In 1915, Albert Einstein proposed the general theory of relativity, also known as General Relativity, to accurately describe gravity. This framework treats gravity as a phenomenon resulting from the curvature of spacetime. By accelerating masses, the curvature of spacetime is disturbed and produces gravitational waves (or ‘ripples’ in spacetime), emitting outwards from the source at the speed of light. Simply, gravitational waves squeeze and stretch anything in their path as they pass by and present everywhere. Massive objects moving at very high speeds are the most powerful sources of gravitational wave emitters and are the primary targets. For some examples, asymmetric explosions of stars, massive stars orbiting each other, and compact stars (such as black holes or neutron stars) orbiting each other, and their merges are promising gravitational wave sources. Unfortunately, detecting gravitational waves is extremely difficult due to their weak nature and thus, exceptionally sensitive instruments and techniques are required to catch them.
Scientists detected the first gravitational wave signal in 2015 using a very sensitive ground-based instrument called LIGO (Laser Interferometer Gravitational-Wave Observatory). This signal was produced by merging a pair of black holes with masses more than 30 times the mass of the Sun. The collision happened 1.3 billion years ago, and the gravitational wave reached the Earth in 2015! Since then, more than two dozen gravitational waves have been detected by the LIGO/Virgo collaboration. Not only from black holes, but scientists also detected gravitational waves produced by merging a pair of neutron stars in 2017 for the first time. LIGO/Virgo-type instruments are only sensitive to high frequency (or short period) gravitational waves, and currently detected signals cover frequencies in hertz to kilohertz. The primary targets of these instruments are binary black holes and neutron stars and their mergers. To understand gravity better, scientists need to detect more gravitational wave signals covering other frequency ranges in its spectrum.
Scientists worldwide attempt to detect ‘low frequency’ (or order of yearly periodic) gravitational waves using fast-spinning millisecond pulsars. When a large star with a mass of about ten times the mass of the Sun ends its active life cycle with a supernova explosion, and its core becomes a dead star, a so-called neutron star. Pulsars are rapidly rotating highly magnetized neutron stars that emit beams of electromagnetic radiation. While spinning these dead stars, their emission beams sweep across the sky, and we detect them as pulses separated by their rotational period. One can imagine pulsars as lighthouses! Millisecond pulsars are old neutron stars that have spin periods in milliseconds (in other words, they spin faster than our kitchen blenders!). They are among the most stable known objects in the universe, and their rotational stability is comparable to the stability of atomic clocks. The technique of analyzing the arrival time of the periodic pulses on Earth with quality observations is called pulsar timing. Due to high rotational stability, the timing measurements of millisecond pulsars can be obtained with significant accuracy. For example, the spin period of pulsar J1713+0747 is measured accurately up to 17 decimal places! (period is 0.00457013659815452 +/-0.00000000000000002 seconds) through 22 years of its high-precision timing observations. Indeed, any changes in spacetime at the pulsar and Earth locations due to gravitational wave propagation can be seen in pulsar timing data. Therefore, pulsars have been identified as excellent tools to search for gravitational waves. Interestingly, pulsar timing is sensitive to low-frequency gravitational waves – from nanohertz to microhertz – the natural frequency range of inspiralling supermassive black hole binaries. These are massive black holes with masses more than a million times the mass of our Sun, residing in centers of galaxies. Supermassive black hole binaries are expected to form through galaxy mergers in cosmic history. The addition of all signals produced from these massive binaries throughout the history of the universe results in a cosmological stochastic gravitational wave background, which can be detected using pulsars. In particular, nearby massive systems are likely to result in individual signals rising above this background level.
The gravitational wave signals can create tiny signatures in pulsar timing data, typically with an order of 100 nanoseconds in size. Therefore, ‘Pulsar Timing Arrays’ are formed to achieve this required high timing precision in gravitational wave detection. Pulsar timing arrays monitor a collection of stable millisecond pulsars, spread across the sky, using high-sensitive large radio telescopes worldwide. Globally there are three main pulsar timing arrays – NANOGrav in North America, EPTA in Europe, and PPTA in Australia – and newly formed InPTA in India, and all these collaborate as IPTA to work together with a primary goal of detecting gravitational waves. Due to the gravitational wave background, pulsar timing arrays expect to see a correlated signature in their pulsar timing data. They have been regularly monitored pulsars for more than 15 years so far and have started seeing a correlated signal in their data as expected. This signal is strong enough to exhibit in all their pulsars, and perhaps it could be the first hint of gravitational wave background. Although, this signal does not yet statistically satisfy the required quadrupole signature in the spatial correlation to claim detection. Currently, pulsar timing arrays are significantly improving their sensitivities to gravitational waves by adding more pulsars and collecting more data. Thus, scientists are now in the era of detecting low-frequency gravitational waves, and detection can be expected anytime over the coming years with the current pace and configuration. The detection will open a brand-new window in the gravitational wave spectrum, which is crucial in enhancing our understanding of gravity.
In summary, scientists are almost ready to detect low-frequency gravitational waves using highly stable dead stars, called pulsars. This groundbreaking discovery will revolutionize our insights into supermassive black holes and their binary systems, the evolution of galaxies, and simply the universe.
B. B. P. Perera et al., 2019, MNRAS, 490,4666, The International Pulsar Timing Array; second data release
B. B. P. Perera et al., 2018, MNRAS, 478, 218, Improving timing sensitivity in the microhertz frequency regime: limits from PSR J1713+0747 on gravitational waves produced by supermassive black-hole binaries
Z. Arzoumanian et al., 2020, ApJ, 905, L34, The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background