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Did BICEP2 Detect Gravitational Waves Directly or Indirectly?

30 Apr 2014, 12:35 UTC
Did BICEP2 Detect Gravitational Waves Directly or Indirectly?
(200 words excerpt, click title or image to see full post)

A few weeks ago there was (justified) hullabaloo following the release of results from the BICEP2 experiment, which (if correct as an experiment, and if correctly interpreted) may indicate the detection of gravitational waves that were generated at an extremely early stage in the universe (or at least in its current phase)… during a (still hypothetical but increasingly plausible) stage known as cosmic inflation. (Here’s my description of the history of the early universe as we currently understand it, and my cautionary tale on which parts of the history are well understood (and why) and which parts are not.)
During that wild day or two following the announcement, a number of scientists stated that this was “the first direct observation of gravitational waves”. Others, including me, emphasized that this was an “indirect observation of gravitational waves.” I’m sure many readers noticed this discrepancy. Who was right?
No one was wrong, not on this point anyway. It was a matter of perspective. Since I think some readers would be interested to understand this point, here’s the story, and you can make your own judgment.
I’ll describe

a past observation of gravitational waves that everyone agrees is indirect;
a future observation of ...

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