The most conservative interpretation of the nHz gravitational waves discovered by NANOGrav and other Pulsar Timing Array (PTA) Collaborations is astrophysical, namely that they arise from supermassive black hole (SMBH) binaries. However, alternative cosmological models have been proposed, including cosmic strings, phase transitions, domain walls, primordial fluctuations and "audible" axions....
We are currently witnessing the dawn of a new era in astrophysics and cosmology, started by the first LIGO/Virgo/KAGRA observations of Gravitational Waves (GW). Very recently, also the detection of a stochastic background of GWs at very low frequencies was announced by the Pulsar Timing Array collaborations. In this talk, I will discuss how such signals are produced in cosmological phase...
We discuss physics of a model for electroweak baryogenesis, which satisfies theoretical and current experimental bounds. We then discuss how the model can be tested at future experiments at space-based gravitational wave interferometers, future colliders and also observation of primordial black holes.
In this work, we investigate the relation between higher-dimensional gauge theories and stochastic gravitational wave (GW) spectrums caused by their potential. It is known that the higher-dimensional gauge theories can induce the spontaneous symmetry breaking of the gauge symmetry. If the spontaneous symmetry breaking induces the first-order phase transition, the stochastic GW can be observed...
I describe difficulty of primordial black hole formation in single field inflation and its relevance to stochastic gravitational wave observation.
We present a new way to study cosmic inflation with gravitational waves. The gravitational signal is generated thanks to nonlinear structures in the inflaton field, called oscillons. This novel probe allows us to test models of inflation which are challenging to test with CMB experiments.
I shall discuss gravitational wave signatures coming from a single-field inflation model in which the inflaton couples to the Gauss-Bonnet term. When the scalar potential term and the Gauss-Bonnet coupling term have different signs, a nontrivial fixed point arises, effectively inducing ultra slow-roll inflation. In this case, primordial black holes can form, together with enhanced...
Gravitational Waves are an excellent way to probe the evolution of the universe way back before BBN. In particular, they offer a unique way to test modified Einstein equations. At the level of the equation of motion, the simplest example of Horndeski’s theory containing higher-curvature terms is the dilaton-Einstein-Gauss-Bonnet (dEGB) the- ory, obtained by adding a specific quadratic...
Observational constraints have closed off all but one mass-window for primordial black holes making up all of the dark matter, and there are some specific conditions required for their production in the first place. However, they remain a tantalising dark matter candidate because they require no new beyond the standard model particles and they would additionally provide a lot of information...
After briefly reviewing the basics of primordial black holes, I would like to talk recent interesting works on this topic, mainly focused on those of our own.
In this article we investigate the cumulative stochastic gravitational wave spectra as a tool to gain insight on the creation mechanism of primordial black holes. We consider gravitational waves from the production mechanism of primordial black holes and from the gravitational interactions of those primordial black holes among themselves and other astrophysical black holes. We specifically...
Gravitational-wave observations have delivered a lot of information about black holes and neutron stars. Although most features are consistent with our current understanding about standard physics, severe astronomical uncertainties leave the room of possible deviations. Future detectors with high sensitivity will also allow us to investigate more extreme conditions, e.g., ultrahigh density. In...
Gravitational waves are a new observational probe that can enable data-driven tests of the fundamental laws of physics that govern the Universe. I will discuss how this avenue can explore new frontiers that can shape our understanding of astrophysics and cosmology. I will show some latest findings using the current gravitational wave data and discuss novel techniques for discovering new...
It is known that black holes have unsolved problems such as the singularity problem and the information loss problem. To solve these problems, as one of the black hole mimicker, gravastar is proposed. Gravastar is filled by cosmological constant, and therefore metric is de-Sitter inside the star. In usual, gravastar solutions are constructed by using Israel's junction condition. By the way, in...
I will review constraints on fundamental physics that have been inferred from observations of gravitational waves, and look forward to see what new results might be obtained in the near future, and what might be possible with future generations of gravitational-wave detectors.
KAGRA is a ground-based interferometric gravitational-wave detector with kilometer-scale arms as well as LIGO and Virgo. There are two unique features in KAGRA: one is constructed at the underground site and the other is using cryogenic sapphire mirrors for the main mirrors. Underground site provides a quite environment and is effective to reduce the seismic noise. Utilizing cryogenic sapphire...
Gravitational waves from compact binary coalescences provide a unique laboratory to test properties of compact objects. As alternatives to the ordinary black holes in general relativity, various exotic compact objects have been proposed. Some of them have largely different values of the tidal deformability and spin-induced quadrupole moment from those of black holes, and their binaries could...
The DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is a future Japanese space gravitational-wave antenna. There are many science targets that DECIGO aims to achieve, including the detection of primordial gravitational waves, direct measurement of the acceleration of the Universe, the revelation of the formation of massive black holes, and many others. DECIGO consists of four...
Newtonian noise (or gravity gradient noise) is one of the principal noises for ground-based GW detectors, especially below 10 Hz. In this study, we estimate the NN caused by the acoustic field inside an underground facility for the KAGRA and further detectors based on the infrasound sensors' data at the KAGRA site.
The induced gravitational wave signal can be affected by the modification of the sound speed $c^2_s$ and the equation of state parameter $w$ at horizon reentry. Inspired by the occurrence of phase transitions in various theories beyond the Standard Model (SM), we conducted numerical simulations to assess the induced gravitational waves generated by a hypothetical smooth transition beyond the...
In 2015, gravitational wave was first detected and it was from binary black hole. After this event, many gravitational wave events from binary black hole were detected. However, including first detected gravitational event, origin of binary black hole observed by gravitational wave is not solved. To solve this, first we made mass distribution of 1year gravitational wave observation and mass...
Circular polarized cosmological gravitational wave background may provide evidence of possible parity violation predicted by the e.g., Chern-Simon theory. However, Circular polarized astrophysical gravitational wave background may also exist with detectable amplitudes, if their source distribution is anisotropic. Therefore, we need to develop a method for all-sky search for possible circular...
Pulsar timing array is an experiment to detect gravitational waves with a frequency of nanohertz by accurate long-term observation of pulsars. The pulsar timing array has various systematic errors such as dispersion delays caused by interstellar plasma and uncertainty in the position and motion of solar system objects, which inhibit detection of gravitational wave signals. Recently, there have...
We explore the possibility that the new results from the pulsar timing array (PTA) observations could come from the secondary gravitational wave sourced by curvature perturbations generated by a first-order phase transition during the inflation. Based on the results of a field-theoretic lattice simulation of the phase transition process, we show that the gravitational wave signal generated...
Pulsar timing arrays (PTAs) provide a way to detect gravitational waves (GWs) at nanohertz frequencies. To ensure the detection of GWs, observational data must exhibit the Hellings-Downs angular correlation. It is also known that PTAs can probe ultralight dark matter. In this talk, we consider possible contamination of the Hellings-Downs angular correlation by the ultralight dark matter. We...
The recent pulsar timing array (PTA) data show evidence of the stochastic gravitational wave (GW) background around the nanohertz frequency range. With the power-law fit, the data, particularly those of the NANOGrav, favor the spectral index of the cosmological strength of the GWs, Omega_GW(f), around 2. We discuss explanations of the PTA data by GWs induced by curvature perturbations. In...
In this talk, I will describe a quantum field theoretic phenomenon called the gauge ``slingshot" effect. The effect occurs when a source, such as a magnetic monopole or a quark, crosses the boundary between the Coulomb and confining phases. The corresponding gauge field of the source, either electric or magnetic, gets confined into a flux tube stretching in the form of a string (cosmic or a...
In this study, we propose a novel method for distinguishing spin of ultralight dark matter (ULDM) through the ULDM search with interferometric gravitational wave detectors. ULDM, presenting potential spin states of 0, 1, and 2, induces distinctive signatures in GW signals. We found that the finite-time light-traveling effect dominates for spin-0 and spin-1 ULDM, resulting in unique overlap...
The string theory suggests the possible existence of scalar fields with tiny masses called string axions. Such a scalar field grows around a rotating astrophysical black holes by extracting rotation energy of that black hole due to the mechanism called the superradiant instability and forms an axion cloud. The self-interaction of the scalar field becomes important at the final stage of that...
Cosmic strings, one-dimensional topological defects that may have formed in the early universe, predict a wide variety of gravitational wave features. I will provide an overview of the gravitational wave signatures associated with cosmic strings.
I will explain the formation of cosmic superstrings following the confinement phase transition in pure Yang-Mills theory, without invoking string theory or extra dimensions, and their significance in the production of GWs. Moreover, in pure SO(2N) gauge theory, a "baryonic glueball" is predicted as a potential candidate for dark matter. This model offers a way to potentially explain both the...
Recent gravitational wave observations suggest the existence of cosmic strings except stable local strings. Embedded string are candidates of such the string, and the Z string is one of them. We have generalized the Z string for the case of $SU(N)\times U(1)$ gauge theory and found that classical stability of the string only depends on two mass ratios of Higgs and the gauge bosons. We show the...
First order phase transitions are well-motivated and extensively studied sources of gravitational waves (GWs) from the early Universe. The vacuum energy released during such transitions is assumed to be transferred primarily either to the expanding walls of bubbles of true vacuum, whose collisions source GWs, or to the surrounding plasma, producing sound waves and turbulence, which act as GW...
