arXiv subjects

Misao Sasaki

Publications and source records attributed to Misao Sasaki.

At least 19 recordsLinked to original sources

arXiv2026

A Unified Fermionic Origin for Density-Driven Waterfall Inflation and its Multi-Messenger Signatures

Inflation is conventionally described by fundamental scalar fields whose microscopic origin remains unknown. We propose a different mechanism in which inflation emerges from fermion condensates generated by torsion-induced four-fermion interactions in Einstein-Cartan-Holst gravity. Starting from a purely fermionic theory, we derive a two-condensate effective description whose dynamics resembles hybrid inflation but exhibits a new exit mechanism: a density-driven waterfall transition. During inflation, axial charge production continuously increases the fermion density and associated chiral chemical potential. Solving the finite-density gap equation, we show that above a critical axial density the condensate no longer admits a symmetry-broken solution. Inflation therefore ends through evaporation of the condensate itself rather than the inflaton crossing a critical field value. This mechanism naturally triggers rapid energy transfer to fermionic degrees of freedom, providing a microscopic realization of instant preheating. The post-transition condensate is unstable to fragmentation into charged non-topological solitons. These objects can subsequently collapse into primordial black holes, generating a calculable relic population whose properties follow from the same condensate dynamics that drives inflation. Because the underlying theory is intrinsically chiral and connected to dynamical Chern-Simons gravity, the scenario also predicts parity-violating signatures. The framework unifies inflation, reheating, soliton formation, primordial-black-hole production, and parity-violating phenomenology within a single gravitationally induced fermion-condensation mechanism. It provides a composite alternative to scalar-field inflation and identifies correlated observational signatures that can test the role of spacetime torsion in the early Universe.

hep-th↗

arXiv2026

Bumpy inflation from explosive particle production

We present a first-principles derivation of an effective inflaton potential with a bumpy feature. We consider the coupling of a spectator field χto the inflaton ϕsuch that χis massive enough for almost all values of ϕ, but becomes massless when the inflaton crossed the value ϕ_\star. This gives rise to a burst of χ-particle production at ϕ=ϕ_\star. By taking the backreaction of the particle production, we obtain a bumpy feature in the effective potential. We discover that for certain choices of the interaction constants and field scale relevant for particle production, the backreaction-induced bumpy effective potential can lead to a sizable enhancement in the curvature perturbation. In particular, the enhancement can be large enough to generate observationally relevant amounts of primordial black holes and stochastic gravitational wave backgrounds.

astro-ph.CO↗

arXiv2026

Oscillations and parity violation in gravitational wave background from extra tensor modes

Spectator fields which provide additional tensor degrees of freedom, on top of the standard metric tensor perturbations, can produce significant amounts of gravitational waves (GWs). Employing the effective field theory approach for spin-2 fields, we identify a characteristic prediction of this class of scenarios: whenever the spin-2 sector undergoes a localized non-adiabatic evolution during inflation, the linear mixing between the metric and extra tensor modes transmits this feature to the GW spectrum as oscillations in scale. The leading oscillation period is determined analytically by the time at which the localized feature occurs and by the propagation speed of the extra tensor mode outside the feature. The same coupling, when sufficiently strong, also drives an in-time oscillation of the superhorizon modes that is reminiscent of neutrino flavor oscillations. Moreover, parity-violating operators in the spin-2 EFT can imprint chiral signatures on the resulting GW background. Such chirality is an important possible signature for features in the non-minimal kinetic coupling, and becomes closely tied to large observable enhancement for features in the sound speed of the extra tensor mode. We consider a concrete scenario in which the spin-2 field generates observable chiral GWs with characteristic oscillatory patterns. These results identify robust signatures that can be probed with future GW detectors, while the exact amplitude, peak position, and parameter values remain model dependent.

astro-ph.CO↗

arXiv2026

Quantum Matter Makes Lightcones Quantum

In gravitational physics, matter does not merely move within spacetime; it also determines the light cones that define causal relations. What happens when the matter that determines these light cones is itself in a quantum state? We address this question in a controlled low-energy setting: a massless scalar field propagating in the spacetime with the Newtonian gravitational potential sourced by a non-relativistic quantum particle. We show that the light cones are affected by an operator-valued Shapiro delay, with the three consequences: (i) causal-boundary shifts are promoted to noncommuting observables, giving the causal structure an irreducible quantum uncertainty; (ii) the causal relation between two fixed spacetime points can become a superposition of timelike and spacelike configurations; and (iii) tracing out the source smears the Wightman light-cone singularity, producing an effective UV cutoff. Thus, quantum matter does not merely fluctuate within spacetime; it makes the causal structure itself quantum, even without including propagating graviton modes.

gr-qc↗

arXiv2026

$δn$ formalism: A new formulation for the probability density of the curvature perturbation

$δN$ formalism is a useful method to calculate the curvature perturbation. Contrary to what it is typically done in the literature, we re-formulate the $δN$ formalism by using the $e$-folding number $n$ counted forward in time. For a fixed initial time $\bar{n}_0$, the probability density function (PDF) of the field perturbation $δϕ_0$ and its velocity $δπ_0$ are specified by the solutions of the perturbation equation on subhorizon scales. As $δπ_0$ is fully correlated with $δϕ_0$ after horizon exit, we find a novel $δn$ formalism to calculate the curvature perturbation as well as its PDF. It can naturally incorporate those trajectories for which inflation never ends, which are lost when counting $N$ backward in time.

astro-ph.CO↗

arXiv2026

Accelerating Universe from Constraints

We introduce a framework of constrained scalar fields that can give rise to cosmological evolution of the Universe independent of the values of the cosmological constant. Focusing on the simplest realization involving a scalar field with non-minimal coupling, we first study the analytical solutions in the Jordan frame. We show that such solutions include evolution from a radiation dominated-like universe to an exponential expansion, as well as evolution that starts with super-Hubble expansion and then relaxes towards an exponential expansion of the Universe, while describing well-behaved scalar perturbations. We then also relate the theory to the Einstein frame, and analyze the corresponding accelerating solutions. We then consider the model in the presence of external matter. We find that the matter does not affect the evolution of the universe if minimally coupled to the scalar field. In other words, in the Jordan frame, in order to influence the evolution of the space-time, the matter should be non-minimally coupled to the constrained scalar, while it may be minimally coupled to gravity. We show that in this case, the solutions are similar to the free case, and, in addition, allow for the phantom-like equation of state. Finally, we introduce the minimal frame, a frame in which the matter is minimally coupled to both gravity and the constrained scalar, and show that among other possibilities, the phantom-like equation of state can still be realized.

hep-th↗

arXiv2026

Constraining the inflaton potential with gravitational waves from oscillons

Under certain conditions, the oscillating inflaton condensate filling the Universe after inflation can fragment and form so-called oscillons. These long-lived soliton-like field configurations can dominate the Universe for several $e$-folds of expansion, leading to an early matter-dominated phase preceding the standard radiation era. In this paper we show how the rapid final decay of the oscillons leads to an enhanced production of induced gravitational waves, whose energy density can saturate the observational bound on the effective number of relativistic species. We leverage this bound to constrain the inflaton mass, cubic, and quartic self-coupling in generic models that admit oscillon formation, providing novel and complementary constraints in regions of parameter space that are inaccessible with cosmic microwave background observations alone.

astro-ph.CO↗

arXiv2026

Universal Suppression of Gravitational Waves from Black Hole Evaporation Dynamics

Evaporating black holes can leave distinct imprints on gravitational wave (GW) backgrounds. We show that black hole populations with finite width mass distributions exhibit a universal late time evolution governed by the evaporation dynamics rather than the details of the initial mass distribution, leading to a characteristic power law suppression of the induced GWs. We demonstrate this for a broad class of mass functions in primordial black hole (PBH) scenarios featuring an early Universe matter-dominated era, and identify the suppression of PBH-induced GWs found for critical collapse distributions as a manifestation of this general phenomenon. Our results establish a direct connection between the asymptotic GW spectrum and the underlying law of black hole evaporation.

astro-ph.CO↗

arXiv2026

Fundamental Physics and Cosmology with TianQin

The exploration of the surrounding world and the universe is an important theme in the legacy of humankind. The detection of gravitational waves is adding a new dimension to this grand effort. What are the fundamental physical laws governing the dynamics of the universe? What is the fundamental composition of the universe? How has the universe evolved in the past and how will it evolve in the future? These are the basic questions that press for answers. The space-based gravitational wave detector TianQin will tune in to gravitational waves in the millihertz frequency range ($10^{-4} \sim 1$ Hz, to be specific), opening a new gravitational wave spectrum window to explore many of the previously hidden sectors of the universe. TianQin will discover many astrophysical systems, populating the universe at different redshifts: some will be of new types that have never been detected before, some will have very high signal-to-noise ratios, and some will have very high parameter estimation precision. The plethora of information collected will bring us to new fronts on which to search for the breaking points of general relativity, the possible violation of established physical laws, the signature of possible new gravitational physics and new fundamental fields, and to improve our knowledge on the expansion history of the universe. In this white paper, we highlight the advances that TianQin can bring to fundamental physics and cosmology.

gr-qc↗

arXiv2026

Detecting Chiral Gravitational Wave Background with a Dipole Pulsar Timing Array

The pulsar timing array (PTA) is a powerful technique for detecting nanohertz gravitational wave backgrounds (GWBs). However, conventional PTAs lack sensitivity to parity violation in the GWB. In this work, we propose a dipole pulsar timing array system (dPTA). By deriving the overlap reduction functions (ORFs) from the cross-correlation of timing signals, we find that this system exhibits sensitivity to chiral GWBs in the nanohertz regime. Furthermore, through numerical calculations of its sensitivity curves, we demonstrate that the dPTA extends the detectable frequency range of PTAs for GWBs from the nanohertz to the microhertz regime.

gr-qc↗

arXiv2026

Imprints of gravitational-wave polarizations on projected tidal tensor in three dimensions

Gravitational waves (GWs) distort galaxy shapes through the tidal effect, offering a novel avenue to probe the nature of gravity. In this paper, we investigate how extra GW polarizations beyond those predicted by general relativity imprint observable signatures on galaxy shapes. Since galaxy shapes are measured as two-dimensional images projected onto the celestial sphere, we present three-dimensional statistical quantities of the projected tidal tensor sourced by the tensor perturbation. We show that the presence of extra polarization modes modifies both the amplitude and angular dependence of the correlation functions. Furthermore, we identify a distinct observational channel for probing parity violation in helicity-two and helicity-one modes. In particular, we show that if they propagate at different speeds, galaxy surveys can disentangle the source of parity violation. Our findings establish a theoretical framework for using upcoming large-scale galaxy surveys to test modified gravity theories through the polarization content of GWs.

astro-ph.CO↗

arXiv2026

The recipe for the degrees of freedom

We consider the question of counting the degrees of freedom in theoretical models, with an emphasis on theories of fields and gravity. Among the possible approaches, the Hamiltonian formulation remains one of the most systematic and robust tools. However, it can easily become long and technically involved. In this work, we present a broadly applicable recipe to find the degrees of freedom directly, based on the Lagrangian formulation. We compare it to the standard approaches, highlight the challenges that may arise in the latter, and demonstrate that the proposed method leads to transparent insights about the dynamical nature of theory in a quick, simple, and straight-forward way.

hep-th↗

arXiv2025

Unveiling Primordial Black Hole Relics Through Induced Gravitational Waves

Black hole relics are of significant interest in cosmology and theoretical physics. In this work, we consider tiny primordial black holes (PBHs) ( $M_{\text {PBH }} \lesssim 10^7 \mathrm{~g}$ ) which are generated soon after the end of inflation and evaporate and reheat the Universe before big bang nucleosynthesis (BBN), but leave their remnants due to incomplete evaporation. These PBHs remnants may contribute as part or all of the dark matter (DM) today. Assuming that there exist PBH relics, we point out that the number density of PBH today can be directly read from the peak positions of the induced gravitational waves due to the inhomogeneous PBH distribution. If PBH relics are of Planck mass and they forms all the DM today, the PBH number density would be of $10^{-25} \mathrm{~cm}^{-3}$ with the peak frequency 60 Hz . The peak frequency scales as $f_{\text {relic }}^{1 / 3}$ where $f_{\text {relic }}$ is the fraction of the PBH relics in the total DM density. The peak amplitude carries the information of initial PBH abundance. For monochromatic-mass PBH with the current number density $10^{-41} \sim 10^{-25} \mathrm{~cm}^{-3}$ and initial abundance $10^{-13} \sim 10^{-7}$, the amplitude may be large enough to be detected by planned gravitational wave experiments in the near future.

hep-ph↗

arXiv2025

Primordial Black Hole Formation from Power Spectrum with Finite-width

Primordial black holes (PBHs) can form from gravitational collapse of large overdensities in the early Universe, giving rise to rich phenomena in astrophysics and cosmology. We develop a novel, general, and systematic method based on theory of density contrast peaks to calculate the abundance of PBHs for a broad power spectrum of curvature perturbations with Gaussian statistics. We introduce a window function to account for the relevant perturbation scales associated with PBHs of different masses, along with a filter function that removes unphysical contributions from super-horizon-scale overdensities. While some uncertainties remain due to the limited understanding of the nonlinear collapse process, our approach substantially reduces the discrepancy previously observed between peaks theory and the Press-Schechter formalism.

astro-ph.CO↗

arXiv2025

Beyond Coleman's Instantons

In the absence of gravity, Coleman's theorem states that the $O(4)$-symmetric instanton solution, which is regular at the origin and exponentially decays at infinity, gives the lowest action. Perturbatively, this implies that any small deformation from $O(4)$-symmetry gives a larger action. In this letter we investigate the possibility of extending this theorem to the situation where the $O(4)$-symmetric instanton is singular, provided that the action is finite. In particular, we show a general form of the potential around the origin, which realizes a singular instanton with finite action. We then discuss a concrete example in which this situation is realized, and analyze non-trivial anisotropic deformations around the solution perturbatively. Intriguingly, in contrast to the case of Coleman's instantons, we find that there exists a deformed solution that has the same action as the one for the $O(4)$-symmetric solution up to the second order in perturbation. Our result implies that there exist non-$O(4)$-symmetric solutions with finite action beyond Coleman's instantons, and gives rise to the possibility of the existence of a non-$O(4)$-symmetric instanton with a lower action.

hep-th↗

arXiv2025

Dark matter from inflationary quantum fluctuations

We explore a scenario in which dark matter is a massive bosonic field, arising solely from quantum fluctuations generated during inflation. In this framework, dark matter exhibits primordial isocurvature perturbations with an amplitude of ${\cal O}(1)$ at small scales that are beyond the reach of current observations such as those from the CMB and large-scale structure. We derive an exact transfer function for the dark matter field perturbations during the radiation dominated era. Based on this result, we also derive approximate expressions of the transfer function in some limiting cases where we confirm that the exact transfer function reproduces known behaviors. Assuming a monochromatic initial power spectrum, we use the transfer function to identify the viable parameter space defined by the dark matter mass and the length scale of perturbations. A key prediction of this scenario is copious formation of subsolar mass dark matter halos at high redshifts. Observational confirmation of a large population of such low-mass halos will support for the hypothesis that dark matter originated purely from inflationary quantum fluctuations.

astro-ph.CO↗

arXiv2025

When Tiny Halos Stir Spacetime: Gravitational Waves from Fifth-Force Mergers

Dark matter fermions interacting via attractive fifth forces mediated by a light mediator can form dark matter halos in the very early universe. We show that bound systems composed of these halos are capable of generating gravitational wave (GW) signals detectable today, even when the individual halos are very light. The Yukawa force dominates the dynamics of these halo binaries, rather than gravity. As a result, large GW signals can be produced at initially extremely high frequencies, which are then redshifted to frequency bands accessible to current or future GW observatories. In addition, the resulting GW signals carry distinctive features that enable future observations to distinguish them from conventional ones. Notably, even if only a tiny fraction of dark matter experiences strong fifth-force interactions, such effects provide a new avenue to discover self-interacting dark matter through GW observations.

astro-ph.CO↗

arXiv2025

Hybrid Inflation from Fermion Condensate

We investigate how inflation can emerge from four-fermion interactions generated by spacetime torsion, eliminating the need for additional scalar fields beyond the Standard Model. We partition fermions in two sectors and introduce two bound fields. In the effective theory approach, once all the fermions have been integrated out, the bound fields serve as the inflaton and the auxiliary field, in analogy to the hybrid inflation and accounting for a waterfall (hybrid) mechanism. The inclusion of an axial chemical potential naturally facilitates the end of reheating. During the waterfall regime, the effective potential governing the fermion condensate supports the formation of non-topological solitons, known as Q-balls, which can be accounted for seeding primordial black holes (PBHs).

gr-qc↗