enter as a noise term. We solve the Langevin equation for a inflaton field with Dirac Born Infeld (DBI) kinetic term perturbatively in the noise and use the result to determine the field value's Probability Density Function (PDF). In this calculation, both the shape of the potential and the warp factor are arbitrary functions, and the PDF is obtained with and without volume effects due to the finite size of the averaging domain. DBI kinetic terms typically arise in string-inspired inflationary scenarios in which the scalar field is associated with some distance within the (compact) extra dimensions. The inflaton's accessible range of field values therefore is limited because of the extra dimensions' finite size. We argue that in a consistent stochastic approach the distance-inflaton's PDF must vanish for geometrically forbidden field values. We propose to implement these extra-dimensional spatial restrictions into the PDF by installing absorbing (or reflecting) walls at the respective boundaries in field space. As a toy model, we consider a DBI inflaton between two absorbing walls and use the method of images to determine its most general PDF. The resulting PDF is studied in detail for the example of a quartic warp factor and a chaotic inflaton potential. The presence of the walls is shown to affect the inflaton trajectory for a given set of parameters.

hep-th↗

arXiv2010

Phantom boundary crossing and anomalous growth index of fluctuations in viable f(R) models of cosmic acceleration

Evolution of a background space-time metric and sub-horizon matter density perturbations in the Universe is numerically analyzed in viable $f(R)$ models of present dark energy and cosmic acceleration. It is found that viable models generically exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$. Furthermore, it is shown that, as a consequence of the anomalous growth of density perturbations during the end of the matter-dominated stage, their growth index evolves non-monotonically with time and may even become negative temporarily.

astro-ph.CO↗

arXiv2010

Phantom behaviour and growth index anomalous evolution in viable f(R) gravity models

We present numerical calculation of the evolution of a background space-time metric and sub-horizon matter density perturbations in viable $f(R)$ gravity models of present dark energy and cosmic acceleration. We found that viable models generically exhibit recent crossing of the phantom boundary $w_{DE}=-1$. Moreover, as a consequence of the anomalous growth of density perturbations during the end of the matter-dominated stage, their growth index evolves non-monotonically with time and may even become negative.

astro-ph.CO↗

arXiv2010

Analytic solution for matter density perturbations in a class of viable cosmological f(R) models

For a class of viable cosmological models in $f(R)$ gravity which deviation from the Einstein gravity decreases as a inverse power law of the Ricci scalar $R$ for large $R$, an analytic solution for density perturbations in the matter component during the matter dominated stage is obtained in terms of hypergeometric functions. An analytical expression for the matter transfer function at scales much less than the present Hubble scale is also obtained.

astro-ph.CO↗

arXiv2009

Constraining the time variation of the coupling constants from cosmic microwave background: effect of Λ_{QCD}

We investigate constraints on the time variation of the fine structure constant between the recombination epoch and the present epoch, Δα/α\equiv (α_{rec} - α_{now})/α_{now}, from cosmic microwave background (CMB) taking into account simultaneous variation of other physical constants, namely the electron mass m_{e} and the proton mass m_{p}. In other words, we consider the variation of Yukawa coupling and the QCD scale Λ_{QCD} in addition to the electromagnetic coupling. We clarify which parameters can be determined from CMB temperature anisotropy in terms of singular value decomposition. Assuming a relation among variations of coupling constants governed by a single scalar field (the dilaton), the 95% confidence level (C.L.) constraint on Δα/αis found to be -8.28 \times 10^{-3} < Δα/α< 1.81 \times 10^{-3}, which is tighter than the one obtained by considering only the change of αand m_{e}. We also obtain the constraint on the time variation of the proton-to-electron mass ratio μ\equiv m_{p}/m_{e} to be -0.52 < Δμ/μ< 0.17 (95% C.L.) under the same assumption. Finally, we also implement a forecast for constraints from the PLANCK survey.

astro-ph.CO↗

arXiv2009

Cosmic Discordance: Detection of a modulation in the primordial fluctuation spectrum

As a test of the standard inflationary cosmology, which generically predicts nearly scale-invariant spectrum of primordial curvature fluctuations, we perform Markov-Chain Monte-Carlo analysis to search for possible modulations in the power spectrum and determine its shape together with the cosmological parameters using cosmic microwave background radiation data. By incorporating various three-parameter features on the simple power-law spectrum, we find an oscillatory modulation localized around the comoving wavenumber $k\simeq 0.009\mathrm{Mpc}^{-1}$ at 99.995% confidence level which improves the log-likelihood as much as $-Δ2\ln {\cal L}\equiv Δχ^2_{\rm eff}=-22$. This feature can be detected even if we use only the cross correlation between the temperature and the E-mode polarization anisotropies.

astro-ph.CO↗

arXiv2009

Gravitational Wave Background and Non-Gaussianity as a Probe of the Curvaton Scenario

We study observational implications of the stochastic gravitational wave background and a non-Gaussian feature of scalar perturbations on the curvaton mechanism of the generation of density/curvature fluctuations, and show that they can determine the properties of the curvaton in a complementary manner to each other. Therefore even if Planck could not detect any non-Gaussianity, future space-based laser interferometers such as DECIGO or BBO could practically exhaust its parameter space.

astro-ph.CO↗

arXiv2009

Gravitino Dark Matter and Non-Gaussianity

We investigate density fluctuations in a scenario with gravitino dark matter in the framework of modulated reheating, which is known to generate large non-Gaussianity. We show that gravitino dark matter is disfavored if primordial fluctuations have large local-type non-Gaussianity in this framework. We also briefly discuss the case with the curvaton mechanism and some other possible dark matter scenarios.

astro-ph.CO↗

arXiv2009

Constraints on moduli cosmology from the production of dark matter and baryon isocurvature fluctuations

We set constraints on moduli cosmology from the production of dark matter -- radiation and baryon -- radiation isocurvature fluctuations through modulus decay, assuming the modulus remains light during inflation. We find that the moduli problem becomes worse at the perturbative level as a significant part of the parameter space m_σ(modulus mass) -- σ_{inf} (modulus vev at the end of inflation) is constrained by the non-observation of significant isocurvature fluctuations. We discuss in detail the evolution of the modulus vev and perturbations, in particular the consequences of Hubble scale corrections to the modulus potential and the stochastic motion of the modulus during inflation. We show, in particular, that a high modulus mass scale m_σ> 100 TeV, which allows the modulus to evade big-bang nucleosynthesis constraints is strongly constrained at the perturbative level. We find that generically, solving the moduli problem requires the inflationary scale to be much smaller than 10^{13} GeV.

astro-ph.CO↗

arXiv2009

The moduli problem at the perturbative level

Moduli fields generically produce strong dark matter -- radiation and baryon -- radiation isocurvature perturbations through their decay if they remain light during inflation. We show that existing upper bounds on the magnitude of such fluctuations can thus be translated into stringent constraints on the moduli parameter space m_σ(modulus mass) -- σ_{inf} (modulus vacuum expectation value at the end of inflation). These constraints are complementary to previously existing bounds so that the moduli problem becomes worse at the perturbative level. In particular, if the inflationary scale H_{inf}~10^{13} GeV, particle physics scenarios which predict high moduli masses m_σ> 10-100 TeV are plagued by the perturbative moduli problem, even though they evade big-bang nucleosynthesis constraints.

astro-ph.CO↗

arXiv2009

Band-power reconstruction of the primordial fluctuation spectrum by the maximum likelihood reconstruction method

The primordial curvature fluctuation spectrum is reconstructed by the maximum likelihood reconstruction method using the five-year Wilkinson Microwave Anisotropy Probe data of the cosmic microwave background temperature anisotropy. We apply the covariance matrix analysis and decompose the reconstructed spectrum into statistically independent band-powers. The prominent peak off a simple power-law spectrum found in our previous analysis turn out to be a $3.3σ$ deviation. From the statistics of primordial spectra reconstructed from mock observations, the probability that a primordial spectrum including such excess is realized in a power-law model is estimated to be about 2%.

astro-ph↗

arXiv2008

Reconstruction of the primordial fluctuation spectrum from the five-year WMAP data by the cosmic inversion method with band-power decorrelation analysis

The primordial curvature fluctuation spectrum is reconstructed by the cosmic inversion method using the five-year WMAP data of the cosmic microwave background temperature anisotropy. We apply the covariance matrix analysis and decompose the reconstructed spectrum into statistically independent band-powers. The statistically significant deviation from a simple power-law spectrum suggested by the analysis of the first-year data is not found in the five-year data except possibly at one point near the border of the wavenumber domain where accurate reconstruction is possible.

astro-ph↗