Engineering topics

Yokoyama, Shuichiro

Publications and source records attributed to Yokoyama, Shuichiro.

At least 19 records

DOE OSTI2016

Cosmology with the Square Kilometre Array by SKA-Japan

In the past several decades, the standard cosmological model has been established and its parameters have been measured to a high precision, while there are still many fundamental questions in cosmology; such as the physics in the very early universe, the origin of the cosmic acceleration, and the nature of dark matter. The forthcoming radio telescope, the Square Kilometre Array (SKA), which will be the world's largest, will be able to open a new frontier in cosmology and will be one of the most powerful tools for cosmology in the coming decade. The cosmological surveys conducted by the SKA would have the potential not only to answer these fundamental questions but also deliver precision cosmology. In this article we briefly review the role of the SKA from the viewpoint of modern cosmology. Furthermore, the cosmological science led by the SKA-Japan Consortium (SKA-JP) Cosmology Science Working Group is also discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Anisotropic CMB distortions from non-Gaussian isocurvature perturbations

We calculate the CMB μ-distortion, (μ), and the angular power spectrum of its cross-correlation with the temperature anisotropy, (μT), in the presence of the non-Gaussian neutrino isocurvature density (NID) mode. While the pure Gaussian NID perturbations give merely subdominant contributions to (μ) and do not create (μT), we show large (μT) can be realized in case where, especially, the NID perturbations S(x) are proportional to the square of a Gaussian field g(x), i.e. S(x)∝ g{sup 2}(x). Such Gaussian-squared perturbations contribute to not only the power spectrum, but also the bispectrum of CMB anisotropies. The constraints from the power spectrum is given by P{sub SS}(k{sub 0})∼P{sub g}{sup 2}(k{sub 0})∼<10{sup −10} at k{sub 0}=0.05 Mpc{sup −1}. We also forecast constraints from the CMB temperature and E-mode polarisation bispectra, and show that P{sub g}(k{sub 0})∼<10{sup −5} would be allowed from the Planck data. We find that (μ) and |l(l+1)C{sup μT}{sub l}| can respectively be as large as 10{sup −9} and 10{sup −14} with uncorrelated scale-invariant NID perturbations for P{sub g}(k{sub 0})=10{sup −5}. When the spectrum of the Gaussian field is blue-tilted (with spectral index n{sub g}≅1.5), (μT) can be enhanced by an order of magnitude.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Blue-tilted tensor spectrum and thermal history of the Universe

We investigate constraints on the spectral index of primordial gravitational waves (GWs), paying particular attention to a blue-tilted spectrum. Such constraints can be used to test a certain class of models of the early Universe. We investigate observational bounds from LIGO+Virgo, pulsar timing and big bang nucleosynthesis, taking into account the suppression of the amplitude at high frequencies due to reheating after inflation and also late-time entropy production. Constraints on the spectral index are presented by changing values of parameters such as reheating temperatures and the amount of entropy produced at late time. We also consider constraints under the general modeling approach which can approximately describe various scenarios of the early Universe. We show that the constraints on the blue spectral tilt strongly depend on the underlying assumption and, in some cases, a highly blue-tilted spectrum can still be allowed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Thermal effects and sudden decay approximation in the curvaton scenario

We study the impact of a temperature-dependent curvaton decay rate on the primordial curvature perturbation generated in the curvaton scenario. Using the familiar sudden decay approximation, we obtain an analytical expression for the curvature perturbation after the decay of the curvaton. We then investigate numerically the evolution of the background and of the perturbations during the decay. We first show that the instantaneous transfer coefficient, related to the curvaton energy fraction at the decay, can be extended into a more general parameter, which depends on the net transfer of the curvaton energy into radiation energy or, equivalently, on the total entropy ratio after the complete curvaton decay. We then compute the curvature perturbation and compare this result with the sudden decay approximation prediction.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

CDM/baryon isocurvature perturbations in a sneutrino curvaton model

Matter isocurvature perturbations are strictly constrained from cosmic microwave background observations. We study a sneutrino curvaton model where both cold dark matter (CDM)/baryon isocurvature perturbations are generated. In our model, total matter isocurvature perturbations are reduced since the CDM/baryon isocurvature perturbations compensate for each other. We show that this model can not only avoid the stringent observational constraints but also suppress temperature anisotropies on large scales, which leads to improved agreement with observations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Curvaton in large field inflation

We comprehensively explore the quadratic curvaton models in the chaotic inflation. In the light of the BICEP2 result r ≈ 0.2, all model parameters and relevant observables are computed. It is found the curvaton field value is constrained into a narrow range, σ{sub *} = O(10{sup −2}–10{sup −1}) and the running of the spectral index is n{sub s}'∼>−10{sup −3}. We show that if the curvaton is added, the models are heavily degenerated on the n{sub s} - r plane. However, introducing a new plane, the degeneracy can be resolved. To distinguish the curvaton models, precise measurements of not only r but also n{sub s}' and the tensor tilt n{sub T} are required.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2014

Isocurvature perturbations and tensor mode in light of Planck and BICEP2

We investigate the degeneracy of the isocurvature perturbations and the primordial gravitational waves, by using recent observations of the cosmic microwave background (CMB) reported by Planck and BICEP2 collaborations. We show that the tension in the bound on the tensor-to-scalar ratio r between Planck and BICEP2 can be resolved by introducing the anti-correlated isocurvature perturbations. Quantitatively, we find that with the anti-correlated isocurvature perturbations the constraints on r from Planck alone and BICEP2 results can be consistent at 68 % C.L.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Compensation for large tensor modes with iso-curvature perturbations in CMB anisotropies

Recently, BICEP2 has reported the large tensor-to-scalar ratio r = 0.2{sup +0.07}{sub −0.05} from the observation of the cosmic microwave background (CMB) B-mode at degree-scales. Since tensor modes induce not only CMB B-mode but also the temperature fluctuations on large scales, to realize the consistent temperature fluctuations with the Planck result we should consider suppression of scalar perturbations on corresponding large scales. To realize such a suppression, we consider anti-correlated iso-curvature perturbations which could be realized in the simple curvaton model.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Critical constraint on inflationary magnetogenesis

Recently, there are several reports that the cosmic magnetic fields on Mpc scale in void region is larger than ∼ 10{sup −15}G with an uncertainty of a few orders from the current blazar observations. On the other hand, in inflationary magnetogenesis models, additional primordial curvature perturbations are inevitably produced from iso-curvature perturbations due to generated electromagnetic fields. We explore such induced curvature perturbations in a model independent way and obtained a severe upper bound for the energy scale of inflation from the observed cosmic magnetic fields and the observed amplitude of the curvature perturbation , as ρ{sub inf}{sup 1/4} < 300MeV × (B{sub obs}/10{sup −15}G){sup −1} where B{sub obs} is the strength of the magnetic field at present. Therefore, without a dedicated low energy inflation model or an additional amplification of magnetic fields after inflation, inflationary magnetogenesis on Mpc scale is generally incompatible with CMB observations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Higher order statistics of curvature perturbations in IFF model and its Planck constraints

We compute the power spectrum P{sub ζ} and non-linear parameters f{sub NL} and τ{sub NL} of the curvature perturbation induced during inflation by the electromagnetic fields in the kinetic coupling model (IFF model). By using the observational result of P{sub ζ},f{sub NL} and τ{sub NL} reported by the Planck collaboration, we study the constraint on the model comprehensively. Interestingly, if the single slow-rolling inflaton is responsible for the observed P{sub ζ}, the constraint from τ{sub NL} is most stringent. We also find a general relationship between f{sub NL} and τ{sub NL} generated in this model. Even if f{sub NL} ∼ O(1), a detectable τ{sub NL} can be produced.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Gravitational waves from a curvaton model with blue spectrum

We investigate the gravitational wave background induced by the first order scalar perturbations in the curvaton models. We consider the quadratic and axion-like curvaton potential which can generate the blue-tilted power spectrum of curvature perturbations on small scales and derive the maximal amount of gravitational wave background today. We find the power spectrum of the induced gravitational wave background has a characteristic peak at the frequency corresponding to the scale reentering the horizon at the curvaton decay, in the case where the curvaton does not dominate the energy density of the Universe. We also find the enhancement of the amount of the gravitational waves in the case where the curvaton dominates the energy density of the Universe. Such induced gravitational waves would be detectable by the future space-based gravitational wave detectors or pulsar timing observations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Implications of Planck results for models with local type non-Gaussianity

We discuss implications of Planck results for models with local type non-Gaussianity. In light of the recent results of the Planck satellite, we constrain model parameters of several representative models and give the prediction of trispectrum, in particular, g{sub NL}. We also consider interesting possibilities that trispectrum appears as the first signature of the non-Gaussianities of the curvature perturbations, that is, f{sub NL} is small while g{sub NL} can be significantly large.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Statistics of general functions of a Gaussian field-application to non-Gaussianity from preheating

We provide a general formula for calculating correlators of arbitrary function of a Gaussian field. This work extends the standard leading-order approximation based on the δN formalism to the case where truncation of the δN at some low order does not yield the correct answer. As an application of this formula, we investigate 2, 3 and 4-point functions of the primordial curvature perturbation generated in the massless preheating model by approximating the mapping between the curvature perturbation and the Gaussian field as a sum of the many spiky normal distribution functions as suggested by lattice calculations. We also discuss observational consequences of this case and show that trispectrum would be a key observable to search signature of preheating in the CMB map. It is found the forms of the curvature correlation functions for any δN, at the leading order in the correlator of the Gaussian field, coincide with the standard local type ones. Within this approximation, it is also found that the standard formula for the non-linearity parameters given by the product of the derivatives of the e-folding number still holds after we replace the bare e-folding number appearing in the original δN expansion with the one smoothed in the field space with a Gaussian window function.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

CMB power spectra induced by primordial cross-bispectra between metric perturbations and vector fields

We study temperature and polarization anisotropies of the cosmic microwave background (CMB) radiation sourced from primordial cross-bispectra between metric perturbations and vector fields, which are generated from the inflation model where an inflaton and a vector field are coupled. In case the vector field survives after the reheating, both the primordial scalar and tensor fluctuations can be enhanced by the anisotropic stress composed of the vector fields during radiation dominated era. We show that through this enhancement the primordial cross-bispectra generate not only CMB bispectra but also CMB power spectra. In general, we can expect such cross-bispectra produce the non-trivial mode-coupling signals between the scalar and tensor fluctuations. However, we explicitly show that such mode-coupling signals do not appear in CMB power spectra. Through the numerical analysis of the CMB scalar-mode power spectra, we find that although signals from these cross-bispectra are smaller than primary non-electromagnetic ones, these have some characteristic features such as negative auto-correlations of the temperature and polarization modes, respectively. On the other hand, signals from tensor modes are almost comparable to primary non-electromagnetic ones and hence the shape of observed B-mode spectrum may deviate from the prediction in the non-electromagnetic case. The above imprints may help us to judge the existence of the coupling between the scalar and vector fields in the early Universe.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Optimal limits on primordial magnetic fields from CMB temperature bispectrum of passive modes

We investigate bounds on the strength of the primordial magnetic field (PMF) from the cosmic microwave background (CMB) bispectra of the intensity (temperature) modes induced from the auto- and cross-correlated bispectra of the scalar and tensor components of the PMF anisotropic stress. At first, we construct a general formula for the CMB intensity and polarization bispectra from PMFs composed of any type of perturbation. Then we derive an approximate expression which traces the exact shape of the CMB bispectrum in order to reduce the computation time with respect to a large number of the multipole configurations, and also show that the non-Gaussian structure coming from PMFs is classified as the local-type configuration. Computing the signal-to-noise ratio on the basis of the approximate formula with the information of the instrumental noises and resolutions, we find expected upper bounds on the magnetic field strength, when the magnetic spectrum is nearly scale invariant (n{sub B} = −2.9), smoothed on 1Mpc scale at 95% confidence level from the WMAP and PLANCK experiments as B{sub 1Mpc} < 4.0−6.7nG and 3.8−6.5nG, respectively, depending on the energy scale of the magnetic field production from 10{sup 14}GeV to 10{sup 3}GeV. Our new consequences imply slight overestimations by the previous rough discussions.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Fine features in the primordial power spectrum

A possible origin of the anomalous dip and bump in the primordial power spectrum, which are reconstructed from WMAP data corresponding to the multipole l = 100 ∼ 140 by using the inversion method, is investigated as a consequence of modification of scalar field dynamics in the inflation era. Utilizing an analytic formula to handle higher order corrections to the slow-roll approximation, we evaluate the relation between a detailed shape of inflaton potential and a fine structure in the primordial power spectrum. We conclude that it is unlikely to generate the observed dip and bump in the power spectrum by adding realistic features in the inflaton potential. In order to generate the fine structure, we find that the slow-roll parameters have to become about O(100) and the number of oscillations in the potential is about O(10{sup 2}), on the scale of the structure.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Scale-dependent bias from the primordial non-Gaussianity with a Gaussian-squared field

We investigate the halo bias in the case where the primordial curvature fluctuations, Φ, are sourced from both a Gaussian random field and a Gaussian-squared field, as Φ(x) = φ(x)+ψ(x){sup 2}−(ψ(x){sup 2}ångle, so-called ''ungaussiton model''. We employ the peak-background split formula and find a new scale-dependence in the halo bias induced from the Gaussian-squared field.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Extension of local-type inequality for the higher order correlation functions

For the local-type primordial perturbation, it is known that there is an inequality between the bispectrum and the trispectrum. By using the diagrammatic method, we develop a general formalism to systematically construct the similar inequalities up to any order correlation function. As an application, we explicitly derive all the inequalities up to six and eight-point functions.

79 ASTRONOMY AND ASTROPHYSICS↗