Tohoku University · Physics and Astronomy
Professor Kai Murai's research lab focuses on theoretical particle physics and early Universe cosmology, with a central theme of exploring fundamental physics beyond the Standard Model through cosmological observations. The lab investigates phenomena such as cosmic birefringence, primordial gravitational waves, axion and Q-ball dynamics, and lepton asymmetry generation, often linking these to current and future observational probes like pulsar timing arrays and CMB polarization. Key research directions include the interplay between dark energy, early universe phase transitions, and stochastic gravitational wave backgrounds, as well as mechanisms for generating matter-antimatter asymmetries via non-thermal fields and solitonic objects. The lab emphasizes model-building that connects high-energy physics with observable cosmological signals.
Figures are computed from collected data and may differ slightly.
A tantalizing hint of isotropic cosmic birefringence has been found in the $EB$ cross-power spectrum of the cosmic microwave background (CMB) polarization data with a statistical significance of $3\ensuremath{\sigma}$. 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 recombina
Abstract The recent observation of 4 He implies that our universe has a large lepton asymmetry. We consider the Affleck-Dine (AD) mechanism for lepton number generation. In the AD mechanism, non-topological solitons called L-balls are produced, and the generated lepton number is confined in them. The L-balls protect the generated lepton number from being converted to baryon number through the sphaleron processes. We study the formation and evolution of the L-balls and find that the universe with
A bstract A new era of exploring the early Universe may have begun with the recent strong evidence for the stochastic gravitational wave (GW) background from the data reported by NANOGrav, EPTA (including InPTA data), PPTA, and CPTA. Inspired by this, we propose a new potential source of stochastic GWs in the minimal supersymmetric standard model (MSSM), which could be the theory at a very high energy scale. This source is the “axion” field in the Higgs multiplets when the Higgs field takes a la
Abstract The existence of a stochastic gravitational wave background is indicated by the recent pulsar timing array (PTA) experiments. We study the enhanced production of second-order gravitational waves from the scalar perturbations when the universe experiences a transition from the early matter-dominated era to the radiation-dominated era due to Q-ball decay. We extend the analysis in previous work by including the frequency range where density perturbations go non-linear and find that the re
Abstract The recent observation of 4 He favors a large lepton asymmetry at the big bang nucleosynthesis. If Q-balls with a lepton charge decay after the electroweak phase transition, such a large lepton asymmetry can be generated without producing too large baryon asymmetry. In this scenario, Q-balls dominate the universe before the decay and induces the sharp transition from the early matter-dominated era to the radiation-dominated era. In this transition, the gravitational waves (GWs) are enha
In an axiverse with numerous axions, the cosmological moduli problem poses a significant challenge because the abundance of axions can easily exceed that of dark matter. The well-established stochastic axion scenario offers a simple solution, relying on relatively low-scale inflation. However, axions are typically subject to mixing due to mass and kinetic terms, which can influence the solution using stochastic dynamics. Focusing on the fact that the QCD axion has a temperature-dependent mass, u
Abstract 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,
A bstract We investigate the level-crossing phenomenon in two-axion systems, where the mass eigenvalues intersect as the mass of one axion increases with the cooling of the universe. This phenomenon can significantly alter the abundance of axions in the early universe. Our study focuses on its impact on the QCD axion and an axion-like particle, identifying viable regions of axion mass and decay constant that explain the observed dark matter. We demonstrate the equivalence of two different bases
Abstract We present a new mechanism to generate a coherently oscillating dark vector field from axion-SU(2) gauge field dynamics during inflation. The SU(2) gauge field acquires a nonzero background sourced by an axion during inflation, and it acquires a mass through spontaneous symmetry breaking after inflation. We find that the coherent oscillation of the dark vector field can account for dark matter in the mass range of 10 -13 – 1 eV in a minimal setup. In a more involved scenario, the range
A bstract We propose a novel mechanism of primordial black hole (PBH) formation through inverted bubble collapse. In this scenario, bubbles nucleate sparsely in an incomplete first-order phase transition, such that they remain isolated and do not percolate or collide with each other due to the extremely low nucleation rate. This is followed by a bulk phase transition in the rest of the universe that inverts these pre-existing bubbles into false vacuum regions. These spherically symmetric false-v
A bstract We revisit the minimal Nelson-Barr model for solving the strong CP problem through the idea of spontaneous CP breaking. The minimal model suffers from the quality problem, which means that the strong CP angle is generated by higher-dimensional operators and one-loop effects. Consequently, it has been considered that there is a cosmological domain wall problem and that leptogenesis does not work. We point out that just imposing an additional approximate global symmetry solves the qualit
We propose a new mechanism for isotropic cosmic birefringence with an axionlike field that rapidly oscillates during the recombination epoch. In conventional models, the field oscillation during the recombination epoch leads to a cancellation of the birefringence effect and significantly suppresses the EB spectrum of the cosmic microwave background (CMB) polarization. By introducing an asymmetric potential to the axion, this cancellation becomes incomplete, and a substantial EB spectrum can be p
We study a dark energy model composed of a bare negative cosmological constant and a single ultralight axion, motivated by the string axiverse. Assuming that intelligent observers arise and observe, as in our Universe, the onset of dark-energy-driven acceleration following matter domination, and that this acceleration persists to the present, we derive nontrivial constraints on both the axion mass and the bare cosmological constant. The axion mass is bounded from above to avoid fine-tuning of th
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 $3σ$. 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 c
We show that the baryon asymmetry of the Universe cannot be explained by a large initial value before inflation because it inevitably predicts correlated baryon isocurvature perturbations that are already excluded by cosmic microwave background observations. Similar arguments can generally be applied to some models of dark matter.
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