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Gravitational Waves That Are 'Sounds of the Universe'
Einstein wrote about them, and we're still looking for them -- gravitational waves, which are small ripples in the fabric of space-time, that many consider to be the sounds of our universe. Just as sound complements vision in our daily life, gravitational waves will complement our view of the universe taken by standard telescopes.

Albert Einstein predicted gravitational waves in 1918. Today, almost 100 years later, advanced gravitational wave detectors are being constructed in the US, Europe, Japan and Australia to search for them.

While any motion produces gravitational waves, a signal loud enough to be detected requires the motion of huge masses at extreme velocities. The prime candidate sources are mergers of two neutron stars: two bodies, each with a mass comparable to the mass of our sun, spiraling around each other and merging at a velocity close to the speed of light.

Such events are rare, and take place once per hundreds of thousands of years per galaxy. Hence, to detect a signal within our lifetime the detectors must be sensitive enough to detect signals out to distances of a billion light years away from Earth. This poses an immense technological challenge. At such distances, the gravitational waves signal would sound like a faint knock on our door when a TV set is turned on and a phone rings at the same time.

Competing noise sources are numerous, ranging from seismic noise produce by tiny quakes or even a distant ocean wave. How can we know that we have detected a gravitational wave from space rather than a falling tree or a rambling truck?

Therefore, astronomers have been looking for years for a potential electromagnetic light signal that would accompany or follow the gravitational waves. This signal would allow us to "look through the peephole" after hearing the faint knock on the door, and verify that indeed "someone" is there. In their new article just published in Nature, Prof. Tsvi Piran, Schwarzmann University Professor at the Hebrew University of Jerusalem, and Dr. Ehud Nakar from Tel Aviv University describe having found just that.

They noticed that surrounding interstellar material would slow debris ejected at velocities close to the speed of light during the merger of two neutron stars. Heat generated during this process would be radiated away as radio waves. The resulting strong radio flare would last a few months and would be detectable with current radio telescopes from a billion light years away.

Search after such a radio signal would certainly take place following a future detection, or even a tentative detection of gravitational waves. However, even before the advanced gravitational wave detectors become operational, as expected in 2015, radio astronomers are geared to looking for these unique flares.

Nakar and Piran point out in their article that an unidentified radio transient observed in 1987 by Bower et al. has all the characteristics of such a radio flare and may in fact have been the first direct detection of a neutron star binary merger in this way.

Dr. Nakar's research was supported by an International Reintegration Grant from the European Union and a grant from the Israeli Science Foundation and an Alon Fellowship. Prof. Piran's research was supported by an Advanced European Research Council grant and by the High Energy Astrophysics Center of the Israeli Science Foundation.


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