arXiv subjects

Keitaro Takahashi

Publications and source records attributed to Keitaro Takahashi.

At least 145 records · Page 8Linked to original sources

arXiv2007

Magnetic Field Spectrum at Cosmological Recombination

A generation of magnetic fields from cosmological density perturbations is investigated. In the primordial plasma before cosmological recombination, all of the materials except dark matter in the universe exist in the form of photons, electrons, and protons (and a small number of light elements). Due to the different scattering nature of photons off electrons and protons, electric currents and electric fields are inevitably induced, and thus magnetic fields are generated. We present a detailed formalism of the generation of cosmological magnetic fields based on the framework of the well-established cosmological perturbation theory following our previous works. We numerically obtain the power spectrum of magnetic fields for a wide range of scales, from $k\sim 10^{-5}$ Mpc$^{-1}$ up to $k\sim 2\times 10^{4}$ Mpc$^{-1}$ and provide its analytic interpretation. Implications of these cosmologically generated magnetic fields are discussed.

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arXiv2006

Gravitational lensing effects on the gamma-ray burst Hubble diagram

Gamma-Ray Bursts (GRBs) offer a potential way to extend the Hubble diagram to very high redshifts and to constrain the nature of dark energy in a way complementary to distant type Ia supernovae. However, gravitational lensing systematically brightens distant GRBs through the magnification bias, in addition to increasing the dispersions of distance measurements. We investigate how the magnification bias limits the cosmological usage of GRBs. We perform Monte-Carlo simulations of Swift GRBs assuming a cosmological constant dominated universe and then constrain the dark energy equation of state neglecting gravitational lens effects. The originally assumed model is recovered with 68% confidence limit even when the dispersion of inferred luminosities is comparable to that of type Ia supernovae. This implies that the bias is not so drastic for Swift GRBs as to change constraints on dark energy and its evolution. However, the precise degree of the bias in cosmological parameter determinations depends strongly on the shape of the luminosity function of GRBs. Therefore, an accurate determination of the shape of the luminosity function is required to remove the effect of gravitational lensing and to obtain an unbiased Hubble diagram.

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arXiv2006

Cosmological Magnetic Field: a fossil of density perturbations in the early universe

The origin of the substantial magnetic fields that are found in galaxies and on even larger scales, such as in clusters of galaxies, is yet unclear. If the second-order couplings between photons and electrons are considered, then cosmological density fluctuations, which explain the large scale structure of the universe, can also produce magnetic fields on cosmological scales before the epoch of recombination. By evaluating the power spectrum of these cosmological magnetic fields on a range of scales, we show here that magnetic fields of 10^{-18.1} gauss are generated at a 1 megaparsec scale and can be even stronger at smaller scales (10^{-14.1} gauss at 10 kiloparsecpc). These fields are large enough to seed magnetic fields in galaxies and may therefore have affected primordial star formation in the early universe.

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arXiv2006

Biermann Mechanism in Primordial Supernova Remnant and Seed Fields

We have studied the generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of the first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We have performed a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ ergs per each supernova remnant, which is strong enough to be the seed magnetic field for a galactic and/or interstellar dynamo.

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arXiv2006

Generation of magnetic field from cosmological perturbations

We discuss generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton and photon) evaluating the collision term between elecrons and photons up to the second order. The collision term is shown to induce electric current, which then generate magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity which is purely second order. We compute numerically the magnitudes of the former contributions and shows that the amplitude of the produced magnetic field is about $\sim 10^{-19} {\rm G}$ at 10kpc comoving scale at present. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

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arXiv2005

Characterizing a cosmic string with the statistics of string lensing

The deep imaging of the field of an observed lensing event by a cosmic string reveals many additional lensing events. We study the statistics of such string lensing. We derive explicit expressions for the distributions of image separations of lensing by a cosmic string and point out that they are quite sensitive to parameters which characterize the cosmic string, such as the redshift and tension of the cosmic string. Thus the statistics of string lensing events add new important information on the cosmic string which cannot be obtained from the detailed investigation of one lensing event.

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arXiv2005

Explosion Mechanism, Neutrino Burst, and Gravitational Wave in Core-Collapse Supernovae

Core-collapse supernovae are among the most energetic explosions in the universe marking the catastrophic end of massive stars. In spite of rigorous studies for several decades, we still don't understand the explosion mechanism completely. Since they are related to many astrophysical phenomena such as nucleosynthesis, gamma-ray bursts and acceleration of cosmic rays, understanding of their physics has been of wide interest to the astrophysical community. In this article, we review recent progress in the study of core-collapse supernovae focusing on the explosion mechanism, supernova neutrinos, and the gravitational waves. As for the explosion mechanism, we present a review paying particular attention to the roles of multidimensional aspects, such as convection, rotation, and magnetic fields, on the neutrino heating mechanism. Next, we discuss supernova neutrinos, which is a powerful tool to probe not only deep inside of the supernovae but also intrinsic properties of neutrinos. For this purpose, it is necessary to understand neutrino oscillation which has been established recently by a lot of experiments. Gravitational astronomy is now also becoming reality. We present an extensive review on the physical foundations and the emission mechanism of gravitational waves in detail, and discuss the possibility of their detections.

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arXiv2005

Magnetic field generation from cosmological perturbations

In this letter, we discuss generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton and photon) evaluating the collision term between elecrons and photons up to the second order. The collision term is shown to induce electric current, which then generate magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity which is purely second order. We estimate the magnitudes of the former contributions and shows that the amplitude of the produced magnetic field is about $\sim 10^{-19} {\rm G}$ at 10Mpc comoving scale at the recombination. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

astro-ph↗

arXiv2005

Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields

We study generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We perform a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ erg per each supernova remnant, which is strong enough to be the seed magnetic field for galactic and/or interstellar dynamo.

astro-ph↗

arXiv2005

Gravity is controlled by cosmological constant

We discuss a Randall-Sundrum-type two D-braneworld model in which D-branes possess different values of the tensions from those of the charges, and derive an effective gravitational equation on the branes. As a consequence, the Einstein-Maxwell theory is realized together with the non-zero cosmological constant. Here an interesting point is that the effective gravitational constant is proportional to the cosmological constant. If the distance between two D-branes is appropriately tuned, the cosmological constant can have a consistent value with the current observations. From this result we see that, in our model, the presence of the cosmological constant is naturally explained by the presence of the effective gravitational coupling of the Maxwell field on the D-brane.

hep-th↗

arXiv2004

An Analytic Model of Superclusters

We construct an analytic model for the mass distributions of superclusters and clusters in each supercluster. Our model is a modification of the Press-Schechter theory, and defines superclusters as the regions that have some overdensity smaller than those for usual virialized objects. We compare the mass functions with a catalog of superclusters in the Sloan Digital Sky Survey Early Data Release and found that they are in reasonable agreement with each other.

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arXiv2004

Asymmetric D-braneworld

In recent papers on Randall-Sundrum D-braneworld model with Z_2 symmetry, it was shown that the effective gravity does not work as usual, that is, the gravity does not couple to the gauge field localised on the brane in a usual way. At first glance there are two possibilities to avoid this serious problem. One is to remove the Z_2 symmetry and another is to consider a non-BPS state. In this paper we analyze the Randall-Sundrum D-braneworld model without Z_2 symmetry by long wave approximation. The result is unexpected one, that is, the gauge field does not couple to the gravity on the brane in the leading order again. Therefore the remaining possibility to recover the conventional gravitational theory would be non-BPS cases.

hep-th↗

arXiv2004

Gravity in Randall-Sundrum two D-brane model

We analyse Randall-Sundrum two D-brane model by linear perturbation and then consider the linearised gravity on the D-brane. The qualitative contribution from the Kaluza-Klein modes of gauge fields to the coupling to the gravity on the brane will be addressed. As a consequence, the gauge fields localised on the brane are shown not to contribute to the gravity on the brane at large distances. Although the coupling between gauge fields and gravity appears in the next order, the ordinary coupling cannot be realised.

hep-th↗

arXiv2004

WMAP Constraints on Decaying Cold Dark Matter

We re-formulate cosmological perturbations in the decaying cold dark matter model, and calculate cosmological microwave background (CMB) anisotropies. By comparing our theoretical predictions with recent observational data from the Wilkinson Microwave Anisotropy Probe(WMAP), we derive a new bound on the abundance and lifetime of decaying dark matter particles. We show that the data of WMAP alone do not prefer the decaying cold dark matter model: the lifetime is constrained to Γ^{-1}> 123 Gyr at 68% C.L. (52 Gyr at 95.4% C.L.) when cold dark matter consists only of such decaying particles. We also consider a more general case in which cold dark matter consists of both stable and decaying particles, and show that the constraint generalizes down to Ω_{DDM}h^2 < -0.5 (Gamma^{-1}/1 Gyr)^{-1}+0.12 for Γ^{-1} > 5 Gyr at 95.4% C.L.. These bounds are robust and widely applicable, because they are derived only from gravitational effects and therefore do not rely on the details of decay channels or decay products.

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arXiv2004

Randall-Sundrum two D-brane model

In Randall-Sundrum two D-brane system we derive the gravitational theory on the branes. It is turned out from the consistency that one D-brane has the negative tension brane under Randall-Sundrum tuning and both gauge fields on the brane are related by scale transformation through the bulk RR/NS-NS fields. As same with the single D-brane case, the gauge field which is supposed to be localised on the brane does not couple to the gravity on the branes.

hep-th↗

arXiv2004

Acoustic causality in relativistic shells

The motion of sound waves propagating in the perfect fluid with inhomogeneous background flow is effectively described as a massless scalar field on a curved space-time. This effective geometry is characterized by the acoustic metric, which depends on the background flow, and null geodesics on the geometry express the acoustic causal structure. Therefore by the effective geometry we can easily study the causality on the flows. In this paper, we consider a spherically symmetric, relativistic outflow and present the maximal causally connected region for a super-sonic flow. When Lorentz factor of the radial velocity of the flow is constant or obeys power-law with respect to the radial coordinate r, we can solve it analytically. As a result we show that in the constant case the maximum angle is proportional to inverse of Lorentz factor and logarithmically increases with respect to r, in contrast, accelerative expansions in power-law case make this angle bounded.

astro-ph↗

arXiv2004

Cosmology and two-body problem of D-branes

In this paper, we investigate the dynamics and the evolution of the scale factor of a probe Dp-brane which move in the background of source Dp-branes. Action of the probe brane is described by the Born-Infeld action and the interaction with the background R-R field. When the probe brane moves away from the source branes, it expands by power law, whose index depends on the dimension of the brane. If the energy density of the gauge field on the brane is subdominant, the expansion is decelerating irrespective of the dimension of the brane. On the other hand, when the probe brane is a Nambu-Goto brane, the energy density of the gauge field can be dominant, in which case accelerating expansion occurs for $p \leq 4$. The accelerating expansion stops when the brane has expanded sufficiently so that the energy density of the gauge field become subdominant.

hep-th↗

arXiv2003

Sunyaev-Zel'dovich power spectrum with decaying cold dark matter

Recent studies of structures of galaxies and clusters imply that dark matter might be unstable and decay with lifetime $Γ^{-1}$ about the age of universe. We study the effects of the decay of cold dark matter on the Sunyaev-Zel'dovich (SZ) power spectrum. We analytically calculate the SZ power spectrum taking finite lifetime of cold dark matter into account. We find the finite lifetime of dark matter decreases the power at large scale ($l < 4000$) and increases at small scale ($l > 4000$). This is in marked contrast with the dependence of other cosmological parameters such as the amplitude of mass fluctuations $σ_{8}$ and the cosmological constant $Ω_λ$ (under the assumption of a flat universe) which mainly change the normalization of the angular power spectrum. This difference allows one to determine the lifetime and other cosmological parameters rather separately. We also investigate sensitivity of a future SZ survey to the cosmological parameters including the life time, assuming a fiducial model $Γ^{-1} = 10 h^{-1} {\rm Gyr}$, $σ_{8} = 1.0$, and $Ω_λ = 0.7$. We show that future SZ surveys such as ACT, AMIBA, and BOLOCAM can determine the lifetime within factor of two even if $σ_{8}$ and $Ω_λ$ are marginalized.

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