Given that all forms of mass and energy couple to gravity, other sources of gravitational waves may exist that are not expected from our current view of the Universe, dominated by electromagnetic observations. LISA is sensitive to signals from the very early Universe (the Terascale frontier), where phase transitions of new forces of nature or extra dimensions of space may have caused catastrophic, explosive bubble growth and efficient gravitational wave production. These phenomena may generate stochastic backgrounds (noise-like, cacophonic ensembles) of gravitational waves incoming from cosmological distances.

The oldest light that can be seen by electromagnetic astronomy is the Cosmic Microwave Background (CMB), which emerged less than 400,000 years after the Big Bang, when the rapidly expanding Universe became thin enough for light to propagate freely. We have no direct information from any time before then, but there is tantalizing evidence in the structure of the CMB suggesting that the dynamics of the early Universe had a profound effect on the Cosmos that we see today. Gravitational waves can propagate freely to us from that very early era, so they provide a way to probe the physics of the early Universe. Many predictions exist for what the Cosmic Gravitational-wave Background might look like, each reflecting different ideas and models for post-Big Bang physics, but any such detection would revolutionize our understanding of the Universe.

String theory, the subject of intense theoretical study as a unified framework for all particles and forces of nature, also predicts the possibility of new fundamental objects called cosmic superstrings, which are stretched to astronomical size by the cosmic expansion, and which lose energy principally through gravitational radiation, with a very broad and uniquely identifiable spectrum. LISA will be a very sensitive probe for these objects, and it may give us the first evidence of their existence. Strings may be detected as backgrounds with characteristic distribution of energy across frequencies, or as short individual bursts of gravitational radiation.