I think gravitational wave astronomy is one of the most exciting breakthroughs we’re tracking on Centauri Dreams. The detection of black hole and neutron star mergers has been a reminder of the tough elasticity of spacetime itself, its interplay with massive objects that are accelerating. Ripples in the fabric of spacetime move outward from events of stupendous energy, which could include neutron star mergers with black holes or other neutron stars. Earth-based observing projects like LIGO (Laser Interferometer Gravitational-Wave Observatory), the European Virgo and KAGRA (Kamioka Gravitational Wave Detector) in Japan continue to track such mergers.
But there is another aspect of gravitational wave work that I’m only now becoming familiar with. It’s background noise. Just as ham radio operators deal with QRN, which is the natural hum and crackle of thunderstorms and solar events, so the gravitational wave astronomer has to filter out what is being called the astrophysical gravitational wave background, or AGWB, as the inevitable acronym would have it. Astronomers also have to consider GW signals associated with events in the early universe, stochastic background ‘static’ that could have originated, for example, in cosmic inflation or the creation of cosmic strings.
The AGWB is the background noise of countless astrophysical events, a ‘hum’ from all sources emitting gravitational waves in the universe. Recent work has been showing that this collective signal, primarily from black hole and binary neutron star mergers, is detectable by the technologies we’ll be deploying in the 2030s in the European Space Agency’s Laser Interferometer Space Antenna (LISA) mission. And it’s clear that for gravitational wave astronomy to proceed, we need to remove the AGWB to uncover underlying signals.
New work now makes the case that, surprisingly, we also have to reckon with the background noise of binary white dwarfs, although I see in the literature that scientists were delving into this as early as 2001 (citation below). In two recent papers, Dutch astronomers have developed models demonstrating that the background noise of white dwarfs would actually be stronger than that produced by black holes. Gijs Nelemans (Radboud University (Nijmegen, the Netherlands), who is working with the software and guidance mechanisms for the LISA mission, is a co-author on two papers on the subject. He sees white dwarf background noise as a way of studying stellar evolution on a galactic scale:
“With telescopes you can only study white dwarfs in our own Milky Way, but with LISA we can listen to white dwarfs from other galaxies. Moreover, in addition to the background noise of black holes and the noise of white dwarfs, perhaps other exotic processes from the early universe can be detected.”
Image: Dutch astronomer Gijs Nelemans. Credit: TechGelderland.
Nelemans has been developing the models described in the two recent papers with students Seppe Staelens and Sophie Hofman. Their work is significant given that until now, the LISA mission had not factored in a noisy white dwarf background problem. In a paper published in Astronomy & Astrophysics, the authors point out:
Given the amplitude of the WD component… it is expected that it can be very well measured by LISA. Furthermore, the relative amplitudes show that, if LISA detects an AGWB signal in the mHz regime, it is likely dominated by the WDs. This means that it is likely hard to make statements about the BH (and NS) population based on a measurement of the AGWB unless there is a way to disentangle the two, or to detect the high-frequency component of the AGWB above 40 mHz.
And in terms of the study of white dwarfs, the paper adds:
This offers an opportunity to study the WD binary population to much larger distances, while hampering the detection of the BH AGWB with missions such as LISA. The WD signal reaches a peak around 10 mHz and at higher frequencies the BH AGWB will become the dominant signal. The detectability of this transition by LISA and other mHz missions ought to be studied in detail.
Image: The LISA mission consists of a constellation of three identical spacecraft, flying in formation. They will orbit the Sun trailing the Earth, forming an equilateral triangle in space. Each side of the triangle will be 2.5 million km long (more than six times the Earth-Moon distance), and the spacecraft will exchange laser beams over this distance. This illustration shows two black holes merging and creating ripples in the fabric of spacetime. Some galaxies are visible in the background. In the foreground, the shape of a triangle is traced by shining red lines. It is meant to represent the position of the three LISA spacecraft and the laser beams that will travel between them. Credit: ESA.
This is indeed a unique kind of probe, because we’re talking about studying white dwarf evolution at high redshift in ways beyond the range of optical astronomy. Realize that only a small selection of gravitational wave sources can be detected with our current technologies. Millions of binaries in the Milky Way will simply merge into the stochastic foreground, a signal that is highly anisotropic (i.e., not uniform in all directions) while unresolved binary sources outside the galaxy produce a background signal that is profoundly isotropic, one that “encodes the combined information about the different source populations,” to quote the Hofman & Nelemans paper.
So we learn that filtering out white dwarf background mergers will be a major part of LISA’s investigations, but that the WD background is also a source of new information. LISA is to be the first dedicated space-based gravitational wave detector, involving three spacecraft in an equilateral triangle 2.5 million kilometers long in a heliocentric orbit. The European Space Agency hopes to launch LISA in 2035 on an Ariane 6.
The papers are Hofman & Nelemans, “On the uncertainty of the white dwarf astrophysical gravitational wave background,” accepted at Astronomy & Astrophysics (preprint); and Staelens & Nelemans, “Likelihood of white dwarf binaries to dominate the astrophysical gravitational wave background in the mHz band,” Astronomy & Astrophysics Vol. 683, A139 (March 2024). Full text. The 2001 paper is “Low-frequency gravitational waves from cosmological compact binaries,” Monthly Notices of the Royal Astronomical Society Vol. 324, Issue 4 (July 2001), pp. 797-810 (abstract).
