Welcome to the 10th International LISA Symposium

The New Worlds, New Horizons decadal survey stated: "It would be unprecedented in the history of astronomy if the gravitational radiation window being opened up by LISA does not reveal new, enigmatic sources." The opening of the gravitational wave window will allow us to explore new physical realms that may reveal exotic physics, such as departures from General Relativity, and phase transitions in the early Universe. I will survey some of the possibilities and discuss how we may uncover exotic phenomena in a detector time series.

Gravitational waves constitute a unique probe of the universe at high redshift. They can provide information about the status of the universe at very high energy, and also about the geometry and the content of the universe at present time. To demonstrate these facts, in this seminar I will first review some of the main cosmological sources of a stochastic background of gravitational waves, and discuss detection prospects in general. I will then concentrate on a given example, namely a primordial first order phase transition. It will be shown how the main features of the gravitational wave spectrum can be predicted by simple, general arguments based on the source properties, such as its time evolution and space structure. Detection prospects will be discussed in particular for the electroweak phase transition, in connection with the eLISA mission.

Compact-binary inspirals and mergers are an ideal astrophysical laboratory for strong-field gravitational physics. A space-based observatory like (e)LISA will observe massive black hole mergers with large signal-to-noise ratio out to large redshifts, and it may provide precise observations of the inspiral dynamics of extreme mass-ratio systems. I will discuss how these observations can be used to test general relativity, and how (if we are lucky) they may even provide hints of the necessity to extend our understanding of gravity beyond Einstein's theory.