Scientists have probably observed gravitational waves, probably caused by the collision of huge black holes at the centers of two colliding galaxies, although there is still work to be done. The waves could have been created when the universe was created. Some of the gravitational wave detectors were earth-based later detectors, but the most recent results come from measuring astronomical signals from other galaxies. The Economist magazine says:

Most gravitational-wave detectors are interferometers. These work by splitting a beam of light in two, and sending each half down one of a pair of long, perpendicular arms. At the end of the arms the light pulses are reflected back towards the source, where they are recombined. If that journey is uninterrupted, the returning beams will cancel each other out when they are put back together. If they do not, then that suggests some disturbance—sometimes a mere seismic tremor, but occasionally a passing gravitational wave—has disturbed them on their journey….

But the latest result concerns waves with frequencies in the nanohertz range, billions of times lower still. To detect those, astronomers must rely on light pulses created by Mother Nature—specifically, by pulsars. These are collapsed, spinning stars that emit flashes of light with metronomic regularity. If a passing gravitational wave distorts a region of spacetime between the pulsar and Earth, then some pulses would arrive earlier or later than expected. Monitoring groups of pulsars can create, in effect, interferometers with arms of interstellar size.

By jwcham

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