High energy physics and the very-early Universe with LISA

Gravitational wave experiments will play a key role in the investigation of the frontiers of cosmology and the structure of fundamental fields at high energies, by detecting, or setting strong upper-limits to, the primordial gravitational wave background produced in the early-Universe. Here we discuss the impact of space-borne laser interferometric detectors operating in the low-frequency window ($\sim 1 \mu$Hz - 1 Hz); the aim of our analysis is to assess the detectability of a primordial background characterized by a fractional energy density $\Omega \sim 10^{-16} - 10^{-15}$, which is consistent with the prediction of "slow-roll" inflationary models.