This figure highlights some events in the history of the Universe as it evolved from the Big Bang.
The early Universe was so hot that atoms could not form, and the strong and weak nuclear forces were not distinguished from electromagnetism (the so-called Grand-Unified Theory era). As the Universe cooled, the forces separated. The Universe also probably underwent a stage of exponential expansion, called Inflation, that made local bubbles of fairly uniform density, like the visible part of the Universe revealed by telescopes.
In the hot, early Universe, light could not propagate freely. Not until the universe cooled enough that protons and electrons formed atoms (called Recombination) could light travel and show images of local structure. Telescopes in the electromagnetic spectrum (radio, infrared, visible, X-ray, gamma-ray, etc.) can never see back into the hot, early Universe.
Gravitational waves do not interact electromagnetically and thus could propagate freely in the early Universe. LISA may see gravitational waves from the early Universe as a background, analogous to the 3 degree radiation background imaged by COBE but much earlier in time. This will reveal new information on the origin of structure in the Universe and the physical processes of extreme states of matter.