Astroparticle Physics

6.5 Gravitational-Wave Astronomy 135a time-varying quadrupole moment. In the same way as adistortion of test masses creates gravitational waves where,e.g., the simplest non-spherical motion is one in which horizontalmasses move inside and vertical masses move apart,a gravitational wave will distort an antenna analogously bycompression in one direction and elongation in the other (seeFig. 6.62).The quadrupole character of gravitational radiationtherefore leads to an action like a tidal force: it squeezes theantenna along one axis while stretching it along the other.Due to the weakness of the gravitational force the relativeelongation of an antenna will be at most on the order ofh ≈ 10 −21 even for the most violent cosmic catastrophes.There is, however, one advantage of gravitational wavescompared to the measurement of electromagnetic radiation:Electromagnetic observables like the energy flux from astrophysicalsources are characterized by a 1/r 2 dependencedue to solid-angle reasons. By contrast, the direct observableof gravitational radiation (h) decreases with distanceonly like 1/r. h depends linearly on the second derivative ofthe quadrupole moment of the astrophysical object and it isinversely proportional to the distance r,h ∼ G ¨Qc 4 (G: Newton’s constant) .rConsequently, an improvement of the sensitivity of a gravitationaldetector by a factor of 2 increases the measurablevolume where sources of gravitational waves may reside bya factor of 8. The disadvantage of not being able to imagewith gravitational radiation goes along with the advantagethat gravitational-wave detectors have a nearly 4π steradiansensitivity over the sky.The most promising candidates as sources for gravitationalradiation are mergers of binary systems, accretingblack holes, collisions of neutron stars, or special binariesconsisting of two black holes orbiting around their commoncenter of mass (like in the radio galaxy 3C66B, which appearsto be the result of a merger of two galaxies).The suppression of noise in these antennae is the mostdifficult problem. There are stand-alone gravitational-wavedetectors, mostly in the form of optical interferometerswhere the elongation of one lever arm and the compressionof the other can be monitored by the technique of Michelsoninterferometry. These detectors can be operated at groundlevel or in space. A convincing signal of gravitational wavesFig. 6.62Oscillation modes of a sphericalantenna upon the impact of agravitational wave causing it toundergo quadrupole oscillationsattenuationof gravitational wavesnoise suppression