The exploration of the universe has recently entered a new era thanks to the multi-
messenger paradigm, characterized by a continuous increase in the quantity and quality of …

Abstract The Laser Interferometer Space Antenna (LISA) has two scientific objectives of
cosmological focus: to probe the expansion rate of the universe, and to understand …

The 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for
Gravitational Waves (NANOGrav) shows positive evidence for the presence of a low …

Abstract The Einstein Telescope (ET), the European project for a third-generation
gravitational-wave detector, has a reference configuration based on a triangular shape …

A number of challenges to the standard Λ CDM model have been emerging during the past
few years as the accuracy of cosmological observations improves. In this review we discuss …

Gravitational waves (GWs) were recently detected for the first time. This revolutionary
discovery opens a new way of learning about particle physics through GWs from first-order …

Pulsar timing arrays (PTAs) have reported evidence for a stochastic gravitational wave (GW)
background at nanohertz frequencies, possibly originating in the early Universe. We show …

We discuss the interpretation of the detected signal by pulsar timing array (PTA)
observations as a gravitational wave background of cosmological origin. We combine …

This Horizon Study describes a next-generation ground-based gravitational-wave
observatory: Cosmic Explorer. With ten times the sensitivity of Advanced LIGO, Cosmic …

We perform a Bayesian analysis of NANOGrav 15-yr and IPTA DR2 pulsar timing residuals
and show that the recently detected stochastic gravitational-wave background is compatible …