Special Topic : Gravitational Wave Astronomy Listening to the Universe with Gravitational Waves

The discovery of gravitational waves by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) on September 14, 2015 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 gravitational wave astronomy, as subsequently manifested by two more events GW151012 and GW151226 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 gravitational waves. Namely, gravitational waves exist at all frequencies in the Universe. The interpretation of all the three LIGO events as being due to coalescing binaries of black holes with the masses of 10-30M⊙ indicates that we have only seen the tip of the iceberg; many more violent events of gravitational waves in the Universe still remain to be explored. Since terrestrial interferometers like LIGO and other similar experiments are sensitive to the wavelength which roughly equals to the arms of interferometers, they can only capture gravitational wave signals from merging binary neutron stars and stellar mass black holes in the nearby Universe corresponding to gravitational waves of about 10-100 Hz in frequency. Actually, the strain of gravitational radiations produced by such merging stellar-mass systems is extremely small over the theoretically anticipated gravitational wave spectrum. Very strong signal should be expected from mergers of more massive black hole systems and even from the very early Universe, though one has to move to a much lower frequency range and employ different techniques compatible with the wavelength of gravitational waves. To detect longer gravitational waves beyond 10 4 km but in the middle and low frequency band (0.1 Hz ~ 10Hz; 100nHz ~0.1 Hz), laser interferometers should be put into Earth or solar orbits, a concept and technology that has been extensively studied and developed since the 1980s. Among many proposed projects with arm lengths ranging from 1000 km to 10 9 km in the past 30 years, progress towards this ambitious goal has been made by the LISA project, a space mission funded by the European Space Agency. The drag-free technology, the key to detect gravitational waves in space, has been successfully demonstrated by the LISA pathfinder launched

in December 2015, which can be regarded as a cornerstone for further development of space detectors for gravitational waves.Yet, it still requires two or three decades worth of effort to build an operational space observatory for detecting gravitational waves, and it is unlikely that the LISA can deliver science data even before 2035.
For even longer gravitational waves beyond the solar system, we need to consider an arm length of the galactic scale.Instead of working with conventional interferometry for short wavelengths or high/middle frequencies, we should focus on an entirely different techniquethe measurement of the arrival times of pulses from neutron stars in the Milky Way.Since their discovery in 1967, pulsars have been regarded as the most precise celestial clocks in nature.About 200 millisecond pulsars among the 2600 known population have been detected in the Galaxy, which are believed to be the ideal clocks for the purpose of detecting gravitational waves by recording their pulse time arrivals accurately and eventually obtaining the so-called timing residuals.By a further analysis of a quadrupolar angular correlation of the timing residuals for a number of pulsars on the sky, we should be able to extract the gravitational wave signature at preferential frequencies of nano Hz.Nearly one decade ago, an International Pulsar Timing Array (IPTA) consortium was thus formed to combine and coordinate eight of largest radio telescopes in the world with diameters over 60m for the detection of gravitational waves by monitoring an array of approximately 30 millisecond pulsars.The current sensitivity has already approached the edge of discovery.It is largely believed that the IPTA might be the next experiment of detecting gravitational waves.In particular, such an experiment may unlock the important clue about the coalescence of supermassive black holes with masses of 10 9 M⊙ residing in galactic centers for the first time.
After leaving the galactic scale gravitational wave observatory, we now enter into the cosmological scale with the hope of detecting the longest gravitational waves in the Universethe primordial gravitational waves from the rapid, exponential expansion of the Universe or inflation 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 such as interstellar dust and synchrotron radiation of the Milky Way.State of the art algorithms have to be developed for foreground removals and systematics control.Moreover, there are probably only four ideal sites on the Earth suited for deployment of CMB detectors including the Antarctic region, unless balloon-borne or satellite telescopes are used.BICEP2/Keck and Planck are the pioneers of the field, followed by several on-going projects such as the Simon Array CMB polarization experiment, the Ali CMB Polarization Telescope, and the balloon-borne Primordial Inflation Polarization Explorer.These next generation CMB polarization detectors may Downloaded from https://academic.oup.com/nsr/article-abstract/doi/10.1093/nsr/nwx021/3058973/Listening-to-the-Universe-with-Gravitational-Waves by guest on 16 September 2017 hopefully allow us to receive the inflationary gravitational waves, 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.
Last but not the least, it is highly expected that astronomical gravitational waves should show a transient electromagnetic and/or neutrino/cosmic ray signature due to energetic outflows in merger processing of binary black holes or neutron stars.
Identification of the electromagnetic counterparts provides valuable messages of astrophysical origin of gravitational waves and physical environment and condition of the corresponding sources.While the lack of electromagnetic counterparts to all three LIGO events pose a challenge to both current observational means and physical explanation of the gravitational wave origins, a worldwide campaign for identifying the electromagnetic counterparts to gravitational wave sources has been launched, coordinating almost all the multi-wavelength astrophysical facilities over the world.
With the rapidly increasing events and reducing errors of their localization to be probed by present and especially next generation gravitational wave observatories, along with the follow-up observations by many large aperture and fast survey speed telescopes across the entire electromagnetic band, potential counterparts to gravitational wave sources will hopefully be discovered in a few years.
Immediately after the LIGO discovery of gravitational waves, the Chinese Academy of Sciences started a 5-year project that aims to pave the way for gravitational wave astronomy in China.The project has covered (1) the study of key technology for the Taiji space mission, (2) pulsar timing with the newly constructed, largest single dish telescope, FAST, along with the participation in the future Square Kilometre Array (SKA) and IPTA, (3) the construction of the Ali CMB polarization Telescope in the Tibet Plateau towards the measurement of the primordial gravitational waves, and finally (4) 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 gravitational waves 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 gravitational wave astronomy, which is to be fully funded and coordinated by the Ministry of Science and Technology of China.