Congratulations to BICEP!
March 17, 2014
These observations demonstrate that gravitational waves reveal aspects of the universe that we cannot learn about in any other way. For example gravitational waves provide unique information about the properties of the universe soon after the big bang.
Measuring the imprint of gravitational waves on the CMB is different from the direct detection of gravitational waves. The waves inferred here are not passing by Earth today. Instead, they left traces of their presence in the radiation and matter present in the universe long before stars and galaxies had formed. Their very long periods, comparable with the 14 billion-year age of the Universe, make them impossible to measure directly.
This demonstrates the promise of gravitational wave astronomy, which is working to directly observe waves of much shorter period. This situation is just like electromagnetic astronomy, in which radio waves, light, X-rays, and gamma rays provide complementary information. The spectrum of gravitational waves ranges from long-period cosmological waves, which leave imprints on the CBR, to very short-period waves produced by collisions of black holes and neutron stars.
In the coming years, we expect to make the first direct detection of millisecond-period gravitational waves with ground-based detectors (LIGO, Virgo and GEO). In the future the eLISA space mission and International Pulsar Timing Arrays should also detect gravitational waves with much longer periods. These dedicated instruments will probe contemporary gravitational waves, yielding spectral and sky position data, and a plethora of new and unique information about our universe and its contents.