Astroparticle Physics

6.5 Gravitational-Wave Astronomy 1336.5 Gravitational-Wave Astronomy“What would physics look like withoutgravitation?”Albert EinsteinFinally, it is appropriate to mention the new field of gravitational-waveastronomy. Gravitational waves have been predictedby Einstein als early as 1916. Apart from the observationof Taylor and Hulse concerning the energy loss of abinary pulsar (PSR 1913+16) due to the emission of gravitationalwaves over a period starting from 1974 (Nobel Prize1993) there is no direct evidence of the existence of gravitationalwaves. Nobody doubts the correctness of Einstein’sprediction, especially since the results of Taylor and Hulseon the energy loss of the binary pulsar system by emission ofgravitational radiation agree with the theoretical expectationof general relativity impressibly well (to better than 0.1%).Taylor and Hulse have observed the binary system PSR1913+16 consisting of a pulsar and a neutron star over a periodof more than 20 years. The two massive objects rotatearound their common center of mass on elliptical orbits. Theradio emission from the pulsar can be used as precise clocksignal. When the pulsar and neutron star are closest together(periastron), the orbital velocities are largest and the gravitationalfield is strongest. For high velocities and in a stronggravitational field time is slowed down. This relativistic ef-energy lossby gravitational radiationchange in periastron time [s]−−−−−−0510152025301975Taylor & HulsePSR 1913+16generalrelativityfect can be checked by looking for changes in the arrival 1995time of the pulsar signal. In this massive and compact pulsar Fig. 6.61system the periastron time changes in a single day by the Observed changes in periastrontime of the binary system PSRsame amount for which the planet Mercury needs a century1913+16 over more than 20 yearsin our solar system. Space-time in the vicinity of the binary in comparison to the expectationis greatly warped.based on Einstein’s theory ofThe theory of relativity predicts that the binary system general relativity. The agreementbetween theory and observation iswill lose energy with time as the orbital rotation energy isbetter than 0.1%converted into gravitational radiation. Fig. 6.61 shows theprediction based on Einstein’s theory of general relativity incomparison to the experimental data. The excellent agreementbetween theory and experiment presents so far the best– albeit indirect – evidence for gravitational waves.The direct detection of gravitational radiation wouldopen a new window onto violent astrophysical events andit may give a clue to processes where dark matter or darkenergy is involved.However, as far as the direct observation is concerned,the situation is to a certain extent similar to neutrino physics1985evidencefor gravitational radiation