We will review both theoretical and obsevational aspects of gravitational wave backgrounds of cosmological origin. We will present a classifcation of backgrounds and a quantification of our ability to use them as a probe of early Universe phenomena, opening a new observational window to energy scales well above the reach of any terrestrial means

Simons Observatory (SO) is a new Cosmic Microwave Background telescope currently under construction in the Atacama Desert, close to ALMA and other recent CMB telescopes. It will have six small aperture (42cm) telescopes (SATs), and one large aperture (6m) telescope (LAT), observing at 30-280GHz (1-10mm) using transition edge sensors (TES) and

After introducing the concepts of axions and axion-like-particles (ALPs) and their signals, true axions which can be either much heavier or muchlighter than the canonical axion will be discussed. The implications for dark matter, neutron stars and gravitational waves searches will also be addressed.