Constellations of Space Interferometers: The Key to Unraveling the Stochastic Gravitational-Wave Background
The proposed solution involves using a constellation of three or four space interferometers to map the almost perfectly homogeneous background, searching for subtle fluctuations known as anisotropies. These fluctuations hold the key to...
Every year, the universe witnesses hundreds of thousands of black hole mergers, a cosmic spectacle that releases gravitational waves in all directions. Since 2015, ground-based interferometers like LIGO, Virgo, and KAGRA have made it possible to detect these signals, although only an infinitesimal fraction of these events have been observed. Most gravitational waves remain indistinguishable, merging to form a flat, diffuse background signal referred to as the stochastic gravitational wave background (SGWB). However, new research from SISSA, published in The Astrophysical Journal, offers a promising solution to uncover the mysteries behind these elusive waves.
The proposed solution involves using a constellation of three or four space interferometers to map the almost perfectly homogeneous background, searching for subtle fluctuations known as anisotropies. These fluctuations hold the key to understanding the distribution of gravitational wave sources on the largest cosmological scale. Next-generation detectors, such as the Einstein Telescope and the Laser Interferometer Space Antenna (LISA), are expected to make a direct measurement of the gravitational wave background possible in the foreseeable future.
However, Giulia Capurri, a SISSA Ph.D. student and the study’s first author, explains that measuring these anisotropies will remain incredibly difficult due to the high level of angular resolution needed, which current and next-generation survey instruments lack. Capurri, supervised by Carlo Baccigalupi and Andrea Lapi, suggests overcoming this obstacle by employing a constellation of three or four space interferometers in solar orbit, covering a distance approximating Earth and the Sun. This increased separation between the interferometers would improve their angular resolution and ability to distinguish gravitational wave sources.
Capurri posits that such a constellation of space interferometers orbiting the Sun would enable the detection of subtle fluctuations in the gravitational background signal, allowing for the extraction of valuable information regarding the distribution of black holes, neutron stars, and other gravitational wave sources throughout the universe. Following the successful LISA project’s space mission test, two proposals for space-based interferometer constellations have emerged: the European Big Bang Observatory (BBO) and the Japanese Deci-hertz Interferometer Gravitational-wave Observatory (DECIGO).
Carlo Baccigalupi, professor of theoretical cosmology at SISSA, states that this research is among the first to provide specific predictions for the size of the stochastic background of gravitational waves using a constellation of solar orbit instruments. He adds that similar projects, with details to be published in due course, will play a critical role in developing the optimal design for future observational instruments to be built and commissioned in the coming decades.
In the age of multimessenger astronomy, which began with ground-based interferometers like LIGO and Virgo, understanding the gravitational-wave background could usher in a new era of large-scale cosmic comprehension, similar to the revelations brought about by the cosmic microwave background.
Giulia Capurri et al, Searching for Anisotropic Stochastic Gravitational-wave Backgrounds with Constellations of Space-based Interferometers, The Astrophysical Journal (2023). DOI: 10.3847/1538-4357/acaaa3