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Mar 16, 2014

Provided they are confirmed by other experiments, the BICEP observations are exciting news for cosmology and gravitational wave science. By measuring the imprint of gravitational waves on the Cosmic Microwave Background (CMB) our colleagues will have provided strong evidence for the inflation of the universe.

This measurement would illustrate the fact that through gravitational waves we can learn things about the universe we cannot learn in any other way. Its implications would go far beyond a simple proof that "Einstein was right". If confirmed, the measurement shows how gravitational waves that interact with radiation emitted by the hot plasma more than 13 billions years ago can provide unique clues about the history and evolution of the universe.

Measuring the imprint of gravitational waves on the CMB is different from the direct detection of the space-time distortions produced by gravitational waves of astrophysical origin: the waves inferred here are not passing through the Earth at the time of the measurement, but instead are observed interacting with matter long before stars and galaxies had formed. Their very long periods, comparable with the 14 billion-year age of the Universe, are making them impossible to measure directly.

This result would also be a major step on the road to gravitational wave astronomy, which will become reality as waves with much shorter periods than these are opened up to observation. Just as with electromagnetic waves, where astronomy is done with radio waves, light, X-rays, and gamma-rays, so also will gravitational wave astronomy provide rich information across its spectrum, from the long-period cosmological waves that BICEP is exploring to very short-period waves produced by black holes and neutron stars.

We are now looking forward to the first direct detection of gravitational waves with millisecond periods by the ground-based GEO600, LIGO, and Virgo detectors, expected a few years from now. In the future the LISA space mission and international pulsar timing arrays will measure gravitational waves with periods of hours to years, which are not accessible to LIGO and its partners. All these dedicated gravitational wave detectors will probe gravitational waves as they exist today, yielding spectral and sky position information. This will provide a plethora of new and unique information about gravitational wave sources and the evolution of the universe.

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