We provide a review on the state-of-the-art of gravitational waves induced by primordial
fluctuations, so-called induced gravitational waves. We present the intuitive physics behind …

It has been a half-decade since the first direct detection of gravitational waves, which
signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave …

Abstract Recently, the NANOGrav, PPTA, EPTA, and CPTA Collaborations independently
reported their evidence of the Stochastic Gravitational Waves Background (SGWB). While …

Multiple pulsar timing array (PTA) collaborations recently announced the evidence of
common-spectral processes caused by gravitational waves (GWs). These can be the …

The stochastic signal detected by pulsar timing arrays (PTAs) has raised great interest in
understanding its physical origin. Assuming the signal is a cosmological gravitational-wave …

Inflationary models predicting a scale-dependent large amplification of the density
perturbations have recently attracted a lot of attention because the amplified perturbations …

We identify a characteristic pattern in the scalar-induced stochastic gravitational wave
background from particle production during inflation. If particle production is sufficiently …

Primordial black holes (PBHs) and the violation of the null energy condition (NEC) have
significant implications for our understanding of the very early Universe. We present a novel …

Scalar metric fluctuations generically source a spectrum of gravitational waves at second
order in perturbation theory, poising gravitational wave experiments as potentially powerful …

Gravitational waves (GWs) in the MHz–GHz frequency range are motivated by a host of early
Universe phenomena such as oscillons, preheating, and cosmic strings. We point out that …