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 (∼ 1μHz 1 Hz); the aim of our analysis is to assess the detectability of a primordial background characterized by a fractional energy density h100 Ω ∼ 10−16 − 10−15, which is consistent with the prediction of ”slow-roll” inflationary models. We analyze the capabilities of the planned single-instrument LISA mission (Laser Interferometer Space Antenna), and the sensitivity improvements that could be achieved by cross-correlating the data streams from a pair of detectors of the LISA-class. We show that the two-detectors configuration is extremely powerful, and leads to the detection of a stochastic background as weak as h100 Ω ∼ 10−14. However, such instrumental sensitivity can not be exploited to achieve a comparable performance for the detection of the primordial component of the background, due to the overwhelming power of the stochastic signal produced by short-period solar-mass binary systems of compact objects, that can not be resolved as individual sources. We estimate that the primordial background can be detected only if its fractional energy density h100 Ω is greater than a few times 10 −12, which is about two orders-ofmagnitude better than what is envisaged for ground-based interferometers of the ”advanced” generation, but still too far from the ambitious goal that we have set. On the other hand, cross-correlation experiments allow us to detect the astrophysically generated background at a signal-to-noise ratio ∼ 100. Electronic address: [email protected] Electronic address: [email protected]