Figure 2: Cosmic evolution
The universe has undergone several dramatic changes in its 13.7 billion-year history, although our knowledge of the early universe contains some gaps. The cosmic microwave background reveals the intensity and polarization of primordial light as it was 380,000 years after the Big Bang when the universe became transparent to light. We therefore cannot use electromagnetic radiation to directly study the universe before this time, although fluctuations in the temperature and polarization of the microwave background on very large scales do preserve much earlier events that took place during cosmic inflation. Gravitational waves, on the other hand, propagate directly to our detectors from the very beginning of time itself, carrying information about cosmic events on all scales throughout cosmic history. Source: NASA.