Details of the Researcher

PHOTO

Fumihiro Naokawa
Section
Graduate School of Science
Job title

Education 4

  • The University of Tokyo　Graduate School of Science

    2023/04 - 2026/03

  • The University of Tokyo　Graduate School of Science　Department of Physics

    2021/04 - 2023/03

  • The University of Tokyo　School of Science　Department of Physics

    2019/04 - 2021/03

  • The University of Tokyo

    2017/04 - 2019/03

Professional Memberships 2

  • Astronomical Society of Japan

    2022/12 - Present

  • The Physical Society of Japan

    2022/12 - Present

Research Interests 6

  • 時間軸天文学

  • 電波銀河

  • 宇宙論パラメータ

  • アクシオン

  • 宇宙マイクロ波背景放射

  • 宇宙論

Awards 5

  1. 第38回 理論懇シンポジウム 若手発表賞

    2025/12　理論天文学 宇宙物理学懇談会 運営委員会

  2. Final Examination 優秀賞

    2025/09　変革を駆動する先端物理・数学プログラム (FoPM)

  3. 日本物理学会 学生優秀発表賞 宇宙線宇宙物理

    2023/03　日本物理学会

  4. 研究奨励賞

    2023/03　東京大学理学系研究科

  5. オーラルアワード 素粒子・重力・宇宙論分科会 第1位

    2022/08　第52回 天文・天体物理若手夏の学校

Papers 7

  1. Universal Profile for Cosmic Birefringence Tomography with Radio Galaxies Peer-reviewed

    Fumihiro Naokawa

    Physical Review Letters　2026/01/27

    DOI： 10.1103/srfg-9fdy 　

  2. nπ Phase Ambiguity of Cosmic Birefringence Peer-reviewed

    Fumihiro Naokawa, Toshiya Namikawa, Kai Murai, Ippei Obata, Kohei Kamada

    Physical Review Letters　2026/01/27

    DOI： 10.1103/6z1m-r1j5 　

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    We point out that the rotation angle $\beta$ of cosmic birefringence, which is a recently reported parity-violating signal in the cosmic microwave background (CMB), has a phase ambiguity of $n\pi \,(n\in\mathbb{Z})$. This ambiguity has a significant impact on the interpretation of the origin of cosmic birefringence. Assuming an axion-like particle as the origin of cosmic birefringence, this ambiguity can be partly broken by the anisotropic cosmic birefringence and the shape of the CMB angular power spectra. We also discuss constraints on $\beta$ from existing experimental results.

  3. Planck constraints on axionlike particles through isotropic cosmic birefringence Peer-reviewed

    Toshiya Namikawa, Kai Murai, Fumihiro Naokawa

    Physical Review D　2025/12/05

    DOI： 10.1103/kc89-xjkb 　

  4. Gravitational lensing effect on cosmic birefringence Peer-reviewed

    Fumihiro Naokawa, Toshiya Namikawa

    Physical Review D　2023/05/23

    DOI： 10.1103/PhysRevD.108.063525 　

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    We calculate the effect of gravitational lensing on the parity-odd power spectrum of the cosmic microwave background (CMB) polarization induced by axionlike particles (ALPs). Several recent works have reported a tantalizing hint of cosmic birefringence, a rotation of the linear polarization plane of CMB, which ALPs can explain. In future CMB observations, we can measure cosmic birefringence more precisely to get insight into ALPs. We find that the lensing effect is necessary to fit the observed EB power spectrum induced by cosmic birefringence in future CMB observations, including Simons Observatory and CMB-S4. We also show that the estimated ALPs parameters are biased if we ignore the lensing effect. Therefore, the lensing correction to the parity-odd power spectra must be included in future high-resolution CMB experiments.

  5. Constraint on Early Dark Energy from Isotropic Cosmic Birefringence Peer-reviewed

    Johannes R. Eskilt, Laura Herold, Eiichiro Komatsu, Kai Murai, Toshiya Namikawa, Fumihiro Naokawa

    Physical Review Letters　2023/03/27

    DOI： 10.1103/PhysRevLett.131.121001 　

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    Polarization of the cosmic microwave background (CMB) is sensitive to new physics violating parity symmetry, such as the presence of a pseudoscalar "axionlike" field. Such a field may be responsible for early dark energy (EDE), which is active prior to recombination and provides a solution to the so-called Hubble tension. The EDE field coupled to photons in a parity-violating manner would rotate the plane of linear polarization of the CMB and produce a cross-correlation power spectrum of $E$- and $B$-mode polarization fields with opposite parities. In this paper, we fit the $EB$ power spectrum predicted by the photon-axion coupling of the EDE model with a potential $V(\phi)\propto [1-\cos(\phi/f)]^3$ to polarization data from Planck. We find that the unique shape of the predicted $EB$ power spectrum is not favored by the data and obtain a first constraint on the photon-axion coupling constant, $g=(0.04\pm 0.16)M_{\text{Pl } }^{-1}$ (68% CL), for the EDE model that best fits the CMB and galaxy clustering data. This constraint is independent of the miscalibration of polarization angles of the instrument or the polarized Galactic foreground emission. Our limit on $g$ may have important implications for embedding EDE in fundamental physics, such as string theory.

  6. Isotropic cosmic birefringence from early dark energy Peer-reviewed

    Kai Murai, Fumihiro Naokawa, Toshiya Namikawa, Eiichiro Komatsu

    Physical Review D　2022/09/16

    DOI： 10.1103/PhysRevD.107.L041302 　

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    A tantalizing hint of isotropic cosmic birefringence has been found in the $E B$ cross-power spectrum of the cosmic microwave background (CMB) polarization data with a statistical significance of $3\sigma$. A pseudoscalar field coupled to the CMB photons via the Chern-Simons term can explain this observation. The same field may also be responsible for early dark energy (EDE), which alleviates the so-called Hubble tension. Since the EDE field evolves significantly during the recombination epoch, the conventional formula that relates $E B$ to the difference between the $E$- and $B$-mode auto-power spectra is no longer valid. Solving the Boltzmann equation for polarized photons and the dynamics of the EDE field consistently, we find that currently favored parameter space of the EDE model yields a variety of shapes of the $EB$ spectrum, which can be tested by CMB experiments.

  7. Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen Peer-reviewed

    Masataka Aizawa, Kojiro Kawana, Kazumi Kashiyama, Ryou Ohsawa, Hajime Kawahara, Fumihiro Naokawa, Tomoyuki Tajiri, Noriaki Arima, Hanchun Jiang, Tilman Hartwig, Kotaro Fujisawa, Toshikazu Shigeyama, Ko Arimatsu, Mamoru Doi, Toshihiro Kasuga, Naoto Kobayashi, Sohei Kondo, Yuki Mori, Shin-ichiro Okumura, Satoshi Takita, Shigeyuki Sako

    Publications of the Astronomical Society of Japan　74　(5)　1069-1094　2022/08/08

    DOI： 10.1093/pasj/psac056 　

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    We report a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3-M5 dwarfs with rise times and amplitudes ranging from $5\, \mathrm{sec} \lesssim t_\mathrm{rise} \lesssim 100\,\mathrm{sec}$ and $0.5 \lesssim \Delta F/F_{\star} \lesssim 20$, respectively. The flare light curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with temperatures of $9,000-15,000\,\mathrm{K}$, the peak luminosities and bolometric energies are estimated to be $10^{29}\,\mathrm{erg\,sec^{-1 } } \lesssim L_\mathrm{peak} \lesssim 10^{31}\,\mathrm{erg\,sec^{-1 } }$ and $10^{31}\,\mathrm{erg} \lesssim E_{\rm bol} \lesssim 10^{34}\,\mathrm{erg}$, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than 90\% of the host stars of the detected flares are magnetically active based on their H$\alpha$ emission line intensities obtained by LAMOST. The estimated occurrence rate of the detected flares is $\sim 0.7$ per day per an active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light curves can be explained by the chromospheric compression model; the rise time is broadly consistent with the Alfv\'en transit time of a magnetic loop with a length scale of $l_\mathrm{loop} \sim 10^4\,\mathrm{km}$ and a field strength of $1,000\,\mathrm{G}$, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down to near the photosphere with a temperature of $\gtrsim 10,000\,\mathrm{K}$. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.

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Research Projects 1

  1. 宇宙背景放射の旋光による暗黒成分探査

    直川 史寛

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 特別研究員奨励費

    Institution: 東京大学

    2024/04/23 - 2026/03/31