Listening to the Universe with gravitational waves
Xiangping Wu · National Science Review · 2017
The discovery of gravitational waves (GWs) by the LIGO experiment not only provides a robust test of the predictions of general relativity by Einstein, but also opens a new window to explore the mysterious Universe, which may be inaccessible solely through conventional electromagnetic radiation. Indeed, this first ever detection marks a milestone for the birth of GW astronomy, as subsequently manifested by two more events observed by the LIGO collaboration. It is generally believed that all the accelerating massive celestial objects ranging from double neutron stars to primordial cosmic inflation can produce observational GWs. That is, GWs exist at all frequencies in the Universe. The interpretation of all three LIGO events as being due to coalescing binaries of black holes with the masses of 10–30 M⊙ indicates that we have only seen the tip of the iceberg; many more violent events of GWs in the Universe still remain to be explored. Since terrestrial interferometers such as LIGO and other similar experiments are sensitive to the wavelength that roughly equals the arms of interferometers, they can only capture GW signals from merging binary neutron stars and stellar-mass black holes in the nearby Universe corresponding to GWs of about 10–100 Hz in frequency. To detect longer GWs beyond 104 km but in the middle- and low-frequency band, laser interferometers should be put into Earth or solar orbits—a concept and technology that have been extensively studied and developed since the 1980s. Among many proposed projects with arm lengths ranging from 1000 to 109 km, progress towards this ambitious goal has been made by the LISA project. The drag-free technology, the key to detect GWs in space, has been successfully demonstrated by the LISA pathfinder. Yet, it still requires two or three decades’ worth of effort to build an operational space observatory for detecting GWs. For even longer GWs beyond the solar system, we need to consider an arm length of the galactic scale. Instead of working with conventional interferometry, we should focus on an entirely different technique—the measurement of the arrival times of pulses from neutron stars in the Milky Way. About 200 millisecond pulsars among the 2600 known populations have been detected in the Galaxy, which are believed to be the ideal clocks for the purpose of detecting GWs by recording their pulse time arrivals accurately and eventually obtaining the so-called timing residuals. Nearly one decade ago, an International Pulsar Timing Array consortium was thus formed to combine and coordinate eight of largest radio telescopes in the world by monitoring an array of approximately 30 millisecond pulsars. The current sensitivity has already approached the edge of discovery. After leaving the galactic scale GW observatory, we now enter into the cosmological scale, with the hope of detecting the longest GWs in the Universe—the primordial GWs from the rapid, exponential expansion of the Universe just after the Big Bang. The best place to look for the signature is the cosmic microwave background (CMB), demonstrated by the so-called B-mode polarization with an amplitude of only 10−9 K. This tiny inflation signal imprinted on the CMB is deeply buried in foreground contamination. State-of-the-art algorithms have to be developed for foreground removals and systematics control. Several ongoing and planned CMB polarization detectors may hopefully allow us to receive the inflationary GWs, which will uncover the unique clue of the birth or the very beginning of our Universe and carry information much farther and earlier than the CMB photons. Immediately after the LIGO discovery of GWs, the Chinese Academy of Sciences started a five-year project that aims to pave the way for GW astronomy in China. The project has covered (i) the study of key technology for space mission; (ii) pulsar timing with the newly constructed, largest single-dish telescope, FAST, along with the participation in the Square Kilometre Array; (iii) the construction of the Ali CMB Polarization Telescope in the Tibet Plateau towards the measurement of the primordial GWs; and finally (iv) searching for electromagnetic counterparts with a space–ground integrated network of telescopes in China. Meanwhile, the National Natural Science Foundation of China has supported a very competing research team to explore underlying physics of GWs and theories of gravitation, complementary to the above experimental and observational projects. Yet, all of these pre-researches may eventually integrate into a long-term megaproject of GW astronomy, which is to be fully funded and coordinated by the Ministry of Science and Technology of China.