LISA will be able to detect a gravitational-wave background emitted at the beginning of the universe. This will help us to probe the universe just afte the Big Bang.
Gravitational waves are the next messengers to probe the very early Universe, after the cosmic microwave background. LISA is sensitive to gravitational waves that may have been produced when the early Universe plasma cooled below a billion billion Kelvin at roughly a billionth-billionth of a second after the big bang. Such energies (of hundreds of TeV) are inaccessible to current laboratory experiments, but may be within reach of far future particle colliders.
LISA will be able to gather information on the state of the Universe at much earlier epochs than those directly probed by any other cosmological observation, thereby helping us on our quest to understand the earliest moments of the Universe.