Abstract The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides a good
description of a wide range of astrophysical and cosmological data. However, there are a …

We review numerous arguments for primordial black holes (PBHs) based on observational
evidence from a variety of lensing, dynamical, accretion and gravitational-wave effects. This …

We report multiple lines of evidence for a stochastic signal that is correlated among 67
pulsars from the 15 yr pulsar timing data set collected by the North American Nanohertz …

Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A
background of GWs modulates pulsar arrival times and manifests as a stochastic process …

We present the results of the search for an isotropic stochastic gravitational wave
background (GWB) at nanohertz frequencies using the second data release of the European …

Observing and timing a group of millisecond pulsars with high rotational stability enables the
direct detection of gravitational waves (GWs). The GW signals can be identified from the …

The NANOGrav 15 yr data set shows evidence for the presence of a low-frequency
gravitational-wave background (GWB). While many physical processes can source such low …

Pulsar timing arrays offer a probe of the low-frequency gravitational wave spectrum (1–100
nHz), which is intimately connected to a number of markers that can uniquely trace the …

We study whether the signal seen by pulsar timing arrays (PTAs) may originate from
gravitational waves (GWs) induced by large primordial perturbations. Such perturbations …

Various pulsar timing array (PTA) experiments (NANOGrav, EPTA, PPTA, CPTA, including
data from InPTA) very recently reported evidence for excess red common-spectrum signals …