Clues to the beginning of time have been found in the relic heat, the cosmic microwave background (CMB), from the Big Bang. The CMB, detected as microwaves, has been traveling to us since the Universe was 300,000 years old——before stars or galaxies existed. And, just as we have observed the CMB, we should also be able to observe the cosmic background of gravitational waves. These waves, which are essentially unaffected by intervening matter as they travel across space-time, should allow us to probe the Universe at much earlier times than the CMB——from the first second of the Universe onwards.

Timeline of the cosmic background of gravitational waves.