arXiv subjects

Tomohiro Fujita

Publications and source records attributed to Tomohiro Fujita.

At least 37 records · Page 2Linked to original sources

arXiv2023

Enhancement of quantum gravity signal in an optomechanical experiment

No experimental evidence of the quantum nature of gravity has been observed yet and a realistic setup with improved sensitivity is eagerly awaited. We find two effects, which can substantially enhance the signal of gravity-induced quantum entanglement, by examining an optomechanical system in which two oscillators gravitationally couple and one composes an optical cavity. The first effect comes from a higher-order term of the optomechanical interaction and generates the signal at the first order of the gravitational coupling in contrast to the second order results in previous works. The second effect is the resonance between the two oscillators. If their frequencies are close enough, the weak gravitational coupling effectively strengthens. Combining these two effects, the signal in the interference visibility could be amplified by a factor of $10^{24}$ for our optimistic parameters. The two effects would be useful in seeking feasible experimental setups to probe quantum gravity signals.

gr-qc↗

arXiv2023

Highly asymmetric probability distribution from a finite-width upward step during inflation

We study a single-field inflation model in which the inflaton potential has an upward step between two slow-roll regimes by taking into account the finite width of the step. We calculate the probability distribution function (PDF) of the curvature perturbation $P[{\cal{R}}]$ using the $δN$ formalism. The PDF has an exponential-tail only for positive ${\cal{R}}$ whose slope depends on the step width. We find that the tail may have a significant impact on the estimation of the primordial black hole abundance. We also show that the PDF $P[{\cal{R}}]$ becomes highly asymmetric on a particular scale exiting the horizon before the step, at which the curvature power spectrum has a dip. This asymmetric PDF may leave an interesting signature in the large scale structure such as voids.

astro-ph.CO↗

arXiv2023

Inflation with two-form field: the production of primordial black holes and gravitational waves

Antisymmetric tensor field (two-form field) is a ubiquitous component in string theory and generally couples to the scalar sector through its kinetic term. In this paper, we propose a cosmological scenario that the particle production of two-form field, which is triggered by the background motion of the coupled inflaton field, occurs at the intermediate stage of inflation and generates the sizable amount of primordial black holes as dark matter after inflation. We also compute the secondary gravitational waves sourced by the curvature perturbation and show that the resultant power spectra are testable with the future space-based laser interferometers.

astro-ph.CO↗

arXiv2023

Exploring spin of ultralight dark matter with gravitational wave detectors

We propose a novel method for distinguishing the spin of ultralight dark matter (ULDM) using interferometric gravitational wave detectors. ULDM can be a bosonic field of spin-0, 1, or 2, and each induces distinctive signatures in signals. We find that the finite-time traveling effect causes a dominant signal for spin-0 and spin-1 ULDM, while not for spin-2. By using overlap reduction functions (ORF) of multiple detectors, we can differentiate between the spins of ULDM. Furthermore, we point out that the current constraint on the coupling constant of spin-1 ULDM to baryons becomes 30 times weaker when the finite-time light-travel effect on the ORF is taken into account.

hep-ph↗

arXiv2023

First test of the consistency relation for the large-scale structure using the anisotropic three-point correlation function of BOSS DR12 galaxies (An explanatory video is available at https://youtu.be/Zi36ooLPhss.)

We present, for the first time, an observational test of the consistency relation for the large-scale structure (LSS) of the Universe through a joint analysis of the anisotropic two- and three-point correlation functions (2PCF and 3PCF) of galaxies. We parameterise the breakdown of the LSS consistency relation in the squeezed limit by $E_{\rm s}$, which represents the ratio of the coefficients of the shift terms in the second-order density and velocity fluctuations. $E_{\rm s}\neq1$ is a sufficient condition under which the LSS consistency relation is violated. A novel aspect of this work is that we constrain $E_{\rm s}$ by obtaining information about the nonlinear velocity field from the quadrupole component of the 3PCF without taking the squeezed limit. Using the galaxy catalogues in the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 12, we obtain $E_{\rm s} = -0.92_{-3.26}^{+3.13}$, indicating that there is no violation of the LSS consistency relation in our analysis within the statistical errors. Our parameterisation is general enough that our constraint can be applied to a wide range of theories, such as multicomponent fluids, modified gravity theories, and their associated galaxy bias effects. Our analysis opens a new observational window to test the fundamental physics using the anisotropic higher-order correlation functions of galaxy clustering.

astro-ph.CO↗

arXiv2023

New constraints on cosmological modified gravity theories from anisotropic three-point correlation functions of BOSS DR12 galaxies

We report a new test of modified gravity theories using the large-scale structure of the Universe. This paper is the first attempt to (1) apply a joint analysis of the anisotropic components of galaxy two- and three-point correlation functions (2 and 3PCFs) to actual galaxy data and (2) constrain the nonlinear effects of degenerate higher-order scalar-tensor (DHOST) theories on cosmological scales. Applying this analysis to the Baryon Oscillation Spectroscopic Survey (BOSS) data release 12, we obtain the lower bounds of $-1.655 < ξ_{\rm t}$ and $-0.504 < ξ_{\rm s}$ at the $95\%$ confidence level on the parameters characterising the time evolution of the tidal and shift terms of the second-order velocity field. These constraints are consistent with GR predictions of $ξ_{\rm t}=15/1144$ and $ξ_{\rm s}=0$. Moreover, they represent a $35$-fold and $20$-fold improvement, respectively, over the joint analysis with only the isotropic 3PCF. We ensure the validity of our results by investigating various quantities, including theoretical models of the 3PCF, window function corrections, cumulative ${\rm S/N}$, Fisher matrices, and statistical scattering effects of mock simulation data. We also find statistically significant discrepancies between the BOSS data and the Patchy mocks for the 3PCF measurement. Finally, we package all of our 3PCF analysis codes under the name \textsc{HITOMI} and make them publicly available so that readers can reproduce all the results of this paper and easily apply them to ongoing future galaxy surveys.

astro-ph.CO↗

arXiv2023

Stochastic formalism for U(1) gauge fields in axion inflation

We develop the stochastic formalism for $\mathrm{U}(1)$ gauge fields that has the Chern-Simons coupling to a rolling pseudo-scalar field during inflation. The Langevin equations for the physical electromagnetic fields are derived and the analytic solutions are studied. Using numerical simulation we demonstrate that the electromagnetic fields averaged over the Hubble scale continuously change their direction and their amplitudes fluctuate around the analytically obtained expectation values. Though the isotropy is spontaneously broken by picking up a particular local Hubble patch, each Hubble patch is understood independent and the isotropy is conserved globally by averaging all the Hubble patches.

astro-ph.CO↗

arXiv2023

How does SU($N$)-natural inflation isotropize the universe?

We study the homogeneous and anisotropic dynamics of pseudoscalar inflation coupled to an SU($N$) gauge field. To see how the initially anisotropic universe is isotropized in such an inflation model, we derive the equations to obtain axisymmetric SU($N$) gauge field configurations in Bianchi type-I geometry and discuss a method to identify their isotropic subsets which are the candidates of their late-time attractor. Each isotropic solution is characterized by the corresponding SU(2) subalgebra of the SU($N$) algebra. It is shown numerically that the isotropic universe is a universal late-time attractor in the case of the SU(3) gauge field. Interestingly, we find that a transition between the two distinct gauge-field configurations characterized by different SU(2) subalgebras can occur during inflation. We clarify the conditions for this to occur. This transition could leave an observable imprint on the CMB and the primordial gravitational wave background.

gr-qc↗

arXiv2022

Perturbation theory of LSS in the $Λ$CDM Universe: exact time evolution and the two-loop power spectrum

We derive exact analytic solutions for density and velocity fields to all orders in Eulerian standard perturbation theory for $Λ$CDM cosmology. In particular, we show that density and velocity field kernels can be written in a separable form in time and momenta at each perturbative order. The kernel solutions are built from an analytic basis of momentum operators and their time-dependent coefficients, which solve a set of recursive differential equations. We also provide an exact closed perturbative solution for such coefficients, expanding around the (quasi-)EdS approximation. We find that the perturbative solution rapidly converges towards the numerically obtained solutions and its leading order result suffices for any practical requirements. To illustrate our findings, we compute the exact two-loop dark matter density and velocity power spectra in $Λ$CDM cosmology. We show that the difference between the exact $Λ$CDM and the (quasi-)EdS approximated result can reach the level of several percent (at redshift zero, for wavenumbers $k<1h/$Mpc). This deviation can be partially mitigated by exploiting the degeneracy with the EFT counterterms. As an additional benefit of our algorithm for the solutions of time-dependent coefficients, the computational complexity of power spectra loops in $Λ$CDM is comparable to the EdS case. In performing the two-loop computation, we devise an explicit method to implement the so-called IR cancellations, as well as the cancellations arising as a consequence of mass and momentum conservation.

astro-ph.CO↗

arXiv2022

Spin-2 dark matter from anisotropic Universe in bigravity

Bigravity is one of the natural extensions of general relativity and contains an additional massive spin-2 field which can be a good candidate for dark matter. To discuss the production of spin-2 dark matter, we study fixed point solutions of the background equations for axisymmetric Bianchi type-I Universes in two bigravity theories without Boulware-Deser ghost, i.e., Hassan-Rosen bigravity and Minimal Theory of Bigravity. We investigate the local and global stability of the fixed points and classify them. Based on the general analysis, we propose a new scenario where spin-2 dark matter is produced by the transition from an anisotropic fixed point solution to isotropic one. The produced spin-2 dark matter can account for all or a part of dark matter and can be directly detected by laser interferometers in the same way as gravitational waves.

gr-qc↗

arXiv2022

Effective Field Theory of Large Scale Structure in modified gravity and application to Degenerate Higher-Order Scalar-Tensor theories

In modified gravity, the one-loop matter power spectrum exhibits an ultraviolet divergence as shown in the framework of the degenerate higher-order scalar-tensor theory. To address this problem, we extend the effective field theory of large scale structure to modified gravity theories. We find that new counterterms appear and renormalize the ultraviolet divergence as a natural consequence of non-linearity in the modified Poisson equation. The renormalized one-loop matter power spectrum is useful to test modified gravity theories by comparing to observations.

gr-qc↗

arXiv2022

Can primordial parity violation explain the observed cosmic birefringence?

Recently, the cross-correlation between $E$- and $B$-mode polarization of the cosmic microwave background (CMB), which is well explained by cosmic birefringence with rotation angle $β\approx 0.3$ deg, has been found in CMB polarization data. We carefully investigate the possibility of explaining the observed $EB$ correlation by the primordial chiral gravitational waves (CGWs), which can be generated in the parity-violating theories in the primordial Universe. We found that the CGWs scenario does not work due to the overproduction of the $BB$ auto-correlation which far exceeds the observed one by SPTPol and POLARBEAR.

astro-ph.CO↗

arXiv2022

Effective treatment of U(1) gauge field and charged particles in axion inflation

The axionic inflaton with the Chern-Simons coupling may generate U(1) gauge fields and charged particles simultaneously. In order to incorporate the backreaction from the charged particles on the gauge fields, we develop a procedure to obtain an equilibrium solution for the gauge fields by treating the induced current as effective electric and magnetic conductivities. Introducing mean field approximation, and numerically solving self-consistency equations, we find that the gauge field amplitudes are drastically suppressed. Interestingly, as the production becomes more efficient, the charged particles gain a larger part of the transferred energy from the inflaton and eventually dominate it. Our formalism offers a basis to connect this class of inflationary models to a rich phenomenology such as baryogenesis and magnetogenesis.

hep-ph↗

arXiv2022

Gravitational waves detectable in laser interferometers from axion-SU(2) inflation

Chromo-natural inflation (CNI) is an inflationary model where an axion coupled with SU$(2)$ gauge fields acts as the inflaton. In CNI, the gauge fields have nonzero vacuum expectation values (VEVs), which results in the enhancement of gravitational waves (GWs). The original CNI is ruled out by the Planck observations due to the overproduction of GWs. In this work, we consider an inflationary model where the gauge fields acquire nonzero VEVs after the CMB modes exit the horizon. Moreover, we add to the model another field that dominates the universe and drives inflation after the axion starts to oscillate and the gauge field VEVs vanish. By performing numerical simulations, we find a parameter space where the enhanced GWs do not violate the CMB constraints and can be detected by the future GWs observations such as BBO and ET.

astro-ph.CO↗

arXiv2022

SU($N$)-natural inflation

We study SU($N$) gauge fields that couple to the inflaton through the Chern-Simons term. We provide a general procedure to construct homogeneous, isotropic, and attractor solutions of the gauge fields during inflation. The gauge fields develop various VEVs corresponding to different spontaneous symmetry breaking patterns of SU($N$) where embedded SU($2$) subgroups are broken with the spatial rotation SO($3$) symmetry. As specific examples, we construct the stable solutions for $N = 3$ and $4$. We also numerically solve the gauge field dynamics and confirm that our analytic solutions are complete and attractor. It is straightforward to apply our procedure to the other simple Lie groups.

hep-ph↗

arXiv2022

Universality of linear perturbations in SU($N$)-natural inflation

We prove the universality of predictions for linear perturbations from the entire class of models of inflation driven by a pseudo-scalar field coupled to an SU($N$) gauge boson, where SU($2$) subgroups in the SU($N$) crossed with the background spatial SO($3$) spontaneously break into a single SO($3$). The effect of which SU($2$) subgroup in SU($N$) acquires a VEV through spontaneous symmetry breaking can be quantified by a single parameter $λ$, which always appears in combination with the gauge coupling constant $g$. In the linear perturbations, as well as the background system, the same dynamics and predictions as in the chromo-natural inflation hold for its SU($N$) extension by replacing $g \to gλ$. The latter models thereby draw the same prediction curve on the $n_s$-$r$ plane as the former at the tree level as long as $g λ$ stays constant during inflation. We briefly discuss possible transitions from one value of $λ$ to another during inflation and the observational prospects.

hep-ph↗

arXiv2022

Stochastic effects on observation of ultralight bosonic dark matter

Ultralight bosonic particles are fascinating candidates of dark matter (DM). It behaves as classical waves in our Galaxy due to its large number density. There have been various methods proposed to search for the wave-like DM, such as methods utilizing interferometric gravitational-wave detectors. Understanding the characteristics of DM signals is crucial to extract the properties of DM from data. While the DM signal is nearly monochromatic with the angular frequency of its mass, the amplitude and phase are gradually changing due to the velocity dispersion of DMs in our Galaxy halo. The stochastic amplitude and phase should be properly taken into account to accurately constrain the coupling constant of DM from data. Previous works formulated a method to obtain the upper bound on the coupling constant incorporating the stochastic effects. One of these works compared the upper bound with and without the stochastic effect in a measurement time that is much shorter than the variation time scale of the amplitude and phase. In this paper, we extend their formulation to arbitrary measurement time and evaluate the stochastic effects. Moreover, we investigate the velocity-dependent signal for dark photon DM including an uncertainly of the velocity. We demonstrate that our method accurately estimates the upper bound on the coupling constant with numerical simulations. We also estimate the expected upper bound of the coupling constant of axion DM and dark photon DM from future experiments in a semi-analytic way. The stochasticity especially affects constraints on a small mass region. Our formulation offers a generic treatment of the ultralight bosonic DM signal with the stochastic effect.

astro-ph.CO↗

arXiv2022

Generation of chiral asymmetry via helical magnetic fields

It is well known that helical magnetic fields undergo a so-called inverse cascade by which their correlation length grows due to the conservation of magnetic helicity in classical ideal magnetohydrodynamics (MHD). At high energies above approximately $10$ MeV, however, classical MHD is necessarily extended to chiral MHD and then the conserved quantity is $\langle\mathcal{H}\rangle + 2 \langleμ_5\rangle / λ$ with $\langle\mathcal{H}\rangle$ being the mean magnetic helicity and $\langleμ_5\rangle$ being the mean chiral chemical potential of charged fermions. Here, $λ$ is a (phenomenological) chiral feedback parameter. In this paper, we study the evolution of the chiral MHD system with the initial condition of nonzero $\langle\mathcal{H}\rangle$ and vanishing $μ_5$. We present analytic derivations for the time evolution of $\langle\mathcal{H}\rangle$ and $\langleμ_5\rangle$ that we compare to a series of laminar and turbulent three-dimensional direct numerical simulations. We find that the late-time evolution of $\langle\mathcal{H}\rangle$ depends on the magnetic and kinetic Reynolds numbers ${\rm Re}_{_\mathrm{M}}$ and ${\rm Re}_{_\mathrm{K}}$. For a high ${\rm Re}_{_\mathrm{M}}$ and ${\rm Re}_{_\mathrm{K}}$ where turbulence occurs, $\langle\mathcal{H}\rangle$ eventually evolves in the same way as in classical ideal MHD where the inverse correlation length of the helical magnetic field scales with time $t$ as $k_\mathrm{p} \propto t^{-2/3}$. For a low Reynolds numbers where the velocity field is negligible, the scaling is changed to $k_\mathrm{p} \propto t^{-1/2}\mathrm{ln}\left(t/t_\mathrm{log}\right)$. After being rapidly generated, $\langleμ_5\rangle$ always decays together with $k_\mathrm{p}$, i.e. $\langleμ_5\rangle \approx k_\mathrm{p}$, with a time evolution that depends on whether the system is in the limit of low or high Reynolds numbers.

physics.plasm-ph↗